The paper is designed to give a clearer perspective of the Liquefaction Phenomenon including the causative conditions that will trigger liquefaction as well as the soil mechanics principles involved.
An understanding of the Geotechnical environment that could be susceptible to liquefaction including the groundwater influence, granulometry including fines content, Plasticity of the fines (LL & PL) and the triggering Earthquake intensity are discussed.
Much of the work have been gleaned from state of the art papers on soil liquefaction such as the “Queen Mary Paper” by Seed et al and references from the USGS website as well as the Authors’ own experiences in evaluation of liquefiable ground conditions have been included in this paper.
Significant advances in the body of knowledge and state of practice in liquefaction evaluation have been published in numerous literature and technical papers. This paper in effect is a compilation and summary of the current State of Practice in liquefaction evaluation and mitigation.
Essentially this Paper is a Literature Review of Current knowledge about Liquefaction. The figures and entries most of the time have been copied verbatim and therefore major credit is due to the sources listed in the references and the authors acknowledge this.
1.0 Introduction 3]
Liquefaction is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading. Liquefaction and related phenomena have been responsible for tremendous amounts of damage in historical earthquakes around the world.
Liquefaction occurs in saturated soils, that is, soils in which the space between individual particles is completely filled with water. This water exerts a pressure on the soil particles that influences how tightly the particles themselves are pressed together. Prior to an earthquake, the water pressure is relatively low. However, earthquake shaking can cause the water pressure to increase to the point where the soil particles can readily move with respect to each other.
Liquefaction has been observed in earthquakes for many years. In fact, written records dating back hundreds and even thousands of years describe earthquake effects that are now known to be associated with liquefaction. Nevertheless, liquefaction has been so widespread in a number of recent earthquakes that it is often associated with them.
Because liquefaction only occurs in saturated soil, its effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans. The effects of liquefaction may include major sliding of soil toward the body slumping and of water.
Earthquake shaking often triggers an increase in water pressure, but construction related activities such as blasting and vibratory pile driving can also cause an increase in water pressure.
When liquefaction occurs, the strength of the soil decreases and, the ability of a soil deposit to support foundations for buildings and bridges are reduced as seen in the photo of the overturned apartment complex buildings in Niigata in 1964.
2.0 The Liquefaction Phenomenon
To understand liquefaction, it is important to recognize the conditions that exist in a soil deposit before an earthquake. A soil deposit consists of an assemblage of individual soil particles.
If we look closely at these particles, we can see that each particle is in contact with a number of neighboring particles. The weight of the overlying soil particles produce contact forces between the particles – these forces hold individual particles in place and give the soil its strength.
To understand liquefaction, it is important to recognize the conditions that exist in a soil deposit before an earthquake. A soil deposit consists of an assemblage of individual soil particles.
If we look closely at these particles, we can see that each particle is in contact with a number of neighboring particles. The weight of the overlying soil particles produce contact forces between the particles – these forces hold individual particles in place and give the soil its strength.
However, when ground shaking occurs, these delicate grain to grain contacts are disturbed and the soils grains are dislodged.
This causes a momentary loss of support which transfers the vertical stresses to the porewater. The pore water in turn further increases in pore pressure thus further buoying up the individual soil grains.
Since water has practically no shear strength, collapse of the support happens and thus all superimposed structures and the soil itself slumps due to gravity.
Much of the previous knowledge of liquefaction attributes this phenomenon exclusively to Clean Sands. However, current State of the art have established that even fine grained soils and coarser materials such as Gravels will liquefy given the right conditions and the right earthquake characteristics. This paper seeks to update our current understanding based on the current State of Practice as gathered from various literatures.
3.0 Types of Liquefaction Related Phenomena
There are several types of liquefaction related Phenomena as Follows (Ref 1.0)
Flow Liquefaction– Flow liquefaction can occur when the shear stress required for static equilibrium of a soil mass (the static shear stress) is greater than the shear strength of the soil when it liquefies. Once triggered, the large deformations produced by flow liquefaction are actually driven by static shear stresses. Flow liquefaction produces the most dramatic Effects.
Flow Liquefaction failures are characterized by the sudden nature of their origin, the speed with which they develop and the large distances over which the liquefied materials often move.
Cyclic Mobility– Cyclic mobility in contrast to Flow Liquefaction occurs when the static shear stress is less than the shear strength of the liquefied soil. It is basically driven by both Cyclic and static shear stresses. The deformations produced by cyclic mobility failures develop incrementally during earthquake shaking. A special case of cyclic mobility is level ground liquefaction.
Because horizontal shear stresses that could drive lateral deformations do not exist, level ground liquefaction can produce large chaotic movements, known as ground oscillation during earthquake shaking but produces little lateral soil movement.
Level Ground failures are caused by the upward flow of water when seismically induced excess pore pressures dissipate. Excessive vertical settlements and consequent flooding of low lying land and the development of sand boils are characteristic of level ground liquefaction.
4.0 Liquefaction Susceptibility 2]
Liquefaction Susceptibility is determined by the grain size, fines content (LL), the presence of groundwater, the static state of stress as well as the characteristics (amplitude and length of time of propagation) of the triggering Earthquake Magnitude that could induce adequate ground shaking.
The following are the criteria for identifying possible liquefaction susceptibility:
4.1 Historical Criteria
Soils that have liquefied in the past can liquefy and liquefaction can recur in these same soils when soil and groundwater conditions have not changed. Thus, liquefaction case histories in a particular site can be used to identify specific sites or general areas that are susceptible in future earthquakes.
4.1 Compositional Criteria
Liquefaction susceptibility is influenced by the compositional characteristics that influence volume change behavior. Compositional characteristics associated with high volume change potential tend to be associated with high liquefaction susceptibility. These characteristics include particle size, shape and gradation and % fines.
For many years, only sands were thought to liquefy. Finer grained soils were considered incapable of generating the high pore pressures commonly associated with liquefaction and coarser grained soils were considered too permeable to sustain any generated pore pressure long enough for liquefaction to develop.
However, present day experience on recent liquefaction events, suggest otherwise. More recently, the bounds on the gradation criteria for liquefaction susceptibility have broadened.
The most important work was done by a team headed by R.B. Seed (Ref 2.0) in their seminal paper delivered at the HMS Queen Mary, on April 30, 2003.
Although previous work have been done by the Chinese in 1979, the Queen Mary Paper further moved it several notches to reflect current recorded liquefaction events.
State Criteria
Even if the preceding criteria are all met, it still may or may not be susceptible to liquefaction. Liquefaction susceptibility also depends on initial state of the soil (i.e. its stress and density characteristics at the time of the earthquake).
Since the tendency to generate excess pore pressure of a particular soil is strongly influenced by density and initial stress conditions, liquefaction susceptibility depends strongly on the initial state of the soil.
5.0 Initiation of Liquefaction
Procedures to determine the initiation of liquefaction have now been updated beyond the understanding before the publication of the Queen Mary Paper.
5.1 Cyclic Stress Approach
Research on the on the advances of present day knowledge on Liquefaction was started by H.B. Seed and continued and refined by the son, R. B.
This general approach came to be known as the “Cyclic Stress Approach” as expounded by Seed, the elder.
Seed, initially leading to the determination of the loading conditions that could trigger liquefaction. This loading was described in terms of cyclic shear stresses and liquefaction potential was evaluated on the basis of the amplitude and the number of cycles of earthquake induced shear stresses.
The improvements came in a more reliable prediction of the non-linear mass participation factor rd as well as the reduction in the peak cyclic stress ratio (CSR).
This CSR has since been modified by R. B.Seed et al as follows:
Recommended NOMOGRAPH for Probabilistic SPT based Liquefaction Triggering Correlation for Clean Sands 3
6.0 Effects of Liquefaction
Liquefaction can induce landslides or collapse of structures, including horizontal infrastructures.
Sand boils can be induced when the excess pore water pressure dissipates by exiting into the ground surface bringing with it fine sand particles much like an erupting volcano.
Ground motion characteristics are also altered after liquefaction when originally stiff soils are altered by positive excess pore pressures. A liquefiable soil that is relatively stiff before may be much softer at the end of liquefaction. Thus, the amplitude and frequency of the soils may change considerably.
This can cause increase in Ground motion and can produce large displacements.
Damage to horizontal and vertical structures can occur depending on the type of liquefaction whether Cyclic Mobility or Flow liquefaction.
Dams and slopes are very vulnerable to flow liquefaction because the static stress is highly influenced by gravity forces and large lateral spreading occurs.
Cyclic mobility on the other hand occurs mostly on level ground with very little or no lateral spreading. The main effects are on vertical structures which can settle significantly or tilt and collapse when loads are imbalanced or where the soil stratification is uneven.
7.0 Liquefaction Mitigation Methods
A lot of mitigation techniques are currently available which have been used as ground improvement methods to improve strength or reduce deformations. While not covered in this paper, the methods are enumerated for Brevity as this may not be covered in this Paper.
The author’s hope that a clearer understanding of the liquefaction phenomenon has been presented to the local Engineering community. Most of the contents of this paper have been directly extracted from the listed references as well as the Author’s own experiences in the evaluation of liquefaction problems.
It is suggested that further readings by the Professional are needed in order to achieve a full understanding of the liquefaction phenomenon. This is something which cannot be achieved within the limitations of time and space for this paper.
References
Kramer, S. “Geotechnical Earthquake Engineering “Prentice Hall, New Jersey 1995.
Seed, et al. “Recent Advances in Liquefaction Engineering: A Unified and Consistent Framework” 26th annual ASCE Los Angeles Geotechnical Spring Convention.
University of Washington Liquefaction Website http://www.ce.washington.edu/~liquefaction/html/main.html
Lade et al “Physics and Mechanics of Soil Liquefaction” A.A. Balkema, Rotterdam 1998.
1 Emilio M. Morales, MSCE took his Master of Science in Civil Engineering at Carnegie-Mellon University, Pittsburgh, PA. USA in 1980. He was employed as a Project Geotechnical Engineer and Software Engineer at D’Appolonia Consulting Engineers. Currently, he is the Principal of EM²A Partners & Co. and formerly elected as President for IGS Philippines, Chairman for the PICE & ASEP Specialty Committee. He is a Fellow of ASCE, PICE & ASEP.
2 Mark K. Morales, MSc took his Master of Science in Civil Engineering Major in Earthquake Geotechnical Engineering at University of California – Berkeley, USA in 2004. He is the Technical Manager of Philippine Geoanalytics, Inc. and CEO of PGA Earth Structure Solutions Inc. (PGAESS) a specialty foundation and Geosynthetics and slope protection Organization.
3 Probabilistic Approaches taking into account various acceleration values and Fines content have been evolved in the Virginia Tech Spreadsheet implementation by Gutierrez et al.
4 In the case of Rammed Aggregate Piers ®, the drainage is greatly facilitated by prestressing and lateral compaction due to the method of installation. Thus, time to U 90 is greatly speeded up, sometimes to weeks instead of months.
Earthworks and foundations invariably are part and parcel of any Civil Engineering Project. A significant portion of the overall project cost is often spent on Foundations Systems and Earthwork Compaction. Particularly on sites with poor or marginal soils, there is a need to look into Ground Improvement in order to provide for economical foundation or earthworks. Ground improvement can be in many forms, such as:
Mechanical
Chemical (Lime, Cement, Stabilization, etc.)
Electrical
Thermal
Hydraulic (dewatering, PVD, etc.)
This paper will focus on Mechanical Ground Improvement because it is often the most misunderstood and the most often taken for granted. In the case of mechanical stabilization, there are three zones of application namely:
Near Surface Ground Improvement by Mechanical Compaction (Compactors)
Deep Ground Improvement (Stone Columns & Vibroflotation)
This paper will try to discuss the foregoing zones of application and how applicable ground improvement procedures could be fully appreciated through an intimate understanding of Soil Mechanics Principles particularly as it pertains to Soil Particulate Behavior.
2.1 The Mechanics of Soils
Mechanical Ground Improvement most of the time can be a cost effective foundation solution if the fundamentals of soil mechanics and soil behavior are known to the user. The knowledge gained from this paper can be put to good use when Mechanical Ground Improvement is comtemplated.
For this paper, we shall only limit ourselves to a clear understanding of Particulate Mechanics or the behavior of soils as discrete particles when subjected to ground improvement.
2.2 Soil as a Particulate Material
Under very high magnification even a piece of solid mass of clay appears as an assemblage of particles with some orientation. This orientation surprisingly can be altered by mechanical reworking of the clay, addition of, or removal of moisture or by altering the chemical make-up of the porewater. Under normal conditions, the assemblage includes water and air. The water is either captured or adsorbed water or free water. The process of compaction is nothing but the expulsion of air and/or water (reduction of voids in the soil). Soil can either be:
Coarse Grained (Sand) or Cohesionless
Fine Grained (clay) or Cohesive
The distinction between the two are somewhat obscured by their combinations that could be found in nature. In their unadulterated states, the differences become readily apparent.
2.3 Soil Shear Strength
Particulate materials derive their strength from friction or intergranular contact and/or from bonding forces or cohesion as we know it. These bonding forces and friction prevent the particles from sliding.
The important property that we have to deal with is the soils’ Shear Strength since most of the loading that the soil is subjected to causes the individual soil particles to slide or “shear” one against the other because of their particulate character. The shear strength is either derived from electrical and chemical forces of attraction (cohesion) and repulsion as in clays or by simple grain to grain contact and friction as in Pure Granular Materials. Since shear strength depends on the sliding resistance of the individual soil particles. Therefore, the more compact the soil becomes, the higher the shear strength. To improve strength mechanically is to lessen the interparticle distances by the expulsion of air and/or water.
This leads us to one of the Fundamental Principles in Earth Compaction:
“Increasing Density (Strength) is achieved by decreasing the soil interparticle distance through the expulsion of air or water or both.”
Reduction of interparticle distances would sometimes require addition of more water into the soil in order to dislodge more water. This statement appears to be confusing but its proof reiterates the importance of the understanding of soil particulate behavior in the solution of Earthwork Problems.
2.4 Microstructure
2.4.1 Clay Microstructure
Shown below is a microscopic view of a sample of cohesive or fine grained clay soil .
The clay is composed of submicroscopic platelets surrounded by Electrical charges, a closely held layer of adsorbed water and an outer layer of loosely held water. The minute interparticle distances, are governed not only by the particle orientation but also by the Electrical forces of attraction and the thickness of the adsorbed and free water. It would take a very high input of energy in order to dislodge or remove the adsorbed layer. The loosely held water can be removed in the field by sample air drying or wind rowing. When free water is removed, densification can be attained.
2.4.2 Sand Particles
At a certain Moisture Content Water comes in-between and holds the grains apart by surface tension. When grains of dry sand are gently deposited in a container, they fall into place in a precarious grain to grain contact. A jarring motion imparted on the container causes the grains of sand to assume a denser packing. Addition of water causes dry sand to swell or increase in bulk while saturation with water causes the sand to be compacted into a denser state. This has been known to us as “Hydrocompaction”. Perhaps only the mechanism behind it is not well understood.
3. NEAR SURFACE COMPACTION
3.1 Earth Compaction
Majority of efforts in Mechanical Ground Improvement is focused on general earthwork compaction. Near surface compaction applies to compaction of Fills or subgrade materials by Mechanical Procedures. In this section, we shall deal with compaction characteristics of soils and the applicable methods best suited for each type of soil. We begin with the all too familiar moisture density relationship known as the “Laboratory Proctor Test” for a clay soils.
Familiarity with this simple bell shaped curve and its universal acceptance as the “characteristic” compaction curve cause most of the problems we encounter today in Earthworks compaction. Too often, it has not been realized that this is not the only shape a laboratory Proctor curve can assume and that grain size and moisture play a great part in influencing the shape of the compaction curve.
This bell shaped curve is only applicable for fine grained soils or soils with significant plasticity as to make it perform as a clay like soil.
At the start of the test when the soil is relatively dry, the soil assumes a flocculated structure “A”. Additional mechanical reworking and increasing amounts of water and expulsion of air and closing of the voids tend to produce a semi flocculated structure “B” with increasing density until a peak is attained. This peak is the maximum density that could be attained by that type of soil in the laboratory. The Degree of Compaction of the same soil in the field is expressed in terms of “Relative Compaction” which a percentage of the maximum dry density obtained in the laboratory test.
The moisture content corresponding to this maximum density is known as the Optimum Moisture Content “OMC”. Further compaction and additional water beyond this point results in decrease in density with increasing amounts of water. The soil platelets begin to be oriented and aligned and the interparticle distances tend to widen as more and more water is captured.
The “S” Curve
We now look at the Moisture Density relationship curve for a coarse grained sand with very little or no fines.
The curve is shaped like an “S”.
Since the individual grains are relatively very very large compared to the clay platelets, surface forces play very little influence on the behavior except at a certain moisture content range.
The Moisture Density curve above indicates two density Peaks “P1” and “P2” where density is high. The first Peak P1 occurs when the soil is very very dry (MC–>O) and the other Peak P2 at almost saturation conditions. Between these two Peaks is a valley where density is lowest.
The reduction in density in this valley as defined by a moisture content range is known as the “Bulking Range” for this particular sand.
This reduced density is caused by surface tension forces of the water surrounding the individual grains which tend to drive the adjacent grains farther apart, causing loss of interparticle contact and collapse in the density from the previous high.
Progressive addition of water beyond the bulking range collapses the surface tension and the additional hammer impacts increases the density again to the second Peak “P2”. The laboratory curve shows a downhill movement in density with increasing moisture content beyond saturation levels.
The real field curve shown by the dotted line suggests otherwise. In the laboratory compaction procedure, the water can not drain within the steel compaction mold and thus the soil becomes a soupy mush. In the field, additional water is continually drained and the Peak density is maintained. This curve clearly shows the fallacy of specifying Proctor Compaction Procedures and an OMC for clean coarse grained soils, because definitely, the soils are insensitive to moisture content except at the very dry and very saturated conditions. Unlike clay soils which follow a typical bell shaped curve, clean coarse grained soils exhibit a typical “S” shaped curve with the Peaks P1 & P2 clearly distinguishable.
Peaks P1 & P2 may sometimes be equal but this is more of an exception than the rule and their relative maximum values could shift either way depending on the type of soil.
This characteristic granular soil behavior has been recognized by the ASTM and standardized into two standards ASTM D-4253
“Max. Index Density of Soils using a Vibratory Table” ASTM D-4254 “Min. Index Density of Soils and Calculation of Relative Density” to arrive at a minimum and maximum density. These values are then used to compute the Relative Density DR once the Field Density is obtained. Compaction is specified not in terms of % of Proctor MDD but rather as Relative Density DR .
Recognition of the two characteristic compaction curves (The “Bell” and the “S”) leads us to the realization that clays and sands behave very much differently when compacted and require different approaches and solutions.
We also know now that the behavior of soils can be tempered to suit our requirements. Thus, we are led to the following conclusions:
There is not one but two General characteristic curves for soils depending on their granulometry.
The concept of Optimum Moisture Content generally does not apply to Clean Granular Soils and therefore the Proctor Standard is inappropriate or could lead to problems in the Field. Clean sands either have to be compacte very very dry or very very wet in order to achieve the maximum density.
The microstructure of the soil needs to be considered in the selection of the right compaction equipment.
For Fine Grained Soils, although density is the same for corresponding points left and right of the OMC, the performance and behavior of the soil are different due to the alteration in the microstructure arrangement.
Beyond the maximum density, additional compaction energy would be detrimental to both clays and sands as breakdown can occur causing a decrease in density. Therefore, use no more than what is necessary to attain good compaction.
For intermediate soils, it would be necessary to determine in the laboratory the characteristic behavior from zero MC to saturation levels.
In case the laboratory curve is not clearly defined or when there are doubts as to the behavior in the field, a field compaction trial would be required.
3.2 Compaction Equipment
Knowing the behavioral characteristics of soils we now look at the means to achieve compaction in the Field.
However, try to remember the fundamental response of the two general types of soils to compactive effort:
Loose Clean Granular Soils because of their precarious grain to grain contact are best compacted by causing a jarring motion such as what a vibratory roller would impart.
Fine Grained Soils on the other hand respond better to a kneading type of compactive effort such as that imparted by sheepsfoot rollers and pneumatic type rollers as these tend to reorient the platelets.
We now look at the Chart below to determine the range of applicability of various compaction equipment:
Figure 8.10 Compaction equipment selection guide. This chart contains a range of material mixtures from 100 percent clay to 100 percent sand, plus a rock zone. Each roller type has been positioned in what is considered to be its most effective and economical zone of application. However, it is not uncommon to find them working out of their zones. Exact positioning of the zones can vary with differing mateial conditions.
We can therefore see the effective range for various compaction equipment under differing soil conditions and we recognize right away that this has something to do with the grain size (clay to rock).
3.3 Applications of Knowledge Gained
It is not only enough to know how to select the proper compactor for the job. It is also important to use this in conjunction with our knowledge of Soil Mechanics principles which we now apply:
3.3.1 Sands and Clean Coarse Grained Soils
Vibratory compaction works best. However, we should aid this by liberal application of water immediately ahead of the vibratory compactor. Coarse grained soils compact best at the very very wet condition “P2” or at very very dry condition “P1”. Water serves as a lubricator but its total absence also prevents capillary forces from impeding the rearrangement of soil grains into a denser packing. Speed of the vibratory compactor is also important and needs to be controlled to about 2 to 4 KPH. Vibratory frequency is essential and should nearly approach the natural frequency of the soils. Standard frequencies are in the range of 30 to 40 cps.
The static weight of the vibratory roller is also important since the dynamic force exerted on the soil is a function of the static weight. The wheels of the roller are either ballasted by water, sand or even slag in order to increase the static mass.
Because the vibratory roller causes an “advancing wave” on the top of the lift being compacted, Field Density Testing in order to be fair has to be done below this disturbed layer. It will normally be required to scrape the top 50 cms before seating the Field Density Plate in order to be able to test the actual condition of the specific lift.
Field Density testing using the Sand Cone Method (ASTM D-1556) requires extra care since the means to measure the volume is calibrated clean sand. Any jarring motion results in increased sand intake of the test hole resulting in bigger hole volume computed than the actual resulting in low compacted densities. This condition can also be caused by compaction equipment being allowed to operate very near a Field Density test in progress.
For clean coarse grained sands, moisture is irrelevant except for the total absence of it or its presence at saturation levels.
Thus, there is no such thing as an Optimum Moisture Content for clean granular soils and Proctor criteria is entirely inapplicable in this context.
3.3.2 Clays and Intermediate Soils
For Fine Grained Materials and intermediate soil types possessing significant plastic fines, sheepsfoot rollers or pneumatic tired rollers are best. The kneading action allows reorientation of the grain and allows expulsion of entrapped air. The sheepsfoot was modelled really after the shape of a sheep’s foot perhaps based on the observations of Mr. Mc Adam. The tendency of the sheepsfoot is to walk-up by progressively compacting or densifying the lowermost layers first and walking upwards. Thus we see that topmost layers are slightly less compacted and therefore need to be bypassed when conducting a Field Density Test.
In contrast with clean coarse grained soils, compaction moisture content and the Proctor criteria are fully applicable and the control of moisture during compaction is a crucial factor in the attainment of proper compaction. Static weight is also important as it increases penetration of the sheepsfoot and increases the force pressing the platelets together. Speed is not so critical as it is the kneading action and coverage that is important. Vibratory motion is not necessary and is in fact harmful as it can cause build up in pore pressures in the soil and also cause shear cracking.
4. INTERMEDIATE GROUND IMPROVEMENT
Intermediate Ground Improvement comes in many forms. However, the immediate objective is to produce a shallow or intermediate crust of soil that would be stiffer or denser than the original soil condition. Other mechanisms are involved depending on the procedure.
This crust can then be depended upon to carry most of the load and dissipate the contact pressure to the softer soils beneath it.
4.1 Overexcavation and Replacement
This is the most common but perhaps the most expensive except for relatively very shallow depths. The procedure consist of excavating the very poor soil and backfilling the excavation by the original soil or imported select Fill and compacting the replacement with fill in lifts. The compacted Fill then becomes an Engineered Fill whose compaction has been carefully monitored and the resulting densities are carefully checked by performing Field Density Tests (FDT). The method may prove to be difficult in the presence of a shallow water table or during the rainy season where the excavation can become flooded. Creation of a controlled Engineered Fill or Backfill would entirely or partly eliminate the underlying poor soils and provide a stiffened natural mat which would reduce the contact stresses on the soils underneath it.
4.2 Dynamic Compaction or Heavy Tamping
Dynamic Compaction or Heavy Tamping is a procedure for compacting soils by dropping a heavy weight from a considerable height. The technique was invented by a Frenchman named Louis Menard. At that time, he called it “Dynamic Consolidation”, thinking that the process would be equally applicable to clays as well as granular materials. However, he would be proven wrong and the procedure is now concentrated on ground improvement of granular materials.
A very high impact energy is necessary to densify a thickness of soil to a depth ‘D’ in order to create a Densified Natural mat. The effective depth of treatment is empirically given as:
Heavy tamping, depending on the depth, would require several tamping passes. Each tamping pass requires the dropping of the weight to form a crater which is subsequently filled with Fill material to await the next pass. A sufficient time is allowed to elapse before the next pass is made to allow for pore pressure dissipation. The dissipation of pore pressure is monitored by use of piezometers in the ground installed specifically for this purpose. The method would be economical in large open areas that are not thickly populated or highly developed as the vibrations could affect surrounding areas.
Depth of treatment even from the very large energy machines is seldom beyond 10 meters as the impact energy is dissipated with depth. An ironing pass is required at the end of the procedure as a levelling pass.
4.3 Intermediate Rammed Aggregate Piers (GEOPIER)
The Intermediate Rammed Aggregate Pier or GEOPIER is a proprietary technology developed to address the problem of shallow to intermediate depth ground improvement.
In contrast to overexcavation and replacement or Dynamic Compaction, the method does not rely only on improvement or stiffening of a soil layer. It also introduces prestraining of the surrounding soil though lateral compaction and outward push (straining) of the surrounding soil. The improvement of the soils by prestraining is achieved by introduction of a Rammed Aggregate Pier (The GEOPIER) into the soil by preaugering and compaction using high frequency, low amplitude tamping of successive lifts of ordinary base course aggregates.
The GEOPIER, because of its relatively very large stiffness compared to the surrounding soil, is able to sustain very heavy loads and assome a major share of the load. The stiffened soil surrounding the GEOPIER provides a greatly improved lateral support through Skin Friction.
The improvement in the lateral stress support is represented by the Mohr’s Circle Diagram below;
As can be seen, the initial state of stress is represented by the first circle where the Normal stress (σ1) is greater than the Lateral Stress (σ3). However, during the insertion and compaction of the GEOPIER (2nd Circle), the lateral stress is to increased Ko level, due to prestraining. Further buildup in lateral stresses elevates the lateral stresses allowing for a higher buildup in the normal stress.
Thus, the allowable load (Normal Stress) is substantially increased by the Rammed Aggregate Pier.
However, the load transfer mechanism is not as simple since a major share of the Normal Stress (load) is carried by the GEOPIER because of its relatively large stiffness compared to the surrounding soil matrix. The load is transferred to the GEOPIER and is carried through increased Skin Friction of the prestrained soil. The GEOPIER transfers most of the load to the surrounding prestrained soil in this manner. As a result, it does not have to penetrate through very poor soil. For this reason, the Ground Improvement can be very very shallow but still have increased load capacity.
A load test is always necessary to validate the design assumptions and determine the actual GEOPIER Modulus.
Because of the increased frictional stresses induced along the GEOPIER shaft due to prestraining and prestressing of the surrounding soil, the GEOPIER has considerable uplift resistance. This makes it a very cost effective Tension Anchor with the incorporation of tension steel within the GEOPIER.
The GEOPIER has considerable uplift capacity because the failure cone of the soil is more developed.
Due to the use of common base course aggregates, simple construction equipment and shallow depth of installation, the method is a cost effective substitute to deep Piles, heavy tamping or overexcavation and replacement with Engineered Fill.
5. DEEP GROUND IMPROVEMENT
Deep very loose or poor Granular and Clay Soils are normally improved with the use of Granular Piles (Stone Columns, Sand Piles, Vibroflotation Piles). Generically, the procedure consists of inserting a vibrating probe or a casing into the loose soil with or without the aid of water jetting.
The cavity formed by displacement methods are then filled up with granular soils (sands or gravels) incrementally and compacted in lifts. The granular material is dropped down through the center of the probe with a bottom flap valve or the probe has to be withdrawn and material dumped into the cavity.
Because granular materials respond well to vibratory excitation, a very dense columnar granular Pile is formed with some densification of the surrounding soil being achieved. However, the radius of influence is very limited. These granular piles act as vertical load support elements to transmit the loads to more competent ground by point bearing and partly by Skin Friction. In addition, they can enhance the shearing capacity of the soil mass by providing stiff shear zones of highly compacted material. The composite ground consisting of vertical granular pile elements and the surrounding soil matrix is able to resist shearing better than the ordinary unreinforced soil.
Stress sharing also occurs between the granular piles and the surrounding soil although to a limited degree. The granular piles are normally installed at closer spacings in a triangular pattern as this gives the most dense packing of the granular piles for a given area.1]
A major share of the load is taken up by the pile because of this higher stiffness as compared to the surrounding soil matrix due to stress concentration. Necessarily, the granular pile need to extend beyond the soft soil to a competent layer and therefore, the piles generally have to be deep. Thus, the load capacity is from the contribution of End Bearing and Skin Friction in much the same way as for conventional Piles. The granular piles confer additional advantages such as:
Reduced liquefaction potential as some densification occurs in the surrounding soils.
Reduced pore pressure buildup during earthquakes which reduce potential for liquefaction, as the granular pile act as vertical sand drains to relieve pore pressures.
Increased shearing capacity
Preconsolidation effects due to the vertical sand drain effect.
Generally, the granular piles differ only in materials used and the method of installation. The overall settlements is that of the composite ground and is calculated based on the area replacement ratio As and settlement reduction ratio. There has been good performance record for stone columns particularly in Japan during the Kobe Earthquake. Pier facilities on liquefiable ground that were reinforced by stone columns did not liquefy while the surrounding areas that were unreinforced, liquefied. The granular stone columns served as vents for the excess pore pressure, preventing buildup.
6. CONCLUSION
Mechanical Ground Improvement in order to be fully understood requires an intimate knowledge of soil behavior particularly as it refers to particulate soil mechanics. Properly applied, Mechanical Ground Improvement is a cost effective measure to employ in the improvement of marginal lands. The use of granular or Aggregate Piles also serve a double purpose. Aside from load support, the granular or aggregate pile body can serve as a vent to prevent the development of excess pore pressures. A new proprietary technology – Intermediate Rammed Aggregate Piers or GEOPIERS has been introduced to provide cost effective solutions to soft soil improvement and settlement control.
ABOUT THE AUTHOR
Emilio M. Morales, MSCE took up his masters degree at the Carnegie Institute of Technology, Carnegie -Mellon University, Pittsburgh, PA. USA in 1980. Formerly a Senior Lecturer of the Graduate Division, College of Engineering, University of the Philippines, Diliman, Quezon City. Presently, he is the Technical Manager of Philippine Geoanalytics, Inc. and was Chairman of the Cherry Hills Investigation Team (Joint PICE/ASEP).
He can be contacted at: Philippine Geoanalytics, Inc. PGA Technical Center #85 Kamuning Road, Quezon City. Telephone Nos. 929-33-54 Fax No. 929-33-53.
e-mail addresses: pgamain@pgatech.com.ph and emmorales@hotmail.com.
This paper is a review of the historical evolution of Electrokinetics to what it is today, a viable and effective technology for the remediation or treatment of pollutants in-situ and ex-situ. By far, Electrokinetics is one of the most cost effective treatment processes for numerous hazardous materials both organic and inorganic.
The objectives of this review are as follows:
To provide for a better understanding of the Electrokinetic process, its mechanisms including its advantages and limitations in one single paper.
To provide for a better understanding of its applicability with various ground contaminations and pollutants.
To gain a better understanding of how the Electrokinetic process can be further enhanced individually or in combination with other processes such as Hybrid systems.
To understand the interplay of Soil type, electrochemistry, materials, chemical additives, enhancers and REDOX reactions that drive the Electrokinetic process.
To determine areas for further research which further improve the process.
While the Review does not claim to provide for a very thorough or complete and authoritative report on the state of practice or the state of the Art, it is hoped that this work can be used as a starting or “ jumping off” point by the readers to gain at least a basic or fundamental understanding of the Electrokinetic process and move onwards. Work in this topic is Dynamic and various sources in the Internet can add to and update the body of knowledge already presented herein to augment research in this field.
2.0 INTRODUCTION
Electrokinetics (EK) is a process that separates and extracts heavy metals, radionuclides, and organic contaminants from saturated or unsaturated soils, sludge, and sediments. A low intensity direct current is applied across electrode pairs that have been implanted in the ground on each side of the contaminated soil mass. The electrical current causes Electroosmosis and ion migration (electromigration) and Electrophoresis, which move the aqueous phase contaminants in the subsurface from one electrode to the other. Contaminants in the aqueous phase or contaminants desorbed from the soil surface are transported towards respective electrodes depending on their charge.
The contaminants may then be extracted to a recovery system or deposited at the electrode. Surfactants and complexing agents can be used to increase solubility and assist in the movement of the contaminant. Also, reagents may be introduced at the electrodes to enhance contaminant removal rates [EPA].
The term “electrokinetics” (EK) refers to the introduction of an electrical gradient (as opposed to a hydraulic or pressure gradient) in the soil to mobilize or promote the migration of water and/or various chemical species towards the preferred electrode.
The procedure takes advantage of the electrically charged characteristics of the soil, its contaminants, the pH as well as the water. In the case of water, the bipolar orientation of the H2O molecule and its preferred orientation in the double layer enhance electrokinetic migration to the electrodes. In the process, breakdown products through hydrolysis of water are formed. The main EK mechanism in such a case is Electroosmosis and this phenomenon can occur whether the soil is coarse- grained sands or fine-grained clays.
In the case of chemical species in the subsurface, various mechanisms interact to promote movement or migration, but generally when these species are in ionic form in suspension.
Electrokinetics (EK) as a soil remediation technology is relatively young, having become an alternative procedure for the removal of toxic chemical species in ionic form in the soil (Lageman, R., Pool, W and Seffinga, G., 1989) in the late 1980’s.1]
Electrokinetics is based on the principle that when direct current (DC) is passed through contaminated soil, certain (negatively charged) types of contaminants will migrate through the soil pore water to a place where they can be removed.
The Electrokinetic Remediation (ER) process removes metals and organic contaminants from low permeability soil, mud, sludge, and marine dredging. ER uses electrochemical and electrokinetic processes to desorb, and then remove, metals and polar organics. This in situ soil processing technology is primarily a separation and removal technique for extracting contaminants from soils. [FRTR]
The demand for an alternative technology to replace costly “remove and treat” processes spurred the clamor for in-situ soil remediation as a cost effective method. This became popular, because of the need for “cleaner” technologies in the 1990’s. (Lageman, et. al. 1989) 1]
Numerous researches and field studies as well as practical applications have brought EK to the status of a practical and cost effective technology for the treatment of contaminated soils. However, much still has to be done to understand the mechanisms as a way to optimizing the processes involved as well as to address other issues to increase the efficiency of the Electrokinetic Process.
A typical schematic layout of the EK process is taken from Pack:
Much of the work gathered for this paper came from various published papers and from the “3rd Symposium and Status Report on Electrokinetic Remediation” (EREM 2001) sponsored by AGK (Angewandte Geologie Karlsruhe) Universitat Karlsruhe and the author’s additional sources such as the USEPA, GWRTC Website, and other Soil remediation websites
The author is particularly very thankful to Dr. Kurt Czurda and Mr. Roman Zorn for providing information on current researches and instrumentation set ups during the author’s brief stay at Universitat Karsruhe and to Dr. Dennes Bergado and Dr. Ulrich Glawe for their guidance and assistance in the preparation of this report.
3.0 HISTORICAL BACKGROUND
In 1808, Reuss observed the electrokinetic phenomena when a DC current was applied to a clay-water mixture. Water moved through the capillary towards the cathode under the electric field. When the electric potential was removed, the flow of water immediately stopped. Napier (1846) distinguished electroosmosis from electrolysis, and in 1861, Quincke found the electric potential difference through a membrane resulted from streaming potential. Helmholtz first treated electroosmotic phenomena analytically in 1879. A mathematical basis was provided. Pellat (1904) and Smoluchowski (1921) later modified it to apply to electrophoretic velocity. Out of this treatise of the subject the well known Helmholtz-Smoluchowski (H-S) theory was developed. The H-S theory deals with electroosmotic/electrophoretic velocity of a fluid of certain viscosity and dielectric constant through a surface-charged porous medium of zeta potential (), under an electric gradient. The H-S equation is as follows:
Cassagrande’s studies in stabilizing clays by Electroosmosis started in the early 1930’s. The introduction of an electrical gradient into the soil to stabilize it mainly by removal of the water has its beginnings 70 years ago.
Most of the studies during this early period were directed towards removal of water for soil stabilization and were generally concentrated on the dewatering of fine gravel soils by Electroosmosis. Research on this specific aspect is still continuing with the use of electro conductive elements on PVD drains, which are subjected to an electrical field.
Several Russian researchers used electromigration in prospecting for metals in the 1960’s. The early 1980’s showed marked interest in the exploration of EK Technologies for the removal of toxic chemical species in ionic form in the soil in Europe and the US. Lageman, Pool and Seffinga (1989) 1]
During this period, not all the mechanisms and electrochemical processes were fully understood, particularly insofar as the contribution of the cation exchange capacity of the soil (CEC) to the overall effectiveness of the EK Process. In addition, the various mechanisms have not yet been fully exploited.
Lageman, Pool and Seffinga (1989) 1] focused on the contribution of electromigration and patented the use of circulating electrolytes and the use of ion permeable wells to manage the Anolyte and catholyte. This encouraged further research resulting in the first commercial and successful application of in-situ electro remediation carried out by Geokinetics at the site of a wood impregnation plant to remove Arsenic (As).
This successful application encouraged further researches and field studies resulting in breakthroughs in the understanding of the various processes in EK for in-situ remediation of contaminated soils. While most of the early researches and field studies concentrated on the removal of inorganic pollutants in the soil, more particularly heavy metals, subsequent work were also done on organic pollutants (hydrocarbons) as existing technologies for removal of these contaminants were costly or very time consuming and sometimes stretched out for years instead of days or months. Today, research, field tests and actual applications have shown a dramatic increase as a result of various successes and also as a result of a clearer understanding of EK processes and how these can be further enhanced.
Further research is needed in hybrid systems as well as in making the EK process cost effective particularly directed towards reduction in electrical consumption and the quest for cheaper and more effective electrodes.
4.0 THE EK PROCESS AND ITS MECHANISMS
4.1 FUNDAMENTALS
Although EK can be applied to both coarse and fine grained soils, in the case of the former, the full potential of EK can not be completely mobilized and the beneficial effect is only confined to removal of water as in soil stabilization or preconsolidation. This is due to the absence of a double layer (the “Debye Layer”) in the soil structure as grain sizes are relatively large, thus, preventing the setup of electrical surface charges. These surface charges play a critical role in the remediation of fine-grained soils using electrokinetics EK Procedures.
In order to gain a full understanding of this phenomenon, we would have to review the soil microstructure and the presence of the double layer in fine-grained or Clay soil. The electrochemical processes induced by the introduction of an electrical field in the clay soil results in triggering various mechanisms, all promoting the mobilization of ionic contaminants as well as altering electrical charges in the soil. In addition, redox reactions take place in the electrolytes (Anolyte and Catholyte), which produces chemical changes that promote or inhibit the furtherance of the desired electrochemical reactions and the overall effectiveness of the EK process.
Basic Soil Mechanics points to the presence of an electrically charged clay soil microstructure with preferred charged orientations. The reason behind this is the occurrence of the clay soil particles as submicroscopic with very large surface area to volume (SA/V) ratio thus making electrochemical forces of attraction very significant.
Because of the dipolar nature of water (H2O), the water is captured as “adsorbed” and “absorbed” layers forming in a very simplistic characterization, what is known as a “double layer”.
Because of the critical importance that the double layer plays in the EK process, a detailed description is necessary for a fuller understanding of EK fundamentals since electrochemical reactions can only happen in electrochemically active interfaces having a double layer structure. Doering, F. and Doering, N. (2001) 3]
As described earlier, due to the submicroscopic size of the individual clay platelet, and its high surface area to volume ratio, the clay platelet is a charged particle with an orientation charge effectively making it an “electrode”.
This charged orientation in turn attracts the dipolar water particles with attractive forces that are significantly very strong near the clay platelet and decreases with increasing distance from it. Distance, in this context, being measured in Angstroms or Nanometers.
In the figure above, the attracted ions are assumed to approach the electrode surface and form a layer balancing the electrode charge, the distance of approach is assumed to be limited to the radius of the ion and a single sphere of solvation round each ion. The overall result is two layers of charge (the double layer) and a potential drop, which is confined to only this region (termed the outer Helmholtz Plane, OHP) in solution. The result is absolutely analogous to an electrical capacitor, which has two plates of charges separated by some distance (d) [from the “Electrochemistry Refresher” University of Bath].
The H2O molecule configuration allows it to be captured by the charged clay platelet with attractive forces varying inversely with distance from the core. The innermost layer of water is the hygroscopic water corresponding to the Inner Helmboltz Layer (IHL), surrounded by another layer of charged molecules, the solvation water or the Outer Helmboltz Layer (OHL). In between these layers is the Inner Helmholtz Plane (IHP). The transition layer between the IHL and the OHL is the transition layer to the main solution known as the diffuse layer. The next interface, between the diffuse layer and the OHL is the Outer Helmholtz Plane (OHP) where most of the electrochemical reactions take place. Doering, F. and Doering, N. (2001) 3]
Because of the charged orientation in this double layer, the individual clay structure is analogous to an electrode with polarization occurring naturally.
(Pamukcu 1997) 2] Various researchers have found out, that natural electric fields occur in soil in nature and Schlumberger coined this as “spontaneous polarization”. Doering, F. and Doering, N. (2001) 3]
This Spontaneous Polarization is brought about in natural soils due to the contribution of primary and secondary weathering products of rocks and various minerals. These weathering products essentially consist of metallic minerals such as Iron, Magnesium, manganese or Titanium compounds, carbonaceous materials and humic substances which possess electroconductive properties of varying degrees or intensity.
In Combination with the porewater, these leached out minerals and carbonates as well as other substances provide a clear electric path for the passage of current into the soil. It is this inherent electroconductive property of the combined soil water system, which contributes to the effectiveness of the EK process.
In turn, artificially introducing a direct DC or alternating voltage AC into the soil can further increase electrical fields. It has been found out that these Induced Polarizations (IP) can and do affect silicates, clays and other soil minerals in addition to rocks and metallic ores. (Doering, F. and Doering, N.) 3]
Electrochemical Remediation Technique (ECRT) was a natural follow-on to induced polarization techniques. The main difference being that ECRT required continuous uninterrupted application of DC current for a relatively longer period of time. The induced polarization is brought about by the introduction of physical electrodes into the soil, which causes the set up of a voltage gradient in the soil. However, the individual soil particles possessing Spontaneous Polarization (SP) and IP brought about by EK serves to increase the degree of polarization further. This IP effect causes the set up of oxidation and reduction (redox) processes in each of the individual clay particles or double layer structure.
Induced polarization (IP) causes ions to diffuse to or from the interface resulting in capture or release. The phenomenon is more complex than this; suffice it to say that IP causes changes in the electrical field that induces electrochemical reactions (redox) to occur. Doering, F. and Doering, N. (2001) 3]
Doering and Doering 3] identify the three sources of energy that sustain the electrochemical reactions. These are:
Spontaneous or provoked polarization occurring naturally in soils.
The DC fed into the soil by artific ial means.
Secondary current resulting from soil discharges by the double layer when further excited by an induced electric field.
A voltage range defines the distinction between electrochemical remediation process (ECRT) (where electrochemical synthesis and geo oxidation occurs) and electrokinetics EK or geokinetics (where mobilization of ionic species and mass transport occurs).
Whereas ECRT needs “low current density” applications to initiate electrochemical synthesis at low conductivity ranges of R=0.2 Ω to 10 Ω , higher current densities would be required in the range of R=0.2 Ω to > 40 Ω , in order to initiate mass transport.
At this range, which is the electrokinetic EK application field, immobile heavy metallic compounds can be electrochemically converted into compounds, which could be mobilized. Doering, F. and Doering, N. (2001) 3]
Doering and Doering 3] suggests the name for this process as “induced complexation” (IC) since the chemical transformation mainly covers the conversion of immobile heavy metal compounds into complexations which could be mobilized.
In the electrochemical cell, oxidation and reduction reactions take place at the anode and cathode respectively. These redox reactions take place at every soil interface resulting in electrolysis of the water. The electrolysis produces the oxidizing agent O2 and H2. This Electrochemical Geo-oxidation that results can successfully treat almost all organic pollutants. Doering, F. and Doering, N (2001) 3]
Electrokinetic remediation techniques use low voltage DC on the order of mA/cm2 of cross-sectional area between the electrodes or an electric potential difference on the order of a few volts per cm across electrodes placed in the ground in an open flow arrangement. The groundwater in the boreholes or an external supplied fluid is used as the conductive medium. Open flow arrangement at the electrodes allows ingress and egress of the processing fluid or of the pore fluid into and out of the porous medium. The low-level DC results in physico-chemical and hydrological changes in the soil mass leading to species transport in the porous media. The species input into the system at the electrodes (either by the electrolysis reactions or through the cycling of the processing fluid) and the species in the pore fluid will be transported across the porous media. This will be done by conduction phenomena in soils under electric voltage gradient fields, towards respective electrodes depending on their charge. Non-ionic species will be transported along with the electroosmosis-induced water flow. This transport, coupled with sorption, precipitation and dissolution reactions, comprise the fundamental mechanisms affecting the electrokinetic remediation process.
Extraction and removal are accomplished by several means including electrodeposition (electroplating at the electrode), precipitation or co-precipitation at the electrode, pumping of water near the electrode, or complexing with ion exchange resins Electrokinetics, Inc. (1994). Adsorption into the electrode may also be feasible because some ionic species will change valence near the electrode (depending on the soil pH) making them more likely to adsorb. Murdoch et. al. (1994) 4]
There are several variations of the basic electrokinetic remediation process or implementing strategies:
Electrokinetic bioremediation (or bioelectric remediation) for continuous treatment of groundwater of soil in situ utilizes either electroosmosis or electrochemical migration to initiate or enhance in situ bioremediation (Bioremediation In Situ Groundwater 18]).
Electrokinetically deployed oxidation (ElectroChemical GeoOxidation, ECGO19] and Electrochemical Oxidative Remediation of Groundwater 20]).
Electrokinetically deployed fixation (Fuel Oils, DNAPL’s & Solvents – EH/DPE 21] and Heavy Metals, Arsenic, Cyanide, etc.-Electrokinetic Remediation 22] ).
Periodically reversing the polarity of the field is intended to repeatedly pass contaminants through a degradation zone, while limiting the development of high or low pH conditions in the vicinities of electrodes and reducing fouling of electrodes by precipitation. This approach of in situ remediation is the essence of the “Lasagna” process, which will be discussed in detail later in this report.
Surfactants and complexing agents can be used to increase solubility and assist in the movement of the contaminants.
Reagents, such as metal catalysts (iron particles, etc.) may be introduced at the electrodes to enhance contaminant removal rates.
4.2 EK Mechanisms
The inherent electro conductive nature of soils particularly moist to wet soils (MC>10%) makes them conducive to EK Processes.
However, other than the spontaneous polarization naturally occurring in soils, the introduction of a direct current by artificial means introduces several mechanisms or phenomena in the soil. These, individually or working together, act to induce mobilization or mobilizes water and other species in solution, whether adsorbed or absorbed in the soil particles.
With the low voltage and low amperage, it is the discharge of electricity from the soil that causes the redox reactions in the soil matrix, in effect; the soil acts as a Capacitor.
The soil–ground water system or the sediment system can be considered as an electrochemical cell. Since the soil particles are already prepolarized by natural electric fields (i.e., spontaneous potential), each soil particle is composed of a part charged positively and a part charged negatively in effect becoming a “microelectrode”. In an electrochemical cell, reactions only occur at the electrodes and comprise anodic oxidation or cathodic reduction.
In soils however, in addition to the local electrode reactions, the complete system of redox-reactions takes place simultaneously at any and all soil particles. These render the soil particles to assume a neutral or nearly neutral pH value. The reaction partners for oxidation and reduction are simultaneously generated at the soil particles by water hydrolysis. Doering, F. and Doering, N. (2001)3] Empirical evidence indicates that reaction rates are inversely proportional to grain size, such that the ECRTs remediate faster in clays and silts than in sands and gravels
These mechanisms tend to free or release water or chemical species in solution, speed up the transport of pore water or create electrical gradients that induce diffusion or migration of water and other chemical species to the electrodes (Anode or Cathode). In addition, the resulting hydrolysis of water causes significant changes in the pH concentration in the electrodes. The traveling acidic front serves to further aid in releasing the adsorb ions in the soil further increasing the mobilizing effect.
Higher pH (Basic or Alkaline Solutions) is produced at the Cathode end and (the opposite condition) very low-to-low pH is generated at the anode.
The changes in pH at the electrodes tend to either enhance or degrade the effectiveness of the EK process depending on the contaminant specie being mobilized or immobilized. In the case of metals, precipitation may occur in the cathode, which would generally tend to reduce its electroconductivity but at the same time also enhance capture of metals by electroplating the cathode.
Thus, the addition of facilitating agents may or may not be required depending on the remediation objective. This will be discussed later.
The introduction of a Direct Current (DC) in the soil initiates the following phenomena:
Electroosmosis
Electromigration
Electrophoresis
The Diagram below taken from Pack show the foregoing EK induced processes:
4.2.1 Electroosmosis
Electroosmosis is the movement of liquid containing ions relative to a stationary charged surface. The motion is that of a liquid through a membrane (or plug or capillary) as a consequence of the application of an electric field across the membrane. Non-ionic species, both inorganic and organic, will also be transported along with the Electroosmosis induced water flow.
When an electric field is induced in a free draining or normally free draining soil even in the absence of initial pore pressure or hydraulic gradient, pore pressures developed in the soil due to the presence of nonuniformities in the local electric field intensity and or surface electrochemistry.
The electroosmotic phenomenon is caused by the accumulation of a net electric charge on the solid’s surface that is in contact with an electrolyte solution and the accumulation of counter ions in a thin liquid (double or Debye) layer next to the solid’s surface. Away from the solid’s surface, the electrolyte is neutral. In the presence of an external (driving) electric field, the counter ions in the Debye layer are attracted to the oppositely charged electrode and drag the liquid along. In other words, the electric field, through its effect on the counter ions, creates a body force that, in turn, induces fluid motion. The electric field intensity and surface (Zeta) potential are the driving parameters for Electroosmosis in a soil region.
When these non-uniform parameters vary in space, there is a non-uniform driving force [local fluid momentum] that must be balanced by a decrease or increase in local pore pressure. This can happen when the net pressure drop is zero and when there is no consolidation of the porous media. The magnitude of the pressure increases with increasing spatial non-uniformity in the driving force and with decreasing hydraulic conductivity.
Pamukcu (1997I 2] illustrated the Spiegler friction model (1958) and showed that electroosmotic water transport per unit electrical charge increases with increasing cation/water ratio in the system. Experimental evidence of this theory has been given by a number of researchers in past (Gray and Mitchell, 1967). An extension of H-S theory considers a portion of the electric current transported near the surface of or through the solid phase (Wiedemann, 1856). The resulting equation is often referred to as the current efficiency, time rate of volume of water flow per quantity of electricity, of the system:
Surface current is due to the ionic motion in the diffuse layer. In narrow capillaries with low ionic concentrations, thus thick diffuse layers, a disproportionate fraction of the current flows in this layer due to the low conductivity of the bulk fluid. Experimental evidence shows that current efficiency, Q/I, decreases with increasing ionic concentration in the bulk fluid (Wittle and Pamukcu, 1992).5] This can be explained readily from Equation (2) since and /are expected to decrease, and 0 to increase with increasing ionic concentration of the bulk fluid. The surface conductance also changes with ionic concentration. As the ionic concentration in the bulk liquid increases, the diffuse double layer shrinks toward the particle surface and the shear plane shifts away from the particle surface so that the majority of the charge is now compensated by the immobile Helmholtz layer. Therefore the charge density in the diffuse layer decreases giving rise to a lower surface conductivity, s. As a result of this lowered conductivity, a smaller portion of the current flows on the capillary surface. In contrast, in the presence of low ionic concentration, the diffuse double layer is swollen and much of the charge is compensated by the ions in the diffuse layer. Therefore, the capillary surface conductivity is high and so is the fraction of the current that is transported on the surface.
The significance of surface conductance in the prediction of electroosmotic flow as it relates to contaminant migration was investigated by Khan (1991). He proposed a modified theory of electroosmotic velocity of water through soil. In this theory, the ‘true electroosmotic’ flow is directly proportional to the current carried by the charged solid surfaces in soil. The soil is modeled as parallel resistances of the soil surface and pore fluid, and the zeta potential used in H-S theory is replaced by the surface potential, d, at the OHP (Outer Helmholtz Plane):
With uEO/Is shown to remain fairly constant for clays of different surface conductivity and also pore fluid electrolyte concentrations below 10-2 M, experimentally, equation (3) was further reduced to the following,
The modified theory basically emphasized that the surface conductivity of the porous compact medium is the most essential precondition for electroosmotic water flow, thus uncoupling it from the water drag component of the migrating ions in pore fluid of high ionic concentration. This theory is in agreement with Spiegler’s theory of water /cation ratio, as well as Gray and Mitchell (1967) approach of co-ion exclusion principle based on Donnan theory of membrane equilibrium (1924). Additional evidence to support this finding was presented by Pamukcu and Wittle (1992) 5] for a variety of ion species, where the ionic concentration effect on the measured current efficiency appeared to be most pronounced in clays with high anion retention capacity. At the same concentrations of dilute solutions of electrolytes, kaolinite clay with higher anion retention capacity (poor co-ion exclusion) showed consistently higher electroosmotic flow than montmorillonite clay with lower anion retention capacity (good co-ion exclusion). This observation suggested that the anionic dragging of water toward the anode diminished the net flow toward the cathode compartment in the montmorillonite clay. (Pamukcu 1997) 2]
Esrig showed that excess pore pressures (negative or positive) could be developed in an incompressible material during Electroosmosis if the voltage drop is non- linear even if the boundaries are freely drained.
Mise suggested that high negative pore pressures measured in free draining kaolinite were due to an imbalance in pH distribution in the soil.
4.2.2 Electromigration
Electromigration defines the movement of ions and ion complexes across the porous media. This occurs via conduction phenomena in soils under electric fields. The average mobility of the ions is approximately ten times greater than that of electroosmotic ability. Lageman, Pool and Seffinga (1989). 1] In electromigration, charged particles are transported through the substrate due to the presence of an electric gradient. The induced Movement resulting from electromigration is superimposed or coupled to the movement induced by Electroosmosis thus further complementing each other.
Pamukcu (1997)2] described Electromigration or ion- migration as the primary mechanism of electro remediation when the contaminants are ionic or surface charged. Speciation and precipitation are major factors in mobilization and transport of heavy metal constituents by ion-migration component of electrokinetics. The speciation is dependent upon a number of fairly well understood parameters including pH, redox potential, and ion concentration. These same factors influence the equilibrium conditions relating to both the soil and the contaminants. Charged ions moving toward the oppositely charged electrode relative to solution is called electromigration. In a dilute system or a porous medium with moderately concentrated aqueous solution of electrolytes, electro- migration of ions is the major cause of current conduction. Electromigration velocity measures ion movement in the pore water caused by electric field at infinite dilute solutions:
The direction and rate of movement of ionic specie will depend on its charge, both in magnitude and polarity.
Ions and polar molecules in the pore fluid migrate under an electric field. Under the electric field, cations (metal ions) move to the cathodes whereas anions move to anode at different mobilities influenced by the electric charge and physicochemical properties.(Pamukcu 1997) 2]
One of the more important aspects of electrokinetic soil processing is the migration of an acid front from the anode to the cathode during the treatment. When electrolysis of water takes place in the surface of electrode, hydrogen ions are produced at the anode and hydroxyl ions at the cathode.
At the electrodes, Electrolysis of water takes place as follows:
This electrolysis results in an acid front at the anode and an alkaline front at the cathode, respectively. The propagation of the acid and the base fronts promote the dissolution of metal ions near the anode and the precipitation of the metal ions near the Cathode. These conditions significantly affect the pH and ionic strength of pore water, the mobility and solubility of metal contaminants, and charge conditions of soil particles.
The variation of pH conditions in soils by electrolysis of water in the electrode compartment affects ionic strength of pore water and soil surface properties such as cation exchange capacity, magnitude and sign of the electrokinetic zeta potential.
Furthermore, speciation, mobility and solubility of contaminants are often varied with pH in soils during treatment, which may limit or enhance the treatment efficiencies.
Since electromigration is the movement of ionic contaminants and pore water towards the electrodes under electric field without convective movement, it is independent of the Permeability of soils and enables the removal of contaminants from all types of soil.
However, electromigration removes only ionic contaminants such as metal ions dissolved by organic acids and bases.
Electromigration is the key mechanism in removing inorganic contaminants, especially metal ions. (Kim, 1998).
4.2.3 Electrophoresis
Electrophoresis is the transport of charged colloids or solid particles under applied direct current DC electric field towards the electrodes as a result of their orientation with the electric field. Electrophoresis acts oppositely of Electroosmosis as the charge particle moves relative to a stationary fluid.
Charged particles including microorganisms (bacteria) may be affected by two processes: EO encourages the movement of the cells towards the cathode while EP will move negatively charged cells towards the anode. Movement is therefore determined by the surface charge characteristics of the Bacteria strain and the direction of bacteria movement is manipulated by altering the electrical field. (Oxford University CE Publication on the Web).
In a compact system of porous plug, electrophoresis should be of less importance since the solid phase is restrained from movement. In some cases, however, electrophoresis of clay colloids may play a role in decontamination if the migrating colloids have the chemical species of interest adsorbed on them. An important contribution of electrophoretic movement to
contaminant transport may be when the contaminants are in the form of colloidal electrolytes or ionic micelles. Colloids are made up of ionizable groups attached to large organic molecules, macromolecules, and aggregates of ions. Ionic micelles or colloidal electrolytes also develop electric double layers about themselves. If the particle conductivity is the same as the surrounding liquid and the electrokinetic potential is low (< 25 mV), then the particle mobility can be described by the Smoluchowski equation. For larger values of electrokinetic potential the effects of electrophoretic retardation and relaxation should be considered, similar to the consideration in electrolyte solution. (Pamucku and Wittle, 1992) 5]
4.3 Factors Affecting Electrokinetic Technology
4.3.1 General
Electromigration rates in the subsurface depend upon grain size, ionic mobility, contamination concentration, total ionic concentration, and significantly upon the soil pore water current density and pH. The process efficiency is not as dependent on the fluid permeability of soil as it is on the pore water electrical conductivity and path length through the soil, both of which are a function of the soil moisture content (Wallmann, P.C. 1994) 6]
The direction and quantity of the contaminant movement is influenced by the contaminant concentration (anions versus cations), soil type and structure, pH, interfacial chemistry, and current density of the soil pore water. Electrokinetic remediation is possible in saturated and unsaturated soils. Experimental results indicate that there is a minimum moisture content at which electromigration can take place, which is related to, and can be estimated from, the residual moisture content of a soil, also called “immobile water.” The soil moisture content must be high enough to allow electromigration, but for optimum results, should likely be less than saturation, to avoid the competing effects of tortuosity and pore water content. The direction and rate of movement of an ionic species will depend on its charge, both in magnitude and polarity, as well as the magnitude of the electroosmosis-induced flow velocity. When electroosmosis processes are operative, non-ionic species will be transported along with the induced water flow (Wallmann, P.C. 1994) 6]
The efficiency of extraction relies upon several factors such as the type of species, their solubility in the specific soil, their electrical charge, their concentration relative to other species, their location and form in the soil, and availability of organic matter in the soil. (Electrokinetics, Inc. 1994 23] and LasagnaTM Public-Private Partnership, 1996 24]).
Electrokinetics is applicable in zones of low hydraulic conductivity, particularly with high clay content. The technology is most efficient when the Cation Exchange Capacity (CEC) and the salinity are low.
During electrokinetic treatment, electrolysis results in the formation of H+ and OH-. These migrate toward one another by electrokinetic processes. As these two fronts meet, a rapid transition from low to high pH occurs, creating a region of minimum solubility of metals. These sharp discontinuities in pH induced within the soil mass by electrokinetics could result in a deposition front where minerals are precipitated in soil pores, markedly reducing permeability and inhibiting recovery. This can be prevented by flushing the cathode with water or a dilute acid to arrest the migration of the OH- front into the soil (Cox, et al 1996) 7]
4.3.2 Data Needs and Site Applicability
The EK technology can be deployed in one of the following ways (Geokinetics 1997):
In-situ Remediation- Electrodes are placed directly in the ground and contamination is recovered with minimal disturbance to the site.
Batch Operation- Contaminated media is transported to a batch facility and treated ex- situ.
EK Ring Fence- a chain of electrodes is deployed in-situ to recover ionic contamination from groundwater as it flows past the electrodes.
Use with PEREBAR- EK when used with Permeable Reactive Barriers can enhance the removal efficiency of the PEREBAR by selectively removing contaminants or chemical species, which could otherwise precipitate or clog the reactive barriers.
The Table below taken from ITRC Website lists the data requirements needed to determine applicability of the EK process:
4.3.3 Enhancing the EK Process
In some instances, it would be desirable to enhance the process with the use of water, bio nutrients, surfactants, additives and ion exchange columns and reactive barriers etc., as well as Electrodialytic processes in order to enhance the desired results. Several experiments in the laboratory as well as in the field have been performed for this purpose.
USE of SURFACTANTS
Surfactants are used in order to “condition” or enhance the solubility and mobility of organics in the soil. In a research described by Mansour, et. al., a Hybrid experiment was conducted combining the use of conditioning agents, ion exchange textiles and chelating agents in combination with EK.
In this hybrid experiment, the target contaminants are Lead (Pb), Nickel (Ni) and Phenantrene, which is a polyaromatic Hydrocarbon (PAH). Water is added at the anodic well continuously throughout the experiment and surfactant was added continuously at certain stages also at the anode side. The chelating agent EDTA was added at the Cathode end.
The results of this experiment showed that where the surfactant as well as the EDTA was added, the recovery rate was highest which was in a cell (C6).
The removal rates were remarkable as follows:
Lead Pb 85 %
Nickel Ni 84%
Phenantrene 74%
It was also shown in the experiments that:
Use of EDTA decreased the electrical resistance of the soil.
Sequenced Application of EDTA and then the surfactant was the most effective procedure.
Introduction of the surfactant at the onset of the test increased the soil’s electrical resistance.
USE of BIOSURFACTANTS
In another experiment conducted by Electorowicz, et. al. (1994)8] biosurfactants were used in order to increase the solubility of PAH’s into the aqueous phase from clayey soil without the introduction of hazardous contaminants.
Biosurfactants are introduced into the soil in order to solubilize the organics from the soil and cause it’s desorption. Bacteria tend to increase the solubilization of organic compounds, which in turn are attacked, by the bacteria for its metabolism. Bacteria produce rhamnolipids and fatty acids, called Biosurfactants that change the surface tension of the pore liquid and form micelles with organic contaminants (Francy, et. al. 1991). 9]
These micelles exhibit increased “bioavailability” and are more easily transported through the soil. However, the natural production of Biosurfactants in clay is very limited (Electorowicz 1999), 8] thus a means of introducing additional biosurfactants into the soil through external means would be highly desirable to increase the bioavailability and attack of organic compounds. It was thought that the negatively charged biosurfactants could be transported through the clayey soils through electrokinetics means.
The ensuing experiment (Electorowicz et. al. 1994) 8] showed the effectiveness of the addition of biosurfactants to enhance the mobility and solubility of the Phenantrene and its successful removal to about 80% level in the soil.
USE with PEREBAR
Permeable Reactive Barriers (PEREBAR) have been used independently before to intercept contaminated Groundwater flow in order for the Contaminated groundwater to flow through the permeable barrier material. The reactive barrier usually consists of granulated iron particles. In the process, the elemental iron (Fe) acts as a reducing agent, which reacts with chlorinated hydrocarbons. The redox reaction with iron generates a ferrous ion and two electrons are freed in the process.
Chemical reduction using iron is applicable to the removal of heavy metals.
However, in most instances, fouling up or clogging of the reactive barriers happen, rendering it inefficient due to formation of precipitates.
In order to remedy this, EK is introduced upstream of the PEREBAR in order to reduce the amount of groundwater contaminants and also by increasing the mobility of the groundwater through fine grained soils by the presence of an Electric gradient and activating the EK mechanisms EO, EP and EM to occur.
Thus, the overall process is enhanced with the introduction of EK techniques.
5.0 THE EFFECT OF pH on the PROCESS
5.1 Water Hydrolysis
When electric current is passed through a solution, the hydrolysis of water is initiated. This results in half reactions to occur at the electrodes (either oxidation or reduction) resulting in a change in pH. (Lageman et.al. 1989) 1]
However, in the soil particles, full redox reactions take place resulting in a neutral pH level once the full redox reactions take place.
The electrolysis reactions, caused by the electric current passing through the soil and solution, occurring at the electrodes generate an acidic medium at the anode and an alkaline medium at the Cathode. The H+ ion generated at the anode advances through the soil towards the Cathode by electromigration, electroosmosis and electrophoresis.
The rate of migration from the Anode to the Cathode is slowed down through the soil and therefore more rapid localized evolution of acids and bases at the Anolyte and Catholyte occur in time with the process. This generates high pH values (alkaline) at the Cathode and low pH values (Acidic) at the Anode. Generally, H+ exhibit a higher affinity to the soil than heavy metal ions, thus metal ions are exchanged by the soil with the H+ ion thereby releasing metals in solution which can travel through EK induced mechanism (ACAR et.al 1993). 10]
The reduction reaction at the Cathode (Jy, et.al.) zone hydrolyses water to form H2 and OH- during electrolytic dissociation. The H+ and OH- ions generated from the electrolytic dissociation are moved across the fluid within particles toward either anode or cathode (Reed 1998). 11]
Because of the electrical gradient induced by the passage of current through the electrodes, the Hydronium ion (H30+) evolved in the anode advances towards the Cathode. This results in the advance of an acidic front. Likewise, hydroxyl ions OH- are generated at the Cathode, which migrates towards the anode as a front. An abrupt change in the pH results when these two fronts collide.
The Acid front moves faster than the Basic front because of the higher mobility of the H+ compared to the OH- and because the direction of electroosmotic flow is towards the Cathode. After a while, the soil becomes generally more Acidic throughout its volume except very near the Cathode influence zone except when the soil is buffered or generally Alkaline or if Reactive electrodes inhibit the acidification of the Anolyte. (Page and Page, 2002)12]
When the two fronts collide within the soil mass, water is formed and a sharp change in pH is experienced which affects the solubility and adsorption of the contaminants. (Page and Page, 2002). 12]
In the case of the use of inert (as opposed to reactive metal) electrodes, pH tends to increase in the electrodes and decrease in the anodes.
However, when reactive metal electrodes are used in the anode and as observed by Doering, the steel in the anode is serving as a sacrificial anode and is subject to oxidation and water electrolysis. Water electrolysis unsuccessfully competes with the oxidation of the steel. Thus, electrolysis is stopped and pH change does not occur at the anode.
On the other hand, the use of steel electrodes for the cathode allows electrolysis to occur resulting in an increase in pH.
As will be discussed later, the choice of electrodes and pH control would be important considerations in the conduct of the EK Remediation.
5.2 Generation of the Acid Front
Generation and significant changes in the pH brought about by electrolysis of water significantly affects the contaminant removal process. Because of its faster rate of propagation or travel relative to the Basic front through the soil, the acid front affects the surface charge characteristics of the clays cation retention capacity (CEC).
This creates temporarily a highly acidic condition generally which serves to desorbs metal species in the soil.
The CEC of the clay decrease in a low pH environment and create suitable environments for the metal ions to remain in solution (desorption) in the bulk pore liquid so that they can be extracted more easily (Ricart et.al. 2001). 13] However, the generation of OH- in the Cathode increases the likelihood of precipitation of ionic species, reducing the efficiency of the Cathode and the electrokinetic process in general.
The pH condition may affect different metals in different ways; metal cations such as Zn2+ and oxy-anions under alkaline conditions (ZNO2 2-) are stable under acidic conditions. (Page and Page, 2002) 12]
This points to the critical importance and attention that needs to be paid to pH generation and its control or non-control during the EK process.
Thus, pH as well as the contaminant type has a greater influence on the removal process rather than electroosmotic flow.
5.3 Effects of pH on Electrode Efficiency
The pH condition at the Electrodes has a marked effect on Electrode efficiencies. As earlier discussed, high conditions occur at the Cathode encouraging precipitation of Carbonates as well as electroplating or deposition of metals on the electrode surface. This precipitation and electroplating both tend to degrade the electroconductivity of the Cathode thus directly affecting the EK process. However, this by itself is not totally objectionable, particularly if the objective is to recover precious metals in suspension (electromining) and if the electrode material is inexpensive and is easily replaceable.
Conditioning of the electrode reservoirs has been used by Reed et al (1995) 11] in order to improve performance.
This condition on pH intervention was demonstrated in an experiment conducted by (Ricart et.al. 2001). 13] On sludge subjected to EK process. The pH in the Cathode was adjusted to 2 with concentrated HN03. This prevented the increase in pH in the sludge being treated and permitted the abundant migration of H+ ions from anode to cathode. This resulted in reduction of the pH in the sludge from 7.3 to 4 in 8 days. The anode pH further dropped down to 1.0. The electrical resistance was significantly reduced and current intensity subsequently was increasing continuously. This was due to the abundance of ions in the sludge, which came from the H+ ions and elements dissolved from the sludge.
In general, extraction efficiency or dissolution of metals from mineral solids is enhanced significantly by acid attack advancing through the soil in addition to the corollary increase in current intensity and acid reduction in electrical resistance which could among others work to reduce electricity consumption. A noticeable drop in conductivity has been attributed by (Cambefort and Caron, 1961) as being due to a Sharp pH jump at the region very near the cathode and also due to the precipitation of heavy metal ion contaminants.
5.4 Changing or Maintaining pH Values
Control of pH during the EK process therefore influences the direction of the remediation process in the following ways:
5.4.1 No Control of pH
Allowing the Electrolysis (redox reactions)) to occur without intervention or control of pH results in the generation of acid and basic fronts which eventually collide within the electric field. The soils essentially remain neutral in pH and undergo complete oxidation and reduction processes. The Anolyte becomes very acidic and the Catholyte produces a highly basic solution.
The high pH at the Cathode encourages precipitation of Calcium (Ca) and other heavy metals in the Cathode. This action produces the following results, which may or may not be the desired or objective result:
⦁ Electroplating of the Cathode with metals. (Sometimes desirable as in Electromining)
⦁ Precipitation of calcium and other carbonates in the Cathode region reduces its electroconductivity retarding the EK process and resulting in very high current consumption.
The advancing acidic front in turn (and as earlier indicated) causes the dissolution and desorption of metals and its release into the solution. The acidification of the soils enhances the desorption of the metal from the soil by exchange of metal ion for hydrogen ion.
In some instances, desorption of some metals in the soil is not desirable particularly if these are stable in their original natural state and hence an acid front generated may not be desirable. This is particularly true when only organic pollutant and not stable metal species are targeted for removal.
If the only intended objective in inducing an electric field in the soil is to remove water by an electrical gradient, the rapid advance of the acidic front which causes precipitation at the Cathode can be avoided by “Toggling” or cyclic reversal of polarities between the Anode and the Cathode. Whereas, water is still free to migrate due to the electrical gradient, the acid front is controlled preventing foul up of the Cathode.
5.4.2 pH Control at the Anode
Anolyte – Maintaining the Anolyte pH level to very low values increases the generation of H+ ions thus aggressively generating an advancing acidic front. This is conducive to releasing and recovering metal contaminants from the soil. The abundance of electrons also facilitates electroconductivity and allows current to pass more easily in the field.
On the other hand, the use of a highly reactive metal (steel) in the anode prevents the generation of H+ and thus retards the advance of an acidic front.
This means of controlling the acidic front can be used to immobilize the desorption of metals which are sometimes better left in place allowing only organic pollutants to be removed by EK.
6.0 APPLICATIONS OF THE ELECTROKINETIC PROCESS
6.1 Fields of Application
In his paper, Pamukcu, 1997 2] discussed the research in electrochemical treatment for the purpose of restoring contaminated subsurface which has accelerated in the past two decades. Some of the currently researched methods of electrochemical treatment (Marks et al., 1994, 1995; Ho et al., 1995; Yeung, 1990; Mitchell and Yeung, 1991; Hansen, 1995; Pamukcu et al., 1997; Haran et al., 1995). Have been termed as:
Electro-kinetic extraction;
Electro-kinetic barriers;
Electro-bioremediation;
Electro-stabilization (injection);
Electro-containment
Electrokinetics had been used to induce dewatering of soils through Electroosmosis. Shang, (2000) 14] conducted Laboratory Tests on mine tailings and also on marine clays. The effectiveness of EK Induced Electroosmosis was clearly apparent in these experiments. Electrokinetics have also been employed successfully for consolidation of clays by providing electroconductive geosynthetics to induce dewatering through Electroosmosis (Jones, et. al. 2002) 15] (Karunarathe, et. al. 2002) 16]
The electroosmotic water flow was found out to be greater than what would have resulted under normal consolidation methods. This consolidation was also accompanied by significant increases in shear strength of the treated zone.
The earlier work focused on utilizing the technique for soil densification as an aid in containment facilities Later, others studied the effects of electrolysis on soil chemistry and the use of electrokinetics to contaminant removal from soil The feasibility and cost effectiveness of the contaminant extraction technique have been demonstrated through numerous laboratory studies and some pilot scale studies.
Banarjee and co-workers (1987) published a field feasibility study for the potential application of electrokinetics for chromium removal from subsurface. Acar and coworkers (1989) realized the importance of pH gradients generated from anode through cathode by the process. In the same year, Lageman and co-workers (Lageman et al., 1989) 1] attempted to utilize pH gradient by controlling the chemical environment around the electrodes. Pamukcu et al. (1991) 2] presented the effects of speciation and precipitation on the efficiency of electrokinetic transport of zinc through soil. Other lab studies further substantiated the applicability of the technique to a wide range of contaminants in soils. Among the contaminants which have been shown to react to electrochemical treatment in the laboratory and a few in the field, are non-aqueous phase liquids such as chlorinated hydrocarbons, mononuclear aromatic hydrocarbons (MAHs), polynuclear aromatic hydrocarbons (PAHs), phenols, sulfurous, nitrogenous compounds and, of course, metals. Ho and co-workers (1995) presented an integrated method of soil restoration method that relies on electrochemical technology. Current field demonstration results of this technology, also known as Lasagna; Soil Remediation, are available from US Department of Energy (1996).25]
Past experience with electrochemical treatment of contaminated porous media has shown that the process is most effective when the transported substances are ionic, surface charged or in the form of small micelles with little drag resistance. This is analogous to soil washing whereby the contaminant is extracted from the soil and subsequently collected in aqueous phase in a collection well or deposited at the electrode site. The alkali metals and alkali earth metals such as Na, K, Cs and Sr, Ca tend to remain ionic under a wide range of pH and redox potential values. Therefore they are expected to electromigrate and are extracted from soil readily unless they become preferentially sorbed onto solid surfaces and clay interstices. Under ideal conditions, the predominant cation and its accompanying anion may be caused to separate efficiently by electromigration only, for which little or no electroosmotic water advection may be necessary. Small anions such as chloride and thiosulfate are so mobile that they can migrate toward the anode despite a strong electroosmotic flow toward cathode (Weinberger, 1997). Pamukcu et al. (1991) 2]
When extraction may become ineffective or infeasible, electrochemistry may still be useful to stabilize and/or contain certain groups of metals and some organic compounds in the ground. In conjunction with environmental restoration, stabilization is defined as fixing the toxic substance in place thereby rendering it less likely to move elsewhere under ambient hydro geological conditions. Electrochemical stabilization can be accomplished by delivering an appropriate oxidizing or reducing agent to the contaminant in the soil that subsequently will: (i) degrade the contaminant; or (ii) change it to a non-toxic or immobile species; or (iii) enhance stable sorption and incorporation of the contaminant into the clay minerals. Zero- valent iron enhanced degradation of TCE and Fe (II) degradation of toxic Cr (VI) to less toxic and less mobile Cr (III) are examples of such processes Pamukcu et al. (1991) 2]
Containment may be defined as causing controlled accumulation of the toxic substance by sorption in a small volume of substrate. Electrochemical containment may be accomplished by causing the electro-migration or electroosmotic transport of the contaminants to reactive permeable barriers strategically situated between the electrodes, where they are attenuated and the filtered water is allowed to pass through (Hansen, 1995; Weeks and Pamukcu, 1996). In actual field applications, such permeable structures could be installed at various positions throughout a contaminated site serving as primary and secondary treatment locations. Such structures are referred to as “reactive permeable barriers” (Rael et. al., 1995; Blowes et al., 1995). The basic idea behind these reactive barriers is to allow the flow to advance the contaminant plume through an in-situ structure containing a substance that will react with the contaminant. When a directed flow of contaminants by electroosmosis or electromigration enter a permeable bed of sorbents material situated in the path of the flow, the water may be filtered sufficiently depending on the rate of flow through the bed as well as the attenuation characteristics of the bed. Pamukcu et al. (1991) 2]
6.2 Technology Performances and Case Studies
The Section was directly taken in part from a 1997 State of the Practice report by the GWRTAC (Groundwater Remediation Technologies Analysis Center.) insofar as commercially available technologies are concerned. Additional updates on the emerging technology have been added by the author and embedded into the report to make this relatively more complete or updated.
“Due to the specialized nature of electrokinetic remediation, as well as its innovative status, relatively few commercial vendors apply the technology. Therefore, in this section of the report, several trademarked or patented commercial electrokinetic processes are described, as well as additional extended uses of electrokinetics, such as electrokinetic bioremediation and oxidation, and electro-heating to enhance technologies such as vapor extraction, and others. This information is provided for informational purposes only.”
6.2.1 Electro-KleanTM Electrical Separation
Electro-KleanTM is a process available through Electrokinetics Incorporated of Baton Rouge, Louisiana. The process removes or captures heavy metals, radionuclides, and selected volatile organic contaminants from saturated and unsaturated sands, silts, fine-grained clays, and sediments. It can be applied in situ or ex situ. Electrodes are placed on each side of the contaminated soil mass, and direct current is applied. Conditioning fluids such as suitable acids may be added or circulated at the electrodes to enhance the process electrochemistry. The concurrent mobility of the ions and pore fluid decontaminates the soil mass, as the contaminants migrate to the electrodes. Contaminants are separated on the electrodes or separated in a post-treatment unit (Electrokinetics, Inc.).
Electro-Klean extracts heavy metals, radionuclides, and other inorganic contaminants, and can reduce their concentration to below their solubility limit. Treatment efficiency depends on the specific chemicals, their concentration, and the buffering capacity of the soil. The technique proved 85 to 95% efficient for removing phenol at concentrations of 500 ppm. In addition, the removal efficiency for lead, chromium, cadmium, and uranium at levels up to 2,000 mg/kg, ranged between 75 and 95%.
6.2.2 Electrokinetic Bioremediation
Electrokinetic bioremediation technology is designed to activate dormant microbial populations by use of selected nutrients to promote growth, reproduction, and metabolism of the microorganisms capable of transforming organic contaminants. The bioelectric technology directs the nutrients to the organic pollutant. Normally there is no requirement to add microorganisms (Electrokinetics, Inc.).
The economics of this process are favorable because external microbial populations are not required, and nutrients can be uniformly dispersed over the contaminated volume of soil or directed at a specific location, thus reducing nutrient costs. This process may be extremely valuable because it avoids the problems associated with transport of microorganisms through fine- grained soils.
Electrokinetic bioremediation (or bioelectric remediation) technology for continuous treatment of groundwater or soil in situ utilizes either electroosmosis or electrochemical migration to initiate or enhance in situ bioremediation.
Electroosmosis is the dominant process where a direct current can produce an accelerated flow of groundwater in the soil strata. Electroosmosis flow develops more easily in sands, sandy silts or sandy clays. Biological growth factors, including microbial populations, surfactants, and inorganic and organic nutrients, can be moved and often directed into the soil/groundwater matrix. Electroosmosis in this case can be used to accelerate the natural groundwater movement and increase the efficiency of the biodegradation process by the addition of the biological agent into the coarse soils. The flow in coarse-grained soils, however, will tend to follow natural fissures or high permeability lenses and not be uniform throughout the bed. Thus the possibility exists that organic pollutants present in lower permeability areas may not be remediated.
Electrochemical or ion migration is the dominant process in stiff silty clays and mixed clays. Under these conditions, electroosmosis has limited or no effect on groundwater movement. With electrochemical migration, the electrical field will move more uniformly through the soil and ions will readily pass through the small pores in the clay. Natural biological populations tend to exist around organic pollutant spills in soils. Complex organic compounds, however, are not prime energy sources for microbial populations and biodegradation will not flourish until sufficient food, nutrients, and electron acceptors are available to initiate growth and reduce the organic pollutant concentration. The electrical field in this case spreads charged soluble inorganic and some simple organic nutrients uniformly through the site and directs these nutrients to the spatial locations where the food source (e.g., hydrocarbon pollutant) is located.
Limitations of the process include the following:
The concentration of the organic pollutant may be above the toxic threshold limit of the microbial population.
The bioremediation of mixed organic pollutants may produce by-products, which are toxic to the microorganisms, thereby inhibiting the biodegradation process.
6.2.3 Electrochemical GeoOxidation (ECGO)
ElectroChemical GeoOxidation (ECGO) is a patented in situ technology available from ManTech International Corporation (a license of Geotechnologies of Germany) that remediate soil and water contaminated with organic and inorganic compounds. The ECGO in situ works by applying an electrical current to probes driven into the ground. The process utilizes induced electric currents to create oxidation-reduction reactions, which lead to the mineralization of organic constituents (or the immobilization of inorganic constituents) present in a volume of soil and groundwater between the electrode locations (ElectroChemical GeoOxidation, ECGO 19]).
ECGO relies on the induced polarization of naturally occurring conducting surfaces in soil and rock particles. These conducting surfaces are composed of elements such as iron, magnesium, titanium and elemental carbon. Heavy metal impurities that are also naturally occurring further contribute to the process by acting as catalysts for the redox reactions.
Depending on the site conditions, accessibility, and targeted constituents, the ECGO process may take 60 to 120 days.
6.2.4 Electrochemical Oxidative Remediation of Groundwater
Under contract from the Air Force Armstrong Laboratory at Tyndall Air Force Base, SRI International, a non-profit organization with its headquarter in Silicon Valley, California, is developing an innovative technology for groundwater remediation that uses a permeable electrochemical oxidation reactor (PEOR) that is part of an engineered system placed within an aquifer. A stand- alone wall system comprised of porous carbon electrodes and an iron-based catalyst is being designed for installation in the path of a contaminant plume. Utilizing the natural hydraulic gradient, groundwater flows into the permeable wall, where the electrodes are used to generate hydrogen peroxide, which decomposes to hydroxyl radicals in a reaction catalyzed by the iron-based catalyst. The hydroxyl radicals oxidize the organic contaminants in situ, and purified water flows out of the wall into the aquifer. The technology is suited for active pumping or passive groundwater flow. It combines the advantages of an advanced oxidation process with the ability of electrochemical methods to generate oxidants in situ at a controllable rate (Electrochemical Oxidative Remediation of Groundwater, 1997 20]).
6.2.5 Electrochemical Ion Exchange (EIX)
Geokinetics International Incorporated, (GII), a joint venture of five separate companies, uses a combination of electrokinetics and above ground Electrochemical Ion exchangers (EIX’s) to remove ionic contamination from environmental media (Complementary Technologies – Electrochemical Ion eXchange 26] ).
A series of electrodes are placed in porous casings, which are supplied with circulating electrolytes. Ionic contamination is captured in these electrolytes and pumped to the surface where the recovered solution is passed through the electrochemical ion exchanger, which selectively recovers the contaminants allowing the reuse of some of the contaminants.
Because cleaning effluents containing low levels of contamination can be difficult and expensive, EIX can isolate and recover heavy metals, halides, and certain organic species. Typically, inflow concentrations of target species in the range 10 to 500 ppm can be reduced to less than 1 ppm.
Decontamination costs are expected to be in the range of $200 to $325 per cubic meter ($150 to $250 per cubic yard).
6.2.6 ElectrosorbTM
The ElectrosorbTM technology of Isotron Corporation (New Orleans, Louisiana) uses cylindrical electrode assemblies where the electrode is coated with Isotron’s IsolockTM polymer material. The polymer is impregnated with pH-regulating chemicals to prevent fluctuations in pH. The electrodes are placed in boreholes in the soil and a direct current is applied. Under the influence of the current, ions migrate through the pore water to an electrode, where they are trapped in the polymer matrix. If desired, the polymer can also contain ion exchange resins or other sorbents that can trap and hold ions before they reach the electrode. The electrode assemblies and equipment needed for the operation are all commercially available (Department of Energy 1995 27] and ISOTRON Products and Services 1996 28] ).
6.3 LasagnaTM Process
6.3.1 LasagnaTM Public-Private Partnership
In early 1994, the U.S. Environmental Protection Agency (EPA) signed a Cooperative Research and Development Agreement with a private consortium, consisting of Monsanto, DuPont, and General Electric to jointly develop an integrated in situ remedial technology. In early 1995, with significant funding by the Department of Energy (DOE), the work group initiated a field experiment.
General roles of partnership members are (RTDF LasagnaTM Partnership – Remediation Technology Development Forum, 1996 29]):
DuPont: Anaerobic biodegradation and vertical zone installation;
General Electric: Electrokinetic and physiochemical treatment;
The LasagnaTM process so named because of its treatment layers combines electroosmosis with treatment zones that are installed directly in the contaminated soils to form an integrated in situ remedial process. Electroosmosis is well known for its effectiveness in moving water uniformly through low-permeability soils at very low power consumption. Electrokinetics is used to move contaminants in soil pore water into vertically or horizontally oriented treatment zones where the contaminants can be captured or decomposed. Conceptually, the LasagnaTM process would be used to treat inorganic and organic contaminants, as well as mixed wastes.
Major features of the technology are (U.S. Department of Energy – LasagnaTM Soil Remediation 1996 25]):
Electrodes energized by direct current cause water and soluble contaminants to move into or through treatment layers, and heat the soil;
Treatment zones contain reagents that decompose the soluble organic contaminants or absorb contaminants for immobilization or subsequent removal or disposal;
A water management system recycles the water that accumulates at the cathode (high pH) back to the anode (low pH) for acid-base neutralization. Alternatively, electrode polarity can be periodically reversed to reverse electro-osmotic flow and neutralize pH.
The orientation of the electrodes and the treatment zones depends on the site/contaminant characteristics. In general, a vertical configuration is probably applicable to shallow contamination (within 15 meters [50 feet] of the ground surface), whereas a horizontal configuration, using hydraulic fracturing or related methods, is capable of treating much deeper contamination (U.S. Environmental Protection Agency 1996 – LasagnaTM Public-Private Partnership 24]).
6.3.3 Technology Status
Phase I – Vertical Field Test at the DOE Paducah Gaseous Diffusion Plant (PGDP) in Kentucky focused on in situ TCE (trichloroethylene) remediation. The test operated for 120 days and was completed in May 1995. The zone to be remediated measured 4.5 meters wide by 3 meters across and 4.5 meters deep (15 ft by 10 ft by 15 ft deep). The average contamination was 83.2 ppm, and the highest TCE concentrations (200 to 300 ppm) were found 3.5 to 4.5 meters (12 to 18 feet) below surface.
About 4% of the total TCE was lost through evaporation. Soil samples taken throughout the test site before and after the test indicate a 98% removal of TCE from a tight clay soil (hydraulic conductivity <10-7 cm/sec), with some samples showing greater than 99% removal.
TCE soil levels were reduced from the 100 to 500 ppm range to an average concentration of 1 ppm.
DNAPL (dense non-aqueous phase liquid) locations were cleaned to 1-ppm levels except for a 4.5 meter deep sample that was reduced to 17.4 ppm. Because treatment zones were only 4.5 meters deep, diffusion from untreated deep zones may have contributed to the 17.4 ppm result.
Phase II – Vertical Field Test, also conducted at the DOE PGDP, will modify the Phase I configuration by using zero-valent iron in the treatment zones chemically reduce TCE to non-toxic end products. The zone being remediated measures 6 meters wide by 9 meters across and 13.5 meters deep (20 ft by 30 ft by 45 ft deep). This is approximately 20 times more soil (1,360 tonnes, or 1,500 tons) than was treated in Phase I. Phase II is to help resolve scale-up questions, substantiate technology cost estimates, and evaluate the performance of zero- valent iron in the treatment zones. The test is scheduled to be complete on August 4, 1997.
Various treatment processes are currently being investigated in the laboratory to address other types of contaminants, such as heavy metals and mixed wastes (U.S. Department of Energy – LasagnaTM Soil Remediation 1996 25]):
6.3.4 Process Advantages
Effective in low permeability soils (hydraulic conductivity <10-5 cm/s)
Contaminants can be destroyed underground
Silent operation
Rapid installation, low profile
Relatively short treatment duration (RTDF LasagnaTM Partnership – Remediation Technology Development Forum, 1996 29])
6.3.5 Costs
Direct treatment costs of a 0.4 hectare (one-acre) site similar to that used in the Phase I test are estimated at $105-$120/m3 ($80-$90/yd3) for remediation in one year and at $65-$80/m3 ($50-$60/yd3) of soil if the remediation could occur over a period of three years. Comparable estimates for the Phase II mode of operation are $80-$90/m3 ($60-$70/yd3) for one year, and $50-$60 ($40-$50/yd3) for three years. Deeper contamination, although involving more technically challenging emplacement, costs less because of the larger area of influence per electrode (In Situ Solvent Remediation 30]).
6.3.6 Cost Savings Versus Alternative Technologies
DuPont has benchmarked a number of in situ technologies over the last three years. These include:
In situ treatment zones using iron filling for dehalogenation of chlorinated solvents
Pump and treat of contaminated groundwater
In situ aerobic biological dechlorination
Surfactant flushing
Costs for these technologies, some of which require more than 30 years to remediate a site, are between $35 and $100/m3 ($25-$75/yd3). LasagnaTM is within the range of these competing technologies with an implementation cost (over three years) of about $65m3 ($50/yd3), using the method proposed for Phase II.
Use of treatment zones for in situ destruction of contaminants gives LasagnaTM a competitive advantage over other electrokinetic methods that extract contaminants for aboveground treatment or disposal. Because treatment zones eliminate the need for aboveground waste handling, and are presumably cheaper to make and install than electrodes, their use imparts cost advantages.
Typical costs for full-scale installation to treat a zone measuring 0.4 hectares, 13.5 meters deep (one acre, 45 feet deep): (Cauwenberghe, Liesbet Van, 1997) 17]
Cost estimate includes excavation, transportation and land filling fees. Example assumed 30% of soil is disposed in a hazardous landfill and 70% in sanitary landfill.
Capital includes installation, materials, rectifiers and other fixed costs.
Operation is assumed to last one year. Costs include electricity and labor.
Sampling costs are assumed to be an average of $6.50 per cubic meter ($5 per cubic yard).
7.0 ADVANTAGES AND DISADVANTAGES
7.1 Technology Advantages
Electrokinetics may be utilized for site remediation under conditions, which normally limit in situ approaches, such as in the case of the following:
⦁ Can treat both organic and Inorganic contaminants. Recovery of ionic contaminants by conventional means is complicated due to soil being a powerful ion exchange medium. Ionic contaminants are adsorbed and absorbed in soil particles, and are often not available for removal by the simple flushing action of groundwater. A pH shift must be applied to desorbs and mobilize the contaminants. However, flushing with strong acids usually destroys the basic soil structure, and may thus be self-limiting. Electrokinetics may be applied to mobilize contaminants without such concern, because acids are not pumped directly into the soil. Electrolysis of water in the circulating electrolyte produces the H+ ions at the anodes and the OH- ions at the cathodes. These ions migrate through the soil, generating a localized pH shift, which desorbs contaminating ions (Geokinetics, Inc. “Electrokinetic Remediation of Soil and Groundwater”, 1997 31]).
⦁ Effective even in soils of low hydraulic conductivity. Fine- grained sediments or low permeability soils present the greatest obstacle to in situ remediation at many contaminated sites. In clay and tight soils, hydraulic flow through fine pores is extremely limited, making these soils non-responsive to traditional soil flushing. Accessibility of the contaminants and delivery of treatment reagents have posed problems, rendering traditional technologies, such as vapor extraction and pump- and-treat, rather ineffective when applied to the low permeability soils present at many sites. Electrokinetics is an effective method of inducing movement of water, ions and colloids through fine-grained sediments (Murdoch, et. al 1995) 4]
⦁ There currently are no other viable in situ methods of remediating heavy metals contamination from unsaturated soils. Excavating and processing, or disposal at a licensed landfill, will not always be feasible and will always be expensive (Electrokinetic Remediation of Heavy-Metal- Contaminated Unsaturated Soil 1995). If the total area of metals contamination is relatively small (1 acre or 0. Hectares) and highly concentrated, the application of a conventional technology like excavation or solidification might prove to be more economically feasible. But as excavation does not work well adjacent to buildings, small areas with high concentrations located in close proximity to structures may be more receptive to electrokinetics.
⦁ Allows treatment accessibility to soils not available for excavation.
When cost-effectiveness and technical feasibility of other remedial options prohibit their use, electrokinetic remediation may offer an alternative at sites contaminated with inorganic species.
7.2 TECHNOLOGY LIMITATIONS
Based on the results of laboratory tests and field applications, electrokinetics has been shown to be a promising method of covering ionic and water-soluble contaminants. However, the process has associated limitations, such as: (Murdoch, et. al 1995).4] The electrokinetic process is limited by the solubility of the contaminant and the desorption of contaminants from the soil matrix. Heavy metals in their metallic state have not been successfully dissolved and separated from soil samples.
The process is also not efficient when the target ion concentration is low and non-target ion concentration is high.
Acidic conditions and electrolytic decay can corrode some anode materials.
Conventional electrokinetic remediation requires contaminants to migrate from their initial location to an electrode. In some cases, the migration path could be long or there could be stagnant zones between wells where the rate of migration is particularly slow, both of which result in incomplete remediation of the contaminated zone. Moreover, sharply convergent electrical fields can result in heating and potential losses in the vicinities of electrodes. A pH-related deposition can cause contaminants to be removed from solution prior to arrival at the ground surface of point of removal.
Electrolysis reactions in the vicinity of the electrodes may cause changes in ambient pH that may change the solubility and speciation of the contaminants.
Heterogeneities or subsurface anomalies at sites, such as building foundations, rubble, large quantities of iron or iron oxides, large rocks or gravel, or submerged cover materials such as seashells, can reduce removal efficiencies. Immobilization of metal ions by undesirable chemical reaction with naturally occurring and co-disposed chemicals can also occur.
The presence of buried metallic conductors or insulators in the soil and reduction/oxidation and pH changes induced by the process electrode reactions can reduce the effectiveness of the process.
Precipitation of species close to the cathode has been an impediment to the process. Heavy metals can prematurely precipitate close to the cathode at their hydroxide solubility value if the chemistry of the electrolyte at the electrodes is not altered or controlled (unenhanced electrokinetic remediation). Currently, studies conducted by Electrokinetics Inc. and the U.S. Army Waterways Experiment Station are underway to overcome the problem of precipitation close to the cathode, and the feasibility of employing different techniques to enhance the process are being evaluated. An objective of the studies is to promote transport of the positively charged species into the catholyte where they could be removed by electrodeposition, membrane separation, or ion exchange.
8.0 ELECTRODE EFFECTS
8.1 Corrosion Effects
The use of reactive metal electrodes eventually results in the accelerated corrosion of the electrode metal. The use of steel or copper electrodes causes anodic reactions of the electrode material, which are preferred over that of Hydrolysis of water. Aside from breakdown of the reactive metal to its ionic products, the reaction at the anode causes the suspension of water hydrolysis from taking place thus preventing generation of H+ ions, which in turn prevent the propagation of an acidic front that would have traveled through the soil. The prevention of acid generation through the soil may not be disadvantageous at all, if the objective is not to dissociate stable metals that are adsorbed by the soil and when only organic pollutants are targeted for removal.
In some instances, the use of reactive metals is objectionable, as the metallic breakdown products could in themselves become a source of pollution such as copper and other reactive metals. Thus, selective use of electrode materials could affect the conduct of the electrokinetic process and the generation of other species.
The use of reactive metals as electrodes such as steel produces different reactions. At the anodes, the preferential electrochemical reaction is with the more negative standard potential (voltage). At the cathodes, the opposite is true and preference is towards the more positive standard potential (voltage) Doering & Doering (2001) 3] illustrated this with the following half reactions:
Thus, the reaction involving Iron (Fe) will prevail. The steel anode serves as a sacrificial anode and is oxidized since the (-) voltage predominates. Thus, the half reaction oxidative water electrolysis cannot take place.
In some specific instances, the use of reactive metal electrodes would be desirable in order to suppress generation of the Acidic front which could undesirably mobilize metallic compounds which otherwise could be safely “locked-in” and adsorbed by the soil.
The positive potential E°=+0.401V in equation (5) predominates causing the increase in pH with the generation of 2OH-. However, in the field, it was observed. Doering & Doering (2001) 3]
Those additional reactions take place:
Neutralization by CO2
Formation of hydrocomplexes and decomposition of hydro complexes due to changes in pH.
Natural buffer effects of the soil, which minimize the production of hydrogen.
The foregoing illustrates how by controlling the acid front, the mobilization or immobilization of heavy metals can be controlled through the use of reactive electrodes such as steel.
Also, in simple processes such as dewatering the soil, the EK process is primarily geared to inducing electroosmosis without the need for generating additional electrochemical reactions. In such cases, the use of inexpensive steel electrodes is indicated.
8.2 Use of Various Materials for Electrodes
More noble metals such as Titanium rods or titanium-plated rods have been used in order to prevent corrosion setup by preventing any anodic reactions of the base metal from taking place. Thus, only Hydrolysis reaction occurs allowing the generation of H+ ions and allowing the propagation of the acidic front as in the normal EK process. The same objective could be achieved using non- reactive electrodes such as Graphite or other Carbon electrodes.
In addition, non-reactive Titanium coated wire screens have been used in laboratory bench scale tests for research purposes. The use of Carbon Electrodes in its various forms have also been successfully employed in order to prevent electrode reactions from taking place as well as be a cost effective substitute to more expensive metal electrodes.
9.0 DIRECTIONS FOR FUTURE RESEARCH
Electrokinetics is fast emerging as a cost effective In-situ and Ex-situ Soil remediation technology for the removal of Organic and Inorganic contaminants. Numerous field scale tests have proven the commercial viability and technical effectiveness of the process when compared to other commercially available methods. The ability of EK to enhance the removal process by various mechanisms has been shown to demonstrate its effectiveness in ground remediation technology. In addition, these mechanisms and their effects can be tailored or altered in order to:
Speed up removal with the use of reagents, chemical surfactants etc.
Lock in non-critical contaminants in the soil by immobilization
Enhance removal of target contaminants while retarding some.
Work with other Ground Remediation processes for overall system effectiveness:
Enhance the effects of reactive barriers
Introduce or inject biosurfactants to enhance bioavailability for Bacterial attack of Organic contaminants
Assisted Increased generation of Acids for accelerated desorption of chemical species.
However, there is still a lot of ground to cover in this newly deployed technology, which are subject to further research and investigation. Among these are:
⦁ Electrode Material – Most field technologies use expensive non- reactive metals as electrodes such as Titanium or titanium coated metals. The consideration for the use of Carbon forms (Graphite, Activated Carbon or Carbon Fibers) needs to be further exploited, as these are relatively inexpensive and easier to produce. In addition, Carbon in its various forms is available and indigenous to almost all countries. Particularly in the third world, where Environmental controls in the past have been absent or sadly lacking, use of Locally available and cheap electrodes could render the technology available to the poorest of nations.
⦁ Study of various Electrode Geometries to enhance electroconductivity or allow increased surface area exposure. Use of hollowed out electrodes to allow pumping in and out of absorption media and chemicals to enhance the EK Process.
⦁ Electrical consumption- Electricity usage of the process although still reasonable can be further reduced to increase the cost effectiveness of the process. This can be addressed with the use of more electroconductive electrodes, enhancing the soil’s electroconductivity by addition of chemicals, etc.
⦁ Use in Hybrid technologies- recent advances have been made in this regard such as the use of EK with Reactive barriers, layered treatment systems such as the Lasagna process and use in conjunction with Biodegradation methods in the soils.
⦁ Research into other Reagents and chemical processes that can decompose the soluble organic contaminants or absorb contaminants for immobilization.
⦁ Introduce the use of chemical or active absorbents in the Electrode well such as actuated carbon to capture heavy metals or react with other chemical contaminants.
⦁ Coupling of external contaminant recovery systems into the EK hydraulic circuit to remove contaminants and cycle electrode well water to further enhance the EK Process.
10.0 SUMMARY
This paper has reviewed Electrokinetics from its historical evolution to what it is today as a viable technology for the In-situ and ex-situ remediation of contaminated materials. The review hopefully has provided for a better understanding of the Electrokinetic process including its applicability for various ground contamination situations.
While the Review does not claim to provide for a very thorough or complete and authoritative report on the “state of practice” or the “state of the Art”, it is hoped that this work can be used as a starting or “ jumping off” point by the readers to gain at least a basic or fundamental understanding of the Electrokinetic process and move onwards. Work in this topic is Dynamic and various sources in the Internet and from expected numerous to be published researches can add to the body of knowledge already presented herein to augment and update research in this field.
Significant advances and successful practical applications of the technology have spurred increased interest in this method for the in-situ removal or treatment of contaminated ground. Because of its versatility, Electrokinetics can be combined with other procedures and processes in order to remediate contaminated ground. In addition, Electrokinetics lends itself to enhancements by chemical means depending on the target pollutant.
Electrokinetics can also selectively target specific contaminants for removal, while immobilizing or locking other contaminants, which are best left in the ground where they cannot do significant damage nor be a source of significant groundwater contamination.
Numerous researches are currently ongoing in various parts of the world in order to further enhance the effectiveness and cost performance of Electrokinetics.
A better understanding of the process and its mechanisms are undoubtedly needed to foster more research and practical commercial applications of the Electrokinetic process. A clearer understanding no doubt can point the way to enhanced methods and optimization of the process leading to more widespread commercialization and practical cost effective solutions to serious ground pollution or contamination.
Particularly for third world countries where significant quantities of hazardous materials have been dumped into the ground and which threatens groundwater as well as the health of the populace, a cost effective deployment of Electrokinetics procedures for ground treatment using indigenous materials and technologies can help immensely in the removal of hazardous substances in the ground. It is hoped that the objectives of this paper have been met and a clearer understanding of the Electrokinetic process is realized through this paper.
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The tragic loss of life in the Cherry Hills landslide attracted a lot of media attention and with it a lot of speculations an rumors as to what caused the slide. Several professional organizations and government agencies were also involved in the various studies and investigations to understand what really happened. The Philippine Institute of Civil Engineers (PICE) and the Association of Structural Engineers of the Philippines (ASEP) banded together through a fusion of its Geotechnical committees to offer the services of volunteer members to investigate, document and come out with a factual report on what happened. This report summarizes the work of the committee, including the methodologies and tools employed in reconstructing the conditions before and after the landslide, its findings, conclusions and recommendation. Guidelines adapted from numerous sources are also included herein to help the Engineering community and the public in understanding potential hazards. It is not the intention of this committee to pinpoint responsibility or determine the culpability of any person or organization, public or private. This matter is best left to the courts to decide.
1 Chairman – PICE/ASEP Geotechnical Team Investigating the Cherry Hills Land Slide.
INTRODUCTION
The Investigation of Landslides presents a very interesting yet very difficult Challenge. It involves a more detailed understanding of the facts at hand in order to reconstruct what happened and how it happened. Unlike standard design projects involving slopes, where, the parameters and geometry are known as well as the anticipated environmental conditions, landslide investigations have to deal with a lot of unknowns and involves the formation of several assumptions of hypotheses which need to be tried out, studied, eliminated, validated and only then can conclusions be made and accepted. In the meantime, these assumptions need to be compared with the actual slide conditions to ensure that the theory and assumptions agree with each other, likewise be presented. Findings and conclusions on the results of the study will also be presented.
SLIDE NOMENCLATURE
The diagram illustrates a typical landslide and the components that make up the slide. (See Figure 1.0)
DATA GATHERING PROCEDURES
General
A slide event needs fast response if we need to ensure that critical evidences and telltales are not obliterated or further disturbed by rescue operations or by washouts or additional mudflows or debris flows. Thus it is important to mobilize the investigation team as fast as possible in order to gather as much information as possible before disturbances to the slide affected areas occur.
Field Reconnaissance and Site Interviews
The site reconnaissance is one of the most important field activities if done as soon as possible. A good site reconnaissance could record important data and information which otherwise could have been lost but which would be critical to a proper and adequate understanding of how the slide occurred and the mechanism involved. Interviews with the locals or residents in the slide affected areas is also very important in determining whether there were any telltale signs of an impending slide and what were the conditions immediately predecing the landslide event.
Aerial and Site Photographs
Particularly for slides of large extent, Aerial and site Photographs would be very valuable in establishing the extent of the slide and whether other slides could have been initiated by the main landslide event. Photographs could permanently record conditions at the site, which may not be readily apparent or could have been missed entirely in the initial walkthrough survey. Such visual records would prove invaluable in looking at several hypotheses.
Mapping of Extent of slide Geometry
The extent of the slide needs to be mapped approximately particularly very important if an aerial survey cannot be done due to limitations in time, budget or availability. The slide geometry may not be readily apparent in photographs but sketches and notes taken could sometime be very crucial to the understanding of the slide mechanism. Understanding the slide Geometry could also aid in guiding the analytical and computational approaches or tools that could be used to analyze the slide.
Study of the Geology of the Site and the Nature and Character of the Soil Deposits and Rocks
Understanding the Geology of the area is important in piecing together the various pieces of the puzzle. The following items need to be considered:
Orientation and dip of Bedding Planes
Manner and origin of deposition
Intensity of Jointing and fracturing in the case of rocks
Relic slides
Geochemistry of soils and rocks
Understanding the Environmental factors likely to have influenced the slide
Environmental effects causing destabilization almost always precede landslides. These is due to climatic, hydrogeologic, chemical and other natural environmental effects and changes, in addition to the artificial disturbances caused by man. A very dominant factor in the initiation of landslides is the sudden rise or lowering of the water table, which could induce the same effects.
An increase in the water content of soils or rocks, leading to saturation could reduce effective stresses and also cause degradation of fine-grrained soils. In addition, the original dry mass becomes heavier due to absorption of water and sliding follows particularly after prolonged heavy rains. Similarly particularly in dam embankments where sudden drawdown occurs, the saturated soil loses the buoyancy support from the water and thus slides downslope of the embankment.
ENGINEERING STUDIES
Reconstruction of Slope Geometry
Engineering plans or records of the preslide slope configuration normally would be available from the owners of the affected property for developed areas. In the case of Mountainsides or Natural terrain, the use of Topographic maps such as the NAMRIA series could be used to recreate the slope prior to the slide. Of course when developments or interventions by man are not recorded in plans or maps, the task of reconstructing the slope geometry prior to the slide becomes very difficult and would involve a lot of field measurements and surveys.
Subsurface Soil Exploration and Insitu Tests
Most often and particularly if no such investigation have been done in the past, subsurface borings would be needed to define the nature and character of the soil and rock deposits in order to determine with some degree of certainty the strength condition prior to failure. It is also important to know how the environmental factors have affected the original soil and rock condition.
Necessarily, the soil borings need to be performed in the slide zone to determine the vertical extent of the slide from detection of the extent of disturbance and residual strength and also at the unaffected areas to serve as a benchmark comparison.
Shallow test pits are also an inexpensive means to gather more information about the slide. These field tests would have to be supported by Laboratory testing to classify the soils and rocks and also determine their physical as well as Chemical properties. Geophysical methods such as Electrical resistivity logging and Seismic Refraction surveys could aid in establishing the true vertical extent of the slide by being able to discriminate between disturbed and undisturbed zones.
Analyses Procedures
After all data gathering is nearly complete, trial and preliminary back of the envelope calculations can be done and published nomograph could be used as a “first pass” analysis using simplifying assumptions. Once a general idea is formed more detailed Slope Stability using back-analyses procedures need to be started. As the name implies, back analysis is the reverse of a conventional Slope stability analysis. Because of the highly interactive procedures and computationally intensive tasks, this is best done using dedicated programs. The procedure begins by assuming first a homogeneous soil or rock mass and determining the critical failure Surface (Lowest Factor or Safety) by assuming a set of strength parameters (c and phi) and setting the program to search for the most critical failure surface. (The one with the lowest Factor of Safety). Once the critical slip surface or surfaces is/are identified the following Back Analysis procedure takes place:
A set of strength parameters (c and phi) is assumed and an interactive search is made on the most critical slip surface by setting c constant and varying the value of phi until a factor of safety of 1.0 (impending slide) is obtained.
The value of c is then changed (increased or decreased) and a corresponding value of phi is solved iteratively to yield a factor of safety of 1.0.
The Family of paired values of (c and phi) normally would fit a straight line. This line defines the most probable value sets that could have been present immediately prior to slide initiation.
Of course it must be understood that this exercise has greatly simplified the search routine by assuming a homogeneous mass which would be unrealistic. However, this could also lead to an understanding of the failure mechanism, which could expedite the detailed analyses that follow. Additionally, this leads to a concept known as the “characteristic strength of the slope” which for lack of a better description is a grossly simplistic attempt at quantifying the overall strength of the slope material at slide inception. Knowing what this is could lead to a quantification of the other external factors that have more than likely contributed to the slide.
Use of Empirical Methods and Criteria by Studying Records of Landslides in the past
Empirical methods are also available to determine the most probable set of strength parameters at slide inception by comapring this with historial records of known slides where back analyses have been performed. In addition Bieniawski has provided an empirical procedure in determining the quality of Highly Fractured rocks by a term known as the Rock Mass Rating (RMR). Several authors have further extended the RMR, notably Hoek in order to quantify the strength of these fractured rocks by relating them to the RMR and other qualitative properties to come out with “m” and “s” parameters. These are then in turn substituted into the Hoek-Brown formulation of a curved Failure envelope using Mohr’s circle.
Normally, although not always, the two preceding procedures although independent of each other would find fairly acceptable agreement. Obtaining convergence allows the investigator to go into more detailed analysis by factoring in multilayered slopes with varying strengths, etc.
Factoring in Environmental Influences
The influence of external environmental factors likely to have contributed to the slide to any degree needs to be considered and factored in the detailed Slope stability analyses. These factors are:
Groundwater levels at time of slide
Amount of precipitation
Possible man-made disturbances
Performing the Detailed Slope Stability Analyses to Identify Candicate Failure Modes
Once the results of the two procedures agree to a fairly acceptable degree, a more detailed analysis can be undertaken factoring in all the factors that are likely to have influenced the slide. The detailed analysis may or may not probably represent the true conditions at slide inception. Nevertheless, its generation and the detailed and painstaking studies leading to it would invariably gain for the investigator a better understanding of the slide and attain a more solid foundation to rest on, compared to a study that suffers for lack of a detailed methodology and procedure and the absence of realistic and factual data.
Comparison of Results with Actual Post Slide Geometry
As the saying goes, the “proof of the pudding is in the eating”. Nowhere is this saying more evident than in this stage when the theoretical failure geometry is arrived at and compared or matched with the actual Failure geometry. It goes without saying that these two should match otherwise failure geometries and perhaps revised strength parameters need to be restudied.
CONCLUDING THE POST SLIDE INVESTIGATION
The post slide investigation does not end with the Preparation of Report on Findings and explaining what happened and how it happened.
The real value of the investigation is when it adds to the body of knowledge and contributes to the welfare of society by recommending steps to prevent future similar landslides. Identifying, the need for corrective works and remediation procedures is a critical component of any investigation report n order to prevent the initiation of future landslide from the unstable and weak geometry of the Relic of the slide. Means should be provided in order to stabilize the slide and prevent the formation of other slides in the disturbed areas.
Synopsis: Almost ten years ago today, a landmark Reinforced Concrete Building serving as General Headquarters for the Armed Forces of the Philippines was Burned for the Third Time. This paper discusses the Investigation, Design, Construction and Monitoring Procedures used in the Structural Rehabilitation. Extensive use of Computer Analyses and Design as well as Computer Aided Drafting (CAD) including computer generated 3D details of critical connections, other than speeding up the design process resulted in effective communication of Designer’s intentions in a timely manner to all concerned. The cost of rehabilitation, not to mention the preservation of the Historical Value of the Building and reduced time to occupancy led to the owner’s decision to rehabilitate rather than demolish and reconstruct this Building.
1. INTRODUCTION
The last fire at the AFP General Headquarters Building resulted in an Engineering assessment being commissioned by the client to determine the viability of further rehabilitation efforts and to determine the integrity of the various Structural Elements that were involved or exposed to the Fire. Our office was retained to undertake this study and was subsequently engaged by the owner to undertake the Detailed Engineering needed for the rehabilitation and structural upgrading of the whole Building.
This paper discusses the steps and procedures undertaken by this office from the start of the investigation, to the Detailed Analysis and Design Engineering and the inspection and monitoring during the construction.
2. ENGINEERING BACKGROUND
2.1 Investigation Phase
The Investigation Phase was done over a Three (3) month period in 1988 culminating in the submittal of a report which concluded that although major repairs would be needed, the Historically Valuable Building could be saved despite three fires and two rehabilitations, without demolishing the Historical Facade (Figure 2) which was the focal point of the whole General Headquarters Complex of Camp Emilio Aguinaldo, the Headquarters and Administrative complex of the Armed Forces of the Philippines.
2.2 Methodology
The investigation study required destructive and non-destructive tests. Numerous cores were extracted in suspect concrete after numerous Rebound Hammer Tests were performed to indicate the location of relatively weaker concrete. In addition, steel reinforcement was extracted for tensile tests in some areas to serve as bases for our subsequent Structural Analyses and Investigation.
Reinforcing bars were exposed at critical structural sections such as at Beams, Girders, Column and Slabs to determine spacing and sizes of reinforcing bars for the existing structure. Fortunately, Plans of the First and Second Rehabilitation Programs were still available and were verified to be relatively accurate except in some instances, for which field changes were required during the ongoing rehabilitation. The rehab programs undertaken prior to this study involve buildup in concrete members one on top of the other for affected areas.
2.3 Results of the Investigation
From the results of the numerous rebound hammer readings undertaken, it was already evident that there were large disparities in the strengths of the two concrete layers. Figure 3 shows the existing configuration before rehabilitation.
The lower layer for Beams and Girders is composed of what we now term as the “original” concrete used in the original construction and the “rehab” concrete on top and integrated with the R.C. Slab used in the first and second structural rehabilitation. The original concrete on the underside showed very low concrete strengths, often preventing extraction of intact cores. Evidently, the original concrete in both Wings had been affected by exposure to Fire as borne out by the rebound hammer tests, the core extraction and subsequent tests on the intact cores.
Figure 4 is a chart of the results of Unconfined Compression Tests on intact cores. Of the more than 78 cores extracted, all of the low concrete strengths (below fc’=2500 psi) were obtained on the original concrete. Zero strengths represented concrete cores which disintegrated during extraction mainly from the original concrete.
All of the later rehab concrete showed consistently high concrete strengths averaging approximately 3500 psi. These results were predicted and anticipated by the numerous rebound hammer tests before the corings were performed.
From the foregoing, we were able to make the following conclusions:
The original concrete used in the original building construction was of relatively lower strength compared to more recent Rehab Concrete. This is probably due to the following:
The original concrete had been subjected to at least two previous fires, thus damage was progressive.
Poor quality concrete was used in the original construction.
The original concrete would pose a danger to building occupants if left in its existing condition.
Newer “Rehab” concrete although being subjected to the same fire exposure during the last fire has been relatively unaffected by it.
There was no need to totally demolish or condemn the Building and only structural strengthening of the historically important Building would be required to restore it to its former function.
The strengthening and rehab measures to be implemented, would also be directed towards upgrading the structural performance of the Building and bring it up to Present Day Seismic Code Design Standards.
Other than the foregoing which were purely of technical nature, the decision to go ahead on the structural rehabilitation was also dictated by practical reasons which weighed heavily in favor of rehabilitation rather than outright total demolition and reconstruction.
The practical considerations are as follows:
The cost of structural rehabilitation is approximately thirty four percent (34%) of the cost of a totally new Building with the same floor area.
Based on the winning bid, the cost of the Civil/Structural Works for the rehab scheme amounted to only one third of the cost per square meter for the demolish/construct scheme.
Time – Time to occupancy would be greatly shortened from a minimum of eighteen (18) months for a demolish/construct scheme to about five (5) months for the rehabilitation scheme eventually adopted.
Historical Value -There was a need to preserve the historical value and architectural details of the Building.
3. ENGINEERING ANALYSES AND DETAILED DESIGN
Several findings and engineering decisions were made during the structural analyses and detailed design stages as follows:
The Preliminary Structural Analyses showed that the existing structural system even without the effects of the fire would be inadequate to sustain Seismic Lateral Forces.
The existing details, even for the rehabbed portion, will not comply with Seismic Detailing Standards for Zone 4 existing at the time as mandated by ACI 318-83 and the National Structural Code of the Philippines and therefore structural strengthening would be necessary.
In the structural analyses, the contribution of the original column core concrete was entirely neglected and the new column was idealized as a Hollow or Box Column enclosing the original concrete core. A specific custom program was developed in-house to analyze the hollow column and generate column interaction diagrams for various cases of loading.
3-D Computerized Frame Analyses were conducted. The East Wing, West Wing and Central Core were idealized as independent structures because of the provision of a Seismic Joint in the original construction. The subsequent strengthening details also preserved this Seismic Joint.
The rear portion of the Central Core was totally demolished and replaced by a new structure integrated with the existing core. This was necessary as this part was still supported on Timber Flooring during the last fire and relatively was more severely damaged.
On the basis of the analyses results, a Reinforced Concrete Seismic joint detailed to existing Code was found adequate. The most important feature of this detail is the provision of additional Vertical Reinforcing bars through the joints to provide for continuity. Confining ties were placed in addition to the confinement by additional rehab concrete on the Beams and Girders meeting at the joint. (See Figure 5)
The horizontal beam and girder bars were in turn confined within the column vertical bars. The illustration of Figure 6 shows the Rehab Scheme adopted for the Beams, Columns and Slabs.
It is important to note here that although the East Wing Columns have been earlier rehabilitated after the previous fires, it was still necessary to provide for additional vertical reinforcing bars through the joints to be confined with closely spaced ties as the existing details were found to be inadequate. Again, in the Structural Analyses of this system, the existing core concrete and reinforcing bars were neglected thus imparting a far bigger “Factor of Safety” in reality.
Negative moments for the existing Beam and Girder needed additional reinforcement for continuity and also to increase lateral load resistance. Additional reinforcing bars were also needed to increase positive moment carrying capacity.
The negative Bars were placed and confined within the new column vertical bars and new ties were integrated to confine the reinforcing bars as well as the positive moment reinforcing bars which were threaded in to the existing Beam hoops.
3.1 Computer Analyses and Design
The computer analyses and design was carried out using M-STRUDL, an enhanced microcomputer implementation of the STRUDL Software written in C Language which was popular at the time. In-house developed software for the analysis of the Hollow or Box Column was used as earlier stated as well as spreadsheet programs for generating column interaction diagrams.
4. REHABILITATION PROCEDURES
4.1 General
Great emphasis was placed on the value of effective bonding between the old and new concrete requiring the use Structural Epoxy Bonding agents. In the process, stringent surface preparation procedures were required to ensure the effectiveness of the Bonding.
We assigned a Senior Structural Engineer on full time basis to monitor and ensure that surface preparation and epoxy application are carried in accordance with the specifications and to oversee rehabilitation in general.
In addition to this, detailed structural rehabilitation procedures were included as part of the plans and specifications to set the minimum basis for rehabilitation. This included outline or step by step procedures for chipping, demolition, surface preparation, epoxy application and reinforcing bars installation. In order to forestall any misinterpretation of Plans and Specifications, the Contractor was required to submit a detailed construction methodology for acceptance and approval by the owner through the consultant. This requirement identified a lot of areas that were overlooked during the bidding stage and was very invaluable in this regard as it enabled the owner and engineer to check the Contractor’s intentions and directions for the rehab even before fieldwork started. Revisions and changes in the methodology were made in the course of construction, nevertheless, this requirement helped immensely in identifying possible problems during the implementation.
4.2 Computer Aided Drafting and Detailing
Three Dimensional (3D) drawings and reinforcing bar layouts as in Figure 6 & 7 were prepared to ensure that the intended reinforcing bars details were faithfully carried out in the field by the Contractor. Extensive use of CAD to generate 3D Plots were made. The extent of detailing required and implemented using CAD would defy manual efforts, given the time pressure. Thus, the decision to fully utilize our CAD Graphics facility which was still relatively crude at the time, to support the project and generate 95% of the drawings proved to be a wise one and was effective in ensuring a clearer understanding of the complicated reinforcing bars layouts. In addition, changes necessitated by unforeseen field conditions could be done easily by just revising the electronic files.
4.3 Detailed Rehabilitation Requirements and Procedures
Based on the findings during the investigation phase, it was decided that all original Beam and Girder concrete would have to be removed and replaced as this could fall off or spall off during an Earthquake. Because of the adequacy of the later rehab concrete for the Slabs and Top portions of Beams and Girders, it was decided to retain and integrate these with the new rehab concrete. This was a major cost saving. The existing columns’ concrete cover would have to be removed to expose the old reinforcing bars and a new rehab concrete and reinforcing bar cage were installed to envelope this core concrete. Vertical column reinforcing bars as well as Beam/Girder negative bars and positive moment bars were made continuous through the joints and confined with Hoops and Ties in accordance with ACI 318 and 315. Extensive use of shoring was specified and required as early as the bid stage in all instances to prevent any movements in the Building Frame during rehabilitation when it was very vulnerable.
Due to the fragile and weakened nature of some elements, the use of heavy Pneumatic Jackhammers was prohibited and only small hand held rotary air hammers and bush hammers were allowed to be used to demolish the original concrete. Vibrations during construction operations was carefully avoided and extensive use of shoring was required.
The existing original reinforcing bars for Beams, Girders and Columns were retained and integrated with the new rehab reinforcing bars and concrete. This in a way compensated for the setting up of creep/shrinkage stresses affecting the structural elements due to the bonding of the old and new concrete.
4.4 SURFACE PREPARATION
The original concrete cover was chipped off (Figure 9) to expose the existing reinforcing bars. Cleaning of the reinforcing bars proceeded immediately using a wet type sand blasting equipment until all the reinforcing bars were to bare white metal finish.
High pressure water spraying (Figure 8) was used immediately before epoxy application in order to remove all loose and defective concrete and for final cleaning of reinforcing bars. This utilized a 6,000 psi (41.3 Mpa) High Pressure washer. The quality of bonding surface and rough texture was assured by this final surface preparation procedure.
The tight spaces and reinforcing bars cages required epoxy application by hand brushing. It was therefore necessary to require that the epoxy would have an adequately long “overlay time” of at least six (6) hours to allow application, erection of forms and pouring of concrete. This requirement meant that the forms would have to be collapsible and allow for easy installation. Prefabricated laminated plywood forms were specified to eliminate the need to refinish the stripped surfaces.
4.5 CONCRETING
It was necessary to pour the concrete columns in two lifts with the last lift being poured monolithically with the upper Beam Column joint (Figure 10). Maximum 3/8” (9.5mm) aggregate was specified to ensure that the congested areas are effectively filled by concrete. Breather holes were punched in the slabs to prevent entrapment of air that could block the flow of concrete. Very small diameter hand held electric Driven Pencil Poker Vibrators were utilized in most instances due to the tight spaces involved. The Concrete Mix required the use of a plasticizer because of the low water cement ratio (0.42) required on top of the minimum guide specification of fc’=4000 psi (27.6 Mpa) Concrete Cylinder Compressive Strength. The low water cement ratio was required to reduce shrinkage to a minimum. Wet curing of the poured concrete elements also helped to reduce shrinkage. The more than liberal distribution of new and old reinforcing bars in a way prevented shrinkage movements that would have otherwise caused concrete to crack.
4.6 Construction Methodology
Unlike conventional construction projects, this rehabilitation project is unique in that rehab work can be started in almost all areas. However, due to priorities dictated by owner requirements and in order to further speed up the project, the contractor was required to adhere to a priority schedule which among others required completion of the roof within 40 days from award of contract. Completion of the roof would have allowed an all weather construction although extensive shoring would have been required.
Additionally, the rehab procedures dictated that no two adjacent columns would be rehabbed simultaneously so as not to unduly weaken the Building. The soundness of this requirement was amply proven during the July 16, 1990 Luzon Earthquake which registered Intensity 7.7 in the Richter Scale.
REFERENCES
ASEP (1992) National Structural Code of the Philippines Volume 1. Association of Structural Engineers of the Philippines (ASEP), Manila.
ACI (1996) ACI Manual of Concrete Practice Part 3. American Concrete Institute (ACI), Farmington Hills, Michigan.
ABOUT THE AUTHOR
Emilio M. Morales, MSCE took up his masters degree at the Carnegie Institute of Technology, Carnegie-Mellon University, Pittsburgh, PA. USA in 1980. Formerly Senior Lecturer of Graduate Division, College of Engineering, University of the Philippines, Diliman, Quezon City. Presently, he is the Technical Manager of Philippine Geoanalytics, Inc., Civil Engineering Laboratory and Principal of EM2A & Partners & Company. Committee Member, TC-11, Bureau of Product Standards Technical Committee on Steel Products.
He can be contacted at: EM2A Partners & Co., No. 17 Scout de Guia corner Scout Reyes Streets, Diliman, Quezon City. Telephone Nos. 371-18-04 & 06/ 410-29-23 to 24. Fax No. 924-98-94; E-mail: em2apart@mozcom.com.
Abstract: Testing of Fresh and hardened concrete has become routinary. This “routineness” has led to blind acceptance of the results without a real understanding of what the test results are saying and only “PASS”/“FAIL” conclusions are made on such results. However, the tests tell us what is happening or what is bound to happen if we do not heed the telltale indications that the tests are trying to convey. Oftentimes, lack of understanding of the failure or the causative mechanisms that bring about such failures are the least understood leading to wrong corrective responses or solutions. Problem identification has been said to be 80% of the solution process. Thus, there is a need to know the factors and significance of each test and the variables involved in the tests in order to arrive at solutions that directly address the problems. Most of the contents of this paper was obtained from literature more specifically from the work by Klieger et al on “ ASTM STP 160C – Significance of Tests and Properties of Concrete and Concrete-Making Materials”.and downloads from the Internet. Nevertheless, the authors hope that this paper could lead to a fuller understanding of the significance of the tests and what the test results are really telling us.
1.0 INTRODUCTION
Testing of Concrete both in the plastic and hardened states need to be carefully understood in order that adequate responses or proper corrective measures can be made to developing problems even before these develop.
Oftentimes, the test results and what they tell the end user remain as merely test results because of a lack of understanding on what these test results are really indicating to us and how they could best be used or interpreted to result in adequate and timely corrective measures to address the problem.
This paper references work by several authors from an ASTM Special Technical Publication ASTM STP 160C on the work by Klieger et al on “Significance of Tests and Properties of Concrete and Concrete-Making Materials”.and downloads from the Internet.
2.0 CONCRETE BASICS
Concrete is defined in the ASTM terminology relating to Concrete and Concrete Making Materials (C-125) as:
2.1 A composite material that consists essentially of a binding medium within which are embedded particles or fragments of aggregates.
2.2 In Hydraulic-cement concrete, “the binder is formed from a mixture of Hydraulic cement and water”.
2.3 Hydraulic cement is defined (ASTM C-219) as “a cement that sets and hardens by chemical interaction with water and that is capable of doing so under water”.
3.0 COMPONENT MATERIALS
3.1 Portland Cement
Portland Cement is the primary constituent of Portland cement Concrete.
It is defined as gray, powdery material that meets ASTM Standard C 150 and is composed primarily of tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. When combined with water, Portland Cement reacts with the water to form a paste, which then becomes rigid as the reaction between the cement and water progresses. 3]
Combined with the other components such as the inert fine and coarse aggregates and water, the Portland cement reacts (Hydrates) with water, initially forming a paste which within hours hardens to form the binding medium or cementitious matrix that holds the aggregates together.
3.2 Water
Next to Portland Cement, Water is an important ingredient. Water controls the strength and workability of the mix. The water reacts with the portland Cement in a process known as hydration to form a cementitious paste that hardens with time. The hydration reaction is a complex chemical reaction which physically alters the character and properties of the original Portland cement.
The water cement ratio is a good approximate indicator of the probable strength of the hardened mix. Its control is essential to a proper mix design which will address the job requirements.
Too much water in the mix will:
Cause large shrinkages to occur if concrete is allowed to cure normally
Result in low strengths
Increased workability and flowability
There are real Reasons to use less water and these are to accomplish the following objectives:
Increased strengths
Lowered permeability
Increased resistance to the effects of weather
Better bonding with reinforcement
Less volume change from wetting and drying
Increased resistance to plastic shrinkage
If strength requirements as well as workability demands need to be met concurrently, addition of water while increasing workability can cause a retrogressive effect on the strength and increases shrinkage potential and should thus be avoided. Reduction of water in the mix would increase strength but may make the Concrete unworkable.
An alternative would be to consider the use of superplasticizers which dramatically enhances the workability and flowability of concrete while reducing the water demand and thus results in a concrete mix that will address the requirements of the project for strength and workability.
3.2.1 Water/Cement Ratio
The water/cement ratio is the weight of the total amount of water relative to the weight of the total amount of cement used per cubic meter of concrete. In simple terms, the lower the water/cement ratio or the less water used for a certain weight of paortland Cement in the mix, the better the concrete. This is true to a point. Enough water is needed to be able to place and consolidate the concrete as well as achieve complete hydration reaction.
The binding quality of the cement/water paste is due to the chemical reaction achieved when water is mixed with cement.
This reaction is called hydration. Very little water is needed for hydration. In fact, most concrete would look like a pile of rocks and be unworkable if the only water added was to hydrate the cement. Most of the water in concrete is “water of convenience” to help ease the task of placing concrete.
The more water added to concrete the thinner the paste. The thinner the paste, the less strength in the hardened concrete. The Portland Cement Association suggests using no more water than is absolutely necessary to make the concrete plastic and workable. The graph below shows the effect of the Water Cement Ratio on the the Compressive Strength of Normal Weight Type I Portland Cement concrete at varying times based on tests on American Portland Cement Concrete. 4
Fig 3.2.1 Age vs Compressive Strength for various Water Contents
3.2.2 Water in Aggregates
Water in aggregates may or may not contribute to the hydration water depending on the condition of the moisture in the aggregates.
Fig 3.2.2. Moisture content in Aggregates for different conditions.
Water in the aggregates may also increase the available water as to increase the water cement ratio.
Thus, tests to determine the Moisture content of the aggregates is necessary for mix design purposes. The condition of the aggregates during the moisture content determination is also important as it determines whether the water is available as free water that could increase the water content of the mix or not at all. By the same token, it is necessary to determine the absorption capacity of the aggregates to determine whether hydration water will be reduced by absorption of the aggregates. Essentially, the absorbed water is not available to react with the cement as hydration water. Knowing the absorbed water will allow us to compensate for mix water that is “robbed” by aggregate absorption.
Normally aggregates are tested for absorption by using “Saturated Surface Dry Condition” as it is a measure of the potential absorption when moisture content under SSD is determined by Oven drying. the moisture content determined is then used to determine the absorption loss.
3.3 Air Content
Air content measurement is important particularly for non air entrained concrete because unexpected increases in air content can have a retrogressive effect on compressive and flexural strengths. The entrapped air bubbles displaces the cement matrix thus reducing the strength to some extent.
In tropical climates, air entrainment would normally only be prescribed for marine exposures for increased resistance to water permeability but more and more, this is being replaced by fly ash to provide a denser less permeable mix. However, even in the country, air entrainment for protection of the concrete (and eventually the rebar from corrosion) is done by using Fly Ash to promote a denser packing with minimum of voids in the concrete. The microfine fly ash fills these voids while at the same time increasing mobility and allowing for strength increases although in a much more delayed setting time.
3.4 Fine & Coarse Aggregates
3.4.1 Gradation Tests
Test of fine and coarse aggregates for grain size distribution is known as a Gradation test.
Grading is the particle distribution of granular materials among various sizes. This is usually expressed in terms of cumulative percentages larger or smaller than each of a series of sizes of standard sieves.
Grading and particle size distribution affects the overall performance of concrete as follows:
Determines the relative aggregate proportions
Determines the cement and water content
Affects Workability
Durability
Porosity
Porosity
Shrinkage
The well graded the particles, the more economical is the mix. a well graded aggregate means that all the grain sizes are represented. This allows for economical, denser concrete mixes with increased strength.
Variations in grading from batch to batch can affect the uniformity of concrete.
Generally, aggregates which do not contain a large deficiency or excess of any particular size and give a smooth gradation curve, within the prescribed gradation will produce a satisfactory mix.
Providing a well graded gradation (where all prescribed particle sizes are present) will reduce the total volume of voids which otherwise will be occupied by the cement paste.
An air meter is normally used for measuring air content.
Fig 3.4.1.1 Cement Content vs % Sand Content illustrating the Significance of Sand Content to the
overall economy of the Mix expressed in terms of Cement Content demand
The Chart above shows that control of Sand content to within the prescribed guidelines will result in an economical mix. Too little sand would result in a very harsh mix and would require cement to be used as voids filler. Too much sand captures the cement paste to coat the individual grains resulting in increased cement demand.
For fine and coarse aggregates the fineness modulus (FM) is defined by ASTM C-125. The fineness modulus is obtained by adding the cumulative percentages retained (by weight) on each of the specified sieve sizes and dividing the sum by 100.
The higher the FM, the coarser is the aggregate.
Fig 3.4.1.2 Fineness Modulus Calculation for Fine Aggregates
FM is important in estimating the proportions of fine and coarse aggregate.
3.4.2 Coarse Aggregates
The strength of aggregates, and hence its influence on the concrete, is primarily dependent on its mineralogy.
Beyond this, a smaller sized aggregate may have strength advantages in that internal weak planes may be less likely to exist or would be smaller and discontinuous.
The bond between mortar and coarse aggregates will be stronger for smaller aggregates.
A rough angular surface such as in crushed aggregates will increase the bond strengths.
As the maximum size of aggregate is increased for a given slump, the water and cement content per cubic meter of concrete are decreased. The larger the coarse aggregate proportion is in the total mix, the lesser is the cement needed due to the lesser surface area compared to smaller aggregates. However, workability is affected and the mix becomes harsher with increasing aggregate size.
Flat elongated and angular shapes require more water to produce workable concrete. Hence, cement demand is increased to maintain the same WC ratio.
The larger the aggregate size, the lesser is the cement demand.
For coarse aggregates, the larger size materials tend to affect the strength of concrete particularly if the aggregates have weakened planes or discontinuities. Gap graded aggregates may sometimes be used because of deficiency in coarse aggregate sizes within a certain sieve series. This would still be acceptable provided the percentage of fine aggregates is controlled. Gap graded mixes can produce a harsher mix but adequate vibration may address the problem.
Segregation is a problem in gap grading and therefore over vibration is to be avoided and the slump limited from 0 to 3 inches.
3.4.3 Fine Aggregates
Sand is primarily a filler for the voids in concrete.
Increasing the proportion of sand in the total mix increases cement demand because of the relatively very large surface area that needs to be coated by cement paste.
Flowability and mobility of concrete is enhanced with larger sand proportions but increases cement demand.
Flat elongated and angular shaped sands such as products from crushed sand, also require more water to produce workable concrete. Hence, cement demand is increased to maintain the same WC Ratio.
Generally, the gradation for fine aggregates given by ASTM C-33 would be adequate. However, it would be preferable to limit the % passing for the two smallest sieve sizes (#50 and #100 to 15% and 3% or more respectively. This would depend on workability during placement. The higher the fines content the more workable is the concrete but also increases cement demand.
4.0 FRESH CONCRETE
4.1 SLUMP TEST
The slump test is a measure of the workability of fresh concrete. It should NOT be used for predicting strength even in an approximate way.
Fig 4.1 Diagram of the Slump Test showing how Slump is measured
4.1.1 Additional Information from the Slump Test
More information can be obtained from the Concluded Slump Tests as follows:
After removing the slump cone and measuring the slump, the concrete is tapped on the side with the tamping rod.
Two concretes with the same slump may behave differently as follows:
One may fall apart after tapping which indicates that it is a harsh mix with a minimum of fines. This may have sufficient workability ONLY for placement in pavements or Mass concrete.
Another may be very cohesive with surplus of WORKABILITY, this may be required for more difficult placement condition
4.2 The Schmidt Rebound Hammer
The rebound hammer is an impact device that indicates relative and approximate concrete strength QUALITATIVELY through the rebound of the probe which has been calibrated against concrete strengths.
It is useful in determining or locating weaker or stronger concrete qualitatively and in a relative sense or for locating areas with discontinuities and honeycombs.
It is not an accurate device even when calibrated against concrete cores extracted in the area and is not intended to replace the compression test. It should NOT be used as a basis for acceptance or non-acceptance of a particular pour.
Fig 4.2 The Schmidt Rebound Hammer with Digital Readout and Polishing Stone.
The device when used properly could give useful indications such as to where weaker vs Stronger concrete is present but only in a relative sense.
A lot of consultants think that it is an accurate and absolute determinant of Concrete compressive strength which it is not. Dependence on this as the sole reference and basis for rejection is unwarranted, unsound and NOT VALID.
4.3 Test on Concrete Core sample
Whenever the strength of hardened concrete is put to doubt, destructive testing in the form of testing concrete core samples uniaxially in accordance with ASTM C-42 is performed on the area where the questionable concrete has been laid. Coring is done using Diamond Coring bits that are thin walled. The core diameter should be no smaller than three times (3x) the size of the maximum nominal size of coarse aggregate.
Where possible, the length of the core should be at least twice the diameter (2X) of the core but other height to diameter ratios are permissible to a certain extent and corresponding correction factors are applied to the test results to compensate for the slenderness of the core sample. Reinforcing steel should not be included in the cores to be tested. If rebars do exist, the cores would have to be discarded and replaced.
When cores are taken for the purpose of strength determination, at least three cores should be taken at each location. The strength of the concrete as cored is expected to be lower than the design strength to take into account disturbance and damage effects during the sampling and testing of the cores. Values approaching 0.85 of the design concrete strength f’c or higher would be generally acceptable. 5 ]
4.4 Concrete Compression Test ASTM C-39
A concrete test as technically defined should consist of tests on at least two standard cylinders taken from the same batch or pour.
4.4.1 ACI Requirements for Compressive Strength Test
For a strength test, at least two standard test specimens shall be made from a composite sample obtained as required in Section 16. A test shall be the average of the strengths of the specimens tested at the age specified in 4.1.1.1 or 4.4.1.1 (Note 19). If a specimen shows definite evidence other than low strength, of improper sampling, molding, handling, curing, or testing, it shall be discarded and the strength of the remaining cylinder shall then be considered the test result.
To conform to the requirements of this specification, strength tests representing each class of concrete must meet the following two requirements mutually inclusive(Note 20):
The average of any three consecutive strength tests shall be equal to, or greater than, the specified strength, f’c, and No individual strength test shall be more than 500 psi [3.5 MPa] below the specified strength, f’c.
4.4.2 Failure Mechanism
Concrete failure in the compression test or in service is a result of the development of microcracking through the specimen to the point where it can no longer resist any further load.
The crack propagates through the weakest link whether it is through the aggregates or the cement matrix or both.
For ultra high strength concrete aggregate strength becomes critical and it would be better to have smaller sized aggregates so that internal weaknesses in the aggregates would not be significant as a crack initiator. Also, the use of small sized aggregates increases the aggregate interlock and increases the chances for Crack Arrest.
For normal strength concrete, failure normally propagates through the cement matrix unless internal planes of weakness in the aggregates give in more readily but the random distribution of these would arrest such crack propagation in the normally stronger aggregates.
In the compression test, because of scale effects, the planeness, perpendicularity and surface imperfections critically influence the results.
4.4.3 Factors Affecting Compressive Strength
Retempering of the mix with water in the concrete can cause a decrease in the mortar strength due to uneven dispersion of the retempering water which leads to pockets of mortar having a high water cement ratio.
If concrete is allowed to dry rapidly, the available moisture for hydration reaction will be reduced and hydration ceases.
A smaller sized aggregate may have strength advantages in that the internal weak planes may be less likely to exist.
The bond between the mortar and coarse aggregate particles will be stronger for smaller sized aggregates which have a higher curvature.
When concrete bleeds, the bleed water is often trapped beneath the coarse aggregate thus weakening the bond within the interfacial zone and allowing for weaker stress paths for cracks to initiate. Excessive bleeding will produce a high water cement ratio at the top portion leading to weakened wearing surfaces and dusting.
4.4.4 Break Patterns
Surface imperfections in the sample or the test platform can cause uneven break patterns which signal lower strength results normally.
Fig. 4.4.4 Uniaxial Test on Concrete Cylinder ASTM C-39
4.4.5 Factors Affecting the Compressive Strength Test Results:
Specimen geometry Size End conditions of loading apparatus Rigidity of Test Equipment Rate of load application Specimen moisture conditions
Fig. 4.4.5a Ideal Break Pattern is Conical and Hour Glass ShapedFig. 4.4.5b Some Break Patterns Due to Uneven
The purpose of specifying end condition requirements of planeness and perpendicularity is to achieve a uniform transfer of load to the test specimen.
Fig 4.4.5c Conventional Compression Tester on Mobile Testing LaboratoryFig 4.4.5d Spherical Platen ensures that the load is transferred evenly to the Concrete CylinderFig. 4.4.5e Surface Deformities result in uneven load Distribution causing premature Test Failure = Fictitious Low Strengths.Fig. 4.4.5f A High Capacity UTM for Testing Ultra High Strength Concrete Cylinders
Non-conforming specimens generally cause lower strength test results and the degree of strength reduction increases for higher strength concrete.
4.4.6 Specimen Size and Aspect Ratio
The ASTM Standard test specimen is a 6” Diameter x 12” high cylinder.
Compressive strength generally varies inversely with increasing cylinder size with the 6” dia cylinder as the reference size.
The ratio of specimen diameter to max aggregate should be 3:1, the accuracy of the strength test results decreases as the diameter to aggregate ratio decreases.
The L:D (aspect ratio) requirements is 2. The strength increases with decreasing L/D ratio due to end restraint. However, correction factors are allowed.
4.4.7 Requirements of Testing Machine Properties
The Test Equipment used for the Compression test :
Must be capable of smooth and continuous load application.
Must have accurate load sensing and load indication.
Must have two bearing blocks one fixed and the other spherically seated both satisfying planeness and rigidity requirements.
Distortion of testing machine or of the bearing plates due to inadequate rigidity can cause strength reductions therefore, adequate rigidity needs to be assured.
4.4.8 Rate of Loading
ASTM C-39 requires that the loading rate for hydraulically operated test frames be controlled to within 20 to 50 PSI or about 500 Lbs per second. Other than making the test procedure follow consistent rates of loading and thus remove this as a variable effect on Concrete Strength, there are other reasons for prescribing a constant rate of loading.
The apparent strength of the concrete increases with increasing loading rate and therefore the loading rate must conform to the required standard to produce consistent and accurate results.
Higher strength concrete are more affected by the loading rate.
This dependence on loading rate has been found out to be due to the Mechanism of creep and Microcracking.
Thus, it has also been found out that when subjected to a sustained load of 75% its ultimate strength, concrete will eventually fail without any further load increases.
4.5 Concrete Flexural Strength
4.5.1 Factors Affecting Flexural Strength Test Results
Specimen Size Preparation Moisture Condition Curing Where the beam has been molded or sawed to size Aggregate Size
Fig. 4.5.1 Flexural Test on Concrete
5.0 CONCRETE SHRINKAGE
Concrete shrinkage is primarily due to rapid and uneven loss of water. Therefore, improper curing of Freshly poured concrete and control of environmental factors plays a key roll in shrinkage control.
Drying Shrinkage increases with increasing water content. Therefore control of mixing water to that required only for complete hydration and desired adequate flowability is important. In case such conditions can not be met, it would be better to achieve these requirements by using water reducing plasticizers rather than by addition of more water.
Although control of total water in the mix is the primary objective to control shrinkage Cracking, other factors contribute to the overall shrinkage cracking as the following Equation would suggest.
Shrinkage cracking can be quantified or predicted based on ACI 209R-92 procedures as given in the following formula below. What is significant to note in this Formula is the contribution of other factors to the overall shrinkage magnitude which are controllable and thus points the way to the reduction of Shrinkage effects.
Prediction of Actual Shrinkage Values based on ACI 209R-92
Concrete shrinks due to moisture loss. However, the actual magnitude of ultimate shrinkage is dependent on a lot of factors as contained in ACI 209R-92.
These factors are:
Relative Humidity
Minimum Thickness
Cement Content
Slump
Air Content
Fines Content
The parameter SH is obtained from the Chart below which is dependent on the Total Water Content of the mix when laid. The chart for obtaining the above variables are given in the Appendix of this paper.
Fig. 3.2.1.2 Shrinkage Values in Millistrain vs Water Content
6.0 BLEED EFFECTS
Although not related to shrinkage, bleeding is A related surface defect that is controllable. Bleeding occurs when over-troweling happens which works up more water to the surface.
The increased water at the surface results in higher W/C which causes a low strength layer that can delaminate and cause dusting or powdering.
7.0 CONCLUSIONS
This paper has identified the various Factors affecting tests on fresh and hardened concrete and their significance in influencing test results.
A complete understanding of these Factors can definitely reduce uncertainties in determining the cause/s of “Failed test Resultsˆ and eliminate costly Guesswork and often times solutions that do not address the problem.
The Authors hope that recognition of the various factors that influence the test results could lead to a more positive and responsive solution to problems brought about by the test results.
REFERENCES
Klieger, Paul and Lamond, Joseph F. “Significance of Tests and Properties of Concrete and Concrete- Making Materials”. ASTM STP 160C.
ACI 209 R -92 METHOD OF SHRINKAGE PREDICTION.
1 Emilio M. Morales, MSCE took his Master of Science in Civil Engineering at Carnegie-Mellon University, Pittsburgh, PA. USA in 1980. He was employed as a Geotechnical Engineer at D’Appolonia Consulting Engineers. Currently, he is the Principal of EM²A Partners & Co. and concurrently serves as President for IGS Philippines, Chairman for the PICE Geotechnical Specialty Committee. He has been elevated to PhD Candidacy at the Asian Institute of Technology, Bangkok, 2 Mark K. Morales, MSc took his Master of Science in Civil Engineering at University of California – Berkeley, USA in 2004. He is the Technical Manager of Philippine Geoanalytics, Inc. and President of PGA Earth Structures Solutions
3 www.ces.clemson.edu/arts/glossary.html
4 NOTE: Slightly Lower results are obtained for philippine Cements due to lower cube compressive strengths but the chart is indicative of the relationship between Concrete Strength and W/C
5 ACI 437R-16 “Strength Evaluation of Concrete Buildings” Excerpts
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