Effect of High Fraction Class F Flyash on Lime Stabilization of Soil

International Journal of Geotechnics and Environment (IJGE)1(2) July-December 2009; pp. 105-128

* Corresponding author: [email protected]

Effect of High Fraction Class F Flyash on LimeStabilization of Soil

J. N. Jha*, K. S. GillDepartment of Civil Engineering, G.N.D.E.C., Ludhiana, Punjab, India

A. K. ChoudharyDepartment of Civil Engineering, N. I. T., Jamshedpur, India

ABSTRACT: The focus of the paper is to evaluate the effectiveness of using class F flyash as pozzolanato enhance lime treatment of soil particularly when mixture posses large fraction of ash. A mixturecontaining 25% of flyash by weight of dry soil was considered as base mixture which was subsequentlyincreased upto 45%. To achieve this objective, a battery of tests was conducted on soil –flyash mixturesprepared with lime as activator. Test specimens were subjected to compaction tests, unconfinedcompression test, split tensile tests. Durability characteristics based on mass loss and unconfinedcompressive strength criteria of stabilized soil were also studied using wet-dry tests. Effect of curingtime on stabilized soil was studied by curing the specimens for 7, 14, 28, 56, 90 and 180 days beforeconducting the tests. Soaked CBR tests were also conducted for those mixes only which retains maximumunconfined compressive strength after 12 cycles of wetting and drying. The results show that additionof high fraction class F flyash not only enhances the strength but also improves the durability of limestabilized soil.

Keywords: Flyash, Compaction, Unconfined compressive strength, Split tensile strength, Durability

1. INTRODUCTIONMany procedures have been developed to improve the physical behaviour of soil byincorporating a wide range of stabilizing agents, additives and conditioners. Theeffectiveness of such agents relies on the formation of cementing bonds between theparticles in the soil system. The two most common stabilizing agents are cement andlime. Soil stabilization has been widely recommended for developing countries for theconstruction of various elements of the pavements. The reasons usually put forwardare that the use of locally available materials will lead to lower costs. Lime as an additive,brings several beneficial changes in the engineering properties of soil such as decreasein soil plasticity and shrink swell potential apart from improving strength characteristics.Stabilization of soil by lime is achieved through cation exchange, flocculation andagglomeration, lime carbonation and pozzolanic reaction. Cation exchange and

mailto:[email protected]

106 / INTERNATIONAL JOURNAL OF GEOTECHNICS AND ENVIRONMENT (IJGE)

flocculation agglomeration reaction takes place rapidly and brings immediate change insoil properties, where as, pozzolanic reactions are time dependent. These reactionsinvolve interaction between soil silica and (or) alumina and lime to form various typesof cementing agents thus enhancing the strength. Certain natural substance, such asvolcanic ash reacts to the lime addition much better than do the ordinary soil types. Ifsuch materials are added to soil, the efficiency of lime stabilization may be greatlyincreased. The characteristics of compacted soil, if improved, resulting from residueutilization like flyash, blast furnace slag, rice husk ash etc can be a practical way ofencouraging sustainable development apart from environmental and economic benefits.However use of such industrial wastes has not found wide application in civil engineeringconstruction activities. A major source of generation of power in India is from ThermalPower Plants (TPPs) which typically used pulverized coal as fuel. The by-product fromthese plants typically contain a coarse material type known as bottom ash and finematerial type known as flyash, with both exhibiting variable physicochemical properties(Sridharan et al. 1996). It is reported that Indian coal based TPPs produce around90x106 t of flyash per year which can cover an area of 265 km2 as ash pond (Das andYudbhir 2005). Acquiring open lands for disposal of flyash in developing countries likeIndia is difficult, where the land to population ratio is small. Since the land requirementand the cost of land are increasing day by day, therefore it is essential to find outdifferent ways for gainful utilization of this waste in civil engineering activities. Kamonand Nontananandh (1991) reported that successful waste utilization (combiningindustrial waste with lime for soil stabilization) could result in considerable saving inconstruction cost. Bulk utilization of flyash is possible only by way of its use ingeotechnical applications such as embankment construction, backfill materials andsubbase materials etc (Pandian 2004). The existing literature also suggests that themaximum amount of flyash to be used in a mix should be around 25% (Consoli et al.2001). In order to ensure bulk utilization of fly ash there is an urgent need to find outways and means to use higher volume of fly ash in the mix. The objective of presentstudy is therefore to evaluate the effectiveness of using large fraction class F flyash asa pozzolan to enhance the lime treatment of soil. Mixture possessing large fraction ofash were used in the testing programme. A mixture containing 25% of fly ash by weightof dry soil was considered as base mixture which was subsequently increased to 35%and 45% respectively. To achieve this objective, geotechnical characteristics of specimenprepared with different proportion of soil-flyash-lime were investigated. Test specimenswere subjected to compaction tests, unconfined compression tests and split tensilestrength tests. In any stabilization application since the stabilized material shouldhave the ability to retain its integrity and strength under in service condition, wet-drytests were also conducted to evaluate the durability aspects of the specimen. Since

EFFECT OF HIGH FRACTION CLASS F FLYASH ON LIME STABILIZATION OF SOIL / 107

pozzolanic reactions between lime and clay/flyash particles is a time dependentchemical reaction, effect of curing period on these soil-flyash-lime mixes werealso studied in the present investigation. Specimens were cured for 7, 14, 28, 56, 90 and180 days before testing. California bearing ratio (CBR) tests to evaluate the bearingstrength of stabilized mix were also included for few mixtures in the test programme.The paper describes a series of results based on the experimental study particularlywhen percentage of flyash selected herein is higher than 25%, the one generally used inprevious studies.

2. EXPERIMENTAL PROGRAMME

2.1. MaterialsPhysical and engineering properties of the locally available soil used for present testingare given in Table 1. The flyash was collected from Ropar thermal power plant, Ropar(Punjab). Table 2 and 3 gives the chemical composition and physical and engineeringproperties of flyash. The flyash is classified as class F flyash as per ASTM C 618 (ASTM1993). Lime varies widely in its quality when collected from different sources or collectedin batches from the same source. In order to keep uniformity in quality of lime, highcalcium calcite lime was used throughout the investigation. Its properties and chemicalcomposition, as supplied by the manufacturer, are reported in Table 4.

Table 1Physical and Engineering Properties of Soil

Specific Gravity 2.72Grain Size distribution:Sand (%) 38Silt (%) 55Clay (%) 07

Liquid limit (%) 23Plastic limit (%) 16Plasticity Index (%) 07Classification of Soil CL-MLStandard Proctor CompactionMaximum dry density (kN/m3) 17.55Optimum moisture content (%) 10.6Unconfined compressive strength (kN/m2) 120.8CBR (%)Unsoaked 6.08Soaked 2.5


Table 2Chemical Composition of Ropar Thermal Power Plant Flyash

Chemical component Percentage (%)

Silicon dioxide SiO2 56.80Aluminium oxide Al2O3 26.10Ferric oxide Fe2O3 5.0Calcium oxide CaO 3.8Magnesium oxide MgO 2.3Titanium oxide TiO2 1.4Potassium oxide K2O 0.6Sodium oxide Na2O 0.4Sulphur trioxide SO3 1.6LOI (1000 0C) 1.9Moisture 0.3

Table 3Physical and Engineering Properties of Flyash

Property Values

Grain Size Distribution:Sand (%) 19Silt (%) and Clay (%) 81Specific gravity 1.96Standard proctor compaction:Maximum dry density (kN/m3) 1.33Optimum moisture Content (%) 29.5Unsoaked CBR (%) 29Angle of internal friction (Φ) 34.5

Table 4Chemical Composition and Physical Properties of Hydrated Lime

Composition or property Value

Specific gravity 2.05Normal consistency 43.50Initial setting time (min) 165Final setting time (h) 46.25Fineness (% age by weight on 300 µm sieve) 2.65Soundness [(Lechatlier’s expansion (mm)] 1.8Compressive strength(14days)(N/mm2) 1.45Compressive strength(28days)(N/mm2) 2.18Calcium hydroxide (%) Maximum 95Chloride (%) Maximum 0.01Sulphate (%) Maximum 0.2Aluminum, iron, insoluble matter etc (%) Maximum 1.0Arsenic (%) Maximum 0.0004Lead (%) Maximum 0.0001


2.2. Methods of Mixture PreparationThe general expression for the total dry weight W of a flyash-soil-lime mixture is

W = Wf + Ws + Wl (1)Where Wf, Ws, Wl are the weights of flyash, soil and lime respectively in the mix.

A mixture containing 25% of fly ash by weight of dry soil was considered as basemixture in the present investigation. The minimum lime content adopted in the presentstudy was based on plasticity changes following the “lime fixation point” principleoriginally proposed by Hilt and Davidson (1960) for clayey soil. Table 5 gives thevalue of plastic limit at different lime content (%). It can be observed from Table 5that lime content greater than 4% did not cause major changes in plastic limit. Thisthreshold value, which is supposed to chemically satisfy the soil-flyash demand forlime has often been adopted as the starting content for construction expediencypurposes. Therefore minimum lime content selected was 4%, in addition to 7% and10% which were expected to largely improve the mechanical behaviour of soil-flyashmixture.

Table 5Lime Fixation Point Determination

Flyash (%) in Mix-25Lime content,% 0 2 4 6 8 10

Plastic limit 18 20 22.5 21 22 22.5

The soil collected from the site was pulverized with wooden mallet to break lumpsand then air-dried. Subsequently it was sieved through 2.36 mm IS sieve and then driedin an oven at 105 °C for 24 hours. The following procedure was used to mix the flyashand lime into the soil. Required quantity of soil was first weighed and poured into amechanical mixture. Then the required quantity of lime (slurry form) was mixed intothe soil in small increments to ensure uniform coverage. Now desired quantity of flyashwas added in the soil-lime mix. This mixture was mixed using a ‘two bladed mortarmixing bit’, powered by an electric drill. Due care was taken to ensure a uniform soil-lime flyash mixture. Table 6 provides the summary of tests conducted along withvariables used in the study.

Test specimens were prepared at optimum moisture content and maximumdry density. All desired tests were conducted as per IS methods. In order toimprove reproducibility of test results, tests were conducted in triplicate for a givensoil-flyash-lime composition. The results reported here is the average of three testresults.


3. EXPERIMENTAL PROCEDURE

3.1. Compaction TestsIn the compaction tests, the soil was thoroughly mixed with various moisture contentsand allowed to equilibrate for 24 hours prior to compaction. The dry weight of totalmixture (W) was taken as per Equation (1). Light compaction tests were performed forvarious combinations of soil-flyash-lime mixtures as detailed in Table 6. The dry density-moisture content relationship was determined using the standard compaction methodas per IS specifications (BIS 1980).

Table 6Test Conducted and Variables Used

Tests Flyash (%) Lime (%) Curing period (Days)

Compaction test 0, 25, 35, 45 4, 7, 10Unconfined compressive strength test 0, 25, 35, 45 4, 7, 10 7, 14, 28, 56, 90, 180Split tensile strength test 25, 35, 45 4, 7, 10 7, 14, 28, 56, 90, 180Durability test 25, 35, 45 4, 7, 10 7, 14, 28, 56, 90, 180CBR test (Soaked) 45 10 28

3.2. Unconfined Compressive Strength TestsOne of the popular methods of evaluating the effectiveness of stabilization is theunconfined compressive strength. As a general rule, for a given type of stabilization,the higher the compressive strength, the better is the quality of the stabilized material.Unconfined compression tests were carried out on cylindrical specimens 38.1mmdiameter and 76.2 mm long. The soil-flyash-lime mixtures were compacted at optimummoisture content and maximum dry density in standard moulds. The mixture wascompacted in three layers and each layer was compacted using 2.6 kg rammer under afree fall of 310 mm. Specimens of 38.1 mm diameter and 76.2 mm long were extractedfrom the mould and were cured in groups of three in a thermostatically controlledincubator set at (30 ± 2) °C before being tested in compression. The details of variouscombinations of mixture for which tests were conducted are given in Table 6. Curingtimes adopted were 7, 14, 28, 56, 90 and 180 days and at least three specimens weretested for each case. At the end of each curing period the specimens were soaked inwater for a period of 24h. The unconfined compressive strength was determined at aloading rate of 1.0 mm/min.

3.3. Split Tensile Strength TestsNormally compressive testing is used for evaluating strength of stabilized soil and there islittle information concerning their tensile strength. A knowledge of the tensile strength isneeded in the study of stability of earth dams, highway and airfield pavements. Tensile


stresses are set up due to movement of traffic on pavement, shrinkage of soils, seasonalvariation in temperature and alternate wetting and drying of soil etc. Various tests andmodifications have been developed and used for evaluating tensile strengths of soils andstabilized soils (Anagnos et al. 1970). Direct tensile, bending, double punch tensile andsplit cylinder are some of these tests. Out of all these methods, the split tensile testappears to be the simplest to perform and has been used in this study. For conducting thesplit tensile test, cylindrical specimens of size 38.1 mm diameter and 76.2 mm lengthwere prepared at optimum moisture content and maximum dry density in the same manneras done in the case of unconfined compression tests. After curing, samples were soakedin water for 24h. The soaked specimens were placed horizontally between the bearingblocks of the compression testing machine adjusted for a strain rate of 1.0 mm/min.Strips of mild steel (5mm thick, 5mm wide and 50mm long) curved at the contact surfacewere placed on the upper and lower bearing elements of the cylinder to ensure uniformbearing pressure. A schematic sketch of the specimen for the split tensile test is shown inFig. 1. The split tensile strength is obtained by the following formula:

Split tensile strength = 2P/ðtdWhere P = failure load, t = thickness or length of specimen and d = diameter ofspecimen

The details of various combinations of the mixture for which tests were conductedare given in Table 6.

3.4. Durability TestsDurability which can be defined as the ability of a material to retain stability and integrityover years of exposure to the destructive forces of weathering is one of the most

Figure 1: Schematic Sketch of Specimen for Split Tensile Test


important properties (Dempesy and Thompson 1968).The durability tests on soil-flyash-lime mixture were conducted as per IS: 4332-1968 and reaffirmed in 1995. For thesetests, specimens were prepared at the maximum dry density and optimum moisturecontent and then moist cured for a specific number of days. Subsequently, specimenswere immersed in water for 5 hours followed by air drying for 42 hours at roomtemperature, which completes single cycle of wetting and drying. After each cycle, thespecimens were brushed with a steel wire brush and the loss in the material is recordedas mass loss (brush loss) in percentage. Further triplicate sets of samples were preparedfollowing the same standard test procedure, and were subjected to 12 cycles of wettingand drying but brushing was omitted. Brushing of specimens has been known to causeuncertainty in the results because it is manual and hence could very well be affected bythe consistency of technician’s procedure. Replacing brushing by measuring thecompressive strength of specimens after they are subjected to the 12 cycles of wetting-drying could provide a more consistent and convenient measures of the deteriorationof the mix. Shihata and Baghdadi (2001) also suggested using the residual compressivestrength of durability specimens without brushing as an indicator of resistance potentialsince it gives more consistent results. Thus the samples prepared without brushingwere tested for unconfined compressive strength. This compressive strength was levelledas unbrushed residual strength (URS). Aim of conducting compressive strength testwithout brushing is to explore the possibility of using residual (compressive) strengthof soil-flyash-lime mix as a viable indicator of durability resistance. The durabilityindices [unbrushed residual strength ratio (URSR) in the present study] of the specimenswere obtained as a ratio of the compressive strength after 12 wetting and drying cycleswithout brushing divided by the compressive strength of a sample preparedsimultaneously, but stored under wet conditions during the entire test period. The detailsof various combinations of the mixture for which tests were conducted are given inTable 6.

3.5. CBR TestsComposite strength-stiffness parameters are employed for the subgrade, where thepavement section and subgrade have to resist repetitive loads with very low deformationtolerance. California bearing ratio (CBR) is more commonly used to describe thecomposite strength-stiffness parameters. As reported in the literature, development ofstrength of soil-flyash-lime mix depends on curing period, moisture content andtemperature. The higher the curing temperature, the greater will be the strengthdevelopment. The strength after 7 days curing at 38 °C is usually considered to be thesame as for 28 days curing at 20 °C and the strength after 7 days at 55 °C is about thesame as one-year strength at 20 °C. To evaluate the suitability of soil-flyash-lime mix in


base and subgrade course of flexible pavements, CBR tests were conducted as per IS:2720 (Part XVI) 1979. Test samples were prepared at optimum moisture content andmaximum dry density. Curing time adopted for test samples were 28 days at 30 °C,which was then soaked for 96 hours. Tests were conducted for soaked samples only.The details of various combinations of mixture for which tests were conducted aregiven in Table 6.

4. RESULTS AND DISCUSSION

4.1. Compaction ParametersFigs. 2 and 3 show the variation of maximum dry density (MDD) and optimum moisturecontent (OMC) with lime percentage for different proportions of flyash in the mixtures.It is observed from the results that addition of lime reduces the maximum dry densityof the soil and increases the optimum moisture content. Earlier studies had also observedthat the addition of lime leads to an immediate decrease in the maximum dry densityof soil and an increase in the optimum moisture content for the same compactiveeffort. The reduction in the maximum dry density of the treated soil is reflective ofincreased resistance offered by the flocculated soil structure to the compactive effort(Nagraj 1964). The increase in optimum moisture content is probably a consequenceof additional water held within the flocculated soil structure resulting from limeinteraction. This trend does not change even after the addition of flyash. It is clearfrom the result that at given lime content when flyash content is increased in the mixture,maximum dry density decreases (Fig. 2) and optimum moisture content increases(Fig. 3). Flyash having low specific gravity may be the cause for this reduced dry density.

10

15

20

4 7 10%age of lime

FA (0%)

FA (25%)

FA (35%)

FA (45%)

Figure 2: Variation of MDD with Lime Percentage at Different % Age of Flyash


The increase in moisture content can be attributed to the increasing amount of fineswhich require more water content due to their larger surface area.

4.2. Unconfined Compressive Strength and Split Tensile StrengthFig. 4 to 6 shows variation of unconfined compressive strength (UCS) with curingperiod for different flyash percentage in the mix at 4%, 7% and 10% lime.

Figure 3: Variation of OMC with Lime Percentage at Different % Age of Flyash

10

12

14

16

18

20

22

24

26

4 7 10

%age of lime

OM

C(%

) FA (0%)

FA (25%)

FA (35%)

FA 45(%)

Figure 4: Variation of UCS with Curing Period at 4% Lime

100

300

500

700

900

1100

1300

1500

7 14 28 56 90 180Curing Period (Days)

FA (0%)

FA (25%)

FA (35%)

FA (45%)




0

200

400

600

800

1000

1200

1400

1600

1800

2000


FA (0%)

FA (25%)

FA (35%)

FA (45%)

0

500

1000

1500

2000

2500

3000


FA (0%)

FA (25%)

FA (35%)

FA (45%)

From the result it can be observed that the unconfined compressive strength is185kN/m2 after 7 days curing when the mix contains 4% lime and 25% flyash (Fig. 4).


Increase in curing period from 7 days to 90 days increases the unconfined compressivestrength from 185kN/m2 to 760kN/m2. If curing period is increased further from 90days to 180 days the unconfined compressive strength increases to 1110kN/m2. Similarpattern is observed when the lime content is changed to 7 or 10% in the mix (Fig. 5 and6). Therefore curing time is one of the major variables affecting the behaviour of thestabilized soil. Again it is important to note that the increase in compressive strengthseems to be delayed in the initial stage of curing. Rate of increase in strength was lowerupto 56 days and afterwards it increased at a much faster rate upto 180 days. The reasonfor delayed significant strength gain is due to necessary induction time required for thecompletion of chemical pozzolanic reactions between lime and flyash for the formationof new cementitious compound.

Although treatment with lime improves the strength characteristics of soil, additionof flyash further improves the strength of the lime-stabilized soil. For example, a samplewith 4% lime and 35% flyash (Fig. 4) has 1.47 times strength to that of a sample with7% lime and no flyash after 28 days curing (Fig. 5). This indicates that in a soil- flyash-lime mixture, a lesser amount of lime is required to achieve a given strength as comparedwith soil-lime mixture alone. The gain in strength of lime–stabilized soil is primarily aresult of pozzolanic reactions between silica and (or) alumina from the soil and lime toform various types of cementing agents. By introducing flyash to the soil, additionalamounts of silica are available for reaction with lime resulting in further increase ofstrength. Since flyash is much cheaper than lime, addition of flyash in soil-lime mix canresult in cost reduction of construction.

It can be observed from the results that for a given lime content and curing periodthe strength of the mix increases with the quantity of flyash. When the soil containsonly 7% lime and no flyash (Fig. 5), the unconfined compressive strength after 28dayscuring is 257.7kN/m2. Addition of 25% flyash in the mix increases this strength by14%. For the same curing period if the mix contains now 35% flyash the unconfinedcompressive strength increases by 75% which further enhances by 115% if flyashquantity becomes 45% in the mix. In the present investigation the maximumimprovement in unconfined compressive strength occurs after 180 days curing whenthe mix contains 45% flyash and 10% lime (Fig. 6) whereas the existing literaturessuggest that the mix which contain around 25% flyash only shall only be used (NCHRP1976). It is also being observed in the present study that the strength improvementcontinues to take place if availability of sufficient lime is ensured in the mix to completethe pozzolanic reaction.

From the results it can also be observed that as the quantity of lime increases in themix unconfined compressive strength increases for a given flyash content and curingperiod particularly after 14 days curing. Consoli et al. (2001) reported that increase in


strength was practically independent of carbide lime content upto 90 days. The reasonfor such contrast observation may due the difference in curing temperature. In thepresent investigation the curing temperature is 30 °C whereas the curing temperaturewas 22 °C for Consoli et al. (2001). Earlier studies of lime treatment of soil had alsoindicated that higher temperature accelerated curing resulting in a higher strength (Marksand Haliburton 1972, Mateos 1964). Jalili et al. (1997) also reported that when temperaturewas raised from 20 °C to 65 °C the induction period of soil-lime-flyash specimenreduced from 75days to 4.8 hour.

Figs. 7 to 9 shows the variation of split tensile strength of the specimen containingvarious percentage of flyash with curing period at 4%, 7% and 10% lime. The resultsreported here shows same trend as that for the unconfined compressive strength. It isobserved that addition of flyash to the soil-lime mixture increases the split tensile strengthwith curing period and the maximum improvement is again observed for the mixcontaining 45% flyash and 10% lime after 180 days curing period (Fig. 9).

Figs. 10 to 12 shows the ratio of split tensile strength and unconfined compressivestrength with curing period for different flyash content at 4%, 7% and 10% lime in themix. It can be observed that the ratio is influenced by curing period only. Effect of limecontent is negligible at 7 days of curing. The average increase in the ratio is around 4%for 7 days curing irrespective of lime content which has been considered in theinvestigation. But as the curing period increases from 7 days to 180 days, this ratio also

0

20

40

60

80

100

120

140

160

180


FA (25%)

FA (35%)

FA (45%)

Figure 7: Variation of STS with Curing Period at 4% Lime




0

50

100

150

200

250

300

350

400


FA (25%)

FA (35%)

FA (45%)

0

100

200

300

400

500

600

700

800


FA (25%)

FA (35%)

FA (45%)

increases. The increase in ratio is 14%, 18% and 21% respectively for 4% (Fig. 10), 7%(Fig. 11) and 10% (Fig. 12) lime content.


From the behaviour observed in the Figs. 10 to 12, it can be concluded that thetensile strength is a function of the amount of cementitious compound only whichincreases with curing period. The plot shown in Figs. 4 to 6 and 10 to 12 gives veryuseful information concerning the lime content to adopt during stabilization particularly

Figure 10: Variation of STS/UCS Ratio with Curing Period at 4% Lime

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16


FA (25%)

FA (35%)

FA (45%)


0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2


FA (25%)

FA (35%)

FA (45%)


for the cases where tensile strength is expected to play a main role like pavement structureby assuming that the ratio of split tensile strength to unconfined compressive strengthis independent of lime content.

The importance of flyash is clearly demonstrated by comparing the unconfinedcompressive strength values obtained at different curing period for both the soil-limeand soil-flyash-lime mixture (Fig.-4 to 6). It should be emphasized at this point that thesilty sand used in the present investigation with a plasticity index less than 7, is usuallyconsidered as inappropriate for lime stabilization, because the pozzolanic reactionresponsible for largely improving lime stabilized soil is basically dependent on the amountand physical-chemical activity of the clay minerals present in the soil mass. The purposeof replacing the soil by flyash is therefore to ensure that there will be enough pozzolanicmaterial in the mixture to eventually combine with the lime in both short and long termchemical reactions. In countries like India where flyash is considered as waste material,use of flyash in the construction of roads, airfields and other earthworks may particularlybecome attractive, because of reduced construction and disposal costs apart fromenvironmental benefits.

4.3. Durability TestTo assure that the mixture has the potential to perform satisfactorily in the field; itshould satisfy criteria based on both durability and strength. There are a number of


0

0.05

0.1

0.15

0.2

0.25

0.3


FA (25%)FA (35%)

FA (45%)


such criteria, some of which are related to the type or classification of soil (for durability)while others are related to type of use (for strength). Table 7 gives the durability criteriabased on maximum allowable mass loss (%) depending on the type of soil. The testresults of wet-dry durability in terms of mass losses after 12 cycles have been plottedagainst curing period and are presented in Fig. 13 to 15.

0

5

10

15

20

25

30

35

40


Lime (4%)

Lime (7%)

Lime (10%)

0

5

10

15

20

25

30

35

40

45


Lime (4%)

Lime (7%)

Lime (10%)

Figure 13: Variation of Weight Loss (%) with Curing Period at 25% Flyash



From the results it is observed that lime content and curing period directly influencesthe mass loss. Mass loss is 10.31% for the mix containing 4% lime and 25% flyashhaving curing period 180 days (Fig. 13), which increases to 16% (Fig. 14) and 18% (Fig.15) when the flyash content in the mix becomes 35% and 45% respectively. It can alsobe observed from the results that if the curing period is being decreased mass lossincreases for all the three composition of soil-flyash mix. The reason may be attributedto availability of insufficient quantity of lime in the mix, thus requiring long time tocomplete the chemical pozzolanic reaction which is responsible for the formation ofcementitious phase. But when the lime content in the mix is increased to 7%, the massloss after 56 days curing period for all the three flyash compositions considered in theinvestigation (i.e. 25%, 35% and 45%) are 8.71%, 9.82% and 10.31% respectively(Fig. 13 to 15) thus satisfing all the durability requirements based on mass loss(Table 7). When the lime content in the mix is further increased to 10% than thedurability requirement based on maximum allowable mass is being satisfied for all threesoil-flyash composition even after 28 days curing. The reason for decrease in inductionperiod is due to the availability of sufficient quantity of lime in the mix necessary forpozzolanic reaction.

Durability criteria seem to be quite severe and governed by availability of quantityof lime and curing period in the present study. Kalankamary and Donald (1963) alsonoted that the evaluation of durability by weight loss is overly severe, and this testprocedure does not totally simulate field conditions. Therefore, some highway agencies


0

5

10

15

20

25

30

35

40

45

50


Lime (4%)

Lime (7%)

Lime (10%)


currently require unconfined compression tests in lieu of durability testing. The AmericanCoal Ash Association (ACAA) recommends using unconfined compressive strength(UCS) to assess the durability of pozzolanic stabilized mixtures (PSMs) for flexiblepavement systems. Dempsey and Thompson (1968) also found that UCS appears to bea meaningful measure of the freeze-thaw durability of the lime soil mixtures. Baghdadiand Shihata (1999) concluded that durability of soil-cement, in terms of wetting anddrying effect, could also be evaluated through unconfined compressive strength (UCS)of standard durability samples subjected to the alternating cycles without brushing thesamples. Minimum unconfined strength requirement criteria related to the type of use(strength) has been given in Table 8.

Table 8Minimum Unconfined Compressive Strength (UCS)

Agency/Author Minimum UCS(After 7 days Curing)

USACESubbase course 1720 kN/m2

(Flexible pavement)HRB -1961Silty soil 1700-3450 kN/m2

(Soil-cement mix)Ingles and Metcalf (1972)Road subbase - Light traffic 690-1400 kN/m2

Road subbase – Heavy traffic 1400-5500 kN/m2

(Soil-cement mixture)Road Research Laboratory 1720 kN/m2

Soil-cement mixtureNational Lime AssociationSoil-lime mixture -10 cycles of freeze-thaw 915-1126 kN/m2

(Flexible pavement)

Table 7Durability Based on Maximum Allowable Mass Loss (%)

Soil type Maximum allowablemass soil (%)

SM, SC, SPa 14CL, MLa 10MH, CHa 7Siltb 8Clayb 6Siltc 10Clayc 7aPCA Criteria for soil cementbUSACE CriteriacCPCA Criteria


Figure 16: Variation of URSR (%) with Curing Period at 25% Flyash

0

10

20

30

40

50

60

70

80


Lime (4%)

Lime (7%)

Lime (10%)


0

10

20

30

40

50

60

70

80


Lime (4%)

Lime (7%)

Lime (10%)

The URSR values obtained after running the wet-dry cycles are plotted against thecuring period and presented in Figs. 16 to 18. It has been observed from the result thatthe average strength of the specimens with three different flyash compositions (i.e.


Flyash -25%, 35% and 45%) having lime contents 4%, 7% and 10% and moist curedfor 180 days drops to 32, 65 and 72% of the original strengths respectively, after beingexposed to 12 cycles of wetting and drying. The average unbrushed residual strengthsobtained for three compositions having lime contents 4%, 7% and 10 % are 372kN/m2, 1098kN/m2 and 1682kN/m2 respectively. Specimens prepared with 10 % limecontent having curing period 180 days seems to satisfy the minimum unconfinedcompressive strength criteria (Table 8). It has been further observed from the resultsthat as the curing period decrease the URSR decreases. Shihata and Baghdadi (2001)also reported that as the exposure duration (immersion period) increases unbrushedresidual strength increases. Al-Rawi (1981) reported a loss of over 50% due to saturationfor limestabilized soil. Durability index determined after 4 and 8 cycles, though notreported here, were also determined which showed that there was drastic reduction instrength of specimen after first cycle but it regained the strength in subsequent cycles.Reason for reduction in strength may be due to the combination of factors inducedduring wet-dry cycles and the gain in strength subsequently is due to curing effects.Thompson (1966) also reported that the ratio of soaked and unsoaked strength wereapproximately in the range of 0.7-0.85, when the lime stabilized soil was subjected toprolonged exposure to water. In general the addition of flyash produces not only strongerbut also durable samples as compared with those treated with lime only. This impliesthat maintenance costs for roads, airfields etc. will be reduced considerably.


0

10

20

30

40

50

60

70

80


Lime (4%)

Lime (7%)

Lime (10%)


4.4. CBR TestSoaked CBR tests were conducted only for those mixes which are able to retain maximumunconfined compressive strength after 12 cycles of wetting and drying and results arepresented in Table 9. Similar to unconfined strength, the CBR increases with increasingflyash content. The increase in CBR value after addition of lime is due to the formationof various cementing agents due to pozzolanic reaction between silica present in soiland lime. When flyash was added, additional amount of silica became available forreaction with lime, which further increased the CBR value. Therefore the efficiency oflime stabilization may be greatly increased by the addition of flyash. Sample with 45%flyash content gave the maximum value of CBR. A CBR value 107.29 indicates that themixture may be suitable for use of base layer since a CBR of around 100 is generallyacceptable for bases (Asphalt Institute 2003).

Table 9Soaked CBR (%) of 28 Days Cured Specimens at 10% Lime

Flyash (%) in the Mix CBR (%)-Soaked

25 50.1235 76.8845 107.29

5. CONCLUSIONSOn the basis of present study, the following conclusions can be drawn:

1. Addition of lime in the soil decreases maximum dry density of soil-lime mixesand increases the optimum moisture content. Addition of flyash in the mixturefurther reduces the maximum dry density and increases the optimum moisturecontent.

2. Addition of flyash enhances the unconfined compressive strength of limestabilized soil. The strength of soil stabilized with 7% lime and 45% flyashafter 28 days curing is about 2.15 times to that of soil stabilized with 7% limeonly. The enhancement of strength means that for a given strength of treatedsoil, a lesser amount of lime is required in a soil-flyash-lime mixture comparedwith soil-lime mixture only. Since flyash is much cheaper than lime, it will reduceconstruction cost.

3. Increase in percentage of flyash in the mix increase the strength with curingperiod even if the lime content remains constant. Presence of flyash in soil-lime mix essentially causes large amount of time dependent pozzolanic reactionthus improving the material behaviour. The maximum improvement in


unconfined compression strength is achieved when the mix containing 45%flyash and 10% lime is cured for 180 days.

4. Unconfined compressive strength and split tensile strength test results showedthat the gain in strength largely occurs after 56 days for the curing temperatureselected in the study. It is probably due to induction period for the pozzolanicreactions between lime and flyash. Undoubtedly, this might be a drawback tothe practical utilization of stabilized soil. However at a greater temperature ofcuring, which is reasonable for most of the year in tropical region, a reductionin the induction period is expected to occur.

5. The ratio of split tensile strength/unconfined compressive strength of differentmixes at constant lime content increases with increasing curing time. The averageincrease in the ratio is 4% at 7 days curing for all the lime contents consideredin the investigation but increases to 14%, 18% and 21% respectively after 180dayscuring at 4%, 7% and 10% lime.

6. Durability of lime stabilized soil is enhanced by the addition of flyash. It isobserved from the durability test results that for all the three flyash contentsconsidered in the mix; durability requirement based on mass loss is satisfiedafter 28days curing when lime is 10%. Average unbrushed residual strengthratio of the mix for the above three flyash content (i.e. 25%, 35% and 45%)drops to 32%, 65% and 72% after 180 days curing when it contains 4%, 7%and 10% lime respectively. Specimen prepared with 10% lime content seems tosatisfy the durability requirement based unconfined strength criteria also.

7. A CBR (soaked) value equal to 107.29% is obtained for the mix containing45% flyash and 10% lime thus indicating that mixture can be used as a baselayer since a CBR value of 100% is acceptable for bases.

References[1] American Coal Ash Association (ACAA) (1991), “Flexible Pavement Manual”, Washington, D. C.[2] Asphalt Institute (2003), “Thickness Design – Highway and Streets” Manual Series No. 1, Asphalt Institute,

Lexington, Ky.[3] ASTM (1993), “Specifications for Flyash and Raw or Calcined Natural Pozzolana for use as a Mineral Admixture

in Portland Cement Concrete” C618, West Conshohocken, Pa.[4] Al-Rawi, N. M. (1981), “The Effect of Curing Temperature on Lime Stabilization, Journal of Engineering

Mechanics Division, ASCE, 107 (TE1), 25-35.[5] Anagnos, J. N., Kennedy, T. W. and Hudson, W. R. (1970), “Evaluation and Prediction of Tensile Properties

of Cement Treated Materials” Centre for Highway Research, University of Texas, Austin, 89-99.[6] Bureau of Indian Standards (BIS) (1980), “Methods of Test for Soils: Determination of Water Content Dry

Density Relation using Light Compendium of Indian Standard on Soil Engineering” IS 2720, Part 7, NewDelhi, India


[7] Baghdadi, Z. A., and Shihata, S. A. (1999), “On Durability and Strength of Soil-cement” Journal ISSMF,Ground Improvement, 3(1), 1-6.

[8] Consoli, N. C., Prietto, P. D. M., Carraro, J. A. H. and Heineck, K. S. (2001), “ Behaviour of Compacted Soil-flyash-carbide Lime Mixture” Journal of Geotechnical and Geoenvironmental Engineering, 127(9), 774-782.

[9] Dempsey, B. J. and Thompson, M. R. (1968), “Durability Properties of Lime Soil Mixtures” Highway ResearchRecord, 235, Highwar Research Board, Washionton, D. C., 61-75.

[10] Das, S. K. and Yudbhir (2005), “Geotechnical Characterization of Some Indian Flyashes” Journal of Materialsin Civil Engineering, 17(5), 544-552.

[11] Hilt, G. H., and Davidson, D. T. (1960), “Lime Fixation in Clayey Soil” Highway Research Record, 262,Highwar Research Board, Washionton, D.C., 20-32.

[12] IS: 4332 (Part –IV) (1968) (Reaffirmed 1995) “Methods of Test for Stabilized Soils: Wetting and Drying, andFreezing and Thawing Tests for Compacted Soil-cement Mixtures” Indian Standard Institution, New Delhi,India.

[13] IS: 2720 (Part XVI) (1979), “Methods of Test for Soils: Laboratory Determination of CBR” Indian StandardInstitution, New Delhi, India.

[14] Jalali, S., Abyanch, M. Y., and Keedwell, M. J. (1997), “Differential Scanning Calorimetry Tests Applied toLime-flyash Soil Stabilization “Testing Soil Mixed with Waste or Recycled Materials, STP 1275, ASTM, WestConshohocken, Pa., 181-191.

[15] Kalankamary, G. P. and Donald, D. T. (1963), “Development of Freeze-thaw Test for Design of Soil Cement”Highway Research Record, 36, Highwar Research Board, Washionton, D. C., 77-96.

[16] Kamon, M. and Nontananaandh, S. (1991), Combining Industrial Waste with Lime for Soil Stabilization,Journal of Geotechnical Engineering 117(1), 1-117.

[17] Marks, B. D. and Haliburton, T. A. (1972), “Acceleration of Lime Clay Reactions with Salt” Journal of SoilMechanics and Foundation Division, 98 (4), 327-329.

[18] Mateos, M. (1964), Soil-lime Research at Iowa State University, Journal of Soil Mechanics and Foundation Division,90(2), 127-153.

[19] Nagaraj, T. S. (1964), “Discussion on Soil – lime Research at Iowa State University”ASCE, 90(6), 225-226.[20] National Cooperative Highway Research Program (NCHRP) (1976) “Lime-flyash Stabilized Bases and

Subbasea” Synthesis of Hwy. Pract. No. 37, Transportation Research Board, Washington, D. C.[21] Pandian N. S. (2004), “Flyash Characterization with Reference to Geotechnical Application” Journal, Ind. Inst.

Sc., 84, 189-216.[22] Sridharan, A., Pandian, N. S., and Rajasekhar, C. (1996), “Geotechnical Characterization of Pond Ash” Ash

Pond and Ash Disposal Systems, Narosa, New Delhi, India 97-110.[23] Shihata, S. A. and Baghdadi, Z. A. (2001), “Long Term Strength and Durability of Soil Cement” Journal of

Materials in Civil Engineering, 13(3), 161-165.[24] Thompson, M. R. (1966), “Lime Reactivity of Illinois Soils” Journal of Soil Mechanics and Foundation Division,

ASCE, 92(SM5), 67-92.

Effect of High Fraction Class F Flyash on Lime Stabilization of Soil

Documents

Transcript of Effect of High Fraction Class F Flyash on Lime Stabilization of Soil