Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
COMPARISON OF SHEAR STRENGTHCHARACTERISTICS OF STONE MATRIX ASPHALT
MIXTURE WITH WASTE PLASTICS ANDPOLYPROPYLENE
Bindu C S1* and Beena K S1
The triaxial test measures the mix stability in shear strength and gave better information for fieldperformance prediction. The stresses acting on the specimen simulate pavement stresses.The Stone Matrix Asphalt mixtures (SMA) are investigated using triaxial shear strength testing(50.8 mm/min ram rate loading at 60°C) to investigate the effect of additives, waste plastics andpolypropylene on strength properties. The test was conducted at 0, 50, 75 and 100 kPaconfinements. Analysis using Mohr-Coulomb failure theory shows that the stabilized SMA hadhighest cohesion and shear strength as compared to control mixture (SMA without additive), butalmost similar angle of internal friction. SMA with waste plastics shows the highest cohesionand shear strength at 7% waste plastics. The strain at failure and tangent modulus increaseswith increasing confinement pressure, indicating their stress dependent behavior. The increasein tangent modulus indicates the increased elastic stiffness of stabilized SMA. The stress-straincurves show that the peak stress and the time of its occurrence is higher in waste plasticsmixtures when compared to polypropylene mixtures. For stabilized mixtures, shape change ofstress-strain curves is more gradual with increase in additive content and brittle type failuredoes not seem to occur as in control mixture.
1 School of Engineering, Cochin University of Science & Technology, Kochi-22, India.
INTRODUCTIONThe Marshall test is a kind of unconfinedcompressive strength test. It is a goodindication of the ideal binder content but failsto register the true shear strength of themixture. The triaxial test, on the other hand,measures the mix stability in the form of shear
strength of the mix. The strength of bituminousmixtures is due partly to the friction andinterlocking of aggregates, which increaseswith increasing normal stress, and partly tocohesion or viscous resistance, whichincreases with increasing shear rate.According to (Crockford et al., 2002) the
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Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
characterizations attainable with the properconduct of this testing approach are generallyconsidered to be more closely associated withthe true engineering properties than any othertest. McLeod (1951) used and c from triaxialtests to evaluate the maximum vertical load apavement can carry. Triaxial test can beapplied to flexible pavement design. Triaxialtests can be used as best to simulate thestress, temperature and strain conditionsoccurring in the field. Yoder and Witezak (1975)also suggested that the test offers a goodmeans of evaluating pavement design.
In the laboratory, confining stresses areapplied to simulate stress due to thesurrounding material in a pavement structure.This confinement increases with increasingdepth in the pavement. Thus, varying theconfining pressure in a laboratory testsimulates the material at different depths in thepavement. The deviator stress in the laboratoryrepresents applied wheel loads in the field thatare transmitted through the bituminous layersto the underlying unbound layers. Increasingthe deviator stresses in the laboratorysimulates increasing the applied loadmagnitude in the field.
Triaxial strength testing providesinformation concerning mixture cohesion andinternal friction both of which should contributeto mixture rut resistance (Christensen et al.,2000). Tangent modulus, an indication of theelastic stiffness modulus (Ebels and Jenkins,2007) and stress and strain at failure can beobtained from the stress-strain diagram. Sincebituminous mixtures have little or no tensilestrength, shearing resistance of the mix is usedto develop a load-distributing quality thatgreatly reduces the stresses transmitted to the
underlying layers. The objective of the presentstudy is to study the effect of waste plastics onthe shear strength characteristics of StoneMatrix Asphalt mixtures and to compare theresults with that of the expensive polymeradditive polypropylene.
MATERIALS AND METHODSThe specimen size of 100 mm in diameter and150 mm in height (Pellinen et al., 2004) is usedfor the test. A test temperature of 60o�C isused in this study, which is an acceptabletemperature level by many researchers.(Smith, 1951; Low et al., 1995). Static truckloads represent the severest conditionimposed on a bituminous pavement. Suchloading can result in the accumulation ofsignificant pavement deformation. Endersby(1951) found that in the triaxial test, thecohesion increases with increasing loadingspeed. Goetz and Chen (1957) reported thatthe angle of internal friction was not affectedby the rate of strain, but the cohesion increasessteadily as the rate of strain increases. Aloading speed of 50.8 mm/min is selected forthis study (Pellinen et al., 2004), which is sameas the rate of loading given for Marshall test.
The bitumen content for all the SMA mixtureswere kept as 6.42% which is the optimumbitumen content for the control SMA mixture(without any additive). Samples arecompacted to get a cylindrical sample of 100mm diameter and 150 mm height. Thespecimen is encased in a rubber membraneto allow for confinement pressure to be appliedall around the specimen. Water is used as themedium. Axial load is applied through a platenon the end of the cylindrical specimen, so asto get an unconsolidated undrained test
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Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
condition. Test specimens are loaded beyondthe peak load to understand the post peakbehavior. Four different confinement pressuresof 0, 50, 75 and 100 kPa are used in the testing(Bueno et al., 2003, Ebels, 2008). For eachconfining pressure, three samples are testedand the average value is taken.
RESULTS AND DISCUSSIONAnalysis using Mohr-CoulombFailure Theory
Stone Matrix Asphalt (SMA) mixtures with WPand PP additives are investigated using triaxialshear strength testing. The cohesion andfriction angle are obtained using the Mohr-Coulomb failure theory. The triaxial test resultsare tabulated in Table 1. The table shows themeasured deviator stress (
d) obtained at
each confinement level (3). Figures 1 to 3
shows the plots of the Mohr-Coulomb failureenvelope represented by the cohesion c andangle of internal friction for the control mixtureand stabilized mixtures (3 samples for eachconfinement). Cohesion and friction areestimated using test results from differentconfinement levels to obtain at least threepoints in the failure line.
Cohesion
The computed cohesion values for each
additive for different percentages are shown
in Figure 4. Analysis of test data shows that
the presence of additives has shown
significant effect on cohesion, which increases
approximately from 110 kPa in control mixture
to and to about 145 kPa in SMA mixtures with
waste plastics. All the SMA mixtures give thehighest cohesion at 7% for waste plastics and
Table 1: Triaxial Shear Strength Test Results
Type of Additive Confinement 3 ( kPa) Deviator Stress
d ( kPa) C (kPa)/(Deg.)
No additive 0 418.57 109.06/35
50 553.45
75 613.06
100 681.88
Waste plastics 0 570.26 145.55/35.6
50 707.66
75 776.03
100 845.78
Polypropylene 0 553.25 139.52/35.6
50 691.19
75 752.12
100 830.98
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Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
at 5% for polypropylene. The cohesion values
are found to be decreasing when additivecontents are increased beyond thispercentage. Figure 4 shows that Wasteplastics stabilized SMA mixtures show slightlyhigher cohesion value than the Polypropylenestabilized mixtures. When compared to the
control mixture the percentage increase incohesion value is about 33% and 28%,respectively for SMA mixture stabilized withWP and PP at 7% and 5% of additives. Thelarger the cohesion value, the higher the mixresistance to shearing stresses. This shows
that all the stabilized mixtures have greaterresistance to shearing stresses than the controlmixtures.
Figure 1: Mohr’s Circlefor Control Mixture
Figure 2: Mohr's Circlefor WP Stabilized SMA
Figure 4: Variation of Cohesion withDifferent WP and PP Contents
Angle of Internal Friction
The variations of angle of internal friction ofSMA mix with different percentages ofadditives are given in Figure 5. It can beobserved that the presence of additives inSMA mix will not result in considerablevariation in the angle of internal friction of mixas compared to the control mixture. The valueof is 35.6o or WP and PP stabilized mixtureswhile 35o for the control mixture. The angle ofinternal friction value is an aggregate property,mostly dependent on aggregate properties
Figure 3: Mohr's Circlefor Polypropylene Stabilized SMA
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Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
such as grading and angularity of particles.Therefore no significant variation is expectedsince all mixtures have the same aggregategradations.
A slight increase in angle of internal frictionis occurred for the stabilized mixtures. Thisslight increase in may be due to the influenceof cohesion. The cohesion and friction angleare not entirely independent of each othersince there is some sort of balancing effect asa result of the stabilization of the mixture(Jenkins, 2000).
The parameters c and are the strengthindicators of mixtures. The larger the c value,the larger the mix resistance to shearingstresses. In addition, the larger the value, thelarger the capacity of the bituminous mixtureto develop strength from the applied loads, andhence, the smaller the potential for permanentdeformation. The cohesion values of all mixeswith additives are higher than that of the controlmixture, showing their higher resistance toshearing stresses.
Shear Strength
The cohesion and angle of internal friction
cannot be evaluated and compared inisolation. When comparing the performanceof several mixes, the maximum shear stressthat the mixture can withstand is of importance.This is dependent both on cohesion and angleof internal friction.
Shear strength is computed at 300 kPanormal stresses which represents hypotheticalpavement stress at the edge of the tyre at 75-mm deep in the pavement (Pellinen et al.,2004). The test results are shown in Table 2. Itcan be seen that, with additives, SMA mixtureretained higher shear strength. This suggeststhat the stabilized mixture is less prone torutting by shear and densification comparedto the control mixture. In order to densifymixtures by traffic the rearrangement ofaggregate structure must take place bycoupled action of volumetric and shearstraining. Based on these findings thestabilized mixtures seem to be less prone todilatation and shear compared to the controlmixture. WP stabilized SMA mixture shows themaximum shear strength of about 364 kPa.The percentage increase in strength is about14% with respect to the control mixture,showing their much greater resistance toshearing stresses. The results indicate that theshear resistance is rising mostly from cohesionsince the variation of is observed to bemarginal.
Table 2: Shear Strengthof Various SMA Mixtures
Type of mixture Shear strength (kPa)
Control mixture 319.12
SMA with WP 363.51
SMA with PP 355.63
Figure 5: Variation of Angle of InternalFriction with Different WP and PP Contents
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Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
The test results showed that stabilized mixesare stronger than the control mix. The presenceof additives makes the mixes more flexible.Surface layer built with this stabilized SMA maynot become rigid under traffic loading and,therefore, less susceptible to cracking.
Stress-Strain Curves for SMAMixture
Figure 6 (a) and (b) represents the variationof deviator stress with strain for all stabilizedmixtures at 100 kPa confinement level fordifferent percentages of additives. The plotsrepresent before and after peak stressdevelopment during the test. The peak stress
was obtained by examining the graphs. Forthe stabilized mixture, it is observed that thepeak stress developed and the time of itsoccurrence are higher when compared tothose of the control mixture, a behavior thatwas attributed to the influence of additives inthe mix. The additives provide this additionalreinforcement to bituminous mix in resistingpermanent deformation and retard theoccurrence of shear failure. In all tests, thestabilized mixtures showed higher maximumstress at failure than the control mixture. SMAmixture with additives showed betterresistance to shear deformation as shown bythe triaxial shear strength test results. Notably,post peak failure for the additive reinforcedbituminous mixtures showed gradual drop instrength, an effect that was attributed to theinfluence of the additives in the mix.
By examining the stress-strain curves forthe SMA mixtures, it can be inferred that instabilized mixtures, the shape change of thestress-strain curves is more gradual withincrease in additive content and brittle typefailure does not seem to occur. Also, the failurestrains are slightly higher. The followingphenomena were observed during testingbased on visual observations, inspection ofstress-strain curves and failed specimens forSMA mixtures. For SMA mixtures, at around0.5% axial strain aggregates started to slipinitiating a structural transformation. Thisprocess continued until aggregate particleinterlock was overcome and dilatation (volumeincrease) took place in the specimen.
Strain and Stress at Failure
Strain and stress at failure are parameters that
Figure 6a: Variation of Deviator StressWith Strain of SMA With DifferentPercentages of Waste Plastics
Figure 6b: Variation of Deviator StressWith Strain at Different Percentages
of Polypropylene
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Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
could provide some additional insight into thematerial characterization. These values forstabilized SMA at various additive contents at100 kPa confinement pressures are given inTable 3. The stress and strain values at failureincreases due to the inclusion of additives upto 0.3% fibre content, 5% PP content and 7%WP content in SMA and after that it is found tobe decreasing.
The variation of maximum failure strain andthe corresponding stress for differentconfinement pressures for all SMA mixturesare summarized in Figures 7 and 8.
Table 3: Strain and Stress at Failure for Stabilized SMA at Different Percentagesof WP and PP (100 kPa Confinement Pressure)
Additive (%) Waste plastics Polypropylene
Strain at failure (%) Stress at failure(kPa) Strain at failure (%) Stress at failure (kPa)
0 2.13 352.5 2.13 352.5
1 2.27 486.45 2.27 476.45
3 2.4 512.45 2.4 524.68
5 2.53 542.58 2.67 554.42
7 2.93 559.46 2.4 504.587
9 2.27 454.85 2.13 414.568
Figure 7: Maximum Strain at Failure atDifferent Confinement Pressures
Figure 8: Maximum Stress at Failure atDifferent Confinement Pressures
Figure 9: Tangent Modulus at DifferentConfinement Pressures
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Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
Tangent Modulus
Another material property that may be derivedfrom the triaxial test is the tangent modulus.The maximum tangent modulus of all mixturesincluding the control mixture at differentconfinement pressures are summarized inFigure 9. All stabilized mixes show high tangentmodulus than the control mixture. As theconfining pressure increases, the tangentmodulus value increases. The tangent modulusexhibited a stress dependent behavior. It canbe seen that the tangent modulus generallyvaries between 1.2 kPa for control mixture to2.3 kPa for SMA with 0.3% coir fibre. As thetangent modulus is obtained from the linearpart of the stress- strain diagram, it shouldprovide an indication of the elastic stiffness ofthe material. It is evident that the presence offibre in the mix enhances the elastic stiffnessof the SMA mixture.
CONCLUSIONAnalysis using Mohr-Coulomb failure theoryshows that the SMA stabilized mixtures hadhighest cohesion and shear strength ascompared to the control mixture. But all themixes had almost similar angle of internalfriction value, which is mostly dependent onaggregate properties such as grading andangularity of particles. Therefore no significantvariations, since all mixtures have the sameaggregate gradations.
For stabilized SMA mixtures, the highestcohesion and shear strength results are at 7%WP and 5% PP content respectively. Analysishowever suggests that the high additivecontent beyond this percentage prevents themixtures to develop aggregate interlock andtherefore less cohesion and shear resistance.
By examining the stress-strain curves forthe SMA mixture, it can be inferred that in thestabilized mixture, the peak stress developedand the time of its occurrence is higher whencompared to those of the control mixture. Forstabilized mixtures, the shape change of thestress-strain curves is more gradual withincrease in additive content and brittle typefailure does not seem to occur as in the caseof control mixture.
There is a trend that the strain at failure andtangent modulus increases with increasingconfinement pressure, indicating their stressdependent behavior. The increase in tangentmodulus value of SMA mixture in presence ofadditives indicates the increased elasticstiffness of the stabilized SMA.
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Int. J. Struct. & Civil Engg. Res. 2013 Bindu C S and Beena K S, 2013
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