development of soil resilient modulus testing...

- ASEE-NE Conference 2016 -

DEVELOPMENT OF SOIL RESILIENT MODULUSTESTING SYSTEM FOR GREEN HIGHWAY

K. Wayne Lee, Ph.D., P.E., F. ASCE and Syed Amir Hassan, BSc Civil Engineering.

Abstract: Resilient Modulus (Mr or E) is a fundamental property to characterize subgrade soils, base andsubbase of pavement structure. Since it becomes essential for teaching and research activates in civilengineering, a testing system was developed with an Instron machine at the University of Rhode Island(URI). Before preparing the specimens, moisture content, maximum dry density, grain size and specificgravity need to be determined in the laboratory. Using these results, resilient modulus specimens wereprepared to duplicate field conditions sometimes with the help of mathematical models. To preparespecimen the base plate of the triaxial cell was attached with a split mold which was covered with membraneinside. A specimen was compacted in the split mold with six equal layers of 50mm (2in.) thickness so thattotal height of the specimen not greater than 340mm (13.5 in.). After compacting specimen the split moldwas removed and assembled into the triaxial chamber. The second step of resilient modulus testing includescomputer operation which utilizes two software, i.e., Instron Console (control software for load frame) andInstron Wavematrix (software to control testing sequences). Instron Wavematrix software consists of threephases, i.e. proportional-integral-derivative (PID) setting, conditioning and resilient modulus phase. Airconfining pressure was provided throughout the test according to the testing sequence for specific materialprovided by AASHTO T307-99 procedure. Each testing sequences consists of fifteen cycles with increasingair confining pressure. Load or deviator stress was applied from the top of the triaxial chamber apparatuswith increasing amplitude. Displacement was measured by Linear Variable Differential Transducers(LVDTs) attached to the top of triaxial chamber. Finally results obtained from software were used as aninput for MATLAB software to arrange and calculate the resilient modulus in a simple Microsoft Excelsheet and to generate different graphs against resilient modulus.

Introduction: Design for pavement structuresare mainly based upon the strength of subgradesoils and layer materials. The 1993 AASHTOGuide for Design of Pavement Structures(AASHTO Guide) recommended the use ofeffective soil resilient modulus and assigningappropriate layer coefficients based on resilientmodulus value expected (AASHTO 1993).Resilient modulus is defined as deviator stressover recoverable strain measured during specificincremental loading sequences which attempt torecreate vehicular loading conditions onpavement structures. Resilient modulus testingequipment and procedures have undergone rapidchanges over the last few years. Therefore, statedepartment of transportation (DOT) andeducational institutions requires an upgrading forthe determination of resilient modulus values.

Rhode Island Transportation Research Center(RITRC) at the University of Rhode Island (URI)has done significant research in the area of

materials used for pavement structures (Jin et al.1994 and Lee et al. 1994). Therefore, the URI isin a good position to utilize the above equipmentto train future transportation work forces.However, Transportation Research Board (TRB)revised the AASHTO Guide as Mechanistic-Empirical Pavement Design Guide (MEPDG) in2002, and finally AASHTO provided newAASHTOWare Pavement ME Design recently(TRB 2002; AASHTO 2012). Fortunately, bothGuides recommend the use of effective soilresilient modulus and assigning appropriate layercoefficients based on resilient modulus valueexpected. Thus, there is a need to develop and/orupgrade a testing system at educationalinstitutions.

The states, especially Rhode Island (RI), arehaving a hard time keeping up with and payingfor maintenance and rehabilitation (M&R). Thismeans there will be more wear done to ourhighways than ever before, and the states will


have to do more M&R with less funding. To meetupcoming highway demand, RIDOT has beentesting alternative subbase material strategiessuch as reclamations and has been expandingtheir use. Full-depth reclamation (FDR) is arecycling technique in which all of the existingpavement section and all or a predeterminedportion of the underlying aggregate are uniformlyblended to produce a base course. FDR has beenproposed as a viable alternative in roadconstruction, where asphalt and aggregateresources are conserved, and material andtransportation costs are reduced becauserecycling eliminates the need for hauling newmaterials and disposing of old materials. Themixture of recycled asphalt pavement (RAP) andaggregate produced from full-depth reclamation(Fig. 1) has the potential to have engineeringproperties that exceed those of a 100% aggregatebase material, although little data are available tosubstantiate the claim.

Fig 1 Full Depth Reclamation and RAP

Pavements are located on material layers calledthe base and sub-grade. It has been proven thatthe mechanical properties of the base layergreatly affect the pavement performance.Therefore, it is important to determine stiffness,strength, and permanent deformationcharacteristics of the base. By conducting cyclictriaxial or resilient modulus testing that simulatestraffic load, the recoverable and permanent axialstrain can be measured and used to estimate theperformance of the pavement structure. Theresilient modulus was found to be dependent on anumber of factors: soil type, test method,specimen density, specimen moisture, specimensize, confining pressure, deviator stress, etc. Onesoil specimen may have many different resilientmodulus values depending on the states ofstresses. Conducting the resilient modulus testand selecting an appropriate resilient modulus

value for pavement designs are very complexprocesses.

RITRC research team started developing resilientmodulus testing set up from early 1990s, and wassuccessful to develop one in accordance with theprocedure of AASHTO T 307 for a researchproject sponsored by New EnglandTransportation Consortium (NETC) in early2000s (Lee et al. 2001). However, the dataacquisition system was too old or did not followthe advanced software. Thus, the testing systemneeded new software to control servo-hydraulicequipment and data acquisition. This reportpresents initial effort to develop the testingsystem and recent updated system. Softwareupdating efforts was carried out through aresearch project sponsored by RIDOT (Bradshawet al. 2015).

Current Status of Knowledge: Most pavementmaterials, especially soils, are not pure elasticmaterial, but exhibit elastic-plastic behavior.That means that they act partly elastic under astatic load but experience some permanentdeformation. However, under repeated loads,they express other important properties. At thebeginning, they perform just like they wouldunder a static load. But after certain repetitions,the permanent deformation under each loadrepetition is almost completely recoverable. Bythis point, it can be nearly considered elastic, ifthe repeat load is small enough compared to itsstrength, otherwise the soil structure would bedamaged. Figure 2 illustrates the behavior ofgranular soils or unbounded material under asequence of repeating loads. Resilient modulus(MR) is a measurement of the elastic property ofsoil recognizing certain nonlinear characteristics.Resilient modulus is defined as the ratio of theaxial deviator stress to the recoverable axialstrain, and is presented in the following equation:

Mr = σd /εr

Where σd = axial deviator stress

εR = axial recoverable strain

As exhibited in Figure 2, there are twocomponents to the total deformation, a resilient orrecoverable portion and a permanent portion.


Only the recoverable portion is included in themeasurement of resilient modulus.

Figure 2 Concept of Resilient Modulus of Soils

The MR tests conducted in the laboratorygenerally follow the procedure of AASHTO T307 procedure. Cyclic axial stress (Fig. 3), whichsimulates traffic loading, was applied to acylindrical specimen at a given confiningpressure within a conventional triaxial cell.

Figure 3: Example Cyclic Axial Stress

Experimental to Update the Testing System:

Sample Collection: Reclaimed materials wereobtained from Bridge Town Road, RhodeIsland (Fig. 4). An in-situ blend (the mixtureof RAP and aggregate) was taken during FDRoperation.

Figure 4 RAP, Aggregate, and In-situ BlendProduced from Bridge Town Road Rhode Island

In addition, pure RAP and pure aggregatematerials were sampled at three differentlocations separately namely Reclaimed Base(RB-1), (RB-2) and (RB-3) with various blendedmixtures and different ratios of RAP andaggregates.

Moisture Content Determination: RAP materialobtained from RIDOT of each location wassieved by using ¾ in sieve. Moisture contentswere measured for both materials which retainedand passed ¾ in sieve after the period of oneweek.

Gradation Test Procedure: Sieve tests for eachmaterial were conducted according to theprocedure from the American Society for Testingand Materials (ASTM) Standards C136-01,“Standard Test Method for Sieve Analysis ofFine and Coarse Aggregate. About 65 lbs. ofrepresentative material of each sample(containing coarse and fine grained particles)were collected. Then, the representative materialof each sample was put into the coarse grainedsoil sieve shaker (Fig. 5), and masses on eachsieves were measured after 10 minute shaking.Sieves used for gradation 37.5mm, 10mm,4.75mm, 2.38mm, 0.599mm, 0.075mm, and0.01mm (pan)


Figure 5 Sieves for Course Grained Materials

Gradation curve from the sieve analysis forReclaimed Base (RB-2) is shown in Fig. 6

Figure 6 Grain Distribution Curve for RB-2Material

Modified Proctor Compaction Test: ModifiedProctor compaction tests were performedfollowing the procedure of AASHTO T180, “TheMoisture-Density Relations of Soils” Using a4.53 kg (10 lbs.) Rammer and a 457.2 mm (18 in)Drop. Also, ASTM Standard D1557 wasreferenced which uses a 4-inch-diameter(100 mm) mold and holds 1/30 cubic foot of soiland 5 layers of 25 blows each (Fig. 7). ModifiedProctor test was performed only on materialswhich passed ¾ in. sieve. 2% increment of waterwas added to the material until it achievesmaximum density or optimum moisture content.Moisture content by air dried method of every

trail was recorded by collecting sample from top,middle and bottom of the mold.

Figure 7 Modified Proctor

From the Proctor compaction tests, density atdifferent moisture contents were measured, andthe maximum dry density and optimum moisturecontent for each different mixture were estimated.Proctor curve for Reclaimed Base (RB-2)material is shown in Figure 8.

Figure 8 Proctor Curve for Reclaimed Base(RB-2) Material

Specific Gravity: Specific Gravity for coarseparticles which retained ¾ in. sieve wasperformed by using AASHTO T 209 procedure.Apparatus used for this procedure consists ofvacuum pump, agitating machine and a containerwith lid as shown in Figure 9.

Figure 9 Specific Gravity Apparatus

120

125

130

135

140

145

0 2 4 6 8 10 12

Dry

Den

sity

(pcf

)

Water Content (%)

0.00

20.00

40.00

60.00

80.00

100.00

0.0010.0100.1001.00010.000100.000

% P

assin

g

Opening (mm)

Grain Distribution Curve


Formula used to calculate specific gravity is asfollow

Gs= /( + − )A= mass of dried MaterialD= mass of Container plus waterE= mass of container + water and sample aftervacuum and agitate

Sample Calculations for resilient modulustesting: Optimum moisture content andmaximum dry density measured from modifiedproctor test was converted into field conditionsby applying following equations.

dmax(Mr )

dmax( field )PF (112

" )

100dmax( field )PC(11

2" )

kWhere:dmax(Mr) = maximum dry density for resilientmodulus dmax(field) = maximum dry density in fieldconditionsPF(1 ½”-) = Percent finer of 1 ½”- material fromgrain sizePC(1 ½”+) = Percent finer of 1 ½”+ material fromgrain sizek = water * Gs

wct = corrected water content to be used inresilient modulus sample

wf = optimum water content of ¾”

wc =w f 2 wc1w f 1

wf2 = wf = optimum water content of ¾”- indecimal (Step 11b)wc1 = water content of coarse particleswf1 = water content of fine particlesPF(3/4”-) = Percent finer of ¾”- materialPC(3/4”+) = Percent coarser of ¾”+ material

Material which passed 1 ½ in. sieve was used toprepare resilient modulus sample. After applying

the above mentioned equations optimummoisture content (OMC) for Reclaimed Base(RB-2) material under field condition was foundto be 6.08% and maximum dry density of 124.46lbs/ft3. Total density of sample with moisturecontent was happened to be 132.02 lbs/ft3.Volume of split mold was 0.193ft3 which gave thetotal weight of sample required to make specimenthat was 25.81 lbs and weight of calculated waterin sample was found to be 1.48 lbs. According toAASHTO T 307 procedure sample for resilientmodulus has to compact into six equal 2 in. thicklayers, so weight per lift became 4.31 lbs.

Sample Preparation for resilient modulus testing:Firstly membrane was attached to the base oftriaxial cell using elastics. Split Mold wasattached by tightening screws on oppositedirection. It should be careful about membranenot caught in split mold while tightening.Membrane was stretched over the top of the splitmold and was secured with elastics. Vacuum linewas attached with swage lock fittings toperimeter of split mold. Portable vacuum wasattached to the line so that membrane expands tothe walls of the split mold. A filter paper wasplaced at the bottom of split mold. The first lift ofsoil was put into split mold according tocalculations of OMC and weight per lift ofspecimen and was compacted using impacthammer as shown in Figure 10. When finishedvacuum was released from split mold and smallvacuum was applied to the base of the sample.Membrane should be carefully folded over thetop and bottom of the sample. The secondmembrane was carefully placed on the top of firstone.

Figure 10 Impact Hammer with Split mold

wct w fPF WCPC


The height and diameter of the sample wasmeasured and recorded. The triaxial apparatuswas assembled as shown in Figure 11. Rods weretightened the same amount. Vacuum wasremoved from the base of the sample andtransported to the Instron servo-hydraulic testingmachine.

Figure 11 Triaxial cell with specimen

Resilient Modulus Testing Equipment: Theresilient modulus (MR) test system consisted ofall appropriate sensors and data acquisitionsystem (Figure 12) necessary for conducting thetest, generally following the procedure ofAASHTO T 307. The specimen was locatedinside the chamber, which acts as a pressurevessel. The base contained a port for the airsupply for confinement. Axial load is applied bya servo-hydraulic load frame which was 22.2 KNcapacity and 102 mm stroke. The load framesystem is operated by a controlling programnamed Instron Console. The load cell had a 22.2KN capacity. Load and displacement data werecollected by a Wavematrix Instron where alltesting sequences were created.

Resilient Modulus Testing & ComputerOperation Procedure: Resilient modulus testingand computer operation procedure consists ofthree phases as follows:

Proportional Integral Derivative (PID)Settings

Conditioning Phase Resilient Modulus Testing Phase

Figure 12 Triaxial cell with specimen

Triaxial chamber was placed into the Instrontesting machine load frame. Piston of machinewas adjusted just above the triaxial chamber butnot in contact with the sample. Seating load of -0.050 KN, -0.10KN and -0.20 KN was appliedfrom the load control frame. Confining pressureof 80.18 KN (11.63 psi) was applied on thesample. All drainage valves leading into thespecimen to atmospheric pressure were opened.This was done to simulate drained conditions.New and old PID settings were recorded with dateand time.

The test was began by applying a minimum of500 repetitions of a load equivalent to amaximum axial stress of 103.4 kPa (15 psi)and corresponding cyclic stress of 93.1 kPa(13.50 psi) using a have sine shaped load pulsewith durations. If the sample is still decreasing inheight at the end of the conditioning period,stress cycling shall be continued up to 1000repetitions prior to testing as outlined inSequence No. 0, Table 1


Table 1 Testing Sequence for Base/SubbaseMaterials

The above stress sequence constitutes sampleconditioning, that is, the elimination of theeffects of the interval between compaction andloading and the elimination of initial loadingversus reloading. This conditioning also aids inminimizing the effects of initially imperfectcontact between the sample cap and base plateand the test specimen.

The testing was performed following theloading sequences in Table 1 using a haver sineshaped load pulse as described above. Themaximum axial stress was decreased to 21.0kPa (3 psi) and set the confining pressure wasset to 21.0 KPa (3.0 psi) (Sequence No. 1,Table 1). At this confining pressure three loadingsequences were performed on the sample with 100

cycles of loading with same amplitude. After thecompletion of 300 cycles software allowed 1 min ofbreak time to change confining pressure from 21.0KPa (3 psi) to 34.47 KPa (5 psi) and similarly itcompleted 300 loading cycles under same confiningpressure. After the completion of 15 sequences asshown in Table 1, wave matrix software saved thedata file in the C drive of computer. Data obtainedfrom the test was too large that it was impossible tohandle on Microsoft Excel sheet. Matalb code wasset up to get output from the data obtained fromwave matrix. After running through the code we getresilient modulus values in a simple excel sheet asshown in Table 2. Deviator Stress and resilientstrain relationship is showing in the Figure 13

Figure 13 Deviator Stress VS Resilient StrainChart

Table 2: Resilient Modulus values corresponding to different loading sequences


Resilient modulus value for each sequence isshowing in the Table 2. Reported value ofresilient modulus was calculated by taking theaverage for all values obtained under differentsequences which happened to be 99.93 MPawhich is showing the stiffness of the material.

Conclusions and Summary:

A resilient modulus testing system was graduallydeveloped for educational purpose throughconducting several research programs. This paperdescribed testing equipment and procedures forpreparing and testing untreated subgrade soilsand untreated base/subbase materials fordetermination of resilient modulus (Mr) underconditions representing a simulating the physicalconditions and stress states of materials beneathflexible pavements subjected to moving wheelloads. Reclaimed Base (RB-2) material providedby RIDOT was tested with the developed testingsystem at URI to determine resilient modulusvalue. The testing was conducted in accordancewith the procedure of AASHTO T 307-99, andthe resilient modulus was determined as 99.93MPa.

References:

American Association of State Highway andTransportation Officials (AASHTO), (1993). TheAASHTO Guide for Design of PavementStructures, Washington, D.C.

American Association of State Highway andTransportation Officials (AASHTO), (2003).Determining the Resilient Modulus of Solis andAggregates Materials.

American Association of State Highway andTransportation Officials (AASHTO), (2012).

AASHTOWare Pavement ME Design,Washington, D.C.

Jin, M. Lee, K.W. and Kovacs, W.D.(1994).“Seasonal Variation of Resilient Modulus ofSubgrade Soils,” Vol. 120, No. 4, Journal ofTransportation Engineering, American Society ofCivil Engineers (ASCE), New York, NY.

Lee, K.W., Marcus, A., and Mao, H. (1994).“Determination of Effective Soil ResilientModulus and Estimation of Layer Coefficients forUnbound Layers of Flexible Pavement in RhodeIsland,” a Final Report to RIDOT No.1, URI.

Lee, K. W., Huston, M. Davis, J. and Vajjhalla,S., (2001). Structural Analysis of New EnglandSubbase Materials and Structures.

Transportation Research Board (TRB), (2002).Mechanistic-Empirical Pavement Design Guide(MEPDG), Washington, D.C.

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