TRANSPORTATION RESEARCH BOARDNATIONAL RESEARCH COUNCIL

National Cooperative Highway Research Program

RESEARCH RESULTS DIGESTFebruary 2002—Number 262

Subject Area: IIIB Materials and Construction Responsible Senior Program Officer: Edward T. Harrigan

Field Shear Test for Hot Mix Asphalt

This digest summarizes the findings from NCHRP Project 9-18, “Field Shear Test for Hot Mix Asphalt,” conducted by thePennsylvania Transportation Institute of Pennsylvania State University.This digest was prepared by Dr. Donald W. Christensen Jr.,

formerly of the Pennsylvania Transportation Institute, Dr. Ramon F. Bonaquist of Advanced Asphalt Technologies, LLC, andDr. Titin Handojo, formerly of the Pennsylvania Transportation Institute, from the text of the final report of the research project.

problems. The objective of NCHRP Project 9-18was to refine and evaluate the FST and make itsuitable for QC/QA testing in a construction envi-ronment.

During NCHRP 9-18, the original FST proto-type was evaluated and a second, improved FSTprototype was developed. This prototype is a sig-nificant improvement over the first device. It iscompact, rugged, and extremely quick and easy tooperate. In executing tests with the new FST,gyratory specimens are placed directly in the deviceand are held in place by hydraulic clamps. Thetesting geometry is similar to that used in the SST,though the distribution of stresses and strains isquite complex, and test data generated with theFST should therefore be considered approximate.Average shear strain in the FST can be estimatedby dividing the deflection by the distance betweenthe grips, 21.3 mm in the prototype device. Thebest results with the FST were obtained for fre-quency sweep data at 40°C and 5 to 10 Hz. Underthese conditions, dynamic modulus (|G*|) data gen-erated with the FST were typically about 40 per-cent higher than those produced in the SST, whilephase angles were 5° to 15° lower. Because of thehighly variable nature of modulus data for HMA,these results should be interpreted with care. Addi-tional research is needed to establish correlationsamong data gathered using the FST, SST, anduniaxial test methods. Variability in modulus datafrom the FST is comparable to that seen in othermodulus measurements.

In commercial production, the cost of the FSTis estimated to be between $25,000 and $50,000,depending on the number of units produced and

BACKGROUND

This digest summarizes key results of NCHRPProject 9-18, “Field Shear Test for Hot Mix Asphalt.”Of particular interest, it describes the redesign ofthe field shear test (FST) device developed as apotential quality control and quality assurance(QC/QA) tool in NCHRP Project 9-7 to improveits test configuration and increase the precision ofits measurements. The redesigned FST was foundcapable of performing a variety of tests, but thebest results were obtained from the frequency sweepprocedure described in AASHTO TP7 at high tem-peratures and high frequencies. Under these condi-tions, the equipment produced dynamic modulusvalues and phase angles in the same range as thosegenerated using the full-sized Superpave shear testequipment described in AASHTO TP7; moreover,these results were found to be fairly sensitive tochanges in hot mix asphalt (HMA) composition.However, while the data repeatability of the re-designed FST is similar to other methods formeasuring HMA stiffness, it remains too high forQC/QA testing.

SUMMARY

The field shear test, or FST, was originallydeveloped during NCHRP Project 9-7 as a simpleand inexpensive alternative to the Superpave sheartest (SST), suitable for use in QC/QA testing ofHMA mixtures. The first FST prototype, developedduring NCHRP 9-7 and found to be promising insome ways, nevertheless exhibited several serious

2

sold. For a market size of 1,000 units or more, the cost isestimated to be $25,000 to $30,000—similar to that of agyratory compactor. These prices include the cost of anenvironmental chamber, computer, and other necessaryaccessories. Once a specimen is conditioned, a test can becompleted in less than 10 min, if limited to high-frequencytesting. Specimen conditioning depends on the protocolused. Bringing a specimen to 40°C from 120°C requiresabout 2-1/2 hours in a chamber. Equilibrating to 40°C fromroom temperature requires about 1-1/2 hours. Conditioningtimes are much less if water baths are used; in either case,equilibrium is reached within 30 min. If it is found thatsubmerging FST specimens in water affects the modulusmeasurement, the specimens could be vacuum-sealed inplastic prior to conditioning. By conditioning the specimensin a water bath, the total time from compaction to comple-tion of FST testing can be kept to less than 40 min.

Data gathered during sensitivity testing showed thatFST data are fairly sensitive to changes in asphalt concretecomposition, especially coarse aggregate content and bindercontent. However, variability in HMA modulus data—pro-duced with the FST or any other device—appears too highfor QC purposes. Most of this variability is probably causedby varying amounts of steric hardening among the HMAspecimens, nonuniform distribution of air voids, and largeaggregate particles within specimens. Two simple modifi-cations in the testing protocol should improve the precisionof FST modulus data: (1) keeping the time between mixing,compaction, and FST testing to between 2 and 4 hours, aswould normally be done during QC testing; and (2) report-ing the average of four repeat determinations on a specimen,rotating or flipping the specimen between each measure-ment. Because of the simple operation of the FST, fourdeterminations can be completed in less than 10 min, andwould greatly reduce variability in test data due to specimennonuniformity.

Because the only problem encountered in NCHRP 9-18was the marginal precision of the FST, which likely can beimproved with simple and appropriate modifications in thetest protocol, a future project is recommended. The initialtasks of this project should include verifying that adequateprecision with the FST can be achieved with the suggestedQC protocol and establishing reasonable relationshipsamong FST, SST, and uniaxial modulus data. Additionalresearch is also needed on the effects of specimen height onFST test results. Continuation of the research to fielddemonstrations should be contingent on a successful out-come of the initial laboratory test program demonstratingthe accuracy and precision of the FST.

CHAPTER ONEINTRODUCTION AND RESEARCH APPROACH

Problem and Purpose

Although many of the tests, devices, procedures, andmodels developed during the Strategic Highway ResearchProgram (SHRP) were later found to be of limited value inthe design and construction of more durable pavements, theSST has become widely used. In particular, the frequencysweep test and repeated shear at constant height (RSCH)procedure have been successfully used by many laboratoriesfor predicting the rut resistance of HMA mixtures. How-ever, the SST device is too large, complex, and expensivefor use in QC/QA testing. As a result, NCHRP Project 9-7developed the FST several years ago as a simple method ofproviding test data comparable to those produced by theSST but more quickly and at a substantially lower cost. Theoriginal FST prototype had several shortcomings andrequired further refinement before it would be suitable foruse in QC testing of HMAC. The purpose of this report is todocument research performed during NCHRP Project 9-18to further evaluate and refine the FST.

Scope and Research Approach

NCHRP Project 9-18 consisted of seven tasks: (1) Re-viewing prototype development and performing a literaturesearch, (2) Performing stress analyses, (3) Modifying andrefining FST hardware and procedure, (4) Performing pre-liminary evaluation of the redesigned FST, (5) Compilingan interim report, (6) Conducting sensitivity testing, and(7) Compiling a final report.

The final report includes detailed information on theresults of each of these tasks. This digest summarizes im-portant findings made in completing each task; several fig-ures and tables are included to illustrate and emphasize keyfindings. Detailed documentation of the various projectactivities is provided in the appendices of the agency finalreport. These appendices are available for loan on request toNCHRP.

NCHRP Project 9-18 was planned as a multiphaseproject. Contingent upon successful completion of the firstphase of the project and availability of funding, a secondphase of research would be performed to validate the FSTunder field conditions and to facilitate implementation ofthe test device and method. Thus, the final report includesspecific recommendations for continuing research aimed atvalidating and implementing the FST for use in QC/QA test-ing of HMA paving mixtures.

3

CHAPTER TWOFINDINGS

In this chapter, the key findings of NCHRP Project 9-18are summarized. Detailed documentation of each of thevarious project activities is provided in the appendices of theagency final report. A discussion of the practical ramifica-tions of these findings is given in Chapter Three of thisdigest, while an extension of the findings to general conclu-sions and recommendations is presented in Chapter Four.

The SST

The SST was developed during SHRP, a $50-millionnationally coordinated research project completed in 1993(1). SHRP was geared toward developing improved testsand specifications for HMA paving materials. The SST wasdesigned to perform a variety of performance-related testson HMA, including characterization of the complex modulusand phase angle, determination of the bulk modulus, andevaluation of various aspects of the nonlinear, plasticbehavior typical of granular materials such as HMA at hightemperatures. Data gathered using the SST, along with avariety of other information, were in turn used as input to acomputer program meant to provide performance predic-tions for a given pavement system as a function of time.Unfortunately, a number of serious problems were found insome of the test procedures and in the computer program,which resulted in unreliable predictions of rut depth andfatigue cracking. Work has continued on the conceptsevolved during SHRP within NCHRP Projects 9-19 and1-37A. The completion of these projects will result in a setof more rugged and reliable test methods and models thatcan be implemented in a coherent system of materials char-acterization and pavement design and analysis.

Although many of the tests performed with the SSTwere found to be unreliable, two of the procedures wereshown to be reasonably accurate and to relate well to vari-

ous aspects of pavement performance: (1) the frequencysweep test and (2) the RSCH test (2, 3, 4). The frequencysweep test is a technique for evaluating the complex shearmodulus, G*(ω), of HMA. The shear modulus defines therelationship between shear stress and shear strain and isessential information in analyzing the behavior of a pave-ment system under traffic loading and during changes intemperature. The RSCH test is a repeated load test designedto characterize the resistance of an HMA mixture to perma-nent deformation at high temperatures. Numerous studieshave shown that the maximum permanent shear strain (MPSS)determined after the 5,000-cycle RSCH test is a good pre-dictor of the rut resistance of HMA mixtures test (2, 3, 4).The magnitude of the complex modulus (|G*|) at high tem-peratures has also been related to rut resistance, although therelationship is not as good as that for MPSS.

Both of these tests are described in AASHTO TP7-94:Test Method for Determining the Permanent Deformationand Fatigue Cracking Characteristics of Hot Mix Asphalt(HMA) Using the Simple Shear Test (SST) Device. The SSTtests are usually performed on 50-mm-thick, 150-mm-diameter specimens taken from a 115-mm-high standardspecimen as produced by the Superpave gyratory compactor(SGC). Specimen preparation for the SST is complex andtime-consuming, requiring careful sawing of the gyratoryspecimen, gluing platens onto the specimen, and, in somecases, fastening transducers onto the sides of the specimen.Figure 1 is a sketch showing two different approaches tomounting transducers on SST specimens (5). Furthermore,the SST device itself is a large, complex, and expensiveservohydraulic testing system not suitable for routine use instate highway laboratories or hot mix plants. Research fol-lowing SHRP pointed out the need for a simpler, quickermeans for evaluating the performance-related properties ofHMA.

HMA is a complicated material, exhibiting differingdegrees of viscoelastic and nonlinear plastic behavior,depending upon the temperature and level of applied stress

Figure 1. SST specimens showing two different approaches for mounting transducers (LVDTs) for deflection measurements (5).

-

4

and strain. It is, therefore, critical to determine the properloading conditions for a given test procedure and to controlthese conditions carefully during the test. In the SST simpleshear test, which is similar to a creep or constant stress test,the maximum shear stress applied is 35 ± 1 kPa. In theRSCH test, a pulse loading is used, with duration of 0.1 sand dwell time of 0.6 s. The maximum shear stress gener-ated during the RSCH test is 68 ± 5 kPa. During the SSTfrequency sweep protocol, the shear strain is limited to amaximum of 0.01 percent. These limits were developedduring SHRP to try to ensure linear behavior during testingand were based largely on experience and judgment (1).Later research suggests that these limits were too conserva-tive, and that higher stresses and larger strains can be usedduring these and similar tests (6, 7). This finding is impor-tant to the development of practical tests, since using largerstresses and strains during a test means that simpler, less-expensive equipment can be used, and that the results willprobably be more accurate. As documented in Appendix Aof the final report, researchers in the past have typically usedcompressive stresses of 100 to 200 kPa in creep tests onHMA, significantly higher than the 35 kPa used in the SSTsimple shear test, even after accounting for the differencebetween shear and compressive stresses. Furthermore, researchat Pennsylvania State University suggests that essentiallylinear behavior can be maintained in testing HMA at stressesup to 100 kPa and strains up to about 0.1 percent, both sub-stantially higher than the limits used in the SST (7). Becauseload-controlled testing is easier to perform on servohydraulicand servopneumatic systems, the NCHRP 9-18 researchteam suggested that the FST use an applied shear stress of100 kPa for creep and frequency sweep tests. This increasein stress should result in a simpler, less-expensive test deviceand improved data quality while still maintaining linearbehavior during testing.

NCHRP Project 9-7 and the Original FST

By the mid-1990s, 28 states were using Superpavebinder and mixture tests (8). NCHRP Project 9-7 was un-dertaken in 1993 to develop improved QC/QA proceduresfor Superpave projects. During Phase I of that project, theNCHRP 9-7 research team decided that a simple perfor-mance test was necessary to augment the volumetric dataused for QC of Superpave mixtures; the FST was the result.

A diagram of the original FST is shown in Figure 2 (8).The original FST used an unusual diametral shear geometryin which the specimen was held in two semicircular clampsand sheared across the diameter. This approach was used toallow direct testing of a gyratory specimen without sawing,coring, or gluing, so that the test could be executed simplyand quickly. The original prototype lacked a temperaturechamber; specimens were conditioned prior to testing andthen placed in the test device.

The original FST was found to not provide accuratemeasurements of complex modulus or maximum permanentshear strain (8, 9). Figure 3 is a plot in which |G*| valuesdetermined using the FST and the SST are compared, show-ing typical serious discrepancies in the values (8). This wasprobably, in part, due to poor performance of the servo-pneumatic loading system at high frequencies and loads,combined with relatively poor resolution of the deflectionmeasurements. Additionally, it is likely that the unusualgeometry contributed to the problems observed with the firstFST device.

As part of NCHRP Project 9-18, a survey was con-ducted of seven engineers and technicians involved in thedevelopment and evaluation of the first FST prototype. Thepurpose of this survey was to gain insight into what im-provements these key personnel believed were needed in thetest device and procedure. The results of this survey are

Figure 2. Sketch of the original FST developed during NCHRP Project 9-7 (8).

5

1000

10000

100000

0.001 0.1 10

Frequency, Hz

Co

mp

lex

Mo

du

lus

Mag

nit

ud

e, p

si

SSTFST +shearFST -shear

Figure 3. Plot of complex modulus as measured using the original FST prototype and using the SST (8).

presented in their entirety in Appendix C of the agency finalreport. Although there was, in some cases, significant dis-agreement among those surveyed, most agreed to the fol-lowing findings:

• The FST test geometry should be changed to moreclosely resemble that used in the SST, where the direc-tion of shearing is parallel to the ends of the specimen.

• The FST should retain the ability to perform the fre-quency sweep test, the repeated shear test, and thesimple shear (creep) test commonly performed on theSST. Of these protocols, the frequency sweep test is themost likely candidate for QC/QA testing of HMAmixtures.

• A simpler specimen-mounting procedure is needed.• The device and procedure should be kept as simple and

rugged as possible.• The cost of the FST should be in the range of $35,000 to

$50,000.

Based on the review of the original FST and its develop-ment, it was decided that a major redesign of the FST wasneeded. The design of the new FST would incorporate thefindings of the survey, and would also address other short-comings of the first prototype, including a better specimen-clamping system, better system response, improved trans-ducer resolution, and a good temperature-control system.Above all else, it was felt that the new FST should be simpleand rugged, appropriate for use by technicians within a typi-cal HMA plant. The new FST developed on the basis ofthese findings is described in the section on sensitivity test-ing. Prior to development of the new FST, however, anumber of theoretical stress analyses were conducted to

verify the shortcomings of the original FST geometry, evalu-ate the possible alternatives, and compare the results forthese test geometries with the stresses and strains occurringin the SST.

Stress Analyses

The NCHRP 9-18 research team decided prior to start-ing the stress analyses that the most promising alternativegeometry for the FST involved holding the ends of a gyratoryspecimen in cups or clamps while shearing it parallel to itsends, as shown in Figure 4. The specimen in this proposedFST geometry was assumed to be a 115-mm-high by150-mm-diameter gyratory specimen, with a gap betweenthe grips of 22 mm. In order to evaluate the distribution ofstresses and strains generated in the original FST test, thenew FST geometry, and the SST test, Dr. Chandra Desai of

Figure 4. Sketch of proposed geometry for the new FST,as considered in stress analyses.

6

the University of Arizona performed several finite-elementanalyses. The results of these analyses are discussed in thefinal report, available by loan upon request from NCHRP.To simplify the analysis, plane strain conditions wereassumed, so the results of the analyses should be consideredas approximate. Initial analyses were performed assuming amuch higher stress level than what is normally used whentesting HMA, especially at high temperatures. Not surpris-ingly, this approach resulted in significant nonlinearity inboth the original FST geometry and the proposed alternative.However, it was clear that in the proposed FST geometry,shearing stresses and strains would tend to expand into theends of the specimens, so that deflections would tend to belarger than those determined from equivalent loading in theSST.

The SST test, although exhibiting fairly uniformstresses, did not exhibit ideal behavior. Figure 5 shows themagnitude of shear stresses through the center and acrossthe top of an SST specimen under loading. This was part ofthe first series of analyses; the shear modulus of the speci-men was assumed to be 269 MPa, and the stresses shownwere as calculated for a 1-mm deflection. The shear strainapplied was thus 2 percent, about 200 times what is nor-mally used in the SST, whereas the resulting average shearstress of 5.4 MPa is about 150 times that used in the SST. Inthis particular case, linear behavior was assumed so theresulting stress distribution, though showing unusually largevalues, is representative for the SST. Because of the rela-tively large aspect ratio and lack of complementary shearingon the specimen sides, it was found that the shear stressesfall off substantially toward the edge of the specimen. Thisresults in a lower overall load being generated than wouldoccur under pure shearing. Therefore, modulus measure-ments made with the SST will typically be about 10 percenttoo low. Preliminary evaluation of the stresses in the origi-

0.0

2.0

4.0

6.0

8.0

0 50 100 150

Distance (mm)

Str

ess,

MP

a

Top

Middle

Figure 5. Stresses generated in an SST specimen for the linear-elastic case, with E = 700 MPa and a 1-mm deflection.

nal FST (diametral shearing) showed very poor and non-uniform distribution of stresses, as expected. For this reason,the original FST geometry was not included in subsequentanalyses.

In analyzing preliminary test data from the modifiedFST, as discussed in the next section, it became clear thatdeflections from this test were significantly lower thanexpected based on the results of the initial stress analyses.This finding appeared to be caused by confinement of theHMA mixture within the clamps on the specimen ends. Thisconfinement would significantly increase the stiffness of themixture in the specimen ends, improving the distribution ofstresses in the FST specimen and reducing resulting deflec-tions. Because of this finding, Dr. Desai considered twodifferent cases in the final series of stress analyses: (1) aconstant uniaxial modulus (E) of 100 MPa throughout thespecimen and (2) a uniaxial modulus of 100 MPa in thematerial sheared between the clamps, and 260 MPa for thematerial confined within the clamps. Furthermore, a con-stant load of 1200 N was assumed, which would result in anaverage shear stress under ideal conditions of 68 kPa, as isused in the SST simple shear test. Both linear and nonlinearanalyses were performed. However, subsequent testing dur-ing the project clearly showed that deflections in the FSTwere quite low, and that linear behavior was a more realisticassumption. These data also confirmed that there was sig-nificant specimen confinement within the clamps that, insome cases, even extended into the sheared area between theclamps. (See section on sensitivity testing.) This resulted inthe elastic case being the most accurate of the stress analy-ses performed, with an increased modulus assumed for thematerial confined within the clamps. The resulting stressdistributions are plotted in Figure 6 (center of specimen) andFigure 7 (across the specimen at the edge of the lowerclamp). The stress distribution in the center of the speci-

7

-100

-80

-60

-40

-20

0

20

0 50 100 150

Distance, mm

Str

ess,

kP

a

Shear Stress

Horizontal Stress

Vertical Stress

-100

-80

-60

-40

-20

0

20

40

60

80

100

0 50 100 150

Distance, mm

Str

ess,

kP

a

Shear Stress

Horizontal Stress

Vertical Stress

Figure 6. Stresses through center plane of FST specimen, linear analysis; modulus of 260 MPa within clamps and 100MPa between clamps.

Figure 7. Stresses through plane at top of lower clamp in FST specimen, linear analysis; modulus of 260 MPa withinclamps and 100 MPa between clamps.

men, although far from ideal, is reasonable. The shearstresses rise from the edge to a maximum of about 60 kPa,and then gradually decrease to about 40 kPa in the center ofthe specimen. Vertical and horizontal stresses are negli-gible. The stress distribution across the specimen at theclamp edge is more complex. Shear stresses are fairly con-stant at about 40 kPa, but significant horizontal stresses exist,especially at the specimen edge. These horizontal stressesare a result of the end of the specimen being “pushed” by theclamp, rather than having a more ideal shear stress appliedas it is in the SST.

The results of the stress analysis indicated that the new

FST geometry, although not providing an ideal stress distri-bution, was probably suitable for QC/QA testing. The stressdistribution is, however, more complex and substantially dif-ferent from that in the SST. For practical purposes, the mostimportant part of the stress analysis is determining an effec-tive gage length, which represents the thickness that shouldbe used when calculating average shear strains in the FST:

average shear strain = shear deflection/effective gage length

Table 1 summarizes the results of the stress analysis in termsof effective gage length for the FST. Included in this table

8

TABLE 1 Results of equivalent gage length analysis for FST stress analysis

HMAShear Mod. (MPa)

Between Within Defl. Thick./Gage App. App. Shear Effective GageAnalysis Clamps Clamps (mm) Length (mm) Strain Mod. (MPa) Length (mm)

FST-Ideal 38.5 : 0.038 21.3 0.00177 38.5 21.3Linear 38.5 38.5 0.074 21.3 0.00336 20.2 41.9Linear 38.5 100 0.047 21.3 0.00214 31.8 26.6Non-linear 38.5 38.5 0.128 21.3 0.00582 11.7 72.5Non-linear 38.5 100 0.076 21.3 0.00345 19.7 43.0SST 38.5 N/A 0.010 50.0 0.00200 34.0 56.5

TABLE 2 Key features of the improved FST

Characteristic Values

Size (w × d × h) 30 × 30 × 40 cmWeight 50 kgLoading Mechanism ServopneumaticPower Requirements 110 VAC with 550 kPa airSpecimen Diameter 150 mmSpecimen Height 50 mm to 122 mmMaximum Shear Stress 1260 kPa, with calibrated ranges of

1260, 630, 252, and 126 kPaMaximum Axial Stress 550 kPa (available air)Maximum Shear ±1.25 mm with calibrated ranges of

Displacement ±1.25 and ±0.25 mm)Resolution 12 bit

are the “ideal” case, which assumes that the FST specimenis sheared only between the grip collars, and an analysis ofthe SST. The SST data illustrate that the stress distributionfrom this device, although more uniform than that for theFST, is also not ideal. As mentioned previously, limiteddata suggest that the linear analysis with confinement pro-vides the best agreement with experimental data. Test re-sults gathered during the gage-length experiment indicatethat deflections for the FST are sometimes even smaller thanwould be calculated for the ideal case, resulting in an effec-tive gage length less than the 21.3-mm collar separation.Considering the results of both the stress analyses and thelimited experimental data gathered, the effective gage length(thickness) used in calculating average shear strain for theFST should be the same as the collar separation, 21.3 mm.It should be understood that this approach is an approxima-tion and depends, to some extent, on confinement of theHMA within the FST grips.

Redesigned FST

The new design of the FST incorporated a number ofsignificant changes to address the various problems observedin the performance of the first prototype. Most importantly,the geometry of the new FST involved shearing of a gyra-tory specimen parallel to its ends, as is done in the SST.EnduraTEC Systems Corporation, in consultation with theproject team, developed the new FST device and lent theprototype to Advanced Asphalt Technologies for evaluationand use in sensitivity testing. This prototype is shown inFigure 8. A summary of key features of the new FST ispresented in Table 2.

After an initial evaluation of this device, EnduraTECcorrected several minor problems with the instrumentationand software. Details of this preliminary evaluation areprovided in Appendix E. The evaluation’s objectives wereto verify that the new version of the FST was operatingproperly and to make limited comparisons between datafrom the improved FST and the SST. The testing includedevaluation of the redesigned FST as originally supplied by

Figure 8. Photograph of the new FST device.

9

EnduraTEC, and the final version of the FST after minormodifications had been completed. The following para-graphs summarize the findings of the preliminary evaluationtesting.

1. The redesigned FST is a compact, user-friendly deviceand in these respects is well suited to QC/QA applica-tions. The equipment can conduct frequency sweep,repeated shear, and creep tests directly on standardSuperpave gyratory specimens without the need forspecial specimen preparation. The software logicallyprompts the user through the steps required for speci-men setup and testing. The unique hydraulic clampsgreatly simplify testing operations. Midlevel techni-cians can now easily operate the FST.

2. The limited comparison of data collected during the pre-liminary evaluation showed that specimens of the samematerial have consistently stiffer response in the FSTcompared to the SST. Shear moduli were higher andphase angles were lower for comparable specimenstested in the improved FST compared to the SST. Creepand repeated shear deformations were lower in FSTscompared to SSTs. The stiffer response in the improvedFST appears to be related to specimen confinement pro-duced by the hydraulic clamps. Measured responses inthe new version of the FST are sensitive to clampingpressure. A pressure of 10.3 MPa is recommended forgeneral use in the FST and was used for the sensitivitytesting conducted during this project.

3. Data from the redesigned FST can be related to datafrom the SST and other tests through the use of an effec-tive gage length. The effective gage length decreaseswith decreasing specimen stiffness. At high stiffness,effective gage lengths from experimental data comparereasonably well with those calculated from elasticanalyses. For high-temperature testing using a clamp-ing pressure of 10.3 MPa, an effective gage length of21 mm was calculated from data collected during thepreliminary evaluation.

4. In the preliminary evaluation of the FST, the test resultsshowed a level of repeatability similar to that observedfor the SST and similar tests. Coefficients of variation(CVs) for the small sample size of three ranged from5 percent to 15 percent. This finding is similar to datafrom the SST and other tests such as the uniaxial dynamicmodulus and repeated load tests recommended inProject 9-19. The sensitivity study, with its large num-ber of replicates, will provide more definitive informa-tion on the repeatability of the improved FSTs.

Sensitivity Testing

After the new FST device had been evaluated, modi-fied, and found to be performing reasonably well, a testingprogram was performed to evaluate the sensitivity of thisdevice to changes in HMA composition. Four different

materials—9.5-mm, 12.5-mm, 19-mm, and 25-mm NMASSuperpave mixtures—were tested. For each of these, thedesign aggregate gradation and binder contents were sys-tematically varied to produce a total of eight mixture varia-tions.

The following factors were varied in the testing: coarseaggregate content (± 6 percent on the 2.36-mm sieve), min-eral filler content (± 2 percent on the 0.075-mm sieve), andbinder content (± 0.5). The FST and RSCH test were per-formed with the new FST, both at a temperature of 40°C.The RSCH test was performed at the maximum 7-day aver-age pavement temperature for Northern Virginia, 54°C. Theresults of the sensitivity testing are provided in Appendix Fof the agency final report.

Analysis of the results of the sensitivity testing demon-strated that the FST is sensitive to changes in mixture com-position. The best sensitivity was found using |G*| data at5 Hz and 10 Hz. Analysis of variance models includingmixture composition as predictor variables for |G*|, alongwith compaction order and test time, gave r2 values rangingfrom 55 percent to 91 percent; values for models for MPSSfrom the RSCH ranged from 47 percent to 68 percent. Thedynamic modulus was quite sensitive to changes in coarseaggregate content and binder content, and in some casesmineral filler and various interactions of these factors. Theresults of the sensitivity testing are presented graphically fortwo of the mixtures in Figure 9 (complete plots are includedAppendix F). This plot shows average |G*| values at 40°Cand 10 Hz for the eight variations of the 25- and 12.5-mmmixtures. Included on the plot are error bars representingtwo standard deviation ±( )2 4s / confidence intervalsabout the mean. The average for all mixture types is alsoshown on each plot. Because the eight mix variations repre-sent significant differences in mixture composition, the errorbars should not overlap for most of the mixtures if the |G*|measurements are in fact sensitive to these compositionalchanges. Conversely, extensive overlap of the error barsindicates that the modulus measurement shows little or nosensitivity to changes in composition. Otherwise, it isunlikely that the test would be useful in QC. For the 25-mmmixture, six of the eight mix types varied significantly fromthe mean value, but for the 12.5-mm mixture, only threeshowed significant variation from the mean. The sensitivityof the 9.5-mm mix, like that for the 25-mm mix, was good.The sensitivity of the 19-mm mix, on the other hand, waspoor—closer to that observed for the 12.5-mm mix.

Although the results of the sensitivity testing showedthat the FST is sensitive to changes in mixture composition,very good precision is needed for effective QC testing, andthe variability in the test data is too high to allow the FST tobe used for QC/QA testing at this time. This is evident, forexample, in Figure 9(b), where the error bars for the variousmix types are quite large compared to the difference amongtheir responses. This is not a problem unique to the FST—the same problem exists for virtually all test methods cur-rently used to measure the stiffness of HMA mixtures. The

10

0

200

400

600

800

1000

LHH HHH LLL HHL LHL LLH HLL HLH

Mix Code

|G*|

at 4

0 C

& 1

0 H

z (M

Pa)

(a) 25-mm Mixture

0

200

400

600

800

1000

LHH HHH LLL HHL LHL LLH HLL HLH

Mix Code

|G*|

at 4

0 C

& 1

0 H

z (M

Pa)

(b) 12.5-mm Mixture

Figure 9. Average complex modulus at 10 Hz and 40°C with 2s-error bars for the mean. (Mix code represents high (H)or low (L) values for coarse aggregate, mineral filler, or binder content respectively. Dash lines represent mean value formixture types.)

CV for |G*| measured with the FST was found in this studyto range from 7 percent to 17 percent. For SST testing onselected companion specimens, the CV was found to be inthe same range, 5 percent to 18 percent, with most valuestending toward the higher end of this range. Mehta andChristensen reported similar values in testing using theSST (7).

Most of the variability in HMA modulus data is prob-ably due not to inherent problems in the FST or other mix-ture tests, but to variability in the HMA specimens them-selves. This variation is believed to have two sources:(1) variations in steric hardening due to differences in thetime between specimen preparation and testing and (2) non-uniformity within specimens in the distribution of air voidsand large aggregate particles. The first of these problemsshould not exist under QC testing conditions, where speci-mens will be mixed, compacted, and tested within a fewhours. The second problem can be effectively addressedwith the FST by performing four repeat determinations on

each specimen, rotating or flipping the specimen betweeneach test. The reported |G*| values would be the average ofthese four measurements. With the ease of operation of theFST system, including the clamps, these replicate tests canbe performed within 10 min total. The initial tasks in anyproject should focus on evaluating variability in FST datausing this slightly modified protocol. If the precision of theFST is not significantly improved by these changes, it is notlikely that it can be successfully implemented as a QC test,and further FST development should be discontinued.

An experiment related to the sensitivity testing was per-formed to more thoroughly compare test data generated withthe new FST to those generated with the SST. The objectiveof this testing was to determine, if possible, an averageeffective gage length for the FST, so that data generatedwith this test can be compared with those generated with theSST and other devices. This experiment involved testingtwo variations of the 9.5- and 25-mm mixtures using theSST frequency sweep and repeated shear tests. These data

11

were then compared directly with the matching data gener-ated during sensitivity testing with the FST. The results ofthis experiment for one of the 9.5-mm mix types are shownin Figure 10, in which |G*| values from the SST and FST arecompared. The effective gage length used in calculatingFST shear strains and modulus values was 21.3 mm, thesame as the grip separation distance. In this case, the modulusvalues for the FST were consistently greater than those foundfrom the SST by about a factor of two. The results for allfour mixtures tested are summarized in Figure 11, in whichthe ratio of |G*| values (FST/SST) is plotted as a function offrequency. The FST always produces significantly highermodulus values than the SST. This effect is most pro-nounced for the 25-mm mixture at low frequencies, wherethe FST modulus exceeds the SST-measured value by asmuch as four times. This may result from the confinementeffects spreading out from the clamps into the sheared re-gion, which would be more pronounced for mixtures with

larger aggregate particles. However, at frequencies of 5 Hzand 10 Hz, there is better agreement, with an average modulusratio of 1.39, indicating that at high frequencies the FST willprovide |G*| values about 40 percent greater than those givenby the SST. This corresponds to an effective gage length of15.8 mm. However, as discussed previously, modulus valuesdetermined using the SST are potentially lower than actualvalues, and modulus tests on HMAC are in general highlyvariable. Because of these limitations, at this time it canonly be concluded that modulus values determined using theFST and SST are in the same range, although there is someindication that the FST might produce somewhat highervalues for |G*|. Any future research should involve a thor-ough comparison of modulus values measured using theFST, SST, and uniaxial compression.

In general, phase angles measured with the FST at 5 Hzand 10 Hz are 5° to 15° lower than those determined withthe SST, again suggesting that confinement causes signifi-

Figure 11. Ratio of |G*| as determined with the FST to that determined with the SST, as a function of frequency.

10

100

1000

1.00E-03 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02

Frequency, Hz

|G*|

, MP

a SST DataFST Data

0.0

1.0

2.0

3.0

4.0

5.0

1.00E-03 1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02

Frequency, Hz

|G*(

FS

T)|

/|G*(

SS

T)|

25mm LHH

25mm HLL

9.5mm LHH

9.5mm HLL

Figure 10. |G*| at 40°C on 9.5-mm HLL mix, using SST and FST devices. (Plots show average and ± 2s error bars forthe mean.)

12

cant stiffening of the mixture during the FST test. Maxi-mum permanent shear strains measured with the FST werelower than those determined using the SST by a factor rang-ing from 4 to 12 times. The RSCH test performed using theFST device does not produce reliable data.

The final experiment associated with the sensitivity test-ing was an evaluation of the effect of different specimenheights on the quality of FST data. The ability to evaluatethin specimens, such as field cores from thin pavement layersusing the FST, is a useful capability for QA testing andforensic studies. In this experiment, gyratory specimens ofthree different thicknesses were tested in the FST: 75, 50,and 38 mm. Two different variations of the 9.5-mm mixturewere used in the study. For one series of tests, both theaverage value of and variability in |G*| were similar for allspecimen heights and agreed well with the original series oftests, performed on 115-mm-thick specimens. In the secondseries of tests, the |G*| values for the three shorter speci-mens were in general agreement, but were lower than for theoriginal (115-mm-thick specimens) test data. This was prob-ably the result of a very low air void level in the thicknessexperiment specimens—less than 1 percent—compared toan air void level of 2.6 percent in the first series of speci-mens. However, in this test series the CV for the 38-mmspecimen was 35 percent, which is quite high. Therefore,caution should be used in testing specimens thinner than50 mm using the FST device. Additional research is neededto verify the effects of specimen height on FST data for9.5-mm mixtures and to extend this part of the study tomixtures with larger aggregate sizes. Furthermore, it shouldbe recognized that the FST probably will not be useful fortesting thin field cores.

CHAPTER THREEINTERPRETATION, APPRAISAL, ANDAPPLICATIONS

The new FST, as produced in prototype by EnduraTECSystems, is a simple, effective device for measurement ofthe dynamic shear modulus of HMA, |G*|. It is small,rugged, and easy to operate. No hardware is mounted on thespecimen. Tests can be performed very quickly if speci-mens are conditioned in a separate chamber prior to testing.The FST is no more difficult to operate than a gyratorycompactor. It is suggested that in the future, four repeatmeasurements be performed on each specimen, rotating orflipping the specimen between each replicate. The averageof these four measurements would be reported. Even withfour repeat measurements, a complete FST test could beperformed in less than 10 min.

The amount of time required for conditioning FSTspecimens depends on the protocol used. Cooling speci-mens in air from 120°C to 40°C requires about 2-1/2 hours.This is perhaps longer than some engineers and technicianswould like, but this is currently necessary in order to per-

form specific gravity measurements. Bringing specimens to40°C from room temperature requires about 1-1/2 hours.These conditioning times could be reduced substantially byusing a water bath to condition the specimens, rather than aconvection oven. If water appears to affect the measure-ment of modulus, the specimen could be vacuum-sealed inplastic during conditioning to protect it from water damage.Conditioning in a water bath from either 120°C or roomtemperature to equilibrium at 40°C takes 30 min. Thus, byusing a water bath for conditioning specimens, a completeFST modulus test could be completed within 40 min ofcompaction.

The FST should be used only for measuring complexmodulus, and not for repeated shear or creep testing. Fur-thermore, the most reliable measurements of |G*| with theFST are obtained at 5 Hz and 10 Hz. Very limited testingindicated that HMA moduli measured with the FST underthese conditions were about 40 percent higher than thosemeasured using the SST, while phase angles were about 5°to 15° lower. Additional testing is needed to morethoroughly establish the relationships among FST, SST, anduniaxial modulus data. Variability in test data using theFST is comparable to that seen in other methods of measur-ing mixture modulus, such as the SST.

The sensitivity testing of the FST was successful.Modulus values measured with the FST are clearly sensitiveto changes in mixture composition, especially coarse aggre-gate content and binder content. However, the level of vari-ability currently found in HMA modulus measurements,whether determined using the FST or any other availabledevice, is currently too high for QC/QA testing purposes. Itis imperative that QC tests be highly repeatable, so thatreasonable control limits can be established to aid the engi-neer in evaluating whether or not the process is in control.Current test methods do not provide the required level ofprecision.

However, most of the variability in FST modulus mea-surements is probably not the result of problems with thetest device, but is caused by variability among and withintest specimens. As discussed in Chapter Two of this report,it is likely that the precision of the FST can be improvedwith two simple modifications in the testing protocol, bothentirely consistent with implementation as a QC test. Thefirst modification is simply to mix, compact, and test FSTspecimens within 2 to 4 hours, eliminating variations insteric hardening. The second modification involves takingthe average of four repeat measurements made with the FST,rather than a single value. Future research should focus onevaluating whether or not this simple change in the protocolwill be effective in improving the precision of FST measure-ments to an acceptable level. Two other related futureexperiments that are needed are an FST/SST/uniaxial testcorrelation experiment and a more thorough evaluation ofthe effect of specimen height on FST data. The researchshould then only be continued if the results of the laboratorytests indicate that FST data have sufficient precision for use

13

in QC testing, and also correlate reasonably well with modu-lus data gathered using the SST and uniaxial methods.

CHAPTER FOURCONCLUSIONS

Conclusions

• The redesigned FST device is small, extremely easy touse, and appears to be fairly rugged and well designed,and in these respects is well suited for QC/QA testing ofHMA mixtures.

• The distributions of stresses and strains in the FST arefairly complex, especially near the grips, but are suit-able for a QC/QA test. Because of the uncertainty inthe exact stresses and strains existing during testing, theFST should be considered an appropriate method fordetermining HMA modulus.

• To calculate average strain for the FST, an effectivegage length, or specimen thickness, of 21.3 mm shouldbe used, which is the same as the distance between thegrips. This appears to provide shear modulus values inthe same range as those produced using the SST, thoughlimited testing indicates that the FST may provide some-what higher values for |G*| than the SST. More researchis needed to develop correlations between data gener-ated using the FST, SST and other test procedures.

• The FST is best suited for measuring complex modulus(G*) at temperatures of about 40°C and frequencies of5 Hz to 10 Hz.

• The variability in |G*| data gathered with the FST at40°C and 10 Hz is similar to that seen in modulus datafound using the SST and other devices, with CVs rang-ing from 7 percent to 17 percent. This variability in|G*| data is too high for use in effective QC.

• The precision of the FST can probably be improvedsignificantly by two slight modifications in the testingprotocol. The first is to keep the time between speci-men mixing, compaction, and testing to within 2 to4 hours, which would normally be done during QC test-ing. The second modification involves testing eachspecimen four times, rotating or flipping the specimenbetween each repeat determination, and reporting theaverage of these four values.

• By using this suggested protocol and conditioning thespecimens in a water bath rather than an oven, a com-plete modulus measurement can be made with the FSTin less than 40 min from the time of specimen com-paction.

UNPUBLISHED MATERIAL

Appendices A–H of the agency final report are not pub-lished herein, but they are available for loan on request to

NCHRP, Transportation Research Board, Box 289, Wash-ington, D.C. 20055.

Appendix A: Literature ReviewAppendix B: Annotated BibliographyAppendix C: Prototype DevelopmentAppendix D: Stress AnalysisAppendix E: Preliminary Evaluation of the Improved

Field Shear TestAppendix F: Sensitivity TestingAppendix G: Calibration and Validation Procedures for

the Field Shear TestAppendix H: Implementation

AUTHOR ACKNOWLEDGMENTS

The research reported herein was performed underNCHRP Project 9-18 by the Pennsylvania TransportationInstitute of The Pennsylvania State University; AdvancedAsphalt Technologies, LLC; and the University of Arizona.The work performed by Advanced Asphalt Technologies,LLC, was performed under a subcontract to The PennsylvaniaState University. EnduraTEC Systems Corporation madesubstantial informal contributions to this project.

Dr. Donald W. Christensen, Jr., senior engineer forAdvanced Asphalt Technologies, LLC, served as principalinvestigator during most of this project. Dr. RamonBonaquist of Advanced Asphalt Technologies, LLC, wasco-principal investigator. Dr. Chandra Desai of the Univer-sity of Arizona was a contributing author for this report.Dr. H. Randolph Thomas of The Pennsylvania State Univer-sity served as acting principal investigator during the finalstages of the project.

Work on this project was performed under the generalsupervision of Dr. Christensen with assistance from Dr. TitinHandojo, of PCS Law. The work at Advanced AsphaltTechnologies, LLC, was performed under the supervision ofDr. Bonaquist, with significant assistance from Messrs.Kevin Knechtel and Donald Jack. Work at the University ofArizona was performed under the supervision of Dr. Desai.Dr. Charles E. Antle, of State College, Pa., served as projectstatistician.

REFERENCES

1. Lytton, R. L., Uzan, J., Fernando, E. G., Roque, R.,Hiltunen, D. and Stoffels, S. M. Development andValidation of Performance Prediction Models andSpecifications for Asphalt Binders and Paving Mixes.Report SHRP-A-357. Strategic Highway Research Pro-gram, National Academy of Sciences, Washington, D.C.(1993).

2. Harvey, J., Monismith, C., and Sousa, J. “An Investiga-tion of Field and Laboratory Compacted Asphalt Rubber,

14

SMA, Recycled and Conventional Asphalt ConcreteMixes Using the SHRP A-003A Equipment.” Journalof the Association of Asphalt Paving Technologists, Vol.63 (1994), pp. 511–548.

3. Romero, P., and Mogawer, W. “Evaluation of theSuperpave Shear Testing Using 19-mm Mixtures fromthe Federal Highway Administration’s AcceleratedLoading Facility.” Journal of the Association of AsphaltPaving Technologists, Vol. 67 (1998), pp. 573–598.

4. Sousa, J. B., and Weismann, S. L. “Modeling Perma-nent Deformation of Asphalt Aggregate Mixes.” Jour-nal of the Association of Asphalt Paving Technologists,Vol. 63 (1994).

5. The Asphalt Institute—National Asphalt TrainingCenter II. Superpave Asphalt Mixture Analysis: CourseNotebook. Federal Highway Administration, Office ofTechnology Applications (May 1996), 272 pages.

6. Mehta, Y. A. Linear Viscoelastic Limits of Asphalt Con-

crete at Low and Intermediate Temperatures. Ph.D. dis-sertation. The Graduate School, The Pennsylvania StateUniversity, University Park (1999), 211 pages.

7. Mehta, Y. A., and Christensen, D.W. “Determination ofthe Linear Viscoelastic Limits of Asphalt Concrete atLow and Intermediate Temperatures.” Journal of theAssociation of Asphalt Paving Technologists, Vol. 69(2000), pp. 281-312.

8. Cominsky, R. J., Killingsworth, B. M., Anderson, R.M., Anderson, D. A., and Crockford, W. W. NCHRPReport 409: Quality Control and Acceptance ofSuperpave-Designed Hot Mix Asphalt. TransportationResearch Board, National Research Council, Washing-ton, D.C. (1998).

9. Killingsworth, B. M., Demonstration of Field ShearTester (FST) for Quality Control of Superpave DesignedHot-Mix Asphalt, Austin, TX: BRE-Fugro (1998), 17pages.