Superpave Shear Tester Research Articles

Shear accumulated equilibrium compliance as permanent deforamtion susceptibility parameter

Described procedure summarizes the derivation of accumulated creep compliance in the sequence of the creep and recovery cycles that can be easily fitted to experimentally measured data. The local minima in each measured cycle after the recovery were selected and combined into a set that characterized the irreversible deformation of the specimen during the test. Further, the interval fitting procedure was utilized. The estimation of the discrete retardation spectrum from the used 4-modes standard Kelvin-Voigt model was discussed and the accumulated compliance calculated. It was proved that the accumulated compliance may be used as the pavement rutting criterion (expressing the performance of asphalt in terms of its susceptibility to permanent deformations). The proposed theory was demonstrated on experimentally measured asphalt properties in the two test devices. Repeated Shear tests were performed in the Superpave Shear Tester and the Uniaxial Shear Tester. It was found that the accumulated equilibrium compliance correlates with the asphalt mix rutting parameters. As it is presented and in the view of the general principles showing that long-time processes are revealed in more detail in the retardation spectrum, the use of the accumulated equilibrium compliance can be useful as a criterion for the long life rutting potential.

Uniaxial Shear Tester – new test method to determine shear properties of asphalt mixtures

The main focus of the paper is to present the concept of a newly developed Uniaxial Shear Tester (UST) and to investigate the correlation between results from the UST and the Superpave Shear Tester (SST), a tool broadly recognised for asphalt mix design and rutting susceptibility evaluation. In this study, the UST testing principles, finite element analysis of stresses and comparison of measured data are presented. The correlation was assessed on the basis of two tests, the repeated shear test and the small amplitude oscillation test also referred as the shear frequency sweep test. It was shown that the material characteristics determined from UST and SST are highly correlated. The dependencies are discussed in the sense of linear correlation and correlation coefficients. Test variability is discussed in the paper.

Investigation of Ultra-Rapid-Setting Emulsion for Tack Coat Applications

The presence and the proper application of bituminous tack coat between pavement layers are critical elements to the effective functionality and longevity of multilayered flexible pavements. This study assessed the performance of a newly developed ultra-rapid-setting (URS) emulsion as a tack coat, with particular emphasis on its adhesive properties. The experimental testing matrix included physical and rheological tests consistent with relevant specifications of the Pennsylvania Department of Transportation and Superpave® binder grading. Bonding performance was evaluated on the basis of results from shear tests conducted with a modified Marshall setup and the Superpave shear tester (SST). The results revealed that the new product set faster than the typical cationic rapid-setting emulsion (CRS-1h) used as a control tack coat in this study. The physical and rheological properties of the URS residue indicated a reduction in density, stiffness, and viscosity. The changes in the properties of the residue, however, were not significant enough to alter its performance grade. Results from the modified Marshall test at 25°C and SST tests at 25°C and 51°C revealed similar values for shear strength for both types of emulsions. The strengths of the specimens with the CRS-1h tack coat were slightly higher than those of the URS when tested in the modified Marshall setup, while the opposite was true for the SST setup at 51°C. This study demonstrated no statistical difference between the performances of the two products regarding adhesion for all conditions and tests performed.

Comparing Fine Aggregate Angularity with Aggregate and Hot-Mix Asphalt Performance Tests

The fine aggregate angularity (FAA) was determined for 13 fine aggregate sources (angular and round) and eight aggregate blends. AASHTO T304 Methods A, B, and C as well as the Kansas Department of Transportation test procedure KT-50 were used to determine the FAA value. The direct shear test and the compacted aggregate resistance (CAR) test were also conducted to provide a more performance-based aggregate parameter for comparison. Eight different aggregate blends were used to construct eight different fine-graded 12.5-mm Superpave® hot-mix asphalt (HMA) mixtures with similar gradation and volumetric properties. The HMA mixtures were tested in the asphalt pavement analyzer and repeated shear and frequency sweep at constant height test modes of the Superpave shear tester to evaluate their respective permanent deformation properties. Results indicate that for the fine aggregate sources used in the study, the FAA and CAR values correlated well, as did the CAR values with the angle of internal friction calcula...

Comparing Fine Aggregate Angularity with Aggregate and Hot-Mix Asphalt Performance Tests

The fine aggregate angularity (FAA) was determined for 13 fine aggregate sources (angular and round) and eight aggregate blends. AASHTO T304 Methods A, B, and C as well as the Kansas Department of Transportation test procedure KT-50 were used to determine the FAA value. The direct shear test and the compacted aggregate resistance (CAR) test were also conducted to provide a more performance-based aggregate parameter for comparison. Eight different aggregate blends were used to construct eight different fine-graded 12.5-mm Superpave® hot-mix asphalt (HMA) mixtures with similar gradation and volumetric properties. The HMA mixtures were tested in the asphalt pavement analyzer and repeated shear and frequency sweep at constant height test modes of the Superpave shear tester to evaluate their respective permanent deformation properties. Results indicate that for the fine aggregate sources used in the study, the FAA and CAR values correlated well, as did the CAR values with the angle of internal friction calculated from the direct shear tests. By a comparison of the FAA value of the aggregate blends with published criteria of the HMA permanent deformation tests, the Superpave-recommended FAA value of 45 was validated. Results of the HMA mixture testing and FAA comparison also allowed for the recommendation of a pass–fail CAR value for use in Superpave mix design.

Field Validation of Superpave Shear Test on NCAT Test Track

Abstract When the National Center for Asphalt Technology Test Track was built in 2000, one of the primary objectives was to determine the ability of a number of laboratory tests to predict the permanent deformation of various mixtures. Repeated Shear at Constant Height (RSCH) was included as one of the tests in the laboratory study. The subject of this paper is field validation of the Superpave Shear Tester (SST) based on the field rutting data from the Test Track. Permanent shear strain, slope of the deformation rate, repetitions to a certain strain level, and shear modulus were the parameters evaluated for RSCH. The sensitivity of SST test to asphalt binder type in the various test track sections was analyzed. The relationship between RSCH test results and field rut depth was poor. This was partly due to the good quality of the track construction, thick pavement structure, and mild summers during the loading of the 2000 track, which did not cause any significant rutting. The sensitivity study indicated that PG 76-22 binder performed better than the PG 67-22 binder in the RSCH test. The criteria developed by the Asphalt Institute seem to be in a reasonable range.

Performance Testing of Segregated Hot-Mix Asphalt Samples to Evaluate Segregation Models

Testing of asphalt concrete samples was conducted by using the asphalt pavement analyzer test, the Superpave® shear tester (SST) frequency sweep test, and the SST repeat shear at constant height test to determine the rutting susceptibility that might be expected from samples that have different levels of segregation. Previous research had divided segregation into four levels: none, light, medium, and high. Test results suggest that samples with different segregation levels might show similar rutting susceptibility. Therefore, performance-related models were established in which segregation was divided into three levels: good, fair, and unacceptable. The performance-related models were applied to field data, and examination indicates that the modified models are capable of assessing segregation in field projects. Examination also found that fine segregation was present. Fine segregation can be predicted by extrapolating the developed model from the coarse part to the fine part. However, more data should be investigated to develop acceptable criteria for fine segregation. A methodology was developed to improve pavement quality by controlling aggregate segregation and poor compaction in placed mats. With the models and the proposed methodology, realistic construction uniformity could be assessed by agencies and contractors, which is the first step in obtaining pavements with high performance and longer service life.

Performance Testing of Segregated Hot-Mix Asphalt Samples to Evaluate Segregation Models

Testing of asphalt concrete samples was conducted by using the asphalt pavement analyzer test, the Superpave® shear tester (SST) frequency sweep test, and the SST repeat shear at constant height test to determine the rutting susceptibility that might be expected from samples that have different levels of segregation. Previous research had divided segregation into four levels: none, light, medium, and high. Test results suggest that samples with different segregation levels might show similar rutting susceptibility. Therefore, performance-related models were established in which segregation was divided into three levels: good, fair, and unacceptable. The performance-related models were applied to field data, and examination indicates that the modified models are capable of assessing segregation in field projects. Examination also found that fine segregation was present. Fine segregation can be predicted by extrapolating the developed model from the coarse part to the fine part. However, more data should be investigated to develop acceptable criteria for fine segregation. A methodology was developed to improve pavement quality by controlling aggregate segregation and poor compaction in placed mats. With the models and the proposed methodology, realistic construction uniformity could be assessed by agencies and contractors, which is the first step in obtaining pavements with high performance and longer service life.

Comparing Superpave Gyratory Compactor Data to Field Cores

Achieving proper compaction of asphalt pavement is crucial to its longevity and acceptable performance. Laboratory compaction is an essential part of mix design. In order for the mix design process to be effective, laboratory compaction must adequately simulate field compaction. This study used mechanical properties measured with the Superpave shear tester to evaluate field compaction and laboratory compaction. The field compaction consisted of three test sections with different compaction patterns. The laboratory compaction used the Superpave gyratory compactor with adjustments to several parameters. Results of this study indicate that current gyratory protocol produces specimens with significantly different mechanical properties than field cores produced with the same material and compacted to the same air voids. Results also show that adjustments to certain parameters of the gyratory can produce specimens that better simulate the mechanical properties of pavement cores.

Model-Fitting the Master Curves of the Dynamic Shear Rheometer Data to Extract a Rut-controlling Term for Asphalt Pavements

Abstract The dynamic shear rheometer is currently being used by the asphalt community for determination of the viscoelastic properties of asphalts following the recommendations of the Strategic Highway Research Program. The Superpave specification parameter |G*|/sinδ was identified as the term to be used for high temperature performance grading of paving asphalts in rating the binders for their rutting resistance. However, this term was found to be inadequate in describing the rutting performance of certain binders, particularly, the polymer-modified ones. A refinement of the Superpave specification parameter for performance grading of asphalt led to the evolution of the term |G*|/(1 − (1/tanδ sinδ)). This performance-based specification term was shown to be more sensitive to the variations in the phase angle δ than the Superpave specification parameter and thus was found to describe the unrecovered strain in the binders more accurately, especially in the case of polymer-modified asphalts. The high specification temperature THS (°C) is specified as the temperature at which the term |G*|/(1 − (1/tanδ sinδ)) takes a value of 1 kPa for the original unaged binder and a value of 2.2 kPa for the RTFOT aged binder. Though this refinement of the Superpave specification parameter has led to a better discrimination between the rutting resistances of various asphalts, it is found that not all asphalts that have been performance-graded as being the same behave identically in their rutting performance. Thus, there is a need to create a rut-controlling term that will distinguish between asphalts that are graded the same but prepared by different modification routes. Such a rut-controlling term can then be used as a performance-related specification. The present work extracts the rut-controlling term by model-fitting the master curve formed from the |G*|/(1 − (1/tanδ sinδ)) or |G*|/(sinδ)9 versus frequency data at various temperatures for each binder. Each master curve is fitted with a constitutive equation from which model parameters are evaluated. The slope B1 in the low frequency region of the master curve is normalized with the term (T/THS), resulting in the rut-controlling term that relates to the permanent strain after 5000 cycles in the repeated shear at constant height experiment on the Superpave shear tester at temperature T.

Coarse- Versus Fine-Graded Superpave Mixtures: Comparative Evaluation of Resistance to Rutting

Both coarse- and fine-graded hot-mix asphalt mixtures can be designed within the gradation control points recommended in Superpave® mix design. Some states have begun to specify only coarse-graded mixtures (below the restricted zone), whereas other states are specifying only fine-graded mixtures (above the restricted zone). A study was conducted to compare coarse-graded Superpave mixtures with fine-graded Superpave mixtures in terms of resistance to rutting to determine whether restrictions on gradations (either coarse- or fine-graded mixtures) are justified. Fourteen mixtures were tested containing two nominal maximum aggregate sizes (9.5 and 19.0 mm), two coarse aggregates (granite and crushed gravel), and four fine aggregates (sandstone, limestone, granite, and diabase). Resistance to rutting of both coarse- and fine-graded mixtures was evaluated by three test methods: asphalt pavement analyzer, Superpave shear tester, and repeated load confined creep test. Statistical analyses of the test data obtained by the three performance tests indicate no significant difference in rutting resistance of coarse- and fine-graded Super-pave mixtures. It has been recommended that mix designs not be limited to designing mixes on the coarse or fine side of the restricted zone.

Superpave Shear Tester as a Simple Standardized Measure to Evaluate Aggregate-Asphalt Mixture Performance

Abstract The present work provides a standardized method by which various mixtures can be compared and their expected performance can be assessed in a uniform manner using the Superpave shear tester. Based on the present findings, it is evident that complete mixture evaluation can be done through the frequency sweep at constant height (FSCH) data from the Superpave shear tester (SST), without the need to generate the repeated shear at constant height (RSCH) data. This would reduce the experimental time without compromising the information obtained. FSCH and RSCH data at different temperatures are generated on the SST for a number of aggregate-asphalt mixture combinations. The shear moduli versus frequency data from the FSCH at different temperatures are unified to form a single curve for each mixture so that a specification parameter Ts (°C) can be determined. Each unified curve is fitted with a constitutive equation from which model parameters are evaluated. The slope B1 in the low frequency region of the unified curve, when normalized with the term (T/Ts), results in a parameter that relates to the permanent strain after 5000 cycles in RSCH at any temperature T. There is a good possibility that the slope B2 in the high frequency region of the unified curve may relate to distresses in the intermediate temperature range, such as fatigue. If this is proven true through future research, then the Superpave shear tester could earn the distinction of being a “simple performance tester.”

Relationship between the Representative Volume Element and Mechanical Properties of Asphalt Concrete

The size of an asphalt concrete test specimen has been traditionally determined based on experience and practical purposes. This study discusses theoretical considerations for selecting the minimum dimensions so that the resulting size of the specimen represents global material properties. The ideal size of the specimen was determined using an image analysis approach and was verified with mechanical tests. The image analysis was accomplished by studying the internal structure of asphalt concrete. The internal structure was captured using a nondestructive X-ray computer tomography technique. Mechanical testing was accomplished by using the Superpave Shear Tester on laboratory-prepared samples. The laboratory samples had dimensions that were both smaller and larger than the theoretically determined representative size. Statistical analysis indicates that the variability of the results is a function of the loading mechanism used in the Superpave Shear Tester. In general, the required representative size increased considerably when the nominal aggregate size was larger than 25.0 mm.

Variability of Asphalt Mixture Tests Using Superpave Shear Tester Repeated Shear at Constant Height Test

Analyses were conducted on data obtained from the Superpave shear tester using the repeated shear at constant height (RSCH) test. The data indicated that even under the most controlled experimental conditions, the resulting parameters showed high variability. Up to six replicate samples were tested using an extra shear linear variable differential transformer. The coefficient of variation of the permanent strain at 5,000 load cycles for six samples was between 10 and 20 percent in most cases. From the different options studied to decrease variability, the trimmed-mean method provided the greatest decrease in the coefficient of variation without affecting the mean of the results. In this method, five specimens are tested, the high and low values eliminated, and the remaining three are used to obtain the mean and standard deviation of the results. Since the suggested trimmed-mean method requires an increase in the number of samples tested to five, it is proposed that the number of load cycles used in the RSCH test be reduced for materials placed in low-volume roads.

Mechanistic Analysis of Asphalt Pavements, Using Superpave Shear Tester and Hamburg Wheel-Tracking Device

A comparative evaluation of permanent deformation and moisture damage of asphalt mixtures using the Superpave shear tester and the Hamburg wheel-tracking device (HWTD) is presented. The damage analysis of simulated asphalt pavements is also included. The selected materials were a PG 64-22 asphalt binder and a sandstone aggregate with and without an antistripping additive. The repeated shear at constant height (RSCH) and HWTD tests were performed for the evaluation of permanent deformation. The results showed that the Superpave mixtures were less susceptible to permanent deformation than the Marshall mixture. The results of the HWTD tests performed at 40°C were consistent with those of the RSCH tests. Two permanent deformation models were compared to relate plastic strain accumulation with the number of load repetitions. The one selected for use in the Strategic Highway Research Program A-005 contract is generally better than the other model. For moisture susceptibility evaluation of these mixtures, the results of the AASHTO T283 tests were consistent with those of the HWTD tests at 40°C. The creep compliances calculated from the frequency sweep at constant height tests were applied to the KENLAYER program for pavement damage analysis. On the basis of the data analyses in this study, the simulated asphalt pavements are considered more of a material and mix design problem than a structural problem. The use of creep compliances of the above mixtures could not successfully predict the pavement performance in comparison with results of the RSCH and HWTD tests.

Washington State Department of Transportation Superpave Implementation

Washington State’s Department of Transportation (WSDOT) has investigated components of the Strategic Highway Research Program (SHRP)/Superpave technology to include performance-grade (PG) binder usage and specification validation, gyratory mix design, the Superpave Shear Tester, and field performance of Superpave mixes. With a focus on field performance, validation of the binder specification in respect to low-temperature cracking was accomplished using binder and field performance data from 28 projects. The results were encouraging: the original SHRP algorithm for binder selection correctly “predicted” field performance in 22 cases. The original SHRP algorithm was subsequently refined as part of FHWA’s Long-Term Pavement Performance (LTPP) Program. The LTPP algorithm used for binder selection correctly predicted field performance in 26 cases. Since 1993, of the 44 WSDOT projects with a Superpave technology component, 17 parallel Hveem and Superpave mix designs have been conducted. In 13, Superpave design asphalt content was equal to or greater than the Hveem design asphalt content, although the difference was insignificant. A conventional Hveem mix design was conducted on 18 projects using a PG binder (Hveem-PG). The remaining 26 projects were considered to be Superpave because materials selection and mix design were established in accordance with the Asphalt Institute’s SP-2, Superpave Level 1 mix design. According to WSDOT practice, the following indices trigger maintenance: pavement structural condition (PSC), rutting, or International Roughness Index (IRI). Although “young,” all 44 projects are performing well. The average values of rutting, PSC, and IRI are all below the “trigger” values. In rutting and PSC, performance of Hveem-PG and Superpave projects is virtually identical. However, ride quality of Superpave projects is rougher than that of Hveem-PG binder projects.

Effect of Mix Gradation on Rutting Potential of Dense-Graded Asphalt Mixtures

A study was conducted to evaluate the effect of mix gradations, both complying with and violating the Superpave restricted zone, on rutting potential of hot-mix asphalt. Superpave gyratory samples of mixes with granite, limestone, and gravel aggregates were used. The following gradations were used: gradation above the restricted zone, gradation through the restricted zone in close proximity to the maximum density line, and gradation below the restricted zone. Rut tests were conducted at 64°C with the asphalt pavement analyzer under 689-kPa contact pressure and 45.2-kg wheel load. Rut depths were measured at the end of 8,000 cycles. Repeated shear tests at a constant height were also conducted with the Superpave shear tester. Statistical analysis of rut data generally indicates a significant difference between rut depths obtained in mixes using different aggregate types and gradations. The gradations violating the restricted zone did not necessarily give relatively higher rut depths compared with the gradations in compliance with the zone.

Volumetric and Mechanical Performance Properties of Superpave Mixtures

This investigation studied the volumetric and mechanical performance properties of the Superpave mixtures SP-1 and SP-2 and a typical Taiwan mixture (TTM). Both the Marshall and the Superpave mix design procedures were used to produce these mixtures, and the Superpave gyratory compactor was used for realistically compacting specimens to densities achieved under actual pavement conditions. The three mixtures were tested using the Superpave shear tester for conducting a simple shear test at constant height, frequency sweep test at constant height, and repeated shear test at constant height. Volumetric analyses of specimens showed that the TTM mixture hardly met the Superpave volumetric requirements and it contained less than 1% air voids at the number of gyrations for design compaction, which strongly suggests that this mixture might be highly prone to rutting. The design asphalt content of each mixture was found very close to the peak of its compaction slope-asphalt content curve. Mechanical test results indicated that the Superpave mixtures were more resistant to permanent deformation than the TTM mixture.

Evaluation of Moisture Susceptibility of Asphalt Mixtures: Conventional and New Methods

Evaluation of a mixture’s moisture sensitivity is currently the final step in the Superpave® volumetric process. This step is accomplished by using AASHTO T-283, which tolerates a range of values in the test variables of sample air voids and degree of saturation. The tensile strength ratios determined for the mixes in this study varied with the air void level and degree of saturation. Although the levels of conditioning were within the specifications for AASHTO T-283, test results both passed and failed the 80 percent criterion, depending on the severity of conditioning. An alternative to measuring indirect tensile strength is a test that evaluates a mixture’s fundamental material properties. A relatively simple test is proposed that measures the cohesion and friction angle for asphalt mixtures. In addition, the Superpave shear tester (SST) was incorporated as a tool in evaluating moisture sensitivity. The proposed axial test determined the cohesion and angle of friction of the mix. The friction angle remained constant for the conditioned and unconditioned samples. Hence, conditioning of the samples had practically no effect on the mixture’s internal friction. The cohesion of the mix decreased when the mix was subjected to conditioning. The reduction in cohesion was greater in the case of the Fountain aggregate, which is known to be highly moisture susceptible. The shear tests to failure performed on the SST confirmed the results of the new apparatus, which provides a simple method for determining a mixture’s cohesion. The loss of cohesion due to conditioning can be used to determine a mixture’s moisture susceptibility. The three antistrip additives used in this study were hydrated lime, a liquid amine, and a liquid phosphate ester.

Specification parameter for asphalt mixtures using frequency sweep data from the Superpave shear tester

ABSTRACT The data generated from the Superpave shear tester (SST) frequency sweep at constant height (FSCH) test is available in terms of the complex shear modulus G* versus frequency at the temperature of measurement. An attempt is made to unify the sets of curves generated at different temperatures on various mixtures. The procedure involved the use of a normalizing frequency parameter. The temperature at which the normalizing parameter becomes equal to one is suggested as a specification parameter for assessing mixture performance. This specification parameter was determined for various mixtures of known performance and found to follow performance ranking in all the studied cases.