Static Creep Behavior of Superpave Mixes
By: Mansour Solaimanian Project Manager
Superpave Asphalt Research Program
Texas Department of Transportation (TxDOT) has been utilizing the static creep test as an integral part of mix design for Coarse Matrix High Binder (CMHB) asphalt mixtures. The test has been successful in ensuring proper design for these mixes. The test resuls are compared with the pass/fail criteria on stiffness, permanent stain, and creep slope.
TxDOT is investigating the possibility of using the static creep as a strength test for Superpave mixes. A comprehensive research is currently pursued by the South Central Superpave Center (SCSC), in cooperation with TxDOT, to address this issue. At present, the static creep test for designing CMHB mixes is conducted on 100-mm specimens compacted with a Texas gyratory compactor. Specimens for the Superpave mix design are produced using a Superpave gyratory compactor and measure 150 mm in diameter. The current study includes a comparison of results for 100-mm and 150-mm specimens, as well as the effect of compactor type on the results. The effect of temperature is also being investigated. The tests are conducted with both the current creep test equipment of TxDOT and the Superpave shear tester (SST) located at SCSC. Part of the research is also dealing with static versus dynamic creep behavior of Superpave mixes.
As an important part of the study, performance of previously constructed projects with Superpave, CMHB, and Texas type C mixes are being evaluated. Both poor and good performing mixtures are included in the study. The original creep test is conducted in the axial mode. Tests are being conducted to compare the creep behavior of mixes in both shear and axial modes.
Results so far indicate that the axial creep test distinguishes between different mixes better when 150-mm specimens are used. Table 1 presents the results for three mixes produced as 150-mm specimens with Superpave gyratory compactor. The results are for tests under 70 Kpa axial stress at 40 ° C. Gradations for mixes A and C are Superpave coarse and fine, passing blow and above restricted zone, respectively. Mix B is a Texas type C mix passing through the restricted zone. As presented, the coarse mix produces the highest stiffness with the lowest permanent deformation. The results are displayed as the average of five replicates for each type of mix.
The study is generating tremendous amount of useful data. Conducting tests, comparison of results against field performance, and analysis of data is currently pursued actively. The results and findings will be submitted to TxDOT where the applicability and use of creep test and developed criteria for design construction of Superpave projects will be decided.
Note: The results on stiffness reported in table 1 are for 100-mm specimens whereas they should have been for 150-mm specimens. The correct stiffness values for 150-mm specimens are: 117,492 KPa for mix A, 95,522 Kpa for mix B, and 81,851 KPa for mix C. We appologize for the error.
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Table 1. Summary of Results for Creep Test at 40 C for 150-mm mixes |
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Mix Type |
Stiffness, KPa |
Permanent Strain, 10-3 |
Creep Slope, 10-8 |
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A(1) |
100,720 |
0.28 |
2.5 |
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B(2) |
93,284 |
0.35 |
3.3 |
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C(3) |
99,566 |
0.32 |
3.5 |
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1: Mix A: Superpave Coarse Mix (above restricted zone) 2: Mix B: Texas Type C Mix (through restricted zone 3: Mix C: Superpave Fine Mix (below restricted zone)) |
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