Evaluation of Indirect Tensile Test (IDT) Procedures for Low-Temperature Performance of Hot Mix Asphalt (2004) / Chapter Skim
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Tensile Creep/Strength Testing (Thermal Cracking Analysis)
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The test temperatures used in the IDT creep and strength tests should, 6 Component General Requirements Range Sensitivity Axial loading device Shall provide a constant load 98 kN maximum load; Displacement rate between 12 and 75 mm/min 5 N minimum Load measuring device Electronic load cell 98 kN minimum capacity 5 N minimum Deformation measuring device(s) Four linear variable differential transducers (LVDTs)
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EXPERIENCE WITH THE IDT TEST AT THE REGIONAL SUPERPAVE CENTERS In the late 1990s, IDT test systems were procured by the FHWA for four of the five Regional Superpave Centers. These test systems were to be used for further evaluation of the IDT creep and strength test procedures.
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8 In summary, the current version of the IDT test and analysis procedure has been substantially improved to address many of the shortcomings found immediately after the conclusion of SHRP. The following changes have been incorporated into the most recent version of the IDT test procedure and Superpave thermal cracking software: • Simplified formulas have been developed for making correction factors for specimen bulging and non-uniform stress and strain distribution across the specimen; • The initial portion of data analysis, which involves developing a "trimmed" mean for the response of a given set of specimens, has been enhanced to avoid problems that occurred when a transducer was not responding and also to provide the user an overall indication of the quality of the data being analyzed; • The procedure used to shift the individual compliance curves to form a master compliance curve has been substantially improved and is more robust and produces reasonable and repeatable master curves even for nonideal data; • Most or all of the minor problems ("bugs")
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Precision Evaluation of the IDT Creep Test As part of Phase III of NCHRP Project 9-29, numerous laboratories that have IDT creep and strength test systems were contacted and asked to provide data for the purposes of evaluating the precision of IDT creep data. These laboratories were told the results of the study would be anonymous, 2 2 so detailed information concerning the various laboratories cannot be provided.
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D2S precision for critical temperature for six laboratories. electro-mechanical test system used there, which, as discussed previously, was one of the prototypes procured for the Superpave Centers that exhibited many software and hardware problems.
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However, the compliance data for Lab L3 was determined at very low temperatures, which is probably the reason for the extreme variability in critical temperatures. This emphasizes the importance of performing compliance measurements at appropriate temperatures.
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, it cannot be concluded with certainty whether RVE requirements have been met for either the IDT or uniaxial creep and strength tests at low temperature. One indication of whether the specimen size for the IDT and uniaxial tests is adequate is the precision of these methods compared to what is possible for similar tests where specimen homogeneity is not an issue.
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In view of the apparent anisotropy in asphalt concrete mixtures, the IDT test geometry is probably the most effective of the available methods for determining low-temperature creep compliance. Poisson's Ratio Another issue in the IDT test procedure is whether it is truly necessary to determine Poisson's ratio when characterizing the mechanical behavior of HMA at low temperature.
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Using the relationship shown in Figure 6, αAC values were estimated for the SHRP mixtures and then used along with Vbe and VMA values to predict αmix values, using Equation 2. These calculations can be combined into one equation for estimating the coefficient of thermal contraction of asphalt concrete mixtures: Where m is the log-log slope of the mixture creep compliance with respect to time (t)
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The procedure described above was used to calculate relaxation modulus using data for six different mixtures, as submitted by Lab L4 of the IDT creep precision study described previously. In fitting the Prony series to the creep compliance data, five evenly spaced relaxation times were assumed, covering a time range slightly larger than that for the entire master curve.
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approximate method for estimating relaxation modulus from creep compliance is used rather than the Prony series approach, and the calculation stops at estimating the critical cracking temperature, Tc, defined as that at which the thermal stress in the pavement reaches the tensile strength. In this case, a typical mixture coefficient of thermal expansion of 1.1 × 10−5 m/m/°C was assumed for all mixtures.
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For mixtures made using softer binders, all test temperatures should be lowered by 10°C; for mixtures made using harder binders or heavily aged binders, all test temperatures should be raised by 10°C. COMPARISON OF COMPLIANCE VALUES AS DETERMINED USING UNIAXIAL TENSION, UNIAXIAL COMPRESSION, AND THE IDT TEST An experimental test program was designed and executed to answer several important questions related to the determination of the low-temperature creep compliance of asphalt concrete mixtures: • Is the low-temperature creep compliance of asphalt concrete similar in tension and compression?
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The procedure used in the uniaxial tests followed as much as possible the same protocol as described in AASHTO T322 for IDT tests, except where the different geometry made changes necessary. The LVDTs used in the IDT creep tests were as described in AASHTO T322 -- two transducers on each face, one vertical and one horizontal, all with a gage length of 37.5 mm.
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The uniaxial tension strength tests were performed at a loading rate of 3.75 mm/min, which provides a strain rate roughly equivalent to that in the standard IDT strength test. The experiment designs for both creep and strength can be considered full factorials.
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A more rigorous comparison of data generated by the three low-temperature test methods is given in Tables 11 through 16, which summarize the result of statistical pair-wise comparisons for creep compliance, master curve parameters, and critical cracking temperature. Table 11 compares the compliance measured in uniaxial compression and uniaxial tension.
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-0.047 0.627 NO M 0.029 1.436 NO d log a(T) /d T 0.008 2.140 YES TABLE 16 Statistical test for equality of master curve parameters and critical temperatures from uniaxial compression data and IDT data Test Lower Compliance Temp.
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Regression line with 95-percent confidence and prediction intervals for relationship between uncorrected IDT strength and direct tension strength. 800700600500400 800 700 600 500 400 300 Corrected IDT Strength, psi D ire ct T en si on S tre ng th , ps i R-Sq = 49 % Y = 256 + 0.452X 95% PI 95% CI Reg.
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The results of these analyses are shown in Figures 20 through 22. In Figure 20, critical cracking temperature from compliance in uniaxial tension is compared to critical temperature determined using compliance data in uniaxial compression.
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Furthermore, it appears that corrected IDT strength can be estimated fairly well from uncorrected IDT strength using Equation 8. Therefore, the overall recommendation from the experimental portion of this study is that the IDT creep and strength test be retained for use in estimating the thermal cracking resistance of asphalt concrete but that IDT strengths obtained from the maximum load should be empirically adjusted to provide more realistic estimates of the actual tensile strengths of mixtures.
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