Test Methods and Specification Criteria for Mineral Filler Used in Hot-Mix Asphalt (2011)

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phalt (HMA) Using the Asphalt Mixture Performance Tester (AMPT). A statistically significant relationship was found between the mastic non-recoverable com- pliance, Jnr, measured with ASTM D7405, Multiple Stress Creep and Recovery (MSCR) of Asphalt Binder Using the Dynamic Shear Rheometer, and the mix- ture FN; however, this relationship is not as impor- tant to mixture rutting resistance as the aggregate gradation. Tentative maximum limits for mastic Jnr were defined to ensure acceptable mixture FN values for coarse- and fine-graded mixtures. A statistical model to estimate mastic Jnr from filler RV and binder Jnr is proposed to ensure acceptable FN values. HMA Fatigue Damage A significant variation in mastic fatigue damage resistance was found across the range of fillers used in the project. The role of fillers in mastic fatigue damage resistance was highly dependent on binder modification type and binder chemistry. Mixture fa- tigue damage resistance was found to be affected mainly by gradation and binder modification, and only marginally by properties of the filler or base binder. Thus, the results of this research were not sufficient to define the role of fillers in mixture fatigue damage resistance or to propose filler specification criteria for this aspect of HMA mixture performance. HMA Low-Temperature Cracking The only experimental variables with a statisti- cally significant relationship to mixture low temper- ature stiffness were gradation and base binder source. Mastic relative stiffness (i.e., the stiffness of the mas- tic compared to that of its base binder) has a signifi- cant effect on mixture strength at low temperatures that is only slightly less important than the effect of gradation. Mastic low temperature stiffness (S) and creep rate (m) measured by AASHTO T 313, Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer (BBR), were found to be sensitive to filler type and binder modification. It was found, as expected, that all fillers increase binder stiffness, but fillers can either increase or decrease (m) value, depending on binder modification type. The statistical analysis found that the mastic low temperature stiffness is dependent on the RV value and calcium content (reported as % CaO) of the filler, and a model was developed to estimate mastic stiff- ness at low temperatures as a function of filler and binder properties. Although relationships were found between mix- ture strength and relative mastic stiffness and between relative mastic stiffness and filler RV and % CaO, it was not possible to propose limits on mastic or filler properties to ensure acceptable low-temperature cracking resistance. Mixture cracking resistance de- pends on stiffness and the capability for stress relax- ation (as indicated by the creep rate, m) in addition to strength, and since no trends were found for mixture stiffness, no specification limits could be proposed here. HMA Moisture Damage The resistance of mastic to moisture damage was found to be highly binder specific; filler properties had limited influence. Therefore, mixture moisture damage testing was conducted on a limited scale as compared to that for other HMA mixture performance characteristics. These limited results indicated that mixture moisture resistance is highly dependent on mastic performance, but that this dependency is mainly related to binder type, rather than to the filler used in the mastic. PROPOSED TESTS AND SPECIFICATIONS Filler Characterization Tests A multi-laboratory experiment was conducted to assess the repeatability and practicality for routine use of the proposed suite of filler characterization tests in Table 4. The results, although limited in scope, indi- cate that the tests are highly repeatable. In particular, test methods for RV and specific gravity, which have been identified as the most important filler properties for mixture performance, were found to be highly repeatable and able to distinguish between different fillers with high accuracy. The tests were rated by op- erators as user-friendly and can produce repeatable results after modest operator training. The tests for measuring calcium content by X-ray florescence and fineness modulus by laser diffraction showed good repeatability but require costly equipment and so may not be practical for routine testing of fillers. Filler Specification Criteria Proposed specification criteria to ensure adequate mixture performance with respect to workability 5

and rutting resistance are presented in Tables 5 and 6. Due to the high filler-binder interactions measured in this study, these specification criteria are based on mastic properties rather than filler properties. The ex- perimental results did not support any proposed spec- ification criteria for resistance to fatigue damage, low temperature cracking, or moisture damage. In instances where mastic properties cannot be measured directly, these specification criteria are sup- plemented by best-fit models (Equations 1 and 2) de- veloped to estimate mastic properties in terms of filler and binder properties. Mastic Vis ity Binder Vis itcos cos= − +8244 4 68.  y RV+ 205 1 ( ) Where Jnr = non-recoverable compliance. The R2 values for Equations 1 and 2 are 0.684 and 0.749, respectively. In addition, the best-fit model (R2 = 0.606) shown in Equation 3 was obtained to predict rela- tive low-temperature mastic stiffness from low- temperature binder stiffness and filler RV and CaO values: Mastic J Binder J RV nr nr = + −1 01 0 160 0 230 2 . . . (   ) 6 Maximum N92 43 Maximum Relative Viscosity 5.0 Mixture Gradation Maximum Mastic Jnr at 3.2kPa (1/kPa) Fine 0.40 Coarse 0.55 Table 5 Proposed workability limits. Table 6 Proposed maximum values for mastic Jnr at 3.2kPa by gradation type. Mastic Stiffness RV relative = + + −2 32 145 4 84 1. . .71 3CaO Stiffnessbinder ⎡ ⎣⎢ ⎤ ⎦⎥ ( )

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