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The needs exists to evaluate a dynamic modulus test apparatus and appropriate test protocols such that the E* test system will be accepted by DOT's as a method to characterize HMA mixes for design purposes. This report out lines the process.
This project evaluated the procedures proposed by the Mechanistic-Empirical Pavement Design Guide (MEPDG) to characterize existing hot-mix asphalt (HMA) layers for rehabilitation purposes. Thirty-three cores were extracted from nine sites in Virginia to measure their dynamic moduli in the lab. Falling-weight deflectometer (FWD) testing was performed at the sites because the backcalculated moduli are needed for the Level 1 procedure. The resilient modulus was also measured in the lab because it is needed for the Level 2 procedure. A visual pavement rating was performed based on pavement condition because it is needed for the Level 3 procedure. The selected cores were tested for their bulk densities (Gmb) using the AASHTO T166 procedure and then for their dynamic modulus in accordance with the AASHTO TP62-03 standard test method. Then the cores were broken down and tested for their maximum theoretical specific gravity (Gmm) using the AASHTO T-209 procedure. Finally an ignition test was performed to find the percentage of binder and to reclaim the aggregate for gradation analysis. Volumetric properties were then calculated and used as input for the Witczak dynamic modulus prediction equations to find what the MEPDG calls the undamaged master curve of the HMA layer. The FWD data, resilient modulus data, and pavement rating were used to find the damaged master curve of the HMA layer as suggested for input Levels 1, 2, and 3, respectively. It was found that the resilient modulus data needed for a Level 2 type of analysis do not represent the entire HMA layer thickness, and therefore it was recommended that this analysis should not be performed by VDOT when implementing the design guide. The use of Level 1 data is recommended because FWD testing appears to be the only procedure investigated that can measure the overall condition of the entire HMA layer.
An accurate measurement or estimation of dynamic modulus (|E*|) of a hot mix asphalt (HMA) mix is important to understand stress-strain behavior of flexible pavements under loading and unloading conditions. The Mechanistic Empirical Pavement Design Guide (MEPDG) (Transportation Research Board, Washington, D.C., 2004) recommends that |E*| be used in all three levels of design (i.e., Level 1, Level 2, and Level 3). For Level 1, |E*| is measured in the laboratory, while it is estimated using the Witczak models for Level 2 and Level 3 designs. The measurement of |E*| in the laboratory is not always feasible because it requires costly equipment and skilled personnel. Consequently, use of empirical models seems to be a reasonable approach to estimate |E*|. Several researchers have reported that the accuracy of the Witczak models varies with local materials and volumetric properties. The present study was undertaken to compare the measured and the estimated |E*| for some commonly used mixes in Oklahoma. Specifically, |E*| of five different HMA mixes, comprised of aggregates from several sources and sizes, binder grades, and air voids, were measured in the laboratory. The Witczak 1999 model (Andrei , 1999) was used to estimate |E*| for each of these mixes. A comparison was made between the measured and the estimated |E*| at four different levels of air voids, namely, 6%, 8%, 10%, and 12%. It was observed that the Witczak 1999 model overestimates |E*| at all four levels of air voids. To address these overestimates, the Witczak 1999 model was calibrated. The calibrated model was similar in form to the Witczak 1999 model but having different numerical coefficients. Verification of this model was done using a mix that was not used in the calibration process. Furthermore, two full depth field cores were obtained to further verify the accuracy of the calibrated model. Three different criteria, namely, goodness-of-fit statistics, matching the measured and the estimated |E*|, and average relative error (%), revealed that the calibrated model exhibits much better performance compared to the Witczak 1999 model. It is expected that the calibrated model would be useful in estimating |E*| for the Level 2 and Level 3 designs for the implementation of the MEPDG in Oklahoma.
TRB's National Cooperative Highway Research Program (NCHRP) Report 691: Mix Design Practices for Warm-Mix Asphalt explores a mix design method tailored to the unique material properties of warm mix asphalt technologies. Warm mix asphalt (WMA) refers to asphalt concrete mixtures that are produced at temperatures approximately 50°F (28°C) or more cooler than typically used in the production of hot mix asphalt (HMA). The goal of WMA is to produce mixtures with similar strength, durability, and performance characteristics as HMA using substantially reduced production temperatures. There are important environmental and health benefits associated with reduced production temperatures including lower greenhouse gas emissions, lower fuel consumption, and reduced exposure of workers to asphalt fumes. Lower production temperatures can also potentially improve pavement performance by reducing binder aging, providing added time for mixture compaction, and allowing improved compaction during cold weather paving. Appendices to NCHRP Report 691 include the following. Appendices A, B, and D are included in the printed and PDF version of the report. Appendices C and E are available only online.