The Effects of Using Alkali-Silica Reaction Affected Recycled Concrete Aggregate in Hot Mix Asphalt
Geiger, Brian James
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The effects of using alkali-silica reaction (ASR) affected recycled concrete aggregate (ASR-RCA) in hot mix asphalt (HMA) were investigated in this study. Dilatometer and modified beam tests were performed to determine the possibility of new ASR occurring in reactive aggregates within the HMA or re-expansion of existing gel. The Lottman test and micro-calorimeter were used to determine the moisture susceptibility of HMA made with ASR-RCA. A differential scanning calorimeter (DSC) with thermogravimetric analysis (TGA) was used to evaluate the drying of an artificial gel and x-ray diffraction (XRD) was used to check for the potential presence of gel in the filler fraction of the ASR-RCAs. Micro-deval and freeze-thaw tests were evaluated for their potential to indicate the presence of excess micro-cracks or ASR gel. Expansion testing indicated that both ASR-RCAs were still reactive with 0.5 N NaOH solution saturated with calcium hydroxide (CH) at 60 degrees C. Dilatometer testing of HMA specimens in NaOH CH solution at 60 degrees C indicated a reaction between the asphalt binder and the solution, but little, if any, ASR. The lack of expansion in the modified beam test supports the binder-solution interaction. However, dilatometer testing in deicer solution at the same temperature indicated that some ASR may have occurred along with the primary binder-solution interaction. The volume change characteristics associated with the binder-solution interaction with and without ASR was supported by the change in pH and alkali concentration of the test solution. DSC/TGA testing indicated that the artificial gel dehydrated at approximately 100 degrees C. XRD analysis of the filler indicated that some gel may have accumulated in this fraction. Moisture damage testing indicated good resistance to moisture damage by HMA mixtures made with ASR-RCA especially compared to a virgin siliceous aggregate. Micro-deval and freeze-thaw tests can detect the presence of micro-cracks due to ASR in ASR-RCAs as higher mass loss than the virgin aggregate. The potential distress mechanisms that may occur when using ASR-RCA in an HMA pavement were identified. Results obtained using accelerated laboratory conditions were extrapolated based on anticipated field conditions. Guidelines for the mitigation of potential distresses in HMA made with ASR-RCA are presented.