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Using reclaimed and recycled materials in asphalt mixtures is one of the most commonly used sustainable strategies, which results in cost savings and environmental benefits because they replace virgin materials. Commonly used recycled, reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) can pose challenges because their inclusion in the asphalt mixture may adversely affect pavement performance. This paper presents the recently developed fracture testing method, the Illinois semicircular bending (IL-SCB) fracture geometry at 25°C with a displacement rate of 50 mm/min. The flexibility index (FI), derived from the IL-SCB test results, was introduced to rank the potential cracking resistance of mixes. The developed testing protocol provides a practical and reliable approach to screen the capacity of mixes to resist cracking. An application of the IL-SCB test method and FI is presented using laboratory-prepared and compacted mixes with varying proportions of RAP and RAS. The mixes were designed and prepared with strict volumetric requirements, enabling comparison of the effects of RAP and RAS only. Fracture testing was conducted at low and intermediate temperatures. Fracture testing results indicate a consistent trend at intermediate temperatures using both fracture energy and the FI. The discrimination potential and sensitivity of the FI to even small changes in the mix design properties enable integration of the IL-SCB test method and FI as a performance quality indicator. Finally, the FI and Hamburg wheel-tracking test results are combined in an interaction plot that can be used as a performance-based mix design tool along with volumetrics requirements.

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A major problem in using ground penetrating radar (GPR) for estimating pavement layer thickness is assuming the dielectric properties of that layer. Pavement dielectric properties may vary significantly due to aggregate type, moisture presence, and other conditions. Therefore, uncertainties in the dielectric constant, which may vary from 3 to 15, will result in misleading thickness determination. Obtaining cores for calibration may reduce the error, but the variation in the dielectric constant along the roadway often leads to errors in the thickness determination. A method was developed to determine the dielectric constant, and therefore the thickness, of the hot-mix asphalt (HMA) layer of a pavement using GPR. Because of the different compositions and ages of the layers forming HMA in older pavements, dielectric constant estimation based on the surface reflection may not be accurate and may lead to wrong thickness estimations. The developed method uses a modified common midpoint technique (usually used in seismic testing) to estimate the dielectric constant, based on the reflections from a common point at the bottom of the layer. Data were collected from a 27-km portion of Interstate 81 and processed with this technique. Comparison between the thickness estimated by this method and that measured on cores extracted from the highway revealed a mean error of 6.8%.

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A study that validated the field performance of ground-penetrating radar (GPR) as a nondestructive tool to predict in situ asphalt mixture density is presented. New overlays using six types of asphalt mixtures were placed at Illinois Route 72 (IL-72) near the Chicago area. Both GPR and nuclear gauge data were collected from the construction site for density estimation. Six cores were extracted for each mixture, and their densities were measured in the laboratory. A density model named Al-Qadi, Lahouar, and Leng (ALL), developed in an earlier study by the authors, was used to predict the asphalt mixture density from GPR measurement. GPR performance was then verified by comparing the GPR-predicted densities with densities of 20 field cores measured with the nuclear gauge and in the laboratory. This study shows that GPR can provide reasonably accurate density prediction when an appropriate model is used. GPR's accuracy of density prediction is comparable with, or better than, that of the nuclear gauge when two calibration cores are used. GPR measurement of an asphalt mixture was not affected by temperature within the range of 90° to 190°F (32° to 88°C). The relationship between the GPR signal reflection amplitude and the number of roller passes could be used to monitor the asphalt mixture compaction process and determine the optimum number of roller passes.