Evaluation of Crumb Rubber Modification on Specification Compliance of Asphalt Mixtures

Since 2008, the Louisiana Department of Transportation and Development (LaDOTD) has allowed the use of crumb rubber modification (CRM) as per the specifications. However, the introduction of CRM has presented unique challenges during design and production. LaDOTD districts have reported early cracking for dense-graded mixtures. Adhesion and stability issues have been reported for open-graded friction course (OGFC) and stone matrix asphalt (SMA) mixtures. Currently, LaDOTD specifications allow the contractor to switch asphalt cement supplies without redesigning the mixture. This study evaluated several CRM-modified mixture designs to address design and performance concerns; it also evaluated the volumetric and mechanistic properties of dense, SMA, and OGFC mixtures designed with unmodified asphalt cement, styrene-butadiene-styrene-modified asphalt cement and five crumb rubber sources. SuperPave volumetric parameters were evaluated to ascertain the impact of CRM on the design of these mixtures. Rutting and intermediate temperature cracking susceptibility were determined using the Hamburg Loaded Wheel Tracker (LWT) and semi-circular bend test (SCB), respectively. Solvent extraction and the ignition furnace were used to determine the asphalt content of the modified mixtures. The results showed that CRM improved the laboratory rutting resistance of the mixtures. However, the laboratory cracking resistance was decreased for the CRM-modified mixtures. Based on statistical analysis, additional asphalt content, reduced CRM dosage, and the use of polymer additives may mitigate the reduced laboratory cracking performance of the CRM-modified mixtures. The volumetric parameters were also affected by CRMs. The mixtures should be re-designed if the contractor anticipates using a CRM-modified asphalt cement.