Recycled asphalt shingles (RAS) have been used in road pavement construction for a number of years primarily on low volume roads. The use of RAS represents economic and environmental opportunities as it provides as good or better performance when processed and proportioned appropriately than commonly used asphalt mixtures. The primary components of RAS are asphalt, mineral filler, mineral granules, and felt. The effect of RAS fibers on an asphalt mixture's dynamic modulus, which is a key input in the Mechanistic-Empirical Pavement Design Guide (MEPDG) and one of the critical properties of asphalt mixtures affecting flexible pavement responses that are related to its performance, are still uncertain.

The National Pooled Fund Study #1208 conducted a series of researches to investigate various issues related to RAS utilization. Thirteen mix designs with RAS contents ranging from zero to six percent were developed and constructed in Indiana, Iowa, Minnesota, and Missouri. Field produced mixtures were procured and sent to Iowa State University Asphalt Lab for laboratory dynamic modulus tests. The testing results are used to evaluate two commonly used dynamic modulus predictive models, the Witczak and Hirsch models. Two versions of Witczak models, which were developed in 1999 and 2006, are evaluated in this research. It was found that the Witczak models were not very effective in estimating the modulus values of RAS mixtures and thus modifications were made to the models to account for the effects of RAS. The study did determine out that the commonly used dosage of RAS in asphalt mixtures does not affect the prediction accuracy of the Hirsch model, however updates were made to improve the Witczak model's accuracy.