CTIS | March 20, 2023
How can current asphalt mix design guidelines be adjusted to create better performing recycled mixtures?
Under TxDOT Project 0-6923, the Center for Transportation Infrastructure Systems (CTIS) at The University of Texas at El Paso (UTEP) developed an Asphalt Concrete (AC) mix design program and guidelines for the Texas Department of Transportation (TxDOT) pavement designers and engineers to determine the optimum recycled asphalt pavement (RAP), recycled asphalt shingles (RAS), and additive content for designing balanced mixes without compromising the quality and performance of the final product. The program allows for mixes with varying aggregate types, asphalt content and grade, and (if feasible) structural design of the pavement. Researchers performed extensive lab and field testing on service sections constructed with and without RAP, RAS, and other additives.
“This research was aimed at trying to increase the performance of our current mixes,” said Travis Patton, Section Director of Flexible Pavements, Materials and Tests Division at TxDOT. “In our traditional mix design, we do what we call volumetrics. Everything is density driven. The bidding for projects is done based on low bid, so you're not always getting the performance out of it. This project was looking at the life of that pavement while still utilizing these materials (RAP and RAS).
“Here we focused not just on rutting, we’re also focusing on the cracking, so the term balanced mix. We put over 30 sections out there that are balanced mix design. Hopefully within a few years we can see how good this balanced mixed design is going to be.”
From this research, a new special specification for TxDOT was developed, “Special Specification 3074, Superpave Mixtures – Balanced Mix Design.” This specification provides the details for the construction of hot-mix asphalt (HMA) surface pavement layers composed of Superpave (SP) mixture of aggregate and asphalt binder mixed hot in a mixing plant using a Balanced Mix Design (BMD) approach.