Low-temperature properties of plant-produced RAP mixtures in the Northeast

Abstract

The impact of reclaimed asphalt pavement (RAP) materials on pavement performance is an important topic of study in the industry due to environmental and cost benefits. The primary concern for increasing allowable RAP percentages in hot mix asphalt relates to the presence of aged materials, which may embrittle the mixture and decrease cracking resistance. Low-temperature cracking is a major distress in cold temperature climates, such as the Northeastern United States. Currently, there are several procedures to analyse low-temperature performance of asphalt binders and mixtures. However, these methods use different starting (initial) temperatures and cooling rates that may not represent actual field temperatures and cooling rates. This paper presents the results of a study on low-temperature performance of plant-produced RAP mixtures. Eighteen mixtures from three states were tested with varying RAP contents (0–40%) and different virgin binder grades. The objectives of the study were to: (1) evaluate the impact of cooling rate and starting temperature on the critical cracking temperature of RAP materials; (2) evaluate the impact of RAP content on the low-temperature properties of mixtures; (3) evaluate the benefit of using softer virgin binder grades to mitigate the impact of the aged RAP binder in the mixture; and (4) to compare the low-temperature cracking properties determined from different mixture and binder tests. Based on the results, warmer starting temperatures and faster cooling rates result in warmer critical cracking temperatures for all mixtures. Through use of the uniaxial thermal stress strain test, it was found that the addition of RAP alters fracture behaviour from ductile failure towards a brittle failure. Based on results from the indirect tensile test, tensile strength increases with RAP content. However, due to a faster-building thermal stress, warmer critical cracking temperatures result. It was also determined that degree of blending may impact the effectiveness of using softer binder grades at higher RAP percentages to improve low temperature cracking resistance. The data also show that analysis procedure and test protocols can have a profound effect on critical cracking temperature. The conclusions presented reinforce the need for more accurate representation of RAP materials, and careful selection of analysis parameters.