Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85910
by Tom Kuennen Contributing Editor RAP and Thermal Cracking Solving the Riddle of Extreme cold triggers the most com- mon type of distress of asphalt pave- ments, low-temperature cracking, says a technical brief – Investigation of Low-Temperature Cracking in Asphalt Pavements – published in 2008 by the Minnesota DOT (MnDOT). "As temperatures drop, the pavement contracts, building up tensile stresses that lead to cracking," states MnDOT's Research Services Section. "Fractures oc- cur every 20 to 30 feet across the lane, allowing water to penetrate the structure, which further weakens the pavement layer and the base beneath. Traffic loads worsen these cracks, leading to shorter pavement life, increased maintenance re- quirements and rougher roads." MnDOT attempts to limit asphalt pavement cracking by specifying Superpave performance-graded asphalt binders that have favorable properties at very low temperatures, the tech brief says. "However, more factors contribute to cracking than just the binder. The various aggregates that make up more than 90 percent (by weight) of a typical asphalt mix may each respond differently to low temperatures," it says. "Crude oils from different geographic locations pro- duce asphalt binders that interact differ- ently with aggregates, and asphalt binder grades and contents can also vary from mix to mix." Complicating characterization of ag- gregates to fight thermal cracking is the increasing use of RAP and its residual asphalt binder as aggregate in mixes. Challenge of RAP Aggregate The use of RAP in recycled asphalt pave- ment is well accepted at the state, city and county levels. Almost all hot-mix asphalt (HMA) contains at least some RAP in many areas. But the RAP that can be added to hot-plant mix asphalt mix- tures is limited to relatively low percent- ages and, in some areas, the use of RAP is prohibited in certain types of mixtures, such as surface courses. Typically, the maximum percentage of RAP allowed is anywhere from 15 to 30 percent by weight of HMA mixture. Yet, recent investigations show HMA materials with percentages in excess of 50 percent can be produced to perform to the same as "virgin" mixes. Agencies that are not currently allowing RAP into their HMA mixtures — and those that are only allowing small percentages of RAP — can safely increase the amount of RAP used without fear of shortening pave- ment life, provided that best practices are followed, according to a Federal Highway Low-temperature cracking (transverse cracks) may occur when asphalt pavements contract in cold temperatures Administration (FHWA) Tech Brief titled High Reclaimed Asphalt Pavement Use. As reported in Better Roads late last year, government agencies from coast to Better Roads February 2012 17 Photo courtesy Minnesota DOT