Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85910
RoadScience By Tom Kuennen, Contributing Editor Crow Wing County, Minn., is fighting low-temperature cracking of virgin and RAP mixes by using a warm-mix asphalt additive. But there are fears that higher quantities of RAP may com- promise mix performance in cold-weather regions. There, where pavement temperatures plunge during the long winters, "thermal" cracking – also widely known as low-temperature cracking – compromises longevity of asphalt pavements. Prolonged frigid temperatures can cause asphalt pavements to crack if the wrong asphalt binder is used for the climate, or if the binder has "aged" prematurely via excessive heat in the plant where it was made. That can lead to premature deterio- ration of a roadway and create the need for costly, congestion- causing repairs. U As the percentages of RAP in asphalt mixes grow, RAP has come under scrutiny as a contributor to low-temperature cracking. While use of 15 percent or more RAP can result in a significant increase in mixture stiffness, which can enhance durability, that same stiffness can negatively affect low- temperature cracking characteristics of the pavement. 16 February 2012 Better Roads se of reclaimed asphalt pavement (RAP) in hot-, warm- and cold-mix asphalt is growing dramatically, as tests validate its use in ever-increasing quantities. But the public and private sector are working diligently to overcome these negatives. What is Low-Temperature Cracking? This distress is manifested as a series of transverse cracks that extend across the pavement surface in response to cold tem- peratures. Thermal crack intervals of 19 to 30 feet are typical, but may vary widely. Spacing of cracks often is regular over the length of a pavement. Thermal cracking in flexible pavements can exacerbate heav- ing of expansive soil subgrades, causing large cracks that are subject to movement and even faulting of pavements from one side of the crack to the other. In Wichita, Kan., for example, a generation of pavements was plagued by thermal crack- driven fissures so big they could swallow a beer can. There, these expansive bases were stabilized by chemical treatments like pebble lime, soil cement or fly ash, or a mixture of those, which gave excellent bearing values, but were compromised by asphalt thermal cracking, due to climatic variations and freeze/ thaw cycles. Photo courtesy of Evotherm