Better Roads Staff
Solving the Riddle of RAP and Thermal Cracking
By Tom Kuennen, Contributing Editor
Use 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 there are fears that higher quantities of RAP may compromise 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 deterioration of a roadway and create the need for costly, congestion-causing repairs.
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.
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 temperatures. 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 heaving 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.
Extreme cold triggers the most common type of distress of asphalt pavements, 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 occur 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 requirements 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 produce asphalt binders that interact differently with aggregates, and asphalt binder grades and contents can also vary from mix to mix.”
Complicating characterization of aggregates to fight thermal cracking is the increasing use of RAP and its residual asphalt binder as aggregate in mixes.
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