Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/134675
RoadScience by Tom Kuennen, Contributing Editor Beware Premature Aging of Binder Take care with liquid asphalt binder to avoid thermal crack failure T hermal cracking – also known as low temperature cracking – in bituminous pavements appears 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. The spacing of cracks often is regular over the length of a pavement. Prolonged frigid temperatures can cause asphalt pavements to crack if the wrong performance graded (PG) asphalt binder is used for the climate, or if the binder has "aged" prematurely via excessive heat in the plant where it is made. That can lead to premature deterioration of a roadway and create the need for costly, congestion-causing repairs. Highway agencies try to limit low temperature cracking by specifying Superpave performance-graded asphalt binders intended for their climate that have favorable properties at very low temperatures. However, aggregates that make up more than 90 percent (by weight) of a typical asphalt mix may respond in their own ways to low temperatures. Crude oils from which binder is produced will vary and result in liquid asphalt that interacts differently with aggregates. In the design phase, the thermal cracking model included in the new Mechanistic-Empirical Pavement Design Guide (MEPDG) is an engineering tool that can be used to evaluate the potential for thermal cracking during design, says Ramon Bonaquist, Ph.D., P.E., Advanced Asphalt Technologies in Characterization of Wisconsin Mixture Low Temperature Properties for the AASHTO Mechanistic-Empirical Pavement Design Guide, published by the Wisconsin Highway Research Program of the Wisconsin DOT in December 2011. This model performs a thermo-viscoelastic analysis of a constrained asphalt layer to compute stresses within the layer as a function of depth, Bonaquist says. "Pavement temperatures as a function of depth and time for the thermal stress analysis are obtained from the environmental effects model," Bonaquist says. "The computed thermal stresses are used in a linear fracture mechanics model to compute the propagation of a vertical surface crack through the asphalt layer. Finally, the crack spacing at the surface of the pavement is determined from an empirical model that relates the crack spacing observed in the pavement to the average crack depth calculate by the analysis." Getting the right mix design and PG binder notwithstanding, premature aging of liquid asphalt binder that has been damaged in the manufacture of hot mix asphalt (HMA) is receiving much of the blame for low temperature thermal cracking. At a hot mix plant, when liquid asphalt is exposed to high heat in the aggregates dryer, lighter hydrocarbons can be stripped away, leaving a compromised binder that may perform as through it's been part of a mix that's been sitting in the blazing sun season after season, "aging" and becoming more brittle. Thus when the condition appears in a relatively new mix, the binder is said to have been prematurely aged. Premature aging is avoidable. But preventing premature aging in asphalt binder is only one piece in solving the puzzle of building roads not to fail. The addition of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) into new asphalt mix design is on the increase, due to the high cost of petroleum, difficulties in procuring fresh reserves of quality virgin materials at economical distances from where they will be used and limited space in landfills. Yet the use of RAP introduces design challenges because the residual asphalt binder in RAP becomes stiffer due to aging, and that stiffness is imparted to the mix. Stiffness is good because it bolsters a mix against rutting, but can be bad as it's thought to contribute to thermal cracking. 14 June 2013 Better Roads RoadScience_BR0613.indd 14 5/31/13 12:46 PM