PG Binders, CIR Help

Low-temperature cracking proved to be the No. 1 cause of transverse cracks in the vicinity of Ottawa, Ont., but use of appropriate PG binders and cold-in-place recycling helps fight it, according to Low Temperature Cracking Performance of Superpave and Cold In-Place Recycled Pavements in Ottawa-Carleton, by Stephen Q. S. Lee, P. Eng., Anita VanBarnefeld and Michael A. Corbett, P. Eng., Environment and Transportation Department, Regional Municipality of Ottawa-Carleton in Canada.

Historically, the Regional Municipality of Ottawa-Carleton (RMOC) used 85/100 PG binder with typical 58-22 in all conventional hot-asphalt mixes, the authors wrote in 2000. “The low pavement temperature extreme encountered during any given winter in Ottawa-Carleton is generally below minus-26 degrees C (minus-14 degrees F). This has resulted in a high frequency of low-temperature cracking (transverse cracking).”

To mitigate low-temperature cracking, the RMOC was using Superpave PG 58-34 asphalt mixes and the cold-in-place recycling process to mitigate the low-temperature cracking problem. To assess the cost-effectiveness of these pavement alternatives to conventional hotmix, a network-wide evaluation and correlation of costing, performance, serviceability and lifecycle were carried out. A total of 38 regional arterial roads were analyzed, further dynamically sectioned by asphalt type, last rehabilitation method, asphalt age, asphalt thickness, subgrade type and drainage condition.

“[L]imited data on Superpave PG mixes indicated significantly improved initial low-temperature cracking performance and a substantial lifecycle cost advantage when compared to conventional candidates,” they wrote. Cold In-Place Recyling (CIR) rehabilitated candidates “indicate better low-temperature cracking performance than both resurfaced and reconstructed candidates using conventional hot mixes.”

MnDOT Research

MnDOT’s Minnesota Road Research Facility (MnROAD) is conducting a five-year project to study the performance of RAP and determine if the present specified limits on RAP are justified.

Under the project – Cold Climate Performance of RAP under Controlled Testing Conditions – several asphalt concrete test sections will be built and contain 30-percent RAP, but vary in binder grade and fractionated RAP content. It is anticipated that the results of this research will lead to the modification of current specifications to include fractionation of RAP aggregate and/or new percentage requirements for high-RAP asphalt.

A stockpile of RAP from a variety of sources.

Separately, the Minnesota 10-state-pooled fund study – Investigation of Low-Temperature Cracking in Asphalt Pavements – began with a literature search to identify potential test methods and modeling approaches. It then undertook field investigation of 13 HMA pavement sites, documenting performance (good vs. poor), mix properties and traffic, and taking samples (both cores and beams) for laboratory testing.

“Investigators found that standard specifications do not include a specific test for measuring the potential fracture behavior of mixtures,” the tech brief states. “Another key finding was that asphalt binder testing alone does not reliably predict low-temperature cracking. Mixture testing revealed that aggregate type affected resistance to fracture (granite outperformed limestone), as did the use of asphalt binders modified with polymers (modified PG 64-34 binders outperformed PG 58-34 binders).”

New Test for Crack Potential

Also, new devices are making evaluation of low-temperature cracking more predictable. In December 2010, FHWA’s Highways for Life initiative reported that an inter-laboratory study – conducted with the help of an FHWA grant – confirmed the accuracy and usability of a device to pinpoint the temperature at which asphalt binder will crack.

The Asphalt Binder Cracking Device, or ABCD, uses a simple method to determine the susceptibility of various binders to thermal cracking before they are used on paving projects. Highways for Life provided a grant under its Technology Partnership program to help EZ Asphalt Technology of Athens, Ohio, develop and evaluate the ABCD.

The first phase of testing involved conducting a field validation of the ABCD and refining the equipment and analysis software. In the second phase, 31 laboratories participated in a study to evaluate the repeatability, accuracy and simplicity of the testing system.

Unlike conventional test methods, the ABCD creates thermal cracking conditions similar to those in the field. It consists of a metal ring equipped with temperature and strain gauges that fit into a silicone mold. Heated asphalt binder is poured around the ring and the device is cooled.