Better Roads Staff
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.
As the temperature falls, the asphalt binder contracts more than the metal ring, causing the binder to fracture. A computerized data acquisition system captures the temperature and stress level at which the fracture occurs. That information can be used to grade asphalt binders for expected climate conditions, as well as to develop new binders that can better withstand cold temperatures.
The American Association of State Highway and Transportation Officials is reviewing the ABCD test procedure and considering making it a provisional standard, the first step toward final adoption as a standard.
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