State departments of transportation (DOTs) nationwide have long dealt with the problem of asphalt pavement cracking — the primary type of distress that creates the need for pavement rehabilitation. Many cracking tests have been developed, but no single test has been simple, reliable, efficient, repeatable and cost effective — until now.
Over the past four years, researchers at the Texas A&M Transportation Institute (TTI) have developed and validated a single test that does it all: the indirect tension asphalt cracking test (IDEAL-CT).
“Every department of transportation is looking for a simple, practical cracking test for use during mix design, quality control and quality assurance,” says TTI Senior Research Engineer Fujie Zhou, the expert who led development of the IDEAL-CT.
The test was created under projects in the Transportation Research Board’s Innovations Deserving Exploratory Analysis programs. The research was funded by the American Association of State Highway and Transportation Officials as part of the National Cooperative Highway Research Program (NCHRP).
What Makes This Test Special?
The most commonly used traditional cracking tests in the United States require significant specimen preparation before testing; that prep work involves cutting, notching or gluing when using the flexural beam fatigue test, semi-circular bend test, overlay test or other cracking tests.
“This new test has fewer steps in sample preparation, so there’s less potential for human error,” says Zhou, who has worked on cracking-related issues for two decades.
The IDEAL-CT simply requires that a disc-shaped specimen be molded to a commonly used size — with no cutting, notching or gluing required. The specimen can then be tested in a standard indirect tensile strength testing machine.
Because the test doesn’t require traditional prep work or sophisticated user training to achieve accurate results, the IDEAL-CT automatically has two very desirable features — simplicity and practicality. With the loading rate at 50 mm per minute and the testing time taking a minute or less, the test is far more efficient than its predecessors.
Most contractors and DOTs already own the testing equipment needed for performing the IDEAL-CT. And even if a new test machine is purchased, the cost is often less than $10,000, making equipment costs economical over the long term.
“We use exactly the same indirect tensile strength testing machine, the same test, even the same loading speed,” Zhou explains. “The only thing we do differently is to look at the data in a new way.”
The calculation of the cracking tolerance index (CTIndex) requires the whole load versus displacement curve rather than just the maximum load alone that’s normal for that machine. Fortunately, those load-displacement curve readings can be recorded manually, or laboratories can opt to buy an accessory for less than $3,500. The larger the CTIndex, the better the cracking resistance.
The IDEAL-CT has proven a reliable cracking test for fatigue cracking, reflective cracking, top-down cracking and low-temperature cracking. Researchers have validated the test using performance data from the Federal Highway Administration’s accelerated loading facility, Long Term Pavement Performance Special Pavement Study 10, the National Center for Asphalt Technology (NCAT) test track, the Minnesota Department of Transportation’s Road Research facility, and numerous Texas test sections.
How Is the Test Being Implemented?
The IDEAL-CT became ASTM Standard D8225 in 2019 and is being used to evaluate the cracking resistance of asphalt mixes around the world. In the United States, Zhou is leading a national effort sponsored by NCHRP to implement the IDEAL-CT for balanced mix design (BMD) in addition to quality control/quality assurance testing for six states: Texas, Oklahoma, Virginia, Kentucky, Minnesota and Maine.
“The IDEAL-CT is such a simple, practical and reliable cracking test. We have been using it for our balanced mix design implementation initiative, testing pilot BMD sections placed in Virginia over the past three years,” says Virginia Department of Transportation Assistant State Materials Engineer Rob Crandol. “The simplicity and timeliness of the IDEAL-CT sample preparation will be key to our efforts for performing cracking testing during actual plant mix production.”
A recent survey conducted by NCAT showed that at least 14 DOTs around the nation are using the IDEAL-CT as the cracking test for their BMD efforts to improve mix quality and durability.
“The IDEAL-CT is intended for ensuring high-quality mix production at plants in Texas, which is a critical step in the overall process of building and maintaining strong and long-lasting roadways,” says Flexible Pavements Section Director Enad Mahmoud of the Texas Department of Transportation’s Materials and Tests Division.