Two new standards have recently been approved by ASTM International Committee D04 on Road and Paving Materials. The standards, both of which are under the jurisdiction of Subcommittee D04.26 on Fundamental/Mechanistic Tests, are D 6931, Test Method for Indirect Tensile (IDT) Strength of Bituminous Mixtures, and D 7312, Test Method for Determining the Permanent Shear Strain and Complex Shear Modulus of Asphalt Mixtures Using the Superpave Shear Tester (SST).
Complete IDT strength testing fixture
ASTM meeting and staff manager contact information for Committee D04 can be found at the end of this article.
The withdrawal of D 4123, Test Method for Indirect Tension Test for Resilient Modulus of Bituminous Mixtures, led to the development of D 6931 since D 4123 contained a description of an indirect tensile strength test that was referred to in several other standards. It was decided to develop this test into a completely separate standard, D 6931, to stand on its own merit as a method that could be easily referenced by the many other standards.
According to Richard W. May, performance testing laboratory manager, SemMaterials, L.P., and a Subcommittee D04.26 member, D 6931 will serve several purposes for pavement design and analysis engineers, asphalt concrete mix designers and laboratory technicians.
“This test has been used for evaluating many types of asphalt concrete performance, such as fatigue cracking propagation, rutting and moisture susceptibility or stripping, where half of the specimens are subjected to conditioning to evaluate the loss in strength compared to unconditioned specimens,” says May. He also notes that the test covered in D 6931 is not a new test but has been used by many in the pavement field over the last 40 years to predict performance and evaluate reasons for distress, such as stripping and segregation.
The subcommittee is seeking laboratories that can participate in multi-laboratory precision and reproducibility testing. “We will be seeking out mix design laboratories that have the capability of running this simple compressive load testing, which creates the indirect tension through the diametral plane of the specimen,” says May. “Since the test involves no sensor attachment to track specimen deformation and only involves recording the load to a peak value and beyond, there should be plenty of labs that can participate.” //
Technical Information: Richard W. May, SemMaterials, L.P., Tulsa, Okla.
Test Method D 7312 was developed by D04.26 to provide performance-related test procedures for the determination of complex shear modulus (stiffness) and permanent shear strain of asphalt mixtures using the Superpave shear tester.
According to Rebecca McDaniel, technical director, North Central Superpave Center, and leader of the D 7312 task group, Superpave is a system for designing asphalt mixtures to ensure that the right asphalt binder and aggregate structure is used for the traffic and climate where it will be placed on the road.
“Superpave builds on and improves previous mix design methods,” says McDaniel. “In some parts of the United States, implementing Superpave marked a big change in how mixes were designed so there was a major learning curve for contractors and agencies beginning in about 1993.”
To facilitate the implementation of Superpave, the Federal Highway Administration established five Superpave centers across the U.S. as partnerships between a state department of transportation and an engineering university in the state. Each of these labs was equipped with a Superpave shear tester, which was developed during the Strategic Highway Research Program at the University of California, Berkeley. Other laboratories working with Superpave purchased SSTs as well. These labs all wanted to follow the same protocols when working with the SSTs, which led to the development of D 7312.
According to McDaniel, the tests described in D 7312 can be used to evaluate changes in hot mix asphalt mixtures and rank different mixtures in terms of their resistance to permanent deformation (rutting) and, to some extent, to fatigue cracking. Other research applications for the SST include evaluating the effects of using recycled asphalt pavement in new mixes and studying the effects of changing binders, aggregates and additives. In addition to research purposes, the SST can be used as a referee test for mixes placed in the field that have low air voids, as has been done in Indiana in the past.
McDaniel says that the subcommittee welcomes additional input into the continuing development of D 7312. “We will be working on precision and bias testing in the next few years and we welcome labs to participate in that effort,” says McDaniel.
Technical Information: Rebecca McDaniel, North Central Superpave Center, West Lafayette, Ind.
Phone: 765/463-2317, ext. 226
ASTM Staff: Daniel Smith
June Committee Week