Development of Superpave Direct Tension Test Device

The redesign and evaluation of the Superpave direct tension test (DTT) are described. The prototype of the DTT device was developed during the Strategic Highway Research Program (SHRP). The DTT device, an integral part of the new Superpave asphalt binder specification, was developed to test asphalt binders and determine their failure properties, such as the stress and strain at failure. The strain at failure is used in the Superpave asphalt binder specification to determine the lowest temperature at which the asphalt can be used to avoid low-temperature thermal cracking. When the SHRP program was completed, an affordable specification version of the prototype SHRP-DTT device was introduced for field implementation. Unfortunately, this version was plagued with mechanical problems, producing excessive variability and inconsistency in data. The device also was less user-friendly than other test devices that were being implemented for the asphalt binder specification. These problems led to implementation delays and difficulties. The problems were addressed by FHWA in cooperation with Instron Corporation of Canton, Massachusetts (a manufacturer of materials testing equipment). On the basis of recommendations made by FHWA, Instron developed a new, easy-to-use specification-type DTT device. This device is compact (approximately one-tenth the size of the original) and uses a fluid-based temperature control system similar to that used in the bending beam rheometer. The new device also employs feedback elongation (strain) rate control. However, unlike the previous device, the new DTT uses an extensometer between the loading pins instead of a laser to measure elongation (strain). The evaluation and resulting recommendations by FHWA, which led to the development of the new specification-type devices are discussed. The repeatability and testing protocols also are discussed.