The Simple Performance Test Ready for Prime Time By Dan Brown The hot mix asphalt industry has long sought a simple strength test to complement the Superpave volumetric mix design method. Results from WesTrack and other experimental project
he Simple Performance Test Ready for Prime Time
By Dan Brown
The hot mix asphalt industry has long sought a simple strength test to complement the Superpave volumetric mix design method. Results from WesTrack and other experimental projects have raised questions of whether the Superpave volumetric mix design method offers sufficient assurance of reliable mixture performance over a range of traffic and climate conditions.
The search has led to development of the Simple Performance Test (SPT) machine. Three companies manufacture a version of the machine. Since August 2005, InstroTek Inc.,
Sales of the SPT are growing. The Utah Department of Transportation (DOT) bought five IPC machines, reports Ali Regimand, president of InstroTek, which represents IPC Global in the United States and supports the machines. State DOTs in Maryland and Missouri, the Asphalt Institute and the National Center for Asphalt Technology have each bought one. Other machines have been sold to the University of Tennessee, University of Iowa, Louisiana State University and various other universities.
"We have been very encouraged by the test results and performance of our SPT device so far,” says Regimand. “We believe that once the round robin and ruggedness studies conducted at the national level are completed, more customers will realize the potential value of these machines to provide the mixture performance results that everyone has been waiting for.”
At Advanced Asphalt Technologies, Sterling, Virginia, Ray Bonaquist is the principal investigator writing the specifications for the SPT equipment per NCHRP 9-29.
Under NCHRP 9-19, Matthew Witczak and researchers at Arizona State University developed three performance tests for asphalt mixtures. These tests, which led to the development of the SPT, are:
· The dynamic modulus test, which essentially tests for stiffness of the mixture
· The flow number test, which measures permanent deformation of a specimen under a repeated load
· The flow time test, which measures creep, or deformation, under a constant load
Two of these tests, dynamic modulus and flow number, are being used in practice. The dynamic modulus test is performed at the effective fatigue and rutting temperatures, whereas flow number is operated at the effective rutting temperature. To run the SPT tests, an environmental chamber is used to condition the specimens to the proper temperatures. The goal is to accurately measure mixture response characteristics correlated to pavement distress over a range of traffic and climate conditions.
While the tests are not as quick and easy to perform as some practitioners would like, Bonaquist points out that the equipment manufacturers have made considerable progress in simplifying the testing procedures, compared to dynamic modulus tests that have been used in research applications for years.
For example, with the SPT machine, the dynamic modulus test requires the technician to attach gauge points to the side of the cylindrical specimen. All three manufacturers have a device to automatically pin the gauge points, or brass buttons, to the side of the specimen. The brass buttons are used to attach the Linearly Variable Differential Transformers(LVDTs), which are essentially electric strain gauges. “In the past, attaching those gauge points would take hours,” says Bonaquist. “But with the simple perfo.