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., Raleigh, North Carolina, has sold most of the SPT machines—more than 20 of them—made by IPC Global, an Australian company. The other two manufacturers in the United States are Interlaken Technology Corp., Chaska, Minnesota, and Medical Device Testing Services Inc., Minnetonka, Minnesota.
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.”
The IPC machine sells for approximately $60,000, and Interlaken prices its machine at between $70,000 and $80,000. At Advanced Asphalt Technologies, Sterling, Virginia, Ray Bonaquist is the principal investigator writing the specifications for the SPT equipment per NCHRP 9-29.
“We wrote the specifications, but the equipment manufacturers built the machines at what I consider to be a saleable price,” says Bonaquist. “Our target price was roughly $50,000, and we’re a little above that, but not by a factor of two.”
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 performance device, this is done in five minutes.”
Plus, computerized equipment simplifies the data manipulations that a dynamic modulus test requires. “The SPT requires a viscoelastic analysis to reduce the data to a material property, and so computers allow us to do that very quickly,” says Bonaquist. Developers of the SPT tests and machine have sought to design and build equipment that a skilled technician can run. If only graduate engineers can run the tests, it would limit the practical value of the machine.
Technicians do need training to prepare the test specimens, attach the LVDT devices and operate the SPT machine, says Christopher Robinette, now a staff engineer with Granite Construction Inc., Sparks, Nevada. With Chris Williams, now an associate professor of civil engineering at Iowa State University, Robinette ran more than 1,800 dynamic modulus tests at Michigan Technological University. The research testing was con-ducted under contracts with the Federal Highway Administration and the DOTs in Michigan and Wisconsin.
Robinette says it took him nearly a week to compact 12 gyratory specimens, cut and core them down to precise test geometries, dry off excess cooling water, and run a battery of SPT tests. Furthermore, the dynamic modulus testing was performed over a range of frequencies – and at intermediate and high test temperatures. “It took some time to go from an intermediate temperature to a high test temperature,” says Robinette.
“At first, cutting and coring of the SPT specimens was thought to be too laborious,” Bonaquist said. So the NCHRP panel found a manufacturer–Shed Works, of College Station, Texas,–to build an automated machine to core and cut test specimens from the oversized gyratory specimens. “Once you make your over-size gyratory specimen, this machine will saw and core the specimen to size in about 15 minutes,” says Bonaquist.
Still, Jack Weigel of Payne and Dolan Inc., a contractor based in Waukesha, Wisconsin, believes technicians should be well trained. Weigel is on the NCHRP panel guiding the development of the testing equipment. “It takes time to learn how to run the test and run it correctly.” More recent models of the SPT machine are improved and have been refined compared to earlier models. Weigel says, “The $60,000 is still a ‘doable price’ from a contractor’s point of view.”
SPT and the 2002 Design Guide
The development of the SPT is occurring just as states gear up to implement the Mechanistic/Empirical (M/E) Design Guide (also called the 2002 Design Guide), which some observers believe AASHTO will ballot and approve in 2007. Some seventeen states ranging from Florida to Texas to Maine and Washington have joined an AASHTO Lead States Group of those who want to implement the new design guide.
“Already the state of Missouri has designed some 50 projects using the M/E Design Guide,” says John Donahue, a MoDOT pavement engineer who is responsible for implementing the guide and software in Missouri. He says MoDOT bought an IPC Global SPT machine and expects to use actual SPT values in the M/E Design Guide next year.
With mechanistic design, the pavement engineer makes estimated predictions about pavement performance based upon certain applied loadings over the life of the pavement. It is a mathematical model, not a design based upon observed performances of mixes using certain materials and pavement thicknesses. “The M/E Design Guide represents a seismic shift in pavement design,” says Donahue.
“We are the only state using the guide for design, but probably a dozen other states are beginning calibration of it,” says Donahue. “Despite some controversy over the guide, we’re using it for better or worse. We have just been selective about which distress models we use.” Local calibration for the design guide means adjusting the distress model coefficients so that predictions will more closely match the performance of pavements within a given state.
“But that correlated value for your dynamic modulus will not be on the money like direct measurements from the SPT machine will be,” says Donahue. “So far, we’re collecting mixes and compacting them to specified air voids, then running dynamic modulus tests,” says Donahue. “We’ll be able to plug those dynamic modulus numbers into the design guide and it will give you predicted distresses over whatever design life you choose.”
Donahue says Missouri’s SPT results make sense. As one would expect, the dynamic modulus of a mixture decreases with higher temperatures or with slower loading frequencies. “I’m seeing the type of incremental changes I would expect,” Donahue says.
At Iowa State, Williams praises the SPT machine. He says it allows a designer to estimate, but not necessarily predict, the performance of a mix design. “There is some uncertainty in the way construction processes translate what is called for in the design,” he says. “But this is a tool that allows us to assess the performance of a mix on completion of the volumetric mix design.”
Dan Brown is the principal of Technicomm.