Three Tests to Perform
BY Sina Varamini, PhD, PE
Asphalt professionals want the pavements they design and install to perform up to expectations. Pre-testing provides one method to foretell the road’s future performance under ever-increasing traffic and weather changes. Here are three tests to include when attempting to predict the future. Two simulate the combined effect of traffic loadings and climate stresses on asphalt layers, and one tests the resiliency of top road layers in cold weather. Each performs an important role in determining how well your asphalt mix, and eventually your road, will perform under real-world conditions.
Hamburg Wheel Tracking Test (HWTT)
Asphalt mix designers widely use the HWTT to evaluate rutting resistance and moisture susceptibility. The HWTT subjects the asphalt mix to torturous loading and water-submerged conditions under controlled lab conditions. It performs well in raising red flags that should not be ignored. The HWTT can pinpoint uncertainties or risks related to ultimate field performance, or unforeseen changes in summer temperatures, thus qualifying it as one test you shouldn’t skip.
Semi-Circular Bending (SCB) Test
The SCB is another load-simulating test; it calculates the fracture energy of asphalt mixes from a load-displacement curve. It’s often referred to as a “flexibility index test.” It can predict whether the material is resilient enough to meet its intended longevity.
The SCB device positions a semicircular specimen on top of two rollers and then applies a load along the vertical diameter to determine the fracture energy and toughness of the mix. Other fatigue-predicting tests are available, some that apply cyclic loadings, but the SCB is the most widely known and accepted by government agencies due to its simplicity of operation as well as providing enough information to understand the limits of the mix. This makes it an indispensable tool in your arsenal of testing.
Disc Shaped Compact Tension (DCT) Test
The DCT test measures the fracture energy of asphalt pavements. Gyratory-compacted samples of mix designs are tested at various temperatures and loading rates. The test has the capability to capture the transition of asphalt concrete from a brittle material at low temperatures to a more ductile material at higher temperatures.
This index test helps mix designers determine how the pavement will perform under the extreme cold of Canadian winters.
To skip or not to skip?
When it comes to asphalt pavements, balance is key. You don’t want to go the total bodybuilder route and concentrate only on thick layers and strength. At the same time, you don’t want to install the equivalent of the skinny marathoner with limited strength.
The HWTT helps you predict strength, and the SCB reveals a level of long-term flexibility. As an important follow-up, the DCT shows how the road layers will relax under extremely cold weather.
To adequately predict a pavement’s chances of success, you should consider these three tests when developing a new mix design. And since strength and flexibility largely determine the overall life and performance of asphalt pavements, the answer is clear: Don’t skip them.
Sina Varamini is a professional engineer and the director, pavement and materials group, for Engtec Consulting Inc., Toronto, Canada. For more information, visit Engtec Consulting Inc., on LinkedIn.
What these 3 Critical Asphalt Tests Can Do for Your Next Mix Design:
- Increase understanding of how your mix design will behave
- Provide a benchmark for making improvements
- Deliver cost savings in the long run
- Improve pavement performance
- Offer performance results in as little as three to four days