Austroads Shares Long-Term Sprayed Seal Trial Outcomes
In 2011 and 2012, two projects were spray sealed in Australia to evaluate the relative performance of seals constructed using various grades of polymer modified binders (PMBs).
“The idea was that we would monitor how these seals/binders perform over time and collect field-related information about current and future PMB grades,” said Young Choi, senior technology leader at the Australian Road Research Board (ARRB), during a webinar presented by Austroads. Austroads is a collective of Australasian transport and traffic agencies whose focus is to support member organizations as they improve road networks.
“Flexible granular pavements protected by sprayed seal surfacings are the backbone of the Australian and New Zealand road networks,” Choi said. “Road agencies have, therefore, focused on producing quality, long-lasting sprayed seal surfacings.”
According to Choi, a major procurement area involved in this is bituminous binders, with Austroads member agencies collectively purchasing more than $600 million worth of this material annually. To ensure the use of high-performance products, Australia developed a national specification for PMBs, ATS3110. The hope was that the two trials would provide insight for performance-based test methods of PMBs.
The two trials were performed in Coober Pedy in South Australia and Cooma in New South Wales, each divided into sections to test the different binders, which included C170 bitumen as the control, Austroads S Grade PMBs and some proprietary PMBs.
A team of experts performed inspections after six and eight years of service–in October 2017 and November 2019–to investigate the surface texture depth changes of the trial seals to validate a new seal voids parameter, as well as perform rheological and chemical investigations into the aging performance of the trial binders.
Surface texture depth measurements were conducted throughout the trial by Austroads’ sand patch method, which provides information on the changes of texture depth on sprayed seals over time. “We see in pretty much all cases a slow downward progression of surface texture as it becomes a bit smoother,” said Steve Patrick, senior technology leader with ARRB.
“With the sand patch texture depth data we collected from these trials, we were able to investigate if changes in sprayed seal surface texture over time were similar in Australia and New Zealand and whether the different design methods made a large difference in the performance of sprayed seals in an ongoing sense,” Patrick said. Austroads sprayed seal design is an empirically derived method, meaning it was extrapolated from experimental data generated by observation and experience. However, New Zealand’s sprayed seal design method is based on quantitative measurements of surface texture change under traffic.
Results indicated similar changes in surface texture when sprayed seals are trafficked in both Australia and New Zealand. “The empirical method we have matches quite well with the quantitative method used in New Zealand,” Patrick said. “We found that the data we collected demonstrated good potential for the adoption of an updated seal voids design parameter in Australia.”
The second goal of the trials, to perform rheological and chemical investigations into the aging performance of the trial binders, was less conclusive.
Austroads’ PMB specification currently doesn’t include tests that provide information on aging resistance/durability. “We saw this as a good opportunity to investigate binder aging,” Choi said. “Proper understanding of binder aging properties is an essential step towards developing a robust durability performance test.”
The team collected unaged binder samples during construction of the trial sites, prior to spraying. After the six and eight-year periods of service, they collected additional samples to extract binder material for testing at ARRB’s lab. The samples were subjected to Dynamic Shear Rheometer (DSR) stress ratio tests and Fourier Transform Infrared (FTIR) spectroscopy tests.
The DSR stress ratio test discovered that most binders reduced in elasticity and increased in hardness to a similar level of C170 bitumen, with only one highly modified binder (S20E SS) showing greater durability. “All binders hardened with age, without exception, but C170 became the hardest, whereas the PMBs were all softer,” Choi said. “PMBs showed gradual hardening up to six years, but then some started to soften with further aging.” In the end the team concluded that the stress ratio parameter did not appear to be a suitable test for aged binders, as harder binders would be expected to be more prone to cracking.
The FTIR tests provide information on the changes in chemical properties upon aging, particularly the level of binder oxidation. “All binder without exception experienced more oxidation gradually over time, with C170 being the most oxidized and the PMBs, less oxidized,” Choi said. “The general broad trend was that the more highly modified PMBs tend to have less oxidation, with S20E SS the lowest.”
“We found the aging of bitumen to be quite complex,” Choi said. The team concluded that the Australian seal life model developed in the 1980s was not able to accurately predict the age hardening of C170 bitumen at the Coober Pedy site. “That means that even for a simple bitumen material, we don’t have a full understanding of aging.”
The results of the study indicate that simple rheological/chemical characterization tests do not appear suitable for ranking the cracking performance of aged binders. More complex rheological tests, such as repeated loading tests, appear to be needed to develop durability performance tests for PMBs in ATS3110.
“We gained some knowledge [of binder aging], but also raised some questions about our understanding of binder,” Choi said. “Therefore, further in-depth studies appear to be required in order to develop a durability performance test for PMBs in the Austroads PMB spec.”