At their first in-person fall seminar in three years, members of the Ontario Asphalt Pavement Council (OAPC) were urged “to put more asphalt cement into the mix.”
Asphalt cement is a strong, versatile and weather and chemical-resistant binding material, or “glue,” that binds stone and aggregate together to produce asphalt.
But the seminar’s keynote speaker said a rethink about the amount of asphalt cement used in mixes in Ontario is in order.
“Mixes tend to be under-asphalted and will have reduced durability,” said GEMTEC Consulting pavements and materials manager, Steve Goodman.
Noting that roads in Ontario are aging more quickly, which leads to cracking and will ultimately require replacement, the practice can be rectified with some adjustments, he said.
“So why are hot mix asphalt suppliers under-asphalting?” asked Goodman, who then suggested that a “nefarious supplier” might take advantage of the fact they are paid based on Job Mix Formula (JMF) asphalt and not quality assurance acceptance.
But the answer is much more complicated.
An example is the Ontario Provincial Specification 310 which sets thresholds “which are too wide and we are accepting under-asphalted mixes,” he said.
The specification allows for 0.3 per cent difference in what is stipulated in the mix design and what is actually put on the road, “which I think is too much.”
Municipalities also need to conduct more quality testing, said Goodman, adding they should also make better use of what is referred to as Regression Mix Design. Now being used in a number of Ministry of Transportation projects, this approach allows for an increase in asphalt cement and a small reduction in air voids.
A critical balance has to be maintained in mix designs, especially in the amount of air voids. Not enough asphalt cement will lead to cracking, while too much asphalt cement will lead to rutting, which is why air voids can’t be eliminated, he said.
“There has to be some air (in the mix) which acts as a shock absorber. Otherwise, the pavement won’t bounce back and will ultimately fail.”
As part of the seminar, which highlighted several in-depth technical solutions, Goodman encouraged the audience to fully embrace a wide number of technologies and performance tests such as the Hamburg Wheel Tracking Test and the Asphalt Mixture Performance Test.
Another takeaway from the seminar is that Superpave is here to stay. Superpave is an acronym for Superior Performing Asphalt Pavements and is an alternative system to the Marshall method for specifying material components and asphalt mix designs using a gyratory compactor.
The compactor prepares laboratory samples by rotating the mix at a slight angle under a ram that imparts a kneading type compaction effort. Each rotation is called a gyration.
First developed in the late 1980s and 1990s to address rutting, it has replaced the use of the Marshall Mix by the Ontario Ministry of Transportation and other large agencies. But many smaller and medium sized municipalities have not made the switch, he said.
Similar to his recommendation on the need for municipalities to accelerate their use of quality testing, Goodman suggested they should adopt Superpave.
He also discounted some speculation in the industry that the Superpave mix design system is responsible for the reduction of asphalt cement.
Instead of designing to the minimum amount of asphalt, owners and the industry, “should be designing to the proper amount.”
Later in the day a second speaker said Superpave mix designs have significantly improved the performance of pavements in Ontario in terms of resisting rutting and other performance issues.
However, there are concerns that the asphalt cement content of many mix designs may have been reduced to below optimal levels for durability, said the OAPC and the Ontario Road Builders’ Association’s (ORBA), technical director Doubra Ambaiowei.
“We have to be certain that any changes made do not adversely affect other aspects of durability such as rutting.”
In a continuation of the 2018 ORBA/OAPC Quality of Asphalt Review, the council has sought to find a balance between rutting and cracking to improve the cohesive properties of asphalt mix produced and placed for Ontario’s cold climate, said Ambaiowei.
To that end goal, it conducted a study on the impact of lower design gyrations and the impact on durability in partnership with two industry laboratories.
A major finding was that a low design gyration on Superpave CAT E Traprock aggregate mix type “could effectively create mixes that will withstand design traffic loading without rutting and cracking.”
The results support a Quality of Asphalt Review recommendation that asphalt cement contents of 0.5 to one per cent could be beneficial to the production and placement of more durable asphalt pavements, said Ambaiowei.
But the study is an ongoing one and has to investigate other plant-produced granite type mixes, he cautioned.