Sand gritting paves the way to perpetual pavement
June 1, 2012 by Jim Barnes
For several years now, the Ministry of Transportation of Ontario (MTO) has had its sights set on perpetual pavement—highways that will last, if not eternally, at least for the foreseeable future. One of the keys to realizing this dream seemed to be using durable, stone mastic asphalt (SMA) as the surface course. That was, until issues with skid resistance emerged that put the process on hold. Lately, one Canadian roadbuilder seems to have solved the problem.
The notion of “perpetual” pavements started to gain currency in Ontario in about 2003. “The intent is to build the pavement thick, with an elastic layer on the bottom. That way, there is enough material to flex when traffic runs on it. If it cracks, it only cracks at the surface, so it can be repaired easily,” explains Tom Dziedziejko, general manager, AME Materials Engineering, Toronto.
“They were going pretty hard with SMA and perpetual paving,” adds Donn Bernal, general manager, Materials, Aecon Special
Materials Ltd. “They knew it was an excellent product that had been proven in Europe. Everyone was excited about it. There were a lot of trials that showed its benefits.” He adds that even though SMA is a more expensive mix, there were significant advantages in terms of life-cycle costing.
In 2006, the first full-service perpetual pavement highway using SMA as the surface-course mix in Ontario was built: the Red Hill Valley Parkway.
SMA has been used in Europe since the 1960s and has now been used in several projects in Ontario, on highways with vehicle loads of more than three million equivalent single axle loadings a year. The stone-on-stone “skeleton” structure formed by coarse aggregate in the mastic composition is the key to its durability. In addition, its high asphalt cement (AC) content improves binder durability. Bitumen drainage during transport and placement is controlled with cellulose or mineral fibres that are also used in this mix.
Precise compaction is an important concern with SMA; the mix cools rather quickly, and drum rollers must be used to prevent working the binder material to the surface and ruts.
However, of greater concern than compaction was skid resistance. Questions about SMA’s capabilities in this regard have been asked internationally.
“The issue is that there was quite a bit of AC coating the aggregate,” says Bernal. That tended to reduce friction noticeably, compared to conventional asphalt. The effect only lasts for a couple of weeks or so after exposure to traffic.
In the MTO’s eyes, though, that was bad enough. The use of SMA in Ontario was officially paused. A joint MTO/Industry Task Group on SMA mix design, construction and treatment was established to identify a solution.
“As much as the industry might have liked to see ‘reduced speed’ signs go up for a couple of weeks, you have to hand it to the Ministry for their concern with public safety,” says Dziedziejko. “I was on the committee that was brought together to look at this issue. We tried a number of different approaches, including changing the mixes, and published a paper on what we found. In the end, the only real way of doing it was to try gritting—rolling sand into the surface.”
Rubber hits road
The rubber hit the road with Ontario tender 2007-2026, covering grading, drainage, granular base, hot mix paving, illumination, traffic signals, structures and an Advanced Traffic Management System on the Queen Elizabeth Way (QEW) from Burloak Dr. to Third Line. The total value was $88.7 million, according to the tender.
“The section we were working on was to be widened significantly,” notes Dziedziejko. It was a four-year job, with the surfacing completed in 2011. “The SMA was bid into the job, so they couldn’t really tell us not to use it despite the official pause. But this offered the opportunity for the gritting trial, to demonstrate that it would work.”
The MTO put out a non-standard special provision for safe construction with a specification for gritting of stone mastic asphalt, according to Bernal. This was going to be a pioneering project; it had never been tried before in Ontario, he adds.
The first challenge was finding a suitable grit-spreading machine. Dynapac, which had experience with the application in Europe, brought in a special roller for the job.
“In Europe, this kind of gritting is done pretty regularly. Their roads are a little different, and their surface course asphalts use normal aggregates that do not necessarily have a high skid resistance. They incorporate the grit to raise the skid resistance,” says Dziedziejko.
The machine was a fairly typical 55-in. roller with a bin on the front of it, according to Bernal. The bin contains a special grade of sand, which is spread out by rotating brushes at a controlled rate through an adjustable opening in the bottom.
Going to trial
“The machine just kind of showed up here, and we had to figure out how it worked and how to configure it,” says Bernal. “The first thing was figuring out what the spread rate was… We did some trials at the plant with regular sand to see what speeds and settings we could use.”
The operation itself was pretty conventional in concept, according to Dziedziejko. They would place the asphalt as normal, and right behind the paver came the grit spreader. “It spread sand on top of the SMA while it was still hot and rolled it in at the same time. It was part of the compaction process.”
The first step was trials. “We went out on the North Service Rd. and did trial strips on some SMA there,” says Bernal.
Some experimentation was necessary. “There were two different ways we did it,” says Dziedziejko. “We did it with an uncoated grit, which was just the dried sand.” The second was with a coated material, made by adding one per cent AC to the sand. After a little fine-tuning, “That’s what seemed to work the best.”
Without the AC, the sand tended to cling to the roller. “You had a problem with buildup on the rollers and the odd imperfection on the road.” Once the appropriate application of the AC was determined, “It didn’t stick to the rollers and it went down smoothly and uniformly. It went down very, very well,” says Dziedziejko.
The first trials were conducted with one kilogram of sand per square metre of asphalt. “We had to demonstrate to MTO that we could get the one kilogram per square metre” says Dziedziejko. In the final work on the QEW, though, they tried two sections at 0.7kg/m2 and 0.76 kg/m2.
The grit itself took a little pre-treatment. “The Ministry asked us to recommend a few different grits, or sands,” says Dziedziejko. He wound up submitting four different products to them. “They wanted something that was reasonably clean and hard. They settled on a couple of choices. One was a clean, concrete-type sand from Caledon. The other was a manufactured sand from our Marmora quarry.” Ultimately, the material from Caledon was selected.
The SMA mix itself was 79 per cent coarse aggregate, says Dziedziejko. It was designed with 5.4 per cent AC. He notes that 75 per cent of the material was retained on the quarter-inch sieve. The mix incorporated cellulose fibres at 0.3 per cent. The asphalt cement itself was a PG 70-28. The aggregates used were trap rock out of Aecon’s Marmora quarry. “It used the stone and a wash-blend sand, and then there was a filler from E.C. King. Maximum size on the material was 16 millimetres, so everything was smaller than 5/8 of an inch and 95 per cent of it was smaller than a half-inch,” he says. “The mix had good properties all the way t
hrough,” he adds. It was mixed in a drum mix plant in Brampton and hauled from there.
Skid testing obviously had to be a key part of the trials. “We had to determine the skid resistance, before and after, and document the improvement so they brought out a skid tester,” says Bernal.
The gritting “did amazing things,” says Dziedziejko. “The initial numbers on the trial section were significantly higher than they were without it—way above the minimum.”
“The MTO checked the skid resistance, and at our last committee meeting everyone was happy with the skid resistance,” says Dziedziejko. “They plan to recommend to the MTO that they lift the pause on SMA as long as gritting is part of the job.”
“One of the things that surprised us with the gritting operation was just how much of the grit actually wound up getting embedded,” says Dziedziejko. “We were expecting very dusty conditions.”
Even though the uncoated grit was a little dustier, it still was not that significant, notes Dziedziejko. “The coated grit was almost nothing. We were expecting to put down this one kilogram per square metre, and then have to haul three quarters of it away. We found that 75 or 80 per cent of the material actually got embedded and there was hardly any dust.”
Another surprise was the appearance of the pavement after gritting. “The texture of the road after you are finished is not the nice, shiny black surface you’d expect,” says Dziedziejko. “The sand grit gives it what you would think of as a worn appearance, lighter in colour. It’s nice to have a smooth road to drive on—but you want to be able to stop!”
The MTO was satisfied with the results and has allowed SMA back into the specifications with the added gritting specification, notes Bernal. This procedure will probably become the standard in Ontario.
Bernal predicts that the grit spreader they used is just the first of many to be seen in the province. Dynapac already has plans to introduce a model with a 66-in. roller. Other manufacturers can be expected to jump on this bandwagon, too.
The skid resistance committee is going to go back to the different regions and suggest that they give SMA another chance, with the addition of the gritting operation. “With some luck, we will have smoother, safer more durable roads,” says Dziedziejko.
The only real surprise with the job? “It was easy,” says Bernal, once they identified the best parameters for operating the machine.
Jim Barnes is On-Site’s contributing editor. Send comments to firstname.lastname@example.org
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