April 10, 2012
Precast concrete patches make the grade on Ontario highways
First trial for precast concrete patches on Ontario highways was in 2004 on Highway 427
It’s been several years since precast concrete patches were first trialed on Ontario highways as a fast track repair and the data so far shows they’re standing up to the test of traffic and time.
Tom Kazmierowski, manager, materials engineering and research office at the Ministry of Transportation Ontario, says the concept is getting attention, especially in Ontario and Quebec, with Manitoba showing interest and, judging by reaction to a recent presentation at the annual conference of the Transportation Association of Canada, the rest of Canada.
The one exception may be Alberta where concrete roads are a rarity given the abundance of oil and asphalt.
Precast patching is done in one of two main techniques, he said, and was first trialed in 2004 as a demonstration project on Highway 427. Other methods are also in early stage development.
The location was ideal. It is 12 lanes, originally built from 1968-71 with 230 mm of jointed plain concrete pavement (JPCP) over 150 mm of cement-treated base (CTB). Skewed transverse joints, spaced at intervals of 3.7 m, 4.0 m, 5.5 m and 5.8 m, are dowelled with 32 mm dowel bars spaced every 300 mm across the joint.
The pavement selected for repair had not been rehabilitated since original construction and scored 46 to 58 out of 100 on the pavement condition index, meaning it was in fair to poor condition with severe crack spalling and joint failure.
The project used three precast concrete pavement, full-depth repair methods: the so-called Fort Miller super-slab intermittent method, the Fort Miller super-slab continuous method, which are variations on a theme, and the Michigan method.
Each method requires designing and fabricating precast concrete pavement slabs to replace deteriorated concrete pavement.
The advantage is that the repairs can be carried out quickly with minimal lane closure and traffic disruption.
The need for a workable alternative repair for concrete roadways is self-apparent: fast-track concrete mixes, which reach 20 MPa in four hours using accelerating admixtures, are temperamental and have dubious long-term durability, Kazmierowski said.
Each precast method differs in how the base is prepared and how the precast slab is installed and dowelled into the existing concrete.
The trial was a learning experience, Kazmierowski said, and was the first time the technology had been used in Canada.
The only real issues which seem to have emerged from the initial application were likely related to the techniques required to cut and place the slabs, he said. Field inspection revealed the experiments were for the most part doing well.
By 2007 it was clear that there was enough promise in the precast methods to pursue them further with the thinking being that the issues of cracking and degradation could be easily resolved with better preparation and understanding of the technology.
In follow-up inspections, some of the Michigan method slabs were showing severe cracking around the dowel slots by 2006.
The MTO traced this to jack hammering the slots on the existing pavement where it would receive the dowels instead of hand cutting and not preparing the base materials adequately.
On a more positive note, one of the slabs, however, showed no deterioration after five years.
The Fort Miller intermittent method slabs are performing well with some minor cracking around the dowel areas. The Fort Miller continuous method slabs are also performing well, he said, with only hairline cracks around the dowel bars and slight traverse cracking in 2009.
With this experience under their belt and a specification in hand, two more precast contracts were let in 2008 and 2009 on Highway 427 using the Fort Miller intermittent and Fort Miller continuous methods.
For those installations precision grading equipment was used to grade crushed screenings prior to setting the slabs.
“We’ve also carried out a variation on existing precast slab technology that uses the same dowel bar retro-fit techniques on Hwy 417 east of Ottawa to tie the new precast slabs into the adjacent slabs,” he said, and there are two jobs carried over from last year to this year, Hwy 427 and the other on Hwy 401 in Toronto.
The critical element is in the workmanship. Crews now also use a template to ensure precision cutting.
“There’s a little more science involved now,” said Kazmierowski.
“It’s a fascinating production line to watch it in action. Rather than people standing still and the product coming in, trucks come in and then move to the next location, leapfrogging.”
One crew cuts the hole, the next moves in to prepare it, then the next moves with the slab, followed by the grouting crew, each moving up the line to the next patch.
The advantage over poured concrete is absolute and the durability tests show there’s upside to the technology. It’s really catching on in the U.S. also, he added.
“Caltrans (California Department of Transportation) is awarding a job with 4,000 panel repairs,” he said.
The U.S. Federal Highway Administration has also been running precast, pre-stressed concrete pavement projects in several states over the past few years. It was expected to release a major report in March which may touch on precast panel repairs.
The strategic highway research program addresses several strategic focus areas: the role of human behavior in highway safety, reliability and capacity renewal. The latter part is anticipated to include a review of panel installation and its benefits in relation to the other three areas.
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