February 26, 2010

MTO

First the section is cut out with slots for the reinforcing steel to tie it together.

FEATURE | Roadbuilding

Ontario Ministry of Transportation getting concrete results from pre-cast tests

Slabs go down faster with less traffic tie up

PETER KENTER

correspondent

Typically, road crews have a limited range of options to repair concrete roads on busy highways. Conventional concrete takes time to set, impeding traffic. Repairs completed with hot mix asphalt and fast track concrete can get traffic moving quickly but sacrifice durability.

A series of tests begun by the Ontario Ministry of Transportation (MTO) in 2004 took a novel approach to concrete pavement repair. Rather than repair the road surface, why not cut out the damaged section and replace it with a pre-cast concrete slab?

“We were the first agency in Canada to try this method, and the second in North America,” says Becca Lane, Head, Pavements and Foundations Section, Materials Engineering and Research Office, MTO. “With pre-cast concrete you can cure the concrete at a conventional rate in the factory under optimum conditions.”

The slabs, measuring two metres long by a lane-width wide, are lifted by crane and married to existing pavement using steel dowel bars and slots cut or cast into the concrete.

MTO initially placed three trials of precast concrete slabs end-to-end on Toronto’s Highway 427, to replace damaged pavement. The project was designed not only to test the longevity of the slabs, but also to find the best way to prepare the roadbed and anchor the slabs to the existing pavement.

The slabs were installed using three methods:

• The Michigan Method, which involves placing a cementitious flowable material on the roadbed prior to slab placement. Three steel dowels are built into the precast slab, and matching dowel slots are cut into the adjacent concrete to accommodate the dowels. The remaining voids are then filled with grout.

•The Fort Miller Super-Slab Intermittent Method, which involves placing compacted screenings on the roadbed. Pre-cast slabs are designed with slots underneath to sit over dowel bars inserted into the adjacent pavement. Grout is then introduced through precast ports to fill the areas around the dowels and underneath the slab.

•The Fort Miller Continuous Super-Slab Method, which involves the use of several slabs featuring alternating slots and dowels, which are then grouted.

MTO

The pre-cast slab, which is made off site under controlled conditions, is lowered into place.

Dufferin Construction was the successful bidder and awarded the contract to install the trial slabs. The contractor also prepared the precast slabs cast for the non-proprietary Michigan Method, while the Fort Miller slabs were trucked into the province by the patent holder.

“I have to credit them,” says Lane. “The technique was brand new to them and involved a very steep learning curve. They had their entire crew on hand to both observe and to pitch in and help.”

All three panels were assessed annually with a final report released in October, 2009. While the panels performed admirably, the slabs installed using the Michigan Method were replaced during the trial because of damage to the adjacent pavement caused by overcutting of dowel slots.

“The first time the dowel slots didn’t line up very well with the dowel bars,” says Lane. “Part of the reason was that we were in a hurry. Everyone was concerned about the work window, which was from 11:00 p.m. to 5 a.m. As it turned out we had plenty of time because it only takes about five minutes to set the slab. In the areas where the dowel slots were cut too energetically, or the slots not properly aligned, some of the slots failed to perform well. When the slots aren’t properly aligned, the dowels put pressure on the adjacent slab.”

The installation method was later fine-tuned by using a template to specify the locations of the slots and by trading in the jackhammer for a more accurate cut using a gang-saw and lightweight chipping hammers.

“We also learned that we needed to be precise in the way that we saw-cut the damaged pavement sections so that we could achieve a more snug fit,” says Lane. “If the edges of existing pavement are damaged, the precast slab would be surrounded by damaged concrete.”

Precast slabs have been used on two additional Highway 427 contracts, one in 2008 by Dufferin and another in 2009 by Brennan Paving & Construction. MTO is also considering slab repair on sections of the 401 near Yonge Street and Bayview Avenue where the road consists of a concrete base with asphalt overlay.

“The great thing about concrete is that it lasts a very long time,” says Lane. “The trials we conducted have given us a much larger range of options to repair those roads quickly and effectively.”

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