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Campus Station, situated as it is just beside the University of Ottawa, is not surprisingly one of the busiest stops on Ottawa’s Transitway, a 31-kilometre 4-lane road reserved exclusively for buses. Buses are notoriously rough on pavements and Campus Station has to deal with up to 2,400 buses coming and going each day. It is a load that would test any pavement and after twelve years of pounding, Campus Station was getting a failing grade. The buses had worn deep ruts in the asphalt. City engineers resurfaced the pavement but two years later the ruts were back again. In March 2000, the engineers decided to try another approach – ultra-thin whitetopping.

Ultra-Thin WhitetoppingUsing concrete over asphalt is not a new technique. The use of whitetopping, as it is known, has been recorded as far back as 1918 although it was not until the 1970s that it became a more popular rehabilitation technique in the US. Simply spread over the existing asphalt pavements in relatively thick layers, these conventional whitetoppings performed surprisingly well and continued in service for up to twenty years with little or no maintenance.

The idea of whitetopping clearly intrigued engineers who decided that if a relatively simple technique such as this could work well, a bit of refinement might make it work better. The first ultra-thin whitetopping was laid in Kentucky in 1988.

There are two primary differences between conventional whitetopping and ultra-thin whitetopping. First, joints are spaced more tightly than normal and second, the concrete overlay is deliberately bonded to the underlying asphalt to create a composite pavement. Because the combination of bonding and joint spacing lowers the stresses in the concrete, engineers can reduce the thickness of the concrete overlay from the 175mm to 250mm typically used for conventional whitetopping down to thickness of between 50mm to 100mm – hence the designation ‘ultra-thin’.

UTW is designed for low-speed traffic and areas with a lot of stop-and-go traffic, such as street intersections, bus stops, or tollbooths and given its success, agencies are starting to look at UTW as an alternative to asphalt overlays.

About three hundred UTW projects were completed in the US between 1988 and 1998.

Laying a UTW SurfaceUTW works because the concrete and asphalt form a composite pavement. To get the structural integrity needed, there must be at least 75mm to 100mm of residual asphalt and there must be a good adhesion between the two layers. For a good bond between the asphalt and the concrete, the asphalt must be milled to roughen its surface and then thoroughly cleaned

ULTRA-THIN WHITETOPPING FOR ROAD REHABILITATION Measuring the depth of the ruts

Cleaning the asphalt base before applying the UTW

Applying and smoothing the UTW

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to open the “pores” of the pavement. It is extremely important that the surface stays clean and dust free until paving starts.

Concrete mixes for UTW are relatively standard. A typical mix design includes cement, coarse and fine aggregates, polypropylene fibres, air entraining agent, and admixtures (water reducers and plasticizers for a low water-cement ratio). Many UTW projects have been paved with fast track mixes that have either a higher cement content or use high early-strength cement (usually 20MPa in 24-hours).

UTW paving is straightforward using conventional slip-form and fixed-form pavers. Proper curing, however, is critical to avoid shrinkage cracking and debonding between the asphalt and pavement. Curing compound should be applied at twice the normal rate.

The final step is to saw cut the joints using lightweight saws as early as possible to control cracking. Joints are spaced at about 10 times the thickness of the topping, typically between 0.6 and 1.5 metres. The saw depth should be approximately one-fourth to one-third of the total depth of the overlay.

Capital ImprovementBy 1997, the condition of the pavement at Campus Station in Ottawa had deteriorated so badly that it could no longer be ignored. The constant stopping and starting of the buses as they entered and left the station had worn deep ruts in the pavement and in some places, the asphalt pavement had actually been shoved up over the curb.

The first attempt at rehabilitation called for the pavement to be milled and the surface replaced with 45mm of stone mastic asphalt, a premium hot mix known for its strength and durability. In two years, however, the pavement was as badly rutted as it had been before. In March 2000, transportation engineers from the City of Ottawa met with technical staff from the Cement Association of Canada to discuss the possible use of UTW.

“The depth of the ruts on the asphalt pavement was as

much as 170mm in places, far deeper than the Stone Mastic Asphalt surface course that had been laid a couple of years earlier, but there was no rutting of the granular base,” says Rico Fung, Structural Engineer with the Cement Association of Canada. “Based on the volume and loads of the buses, we recommended 100mm of UTW for the southbound lane. On the northbound lane, because of the curb height and thickness of asphalt, we were restricted to just 75mm of UTW.”

The pavement rehabilitation was scheduled to start on a Friday night and finish on Monday morning during which time the station would be closed and buses re-routed. A high early-strength concrete with a compressive strength of 20MPa within 24-hours was specified so that the station could be reopened as quickly as possible.

On June 9, 2000, the contractor started milling the asphalt pavement and had finished the job within about two hours. What the contractor was not prepared for, however, was the high degree of cleanliness needed to ensure a bond between the asphalt pavement and the concrete overlay. Eventually, a high-pressure power-washer truck was brought in to complete the job.

By Saturday morning, the contractor was ready to start pouring the concrete for the two 55 metre long, 3 metre wide bus lanes. A stiff broom finish was applied, shallow grooves 18mm apart were cut across the pavement to provide skid resistance and the surface was sealed with two coats of white-pigmented curing compound.

Joint spacing is critical to ensure that stresses in the UTW are relieved and that the concrete does not crack. For this project, the joints were spaced at 10 times the depth of the UTW (0.75 metres for the northbound lane and 1 metre for the southbound lane). Cuts were made using the “Soff-Cut” system. With Soff-Cut, used primarily to minimize random cracking in concrete, control joints can be cut within one to two hours after the finishing process and are not as deep as traditional methods.

ULTRA-THIN WHITETOPPING FOR ROAD REHABILITATION

Three days from start to completion and the buses are rolling once again!

continued on page 18

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ULTRA-THIN WHITETOPPING FOR ROAD REHABILITATION continued from page 6

By Monday morning, despite a number of heavy rain showers that had delayed construction, the station was reopened for traffic.

The UTW at Campus Station has now seen almost two years of heavy use. Steve Goodman (Pavement Engineer in the Infrastructure Services Branch of the City of Ottawa) recently joined the city and one of his first tasks was to inspect the station.

“This was my first exposure to UTW and I was quite impressed,” he says. “When you have up to 600 buses an hour rushing up to a station, applying the brakes and then taking off again, that is a punishing load for any pavement.

“The southbound lane is holding up very well although the northbound lane is showing some distress. That can probably be explained by the difference in the pavement thickness. The southbound lanes have 100mm of UTW applied over 100mm of asphalt. The northbound lanes, restricted by curb height, have 75mm of UTW over 75mm of asphalt. I am told that the design engineers were hesitant to go with the thinner pavement but we wanted to give it a try.”

There was also, Goodman noted, some cracking at the transition between the asphalt pavement of the Transitway and the UTW at the station due to the restrictions on edge thickening. Edge thickening, normally done with UTW, had to be curtailed to maintain a minimum thickness of 75mm of asphalt. All things considered, however, the distress is relatively minor, Goodman says.

“Considering that at the busiest stations the asphalt pavement was rutting within two years, the UTW pavement looks very clean. And there’s no rutting. If the costs are in-line, it seems a good solution.”

F o r f u r t h e r i n f o r m a t i o n o n t h e s e i s s u e s c h e c k o u t t h e O G R A w e b s i t e a t w w w. o g r a . o r g

Here are just some of the current “hot

topics” that are on OGRA’s agenda:

Electrical SafetyAs a result of deregulation, municipalities must manage the risk to public safety associated with electrical installations for street lighting and traffic signals. Since the Electrical Safety Authority (ESA) is now the sole authority for permitting and inspecting new installations and maintenance, municipalities need to be familiar with ESA’s requirements and the fiscal impact that these requirements will have in 2003. OGRA will be asking the Ministry of Environment and Energy to consider more cost-effective ways to deliver these services. For more information, visit the ESA website at www.esainspection.net.

Smart GrowthThe Central Ontario Smart Growth Panel’s Subpanel on Gridlock will be submitting preliminary recommendations to the Minister of Municipal Affairs and Housing by the end of June. For more information, visit the MMAH website at www.mah.gov.on.ca.

Infrastructure FundingMTO held a series of consultation workshops to identify Smart Growth transportation issues in January and February. Once OGRA has received the summary report of the workshops, it will request a meeting with the new Minister of Transportation, Hon. Norm Sterling, to discuss OGRA’s position on municipal infrastructure funding.

Performance Measurement DataThe Ontario Municipal Roads 2001 Questionnaire gathers condition information about municipal roads and cost to deliver service data that is used by Ontario Municipal CAO’s Benchmarking Initiative and is compatible with MMAH’s MPMP program. The questionnaire is now in circulation to all municipalities and is available on the OGRA website at www.ogra.org.

ON YOUR BEHALF