Pre-Cast Concrete Slab Repair Technology in Canada

Pre-cast Repairs show Potential

The Ontario Ministry of Transportation (MTO) is continually investigating and evaluating emerging concrete technologies in an effort to improve the efficiency of provincial highway repairs. In November 2004, MTO conducted a trial project to evaluate construction techniques for pre-cast concrete pavement slab repairs. The trial, carried out on the heavily trafficked Highway 427, constitutes the first use of this repair technology in Canada.

Highway 427 is a major 12-lane freeway characterized by considerable pavement deficiencies. The existing pavement structure consists of approximately 230 mm of jointed plain concrete pavement over 150 mm of cement treated base. Major distresses included: severe to very severe joint-stepping, severe joint failures, severe joint and crack spalling, severe to very severe cracking, and severe to very severe joint sealant loss. Since its construction, the highway has received only reactive (unscheduled) maintenance activities.

Since Highway 427 is a major commuter route through Toronto, repairs could only be conducted in a narrow overnight timeframe, typically using fast-track concrete repairs or emergency hot mix patches. Fast-track repairs are not ideal, as they are problematic to construct, have a restricted construction timeframe (from May 1 to October 15), require the use of autogenous cylinders, and they may not provide the desired life expectancy of 15 years.

In its search for an alternative method for long-term and emergency repairs, the ministry decided to test and evaluate an innovative technique - pre-cast concrete slab repairs. MTO perceived many advantages to the use of pre-cast repairs, including higher concrete quality, ideal curing conditions, minimal weather restrictions on placement, and a reduced waiting-time for concrete to cure prior to reopening the highway.

The pre-cast slabs were manufactured off-site using three methods: the Fort Miller Super-Slab TM Intermittent Method, the Fort Miller Super-Slab TM Continuous Method (both originating from the Fort Miller Co., New York), and the Michigan Method (originating from the Michigan Department of Transportation). All three methods involve the design and fabrication of pre-cast concrete slabs to replace sections of deteriorated concrete pavement. The methods differ in how the base is prepared and how the pre-cast slabs are installed and dowelled to the adjacent concrete slabs. Three full-depth pre-cast slabs, each measuring 2 m x 3.65 m x 230 mm, were constructed and placed using the Fort Miller Intermittent and Michigan Methods, while a 25 m trial section was used for the Fort Miller Continuous Method.

Non-destructive testing using a Falling Weight Deflectometer (FWD) was conducted after construction to assess load transfer efficiency (LTE), and to detect loss of support underneath the pre-cast slabs. Twelve FWD measurements were taken for each slab. Pre-cast slabs are rejected if FWD results measure less than 70% LTE. Based on the average FWD calculation, each method provided suitable results above this minimum requirement. FWD testing will be carried out annually at the pre-cast trial locations to monitor their performance over time.

Overall, the pre-cast trials produced positive results. Workmanship was a concern; however, this work was undertaken by a contractor carrying out pre-cast slab repairs for the first time under difficult conditions. The condition of the precast slab repairs was evaluated after six months of service - the slabs did not contain any cracking and did not rock. With the exception of slot failure within the Michigan Method, the precast slab repairs are performing well under daily traffic conditions.

The pre-cast repairs are similar in both ride and appearance to fast-track repairs along the same section of highway. MTO will continue to monitor the field performance of these innovative pre-cast technologies, and will assess the cost effectiveness of this alternative to fast-track concrete repairs. Stay tuned to future issues of Road Talk for updates and results.