Case Study 1: Thin Unbonded Overlay in the City of Toronto

Introduction and Design

In 2003, the City of Toronto rehabilitated a composite pavement at an intersection that received a high volume of bus traffic (Kivi et al. 2013). This rehabilitation involved the use of a 6-inch (152 mm) unbonded concrete overlay and a 1-inch (25 mm) asphalt interlayer atop the existing 8-inch (203 mm) concrete pavement. The finished overlay used a panel size of 5 ft by 5 ft (1.5 m by 1.5 m), and dowel bars were used only at turning or stopping locations to provide structural support for static or slow- moving vehicle loading. The design was intended to accommodate over 30,000 vehicles per day (although in the first 10 years of service, the section experienced an estimated 3.5 million ESALs due to transit buses alone). The rehabilitated intersection comprised nearly 280 ft (85m) of street pavement.

The project received extensive pre-overlay preparation and repair. The existing composite pavement was milled to remove asphalt, and cracks in the existing concrete slab were routed and sealed. The overlay was placed using formwork.

Section Performance

The section was monitored by researchers from the University of Waterloo for 10 years after overlay construction. In addition to regular observation, the monitoring by researchers included the use of embedded strain gauges to measure slab response to loading. The general performance of the section was very good, and very little distresses were observed over the first 10 years of service. No additional maintenance or rehabilitation was performed over that time.

In addition, the use of strain gauges led to a surprising finding: the lack of tensile strains at the bottom of the overlay suggested the presence of bond in what was intended to be an unbonded design. However, as evidenced by the performance, the bond formed did not lead to the propagation of distress from the existing pavement into the overlay.

Conclusions

The adoption of this thin overlay in an urban setting was considered a major success by the City of Toronto, which had been accustomed to experiencing recurring rutting and shoving in the asphalt surface course due to bus traffic. The City and researchers maintained that the thin overlay would meet and exceed its 25-year service life target (Kivi et al. 2013).