Have it repaired

News Roads&Bridges December 08, 2003
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Technical experts at the American Concrete Pavement Association (ACPA) and its affiliated chapter/states address thousands of t

Technical experts at the American Concrete Pavement Association (ACPA) and its affiliated chapter/states address thousands of technical issues every year. In response to the many technical questions, we often hear misperceptions about concrete pavements. Some common misperceptions are that concrete costs too much, takes too long or is difficult to repair.

Concrete pavements are easy and inexpensive to repair and restore with proper equipment, materials and procedure. And, when concrete pavement restoration (CPR) methods are no longer viable, there are concrete overlays, which are a cost-effective means of adding structural capacity and returning pavements to a smoother, safer condition. Concrete overlays include bonded, unbonded, whitetopping and ultra-thin whitetopping (UTW).

Unbonded Overlays

The term 'unbonded' is used to describe a concrete pavement placed on top of another with a separation layer that allows them to work independently. The separation layer's primary function is to isolate the movement between the existing concrete pavement and the new concrete overlay. A properly designed and installed separation layer also provides some other key benefits to the overall pavement design.

Using an asphalt separation layer between the two concrete pavements eliminates any reflective cracking from the underlying concrete pavement and ensures no 'keying' action of the joints. Also, offsetting the joint layout by approximately 3 ft improves load transfer and reduces the chance of roughness due to pumping.

The required concrete overlay thickness will depend on several factors, but experience has shown that an unbonded concrete overlay between 4 and 8 in. of plain, unreinforced pavement will perform well for most urban applications.

Using this strategy not only extends the life of the concrete pavement, but also avoids frequent overlays and repairs required when asphalt surfaces are used. Prior to placement of both the separation layer and the new concrete overlay, some minor pre-placement steps or repairs may need to be completed to ensure the overlay has uniform support.

Because the existing concrete pavement's primary function is to act as the base for the new pavement, it must be treated as such to ensure it provides proper support to the overlay, including consideration of some repair.

For more on the story, read the January issue of ROADS & BRIDGES magazine.

Overlay Init