Effectiveness and Limitations

Thin bonded concrete overlays are a proven, cost- effective rehabilitation solution for existing asphalt pavement that are in good structural condition, including asphalt pavements with severe rutting (Han 2005). They are especially effective for cases where the existing asphalt pavement is subjected to heavy traffic and/or braking areas where traffic comes to stop.

For existing concrete pavements, thin concrete overlays can be effective in the following cases:

• If the design period is relatively short (10-15 years), a thin unbonded overlay is a cost-effective alternative to an asphalt overlay.
• For low-to-medium levels of traffic, a thin unbonded overlay is a cost-effective alternative to conventional unbonded overlays.
  
• If grade limitations do not allow for a conventional unbonded overlay, then a thin unbonded overlay may be a cost-effective alternative to reconstruction.
• For rehabilitation cases when an increase in heavy traffic is expected, thin bonded overlays can be effective if the structural capacity is accounted for in the design. However, the long-term performance of a thin bonded overlay relies heavily on the overlay bond with an existing concrete pavement. Thus, while thin bonded overlays can succeed, their demands in both design and construction make them a high-risk/high- reward rehabilitation solution.
  
  

The implementation and successes of thin concrete overlays have grown since their emergence in the late 1990s. However, some of those early overlays did show some premature failures—particularly in the application of ultra-thin overlays—due in part to construction issues but also to the design thickness and inappropriate use. For instance, some industry recommendations point to the use of thin bonded overlays in rehabilitating existing asphalt that has experienced thermal cracking. This should be considered carefully given that the presence of full bond may lead to considerable reflective cracking if the extent of thermal cracking has been even marginally underestimated. Figure 3 illustrates corner breaks in a thin unbonded concrete overlay, which may be due to compromised support conditions due to neighboring asphalt thermal cracking and moisture penetration. Likewise, bonded overlays may be even more sensitive to thermal cracking in the existing pavement (or shoulder).

In general, the pavement evaluation and selection process is most effective when it includes both agency and contractor experience. Each agency may have its own evaluation procedure, and the selection process should consider agency experience with overlays and agency design practices. Furthermore, local contractor experience with thin overlay construction should be taken into account.