Evaluating Existing Pavement Structures for Thin Concrete Overlays

The evaluation of the existing pavement is the first step in determining the appropriate rehabilitation alternative. The evaluation seeks to identify and characterize the existing pavement in terms of distresses (e.g., cracking, rutting), structural condition (i.e., ability to carry load), functional performance (e.g., roughness, noise), and material-related issues (e.g. D-cracking). An understanding of these characteristics will help guide the selection of the appropriate type and thickness of the overlay.

There are a number of resources available that provide detailed procedures to evaluate a pavement prior to placing an overlay (e.g., Harrington and Fick 2014). The evaluation process varies by project but ultimately results in a general condition assessment profile, as illustrated in figure 2. The application of this procedure will result in a full project evaluation that can be used to determine the type and thickness of overlay.

The majority of overlays placed in the field are unbonded overlays. This is due primarily to their ability to accommodate high traffic volumes and most any type of distress in the existing pavement. Thus, in figure 2, the first and third portions of the project describe two candidates for unbonded overlays of different thicknesses. The first portion, given the severity of cracking and high traffic, may be more suited for a conventional unbonded overlay. The third portion may be a candidate for a thin unbonded overlay, given the low traffic volumes and the assumption that the existing pavement is suitable.

In general, when existing concrete pavements present any level of material durability issues, unbonded overlays should be selected. Likewise, in existing asphalt pavements with either higher levels of cracking or rutting that are not restricted to the asphalt layers, unbonded overlays are more appropriate.

However, if an existing pavement is in good structural condition (i.e., it requires minimal pre-overlay repair), one may consider the use of bonded overlays. Therefore, the middle portion of the example project in figure 2 may be a candidate for a bonded overlay, as this portion has little cracking, is exposed to lower traffic volumes, and exhibits fair drainage, which in combination with adequate base layer construction should provide uniform support for the existing slab and bonded overlay.