Interest in concrete overlays, as well as the use of these versatile pavement solutions, has grown significantly in the past decade. More recently, the enactment of the ARRA has created an imperative for roadbuilding and rehabilitation solutions that can be used for shovel-ready projects. Concrete overlays fit the bill for many of those projects, but even before the economic stimulus legislation, many agencies were turning to concrete overlays as budgets declined, traffic volumes and loads increased, and the focus on sustainability sharpened.
The bottom line is state, municipal, and county agencies are expected to do more with less, and to that end, concrete overlays provide a viable means of extending the life of distressed concrete, asphalt, or composite pavements.
Concrete overlays also are attracting the attention of agencies that have seen pricing volatility of asphalt, as well as those who recognize the inherent durability, versatility, and environmental benefits of concrete. With those considerations in mind, this article will provide an overview of concrete overlay types, as well as key considerations in selecting an appropriate overlay type.
Concrete overlay basics
An important step in selecting a concrete overlay is understanding the basic systems, which include bonded and unbonded overlays. Detailed information about these systems can be found in the “Guide to Concrete Overlays: Sustainable Solutions for Resurfacing and Rehabilitating Existing Pavements,” (the Guide) which is the source of much of the information in this article. Authored by Dale Harrington, P.E., principal senior engineer of Snyder and Associates, the Guide was the result of a collaborative effort by a number of organizations and people from the industry, public sector, and academia. The Guide describes the two basic overlay systems.
Bonded overlay systems. In general terms, bonded overlays are used for resurfacing, minor rehabilitation, or increasing structural capacity. To be a candidate for a bonded concrete overlay, the underlying concrete, asphalt, or composite pavement must be in fair to good condition. The bonding of the overlay to the existing substrate is imperative, as the objective is to create a monolithic structure. As such, the surface preparation must be performed correctly prior to resurfacing. Another hallmark of bonded overlays is that they are typically thin layers, usually ranging from 2 to 5 inches, although they can be thicker.
Unbonded overlay systems. Generally speaking, unbonded overlays are used for minor to major rehabilitation. The underlying concrete, asphalt, or composite pavement can be in poor to fair condition, but if the deterioration is too significant or if the pavement has failed, then full reconstruction may be the only viable option. Still, for pavements that have some structural deterioration, unbonded overlays can be viewed as a new pavement over an existing, stable platform, which becomes the base. Unbonded overlays are typically 6 to 11 inches thick.
Figure 2 illustrates how concrete overlays can be used for pavement maintenance and rehabilitation. The expected service life of an overlay will depend on a number of factors, including the overlay structural design; compatibility with expected traffic and other site conditions; and good construction practices. As a general rule, the service life for overlays 2 to 6 inches thick will be 15 to 25 years; for overlays 6 inches or greater, the service life will be 20 to 30 years, or even longer.
A detailed evaluation is necessary to establish the best overlay strategy. The ultimate goal should be to assess the stability and structural capacity of the existing pavement. The survey information also is very important because it will determine whether pre-overlay repairs are needed, and if so, to what extent.
Visual and automated distress surveys should be conducted to determine the overall condition of the roadway including distress types, severity and extent, drainage conditions, grade control issues, and more. On high-volume roads, a falling weight deflectometer (FWD) can be used to estimate the k-value of the existing pavement structure, variability of the support, concrete modulus, load transfer efficiency, the presence of voids, asphalt stability, and other important variables. In addition, coring and materials sampling/testing are desirable for the majority of projects.
With bonded overlays, some repair of the underlying substrate may be necessary to correct identified deficiencies, such as potholes or fatigue cracking in asphalt pavements, which compromise the structural integrity of any overlay. These repairs can be quick, inexpensive, and easy to perform. Typically, a lower grade concrete or flowable fill can be used as a full-depth patching material.