Evaluation of Bonded Concrete Overlays on Asphalt Pavements (2022)

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3   The use of thin (3- to 7-in.) bonded concrete overlays on asphalt (BCOAs) as a rehabilitation treatment first gained momentum in the 1990s (Mack, Cole, and Mohsen 1993; ACPA 1998). Since the first documented thin BCOA application in the United States, in Louisville, Kentucky, in 1991, BCOAs have seen a dramatic increase in popularity. Early adopters, including the Illinois, Iowa, and Minnesota state highway agencies (SHAs), continue to use BCOA technology. Other SHAs, including those in Colorado and Kansas, are more recent implementers, while many other SHAs continue to explore and experiment with the technology. The attractiveness of BCOA technology is based on multiple factors. As a hybrid system, the concrete surface largely relies on the underlying asphalt pavement to provide structural support, while the stiffness of the concrete slab reduces stresses in the underlying layers, adding stability. This makes BCOAs an excellent choice for asphalt pavements exhibiting rutting and shoving from heavy and repeated truck loading. BCOAs are also easy to construct compared with other overlay options: preparation of the existing asphalt pavement can be as simple as sweeping it clean, although localized structural repairs and cold milling are occasionally necessary to cor- rect substantial surface distortions and help provide a good bond (Harrington and Fick 2014). As a result of the reduced concrete section and expedited construction, BCOAs can be a cost- competitive rehabilitation alternative. BCOA performance is strongly linked to the composite behavior developed and maintained with the underlying asphalt pavement and the joint load transfer between slabs. The composite behavior of a BCOA is believed to rely on how well it bonds at the time of construction and remains bonded to the underlying asphalt over the BCOA service life. The time-dependent nature of this bond is not well understood, and the bond has often been observed to degrade over time, especially near joints. Further, concrete–asphalt bonding has recently been shown to be much more related to the asphalt (e.g., condition, properties) than to the concrete (Mateos et al. 2017). The complex nature of concrete–asphalt bonding has typically been oversimplified in BCOA studies and models. A representative example comes from the accelerated load testing of BCOA sections during a Caltrans research project (Harvey et al. 2017). Cracking occurred only from a combination of concrete–asphalt debonding, asphalt moisture damage, and slab warping caused by drying shrinkage. The loss of joint load transfer efficiency (LTE) under traffic was also verified in several sections. However, concrete–asphalt debonding, asphalt moisture damage, slab warping caused by drying shrinkage, and loss of LTE are not directly considered in current BCOA design methods. These findings illustrate how important the structure and condition of the existing asphalt pavement are for subsequent performance. The structure is often known or can be reasonably characterized through preoverlay evaluation. One caution—a minimum asphalt thickness of 3 in., in fair to good condition, has to be present, even after milling. This is discussed in BCOA C H A P T E R 1 Background

4 Evaluation of Bonded Concrete Overlays on Asphalt Pavements guidance, as is the need to repair structural failures such as potholes or medium- to high-severity fatigue cracking (Harrington and Fick 2014; Taylor et al. 2017). Even with the demonstrated advantages of BCOAs, several concerns remain, such as con- crete mixture proportioning (including the use of synthetic macrofibers), effective construc- tion practices, appropriate evaluation tools, effective maintenance treatments and rehabilitation strategies, and a deeper understanding of how BCOAs perform (Mateos et al. 2015). Designers and specification writers need to consider all these factors when using BCOA design methods to determine slab thickness and slab size. Some methods even create mixture considerations, including whether synthetic macrofibers are to be used and in what quantity. Many BCOA design methods have been developed and continue to evolve. These generally predict good long-term BCOA performance, yet their predictive models are based on only a few BCOA projects, largely concentrated in wet-freeze zones of the upper Midwest. As a result, a regional bias may exist, impeding continued and more widespread implementation of this technology. The net result is that the design, material selection, and construction of BCOAs depend heavily on the experience of the designer, specification writer, and paving contractors. In regions where BCOA implementation is advanced, this experience reduces risk to an acceptable level and helps ensure good performance. However, in regions where this experience is lacking, the increased uncertainty results in a reluctance among highway agencies to implement the technology. The problem is equally acute as it pertains to maintenance, repair, and rehabilitation of BCOAs. Little evidence is available on what types of distress might trigger maintenance on a BCOA and what this maintenance would entail. Often, BCOAs are treated as conventional jointed con- crete pavements, yet the design and performance of these two pavement types are considerably different. As this background demonstrates, although great advances have been made in BCOA tech- nology, many unknowns remain. The results of this study address many of these unknowns by • Documenting BCOA practices through a literature review and survey; • Conducting field surveys of selected in-service BCOA projects; • Comparing actual versus predicted BCOA performance using current design methods; and • Compiling best practices for design, construction, performance, maintenance, and rehabilitation. The results of this study indicate BCOAs are a viable rehabilitation option and support wider adoption of this technique.