Concrete overlay

UNDER CONSTRUCTION

Concrete overlays are a sustainable strategy to preserve and extend the service life of a pavement. Concrete overlays can be used for preventive maintenance as well as minor and major rehabilitation for asphalt, composite, and concrete pavements. They can be designed for short-term (5-15 years) as well as long-term (20-35 years) applications. There are two main types of concrete overlay: bonded and unbonded.

History

Concrete has been used to resurface pavements dating back to 1901. They have undergone an impressive growth, which is evident in the number of documented resurfaced highways in service in the last three decades. Between 1980 and 2010, five times as many concrete overlay projects were constructed per decade as were constructed in the previous six decades. The growth is indicative that concrete overlays are a sustainable preservation or rehabilitation alternative. The National Concrete Overlay Explorer is a database that provides historical information on concrete overlays in 46 states with some dating back to before 1905.

Improvements in Concrete Resurfacing Technology

Many of the changes and improvements in the technology foreseen in the earlier reviews have been realized as the highway community has cooperated in working for better design procedures, construction guidelines, and specifications for all types of concrete overlays. Among the major advances has been the better definition of how the existing pavement should be evaluated and prepared for a concrete overlay. Another has to do with improved methods of placement of concrete overlays, improved design methodologies, and synthetic fiber technology. Fiber-reinforced concrete resurfacings have been on the increase because they contribute to the performance of thin concrete resurfacing. The improved performance comes from the increase in concrete structural integrity through improved roughness and durability of the concrete. Major research projects have been completed, providing long-term solutions of bonded and unbonded overlays of concrete and hot mix asphalt (HMA) pavements. These efforts, along with pricing factors and a national focus on training outreach and technical guidance development, have led to more acceptance and increased use of high-quality concrete overlays.

Not surprisingly, concrete resurfacings share two design requirements with on-grade concrete pavements: they require uniform support conditions and management of movement if satisfactory performance is to be realized. Nearly all the documented cases of premature overlay failure can be traced to some violation of these requirements, often a result of incorrect assessment of the existing pavement. For this reason, the evaluation of the existing pavement is paramount to determine if uniform support and movement control of the underlying pavement and interface layer exist or can be cost-effectively made to exist. If so, will a bonded concrete overlay act as a monolithic unit with the underlying pavement and provide the structural capacity, load transfer, and drainage system required to meet the design life? If not, an unbonded overlay will be necessary to meet the same criteria but with a slightly different approach toward overlay thickness, drainage, and vertical constraints.

Concrete resurfacing can be either a preservation fix or a rehabilitation fix. With a preservation fix, the resurfacing is normally completed with a bonded overlay over existing pavement that is in good or fair condition or repaired or milled to bring it to that condition. The preservation fix represents the lowest possible cost with possible small amounts of localized failures (<1 percent) within the design life. For a rehabilitation fix, the resurfacing is normally completed with an unbonded overlay over poor or deteriorated materials. To have a successful overlay, not only should the good and poor characteristics of the existing pavement be understood, but the level of expected success for dollars expended must be realistic. The initial costs can be minimized, but a shorter performance and greater need for additional maintenance in the future is normally the result, or the initial costs can be greater with greater performance and reduced follow-up maintenance.

Minor cracking and localized failures should be expected when placing a concrete overlay on existing pavements. This should not be viewed as a lack of performance, but rather as a cost-effective treatment that may require some further maintenance. In many cases, it will be more cost effective to anticipate some maintenance costs for a concrete overlay rather than go overboard on preoverlay repair costs in an attempt to prevent any cracking or localized failures.

Sustainability

Many agencies are emphasizing sustainability in their pavement management decisions. Quantifying the impact of pavement decisions on the primary sustainability factors of (1) environment, (2) society, and (3) economics is nearly impossible. We can, however, look at the sustainable benefits of concrete overlays from a qualitative perspective and conclude the following:

• Preserving the existing pavement has a minimal impact on the environment (no waste products are produced).
• User delays during construction are reduced as compared to reconstructing a pavement.
• Concrete overlays are capable of maintaining their smoothness for many years, which provides a benefit to society and the environment.
• Concrete overlays typically have a lower life-cycle cost than asphalt overlays of equivalent design life.

Concrete overlay pavement systems can be sustainable for a wide range of design life choices. Rather than removing and reconstructing the original pavement, the owner maintains and builds equity in it, realizing a return on its original investment as long as the original pavement remains part of the system.

For these and other reasons, concrete overlays are cost-effective, sustainable solutions. They provide societal benefits in the form of reliable load-carrying capacity and fewer and shorter disruptions to traffic for pavement resurfacing and rehabilitation.

Asset Management through Resurfacing Solutions

Simply put, asset management involves a strategic and systematic approach to managing pavements; it relies heavily on pavement management data and life-cycle cost analysis. Pavement management and pavement preservation activities have become extremely important in managing and accounting for investments in highway pavements.

First, a little explanation to eliminate confusion. For the last half century, “pavement rehabilitation” has been defined as a functional or structural enhancement of a pavement, which produces a substantial extension in service life, by substantially improving pavement condition and ride quality. Over the last decade, the Federal Highway Administration (FHWA) has been a strong proponent and supporter of the concept of cost effectively preserving the country’s roadway network. This has helped in the recent years to spur a nationwide movement of “pavement preservation” and “asset management” programs.

A number of definitions for the terms rehabilitation and preservation have existed for years, and these terms are constantly misused or incorrectly used interchangeably. The following descriptions are promoted by FHWA.

Pavement preservation is a strategy, a network-level, long-term program to enhance pavement performance by using an integrated, cost-effective set of practices that extends pavement life, improves safety, and meets motorist expectations without reconstruction. Pavement rehabilitation is defined as a structural or functional enhancement of a pavement that produces a substantial extension in service life. To preserve a pavement, it must be maintained and at times rehabilitated. As shown in Figure 5, pavement preservation is considered preventive maintenance plus minor rehabilitation.

What is the difference between concrete resurfacing and concrete overlays? Resurfacing is a generic term for providing a new or fresh surface on the existing pavement and is considered mainly a preservation (preventive maintenance and minor rehabilitation) strategy. Concrete resurfacing consists of both bonded and unbonded concrete overlays. It is an integral component of a comprehensive asset management approach, because it cost effectively extends pavement life and improves both functional and structural characteristics. The variety, flexibility, and cost effectiveness of concrete resurfacing, using overlay options, make resurfacing an excellent solution for a full spectrum of pavement needs. Figure 6 represents a typical pavement condition curve over the life of a pavement. The [preventive maintenance], [minor rehabilitation] (together, the preservation window), and [major rehabilitation] zones are noted where [bonded concrete overlay|bonded] and [unbonded concrete overlay|unbonded] overlays can be used to restore pavement to the original or better condition.

Evaluating Pavements and Selecting Solutions