A growing number of highway agencies in Michigan, Oklahoma, North Carolina, Colorado, Minnesota and Iowa are routinely treating a variety of pavement conditions with concrete overlays. Whether the pavement is in good shape or bad, whether it is asphalt, concrete or a composite, whether it needs preventive maintenance or restoration, concrete overlays have proven to be excellent solutions that do not require extensive repairs of the existing pavement.
In spite of the successes, some agencies are reluctant to use concrete as a resurfacing material. One reason is that, until recently, first costs for concrete overlays were generally higher than those for similar asphalt resurfacing projects. Now, however, initial construction costs for concrete overlays are equivalent to or slightly lower than asphalt alternatives. Consequently, more states are taking a new look at this proven technology.
This article reintroduces concrete overlays using straightforward terminology for the various kinds of overlays. It provides a quick overview of situations in which concrete overlays can be useful pavement solutions. It also includes a basic discussion of design and construction issues for various overlays and points readers to resources for more information.
On the one hand
Cost-effectiveness is one of many potential advantages of concrete overlays:
- Concrete overlays are long lasting. They retain their surface profile for decades without rutting, shoving or pushing;
- Concrete overlays can be constructed quickly and efficiently;
- They can be adapted for a variety of pavement conditions and needs;
- Normal concrete paving equipment can be used. When minimum clearance is needed, the equipment can be modified to accommodate low- or zero-clearance situations;
- Conventional mixtures can be used for most concrete overlays;
- Concrete overlays are recyclable. They can be removed and reused in many applications, including in base material;
- Overlay designs can have the same life expectancy as new construction; and
- Concrete overlays can be constructed in conjunction with pavement widening.
What’s in a name?
Over the years, the paving community’s toolbox of concrete maintenance and rehabilitation strategies has included terms like “overlays,” “whitetopping” and “ultrathin whitetopping,” shuffled around with concepts like “bonded,” “unbonded” and “partially bonded.” To reduce the potential for confusion, the Federal Highway Administration (FHWA) and the American Concrete Pavement Association (ACPA) are adopting a simpler, more straightforward nomenclature. The collective term for these solutions is concrete overlays, and concrete overlays are organized into two families: bonded and unbonded.
Both bonded and unbonded concrete overlays can be constructed on concrete, asphalt and composite (asphalt on concrete) pavements. So, there are basically six types of concrete overlays:
- Bonded on concrete, bonded on asphalt and bonded on composite pavement; and
- Unbonded on concrete, unbonded on asphalt and unbonded on composite pavement.
Bonded concrete overlays are designed to bond with the underlying pavement so that together they perform as if they were one monolithic slab with added structural capacity. As an integral part of the resurfaced pavement, the original pavement continues to carry much of the load.
Bonding is essential. If a good bond is not achieved or if the bond is broken, the overlay will not perform as designed. Many practices contribute to bonding, as discussed later in this article.
Unbonded concrete overlays are designed like new pavements, with the existing pavement performing as a stable base. The overlay and the existing pavement can move independently. Bonding between the overlay and the existing pavement is not required to achieve the desired performance.
Unbonded overlays on asphalt may experience some partial bonding. Because bonding is not essential to the design, loss of the bond does not result in loss of performance. In the case of unbonded overlays on existing concrete, steps are taken to prevent bonding of the two concrete layers. A separator layer is placed that allows the two concrete layers to move independently and prevents or reduces the potential for reflective cracking.
Well designed and constructed bonded and unbonded concrete overlays can be useful tools in an agency’s pavement preservation program. Bonded concrete overlays can provide cost-effective and reliable resurfacing solutions for pavements in fair to good structural condition. Unbonded concrete overlays provide cost-effective and reliable rehabilitation solutions for poor or badly deteriorated pavements needing structural improvement.
These are not hard and fast distinctions, however. Bonded overlays can provide some features of rehabilitation. Some unbonded overlays may be considered reconstruction projects. Both can be used in conjunction with a variety of other improvements, like pavement widening.
Under the ’scope
A professional evaluation of an existing pavement is necessary to determine first, its suitability for a concrete overlay, and second, the appropriate concrete overlay design: bonded or unbonded. The evaluation should include visual examination and analysis of pavement cores. For high-level roadways, like freeways and interstate highways, it may be advisable to obtain more pavement condition information using a falling-weight deflectometer.
The decision to design a bonded or unbonded concrete overlay depends on several factors:
- The owner’s purpose in treating the pavement;
- The condition of the existing pavement; and
- The kind of improvements desired.
Bonded concrete overlays are generally not good solutions if (1) the underlying concrete pavement has widespread, serious distresses like alkali-silica reactivity or D-cracking; subgrade support is inadequate or uneven; or drainage is poor; or (2) the underlying asphalt pavement has significant structural deterioration, inadequate or uneven base or subgrade support or poor drainage conditions.
In these cases, however, unbonded overlays may be considered.
Each of the six types of concrete overlays presents unique considerations. The following discussion provides only a brief overview.
Bonded Concrete Overlays
Bonding is an essential feature of these overlays. A combination of good design and construction practices helps ensure that a bond is achieved and contributes to bonded overlays’ overall performance:
- Overlay thickness. Bonded overlays are relatively thin, generally from 2 to 5 in. (5.1 to 12.7 cm), based on the desired capacity of the new, resurfaced pavement.
- Overlay mixture. Conventional mixtures can be used for bonded concrete overlays. For bonding, a wet, sticky mixture is preferable to a dry one. Using well-graded aggregates in the mixture and good curing practices reduces the potential for curling and warping that might cause stresses to develop at the bond interface.
- Bonded on concrete pavement: Aggregates used in the overlay mixture should have thermal properties similar to aggregates in the existing pavement. The existing pavement and the overlay will then expand and contract similarly in response to temperature changes, minimizing the potential for stresses along the bond interface.
- Repair of existing pavement. Isolated areas of deterioration on the existing pavement should be repaired to promote long-term durability of the resurfaced pavement.
- Bonded on concrete pavement: Unrepaired cracks, especially working cracks, will almost always reflect through the bonded overlay unless joints are sawed directly over them. Wide random cracks may require full-depth repairs. Asphalt patches should be replaced with concrete or simply filled with concrete when the overlay is placed.
- Bonded on asphalt and composite pavements: Potholes, moderate-to-severe alligator cracking and areas lacking base/subgrade support may require partial- or full-depth spot repairs. Milling may be required to remove surface distortions of 2 in. or more or to reduce high spots and ensure a consistent minimum overlay depth. Care should be taken not to mill off too much asphalt and thereby reduce the existing pavement’s load-carrying capacity. Transverse thermal cracks wider than the largest aggregate in the overlay mixture should be cleaned and filled. The concrete overlay can span smaller cracks.
- Bonded on composite pavement: If there is vertical movement of the underlying concrete adjacent to a crack, the movement can be stopped by replacing or retrofitting the joint. Or, the crack can be controlled without repairing the underlying pavement by adding fibers to the mixture, or, in some cases, placing reinforcing steel (rebar) over the joint in the overlay. Rebar is avoided when possible because it makes the overlay harder to recycle later.
- Preparation of existing pavement surface, following repairs and before cleaning. A bonding grout or epoxy is not required or recommended.
- Bonded on concrete pavement: Concrete pavement surfaces must be roughened to promote bonding with the overlay. The most common and effective roughening procedure on concrete surfaces is shot-blasting. On concrete pavements, milling alone often causes microcracking that can weaken the pavement surface and compromise the overlay’s performance.
- Bonded on asphalt and composite pavements: Milling may be considered to roughen the surface, which will likely enhance the bond.
- Surface cleaning. To promote bonding, the existing pavement surface should be thoroughly clean and dry. Pressure washing should be considered only when dust control is an issue or when mud is on the surface. No water should be standing on the pavement when the overlay is placed, because it could prevent bonding.
- Bonded on concrete pavement: Immediately before placing the overlay, the pavement should be swept, followed by cleaning in front of the paver with compressed air.
- Bonded on asphalt and composite pavements: Immediately before placing the overlay, the pavement surface should be swept, followed by cleaning with compressed air.
- Placement temperature.
- Bonded on concrete pavement: The best time to place a bonded overlay on a concrete pavement is when the difference in temperature between the existing slab and the new overlay is minimal.
- Bonded on asphalt and composite pavements: If the existing pavement surface is very hot (120°F or higher), it may pull water from the overlay, making it more susceptible to shrinkage cracking. The surface of the existing pavement can be cooled by spraying it with water, as long as no standing water remains when the overlay is placed.
- Joint pattern and sawing window. Thinner overlays have greater potential for rapid shrinkage, contraction and expansion, and curling and warping, all of which can cause stresses to develop at the bond interface. The joint pattern and timeliness of joint construction are important for relieving the stresses. The joint pattern varies based on the existing pavement type. Although not always necessary, the software HIPERPAV can help users establish ideal joint sawing windows hours or days before constructing overlays on higher-level projects like interstate highways.
- Bonded on concrete pavement: Joints in the overlay should be constructed directly over joints in the existing pavement to help prevent reflective cracking. Joints should be cut full depth plus ½ in.
- Bonded on asphalt and composite pavements: The recommended joint pattern is small, roughly square panels, typically 3-8 ft or 12-18 times the slab thickness in inches. If possible, longitudinal joints should be constructed outside the wheel path.
- Curing. Good curing practices are important for controlling stresses in the overlay and maintaining a good bond. Within 30 minutes of overlay placement, white pigmented curing compound should be applied liberally. Some states apply compound at as much as 1.5 to 2 times the manufacturer’s recommended rate for typical pavements. After application, the finished surface, including the vertical faces of the pavement edges, should appear as a uniformly painted white surface.
Unbonded Concrete Overlays
A variety of good design and construction practices can contribute to the successful performance of unbonded concrete overlays:
- Overlay thickness. Unbonded overlay thickness can range from 4 to 11 in. The appropriate thickness is generally related to anticipated traffic loadings and to the existing pavement’s ability to serve as a uniform and stable base.
- Overlay mixture. Conventional mixtures are typically used. They can be used with accelerating admixtures to provide the early strength required to carry traffic as soon as possible.
- Repair of existing pavement. Most surface distresses do not need to be repaired. Structural-related distresses must be repaired to promote long-term durability.
- Unbonded on concrete pavement: Isolated panels where movements indicate potential nonuniform subgrade support or severe material-related distress, as well as badly shattered panels and tenting panels (early stages of blowups), may require full-depth repair.
- Unbonded on asphalt and composite pavements: Direct placement without milling is appropriate if rutting is less than 2 in. Surface distortions of at least 2 in. should be spot-milled to less than 1 in. to ensure minimum overlay depth throughout the overlay. Two considerations: Care should be taken not to remove too much asphalt and thereby reduce the existing pavement’s load-carrying capacity. Also, milling will roughen the surface and encourage bonding in that area.
- Preparation of existing surface. Preparation of an existing concrete pavement is very different from preparation of an existing asphalt surface.
- Unbonded on concrete pavement: A thin separator layer (usually a 1-in. asphalt layer; in Europe, fabrics containing asphalt have been used) must be placed on the existing concrete pavement to prevent the existing surface from bonding with the overlay. The separator also provides a shear plane that helps prevent reflective cracking up into the overlay.
- Unbonded on asphalt and composite pavements: The pavement surface should simply be swept to remove debris. Remaining small particles are not a problem.
- Placement temperature. If the asphalt surface is very hot (120°F or higher) it may draw water from the overlay, making it more susceptible to shrinkage cracking. The asphalt surface can be cooled by spraying it with water immediately before overlay placement, as long as no standing water
- Joint sawing and curing. Stiff support provided by the existing pavement can increase stresses caused by overlay curling and warping. Spacing joints more closely than on normal concrete pavement projects helps reduce these stresses and the related potential for cracking. Proper timing of saw cutting and thorough, timely curing on all surfaces also help reduce potential stress development.
Repairing concrete overlays is generally easier than repairing a section of conventional pavement. A distressed panel that does not compromise ride quality can be left in place. If ride quality is affected, the panel or an isolated area of panel should be replaced. Thin, unreinforced overlays can simply be cut out and replaced with concrete.
Filling the knowledge gap
The advisory board of the National Concrete Pavement Technology Center (CP Tech Center) at Iowa State University has elevated overlay-related research and technology transfer to high priority. Together with FHWA and ACPA, the CP Tech Center is working to provide owner-agencies with state-of-the-practice information, while helping to enhance both the science and art of overlay design and construction.
Gina Ahlstrom, concrete pavement engineer at FHWA’s Washington, D.C., Office of Pavement Technology, supports the CP Tech Center’s outreach approach. By combining research, hands-on field work, an intensive, national training element, regional expert champions and state-of-the-practice reference tools, she said, the CP Tech Center’s overlay program “has the potential to significantly impact pavement maintenance and rehabilitation practices around the country.”
Training Resources
In January 2007 the CP Tech Center published a 28-page handbook, Guide to Concrete Overlay Solutions. The booklet is unique in that it reflects the input and consensus of about 20 content experts across the country who are experienced with the variety of issues involved in constructing and maintaining concrete overlays. Eight thousand copies of the guide have been distributed, and electronic copies are available online at www.cptechcenter.org/ publications/overlays/guide_ concrete_overlays.pdf.
A second edition of the guide is scheduled to be published in February 2008. The new Guide to Concrete Overlays for Resurfacing and Rehabilitating Existing Pavements will include new tools:
- Decision trees for evaluating an existing pavement: Is it a good candidate for a concrete overlay? If so, which design is most appropriate? Do any preconstruction repairs need to be made? etc.;
- New project-staging and accelerated construction details for timely project completion; and
- Example model specifications for six types of concrete overlay.
Demonstration Projects
The CP Tech Center is seeking eight states willing to work with the center as the states design and construct various concrete overlays in 2008 and 2009. Using the second edition overlay guide, the states will act as a “learning community,” sharing with each other and the CP Tech Center their challenges and lessons learned, then acting as regional experts for surrounding states.
Through a team of experts, the CP Tech Center will provide technical advice to the states, shepherd them through the design process, and visit the sites. The center will widely distribute lessons learned from the eight demonstration projects.
Assimilating lessons from the eight demonstration states with the second edition guide, the CP Tech Center plans to develop a comprehensive design and construction manual and a full workshop curriculum for concrete overlays by 2009.
