Technical experts at the American Concrete Pavement Association (ACPA) and its chapter and state affiliates address thousands of technical issues every year. In response to the many technical questions, we often hear misperceptions about concrete pavements. Some common misperceptions are that concrete costs too much, takes too long or is difficult to repair.
Concrete pavements are easy and inexpensive to repair and restore with proper equipment, materials and procedures. And, when concrete pavement restoration methods are no longer viable, there are concrete overlays, which are a cost-effective means of adding structural capacity and returning pavements to a smoother, safer condition. Concrete overlays include bonded, unbonded, whitetopping and ultra-thin whitetopping.
Unbonded overlays
The term "unbonded" is used to describe a concrete pavement placed on top of another with a separation layer that allows them to work independently. The separation layer's primary function is to isolate the movement between the existing concrete pavement and the new concrete overlay. A properly designed and installed separation layer also provides some other key benefits to the overall pavement design.
Using an asphalt separation layer between the two concrete pavements eliminates any reflective cracking from the underlying concrete pavement and ensures no "keying" action of the joints. Also, offsetting the joint layout by approximately 3 ft improves load transfer and reduces the chance of roughness due to pumping.
The required concrete overlay thickness will depend on several factors, but experience has shown that an unbonded concrete overlay between 4 and 8 in. of plain, unreinforced pavement will perform well for most urban applications.
Using this strategy not only extends the life of the concrete pavement, but also avoids frequent overlays and repairs required when asphalt surfaces are used. Prior to placement of both the separation layer and the new concrete overlay, some minor steps or repairs may need to be completed to ensure the overlay has uniform support.
Because the existing concrete pavement's primary function is to act as the base for the new pavement, it must be treated as such to ensure it provides proper support to the overlay, including consideration of some repair.
During pre-overlay repair, other procedures can occur simultaneously to expedite construction. For example, new drainage systems and pavement widening can be incorporated into the new pavement. After these construction processes are complete, the overlay paving may begin as soon as the separator layer is applied.
Another pre-placement step during an overlay is whitewashing. It is advisable to consider the temperature of the asphalt separator surface, particularly in the summer. The heat of the asphalt, in addition to the heat of cement hydration, can cause the concrete pavement to become very warm. Typical methods of reducing heat buildup are water fogging and whitewashing.
Because of its dark color, asphalt can absorb and store a considerable amount of heat from solar radiation. A hot asphalt surface can track onto paving equipment and cause dowel basket pins or stakes to pull out during paving, resulting in misaligned dowel baskets.
The most common materials used for whitewash are lime slurry or white-pigmented curing compound. A lime slurry mixture is made of water and hydrated lime, which is usually readily available. A 25% hydrated lime-water slurry (one part hydrated lime to four parts of total slurry by weight) has been found to be sufficiently white enough and of the right consistency to "paint" the asphalt surface prior to paving.
ACPA recommends the use of whitewash when the temperature of the bituminous material is expected to exceed 110°F (45°C). Temperature differences between a whitewashed surface and an untreated asphalt surface can be 20°F (10°C) or more.
An unbonded overlay is generally defined as concrete over concrete, which has a thin asphalt layer in between (which may be existing or new). This is an important consideration, because the old concrete adds significant support to the new concrete overlay. By comparison, another overlay strategy--whitetopping--is generally defined as concrete over asphalt.
Fibers and concrete pavements
One of the keys to the use of whitetopping is the use of fibers in the concrete. Fiber-reinforced concrete is increasing in popularity, particularly in ultra-thin whitetopping, as well as thin unbonded overlays and concrete bus pads, which are exhibiting excellent performance.
Although many in the concrete construction industry are familiar with the design, construction, performance and benefits of both steel and synthetic fiber reinforcement, new technologies and research are showing surprising results. Fibers used in concrete pavements are typically made of steel or plastic and are available in a variety of lengths, shapes, sizes and thicknesses. They are added to fresh concrete during the batching and mixing process.
The primary differences between conventional steel reinforcement and fibers are location, length and area of cross section. Fibers are equally distributed throughout the concrete instead of placed at specific pavement locations, are much shorter than continuous steel bars and take up a much smaller cross-sectional area of the pavement than conventional steel reinforcement.
The American Association of State Highway & Transportation Officials (AASHTO) published a guide in 2001 on the use of fiber reinforcement in concrete transportation infrastructure. The document is an excellent resource that includes general information on fibers, as well as guidance on proportioning, mixing and placing fiber-reinforced concrete. Typical applications of fiber-reinforced concrete also are discussed. The report can be obtained from AASHTO at www.transportation.org, publication code TF36-1.
Repairs and utility cuts
Another common misperception about concrete pavements is that repair or utility cuts are either not possible or will compromise the integrity of the pavement. The truth is utility cuts and repairs can be made easily and quickly.
By following a few simple guidelines, utility cut repairs are made simple and do not damage long-term concrete pavement performance. Asphalt appearance and smoothness, on the other hand, are dramatically affected by utility cuts made after the pavement has been placed in service.
Utility cuts in asphalt pavements can reduce the service life of that pavement section by up to 50%. A recent study completed in the Pacific Northwest showed that even if the utility cut was properly backfilled and patched with asphalt pavement, the repaired areas required resurfacing much sooner than the concrete sections. In fact, the pavements initially constructed of concrete and patched with concrete showed no reduction in service life.
The ease of repairs, as well as the long-term effects, are especially important given the drawbacks of closures and the traveling public's growing concern about frequent disruptions.
The repair procedure for a utility cut in a concrete pavement is similar to that of a full-depth concrete pavement repair. By following these six basic steps, the long-term pavement performance is not reduced.
1. Locate and isolate utility for upgrade or repair;
2. Remove the designated pavement section as outlined;
3. Make utility upgrade or repair as required;
4. Replace and prepare subgrade for concrete pavement patch;
5. Place, finish and cure concrete pavement patch; and
6. Open to traffic.
Today there are several products and materials available through the ready-mix concrete industry that both simplify and improve the repair process. First, the use of controlled low-strength material, commonly referred to as flowable fill, can greatly speed up the backfilling process. Primarily, flowable fill consists of a mixture of cement, pozzolan, air entraining admixture, sand and water. The exact proportions depend on local practices and procedures and are readily available from the local ready-mix concrete supplier. Also, conventional granular backfilling procedures may be used to obtain satisfactory performance, if properly compacted.
When selecting a concrete mixture, there are a few options that need to be considered. Using a concrete mixture that was specified during the original construction is a good starting point. If that information is not readily available, then a common mix design with a compressive strength of 4,000 psi at 28 days will perform well. If the utility cut is in a major arterial, the use of a combination of chemical admixtures and cementitious materials will allow the pavement to open to traffic in a matter of a few hours.
To learn more about pavement restoration strategies, read ACPA's "The Concrete Pavement Restoration Guide: Procedures for Preserving Concrete Pavements" (TB020P).
For the complete guide to using unbonded overlays, read ACPA's "Guidelines for Unbonded Concrete Overlays" (T005P).
For additional information about utility cuts, read ACPA's "Utility Cuts and Full-Depth Repairs in Concrete Streets" (IS235P) and "Full Depth Repair and Utility Cuts for Concrete Pavements" (PA169P).
For additional information on bonded concrete overlays, read ACPA's "Guidelines for Bonded Concrete Overlays" (TB007P).
To order ACPA publications, go to the ACPA website (www.pavement.com); call 800/868-6733; or fax requests to 847/966-9666.
