A Mix Match

Group of repair techniques may be the right way to go

Concrete Maintenance Article April 12, 2004
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The concrete pavement industry is setting the record straight on the misconception that concrete is too difficult to rehabilitate. Outreach and educational information from the American Concrete Pavement Association (ACPA) is going a long way in educating agencies and consultants about the best ways to rehabilitate distressed pavements using concrete.

These efforts are aimed at dispelling myths and misinformation that can represent a huge drain on an agency’s resources. For example, we have observed flexible overlays placed over concrete pavements that are still structurally sound, but perhaps need only restoration or resurfacing. Equally disconcerting are the cases when structurally sound concrete pavements have been destroyed prematurely and either resurfaced or reconstructed with flexible materials because better alternatives were never considered.

The concrete pavement industry supports a “mix of fixes,” a number of rehabilitation methods that address both short- and long-term requirements. The concept behind a mix of fixes is that sound planning and pavement assessment will not only help address pavement performance issues as early as possible, but also will help select the right corrective action.

The concrete pavement industry offers a wide variety of pavement renewal solutions, including: concrete pavement maintenance, restoration (CPR), resurfacing and reconstruction techniques. In general, selecting the right option depends on the current conditions of the pavement, traffic requirements, required design life and the relative life-cycle costs.

The most common type of concrete pavement maintenance is joint re-sealing, which also fits into the industry’s broad strategy known as concrete pavement restoration, CPR, a series of engineered procedures used to manage the rate of concrete pavement deterioration.

No. 1 on the list

The first type of rehabilitation procedure applied to concrete pavement ideally is CPR, which is used when the pavement exhibits only slight deterioration. CPR techniques fall into a number of subcategories, which may not all be required during the life of a pavement, but should be followed in the appropriate order. They also may be either preventive measures or corrective techniques.

Applying the right CPR technique requires identification and classification of distresses, noting whether they are structural (which affect the pavement’s ability to carry traffic) or functional (which may affect ride quality and safety, but do not impact the pavement’s load-carrying capability). Examples of structural distresses include cracking or joint deterioration, while functional distresses include roughness or surface polishing.

Preventive CPR techniques include:

  • Joint and crack resealing to minimize the infiltration of surface water and incompressible materials into the joint system;
  • Retrofitting concrete shoulders to decrease edge stresses and corner deflections, as well as to reduce the potential for transverse cracking, pumping and faulting; and
  • Retrofitting edge drains by adding a longitudinal drainage system to assist in the removal of water that may cause pumping, faulting and durability distress.

Corrective and preventive CPR techniques include:

  • Dowel bar retrofit to increase the load transfer efficiency at transverse cracks and joints;
  • Slab stabilization to restore the support to concrete slabs by filling small voids that develop under the concrete slab at joints, cracks or the pavement edge;
  • Full-depth patching, i.e., removing and replacing at least a portion of a slab to the bottom of the concrete, to restore areas of deterioration. Full-depth patches improve pavement rideability and structural integrity and extend pavement service life;
  • Partial-depth patching to restore localized areas of deterioration that do not extend through the slab; and
  • Diamond grinding to remove bumps and re-profile the surface of concrete pavements. This improves the riding comfort to motorists and decreases the severity of dynamic or impact loads from heavier vehicles.

Overlay protective

Concrete pavement resurfacing using overlays is indicated when pavement has medium to high levels of distress and restoration would not be expected to perform effectively.

Concrete overlays fall into two basic categories: concrete overlays for concrete pavements and concrete overlays for asphalt pavements. Within each of these categories are two overlay types: bonded and unbonded.

  • Bonded concrete overlays of concrete pavements are primarily used to increase pavement structural capacity. They consist of a thin concrete layer (4 in. or less) bonded to the top of the existing concrete surface to form a monolithic or composite section. Typically, pavements that have very little deterioration, but are too thin for an increasing traffic volume, are candidates for this technique;
  • Unbonded concrete overlays consist of a thicker concrete layer (5 in. or greater) on top of an existing concrete pavement. Unbonded concrete overlays are more effective—in terms of long-term performance and cost—for deteriorated concrete pavements than asphalt overlay options. Unbonded overlays are generally most cost effective when an existing concrete pavement is deteriorated moderately worse, and removal of existing pavement layers is not desirable.
  • Unbonded overlays require a separating layer between the overlay and old pavement. The separation interlayer is usually a thin asphalt layer of about 0.5 to 1.5 in. thick. The layer is sometimes called a debonding layer or stress relief layer, which separates old and new layers so that they may act independently of each other through temperature cycles and load deflection.

Topped with concrete

Two concrete overlay technologies that have been developed and refined over the years are conventional whitetopping and ultra-thin whitetopping (UTW).

Conventional whitetopping consists of a thick concrete layer (4 in. or more) on top of an existing asphalt pavement. When loaded by vehicles, the new overlay performs like a new concrete pavement on a strong base course. Whitetopping overlays are effective for almost all applications. They have been successfully used on interstate highways, as well as other roadways, intersections and airport pavements.

UTW is a thin concrete overlay; 2 to 4 in. thick placed on top of a prepared surface of an existing asphalt pavement. In addition to being thinner, they differ from conventional whitetopping because they are bonded or partially bonded to the existing asphalt pavement and feature short joint spacings (2 to 6 ft versus 12 to 15 ft).

Bonding the concrete overlay to the asphalt pavement creates a composite section in which the load is shared between the concrete and existing asphalt. The closer joint spacing allows the slabs to deflect instead of bend. This reduces load stresses in the slabs to reasonable values, even at thickness of just 2 in.

Both whitetopping and UTW are designed for existing asphalt surfaces. Standard whitetopping and UTW have well-deserved track records as inexpensive, quick, cost-effective and durable ways to eliminate the constant maintenance and frequent repairs of problems.

The final answer

Pavement reconstruction, the total removal and replacement of worn out or insufficient pavement, is used when the pavement has high levels of distress, when overlays will not solve the problem or when the pavement has simply outlived its design life.

There are many factors to consider prior to reconstruction, but one of the most significant is cost. With the increasing demands on America’s highways, it is important to evaluate more than the initial costs, but also consider what the pavement will cost during its entire design life.

About the author: 
Voigt is senior vice president - technical services and chief operating officer for the American Concrete Pavement Association, Skokie, Ill.
Overlay Init