Recent reports from the Transportation Research Board indicate that every dollar invested in appropriately timed preventive pavement maintenance saves three to four dollars in future rehabilitation costs. This confirms a long-held belief with the American Concrete Pavement Association, which developed concrete pavement restoration (CPR) to manage the rate of deterioration in concrete pavement streets, highways and airports.
CPR is a series of engineered techniques developed as an alternative to overlays during the past 30 years to repair isolated areas of distress in a concrete pavement without changing its grade. This preventive procedure restores the pavement close to its original condition and reduces the need for major and more costly repairs later.
Ideally, CPR is the first rehabilitation procedure applied to concrete pavement, typically applied early and when the pavement is in good condition with only slight deterioration. CPR is typically used to replace isolated sections of deteriorated pavement, to prevent or slow overall deterioration or to reduce the impact loadings on the pavement.
CPR also can be applied to a mildly deteriorated concrete pavement that already has an asphalt overlay. It is quite feasible to remove the existing asphalt, repair the underlying concrete using CPR, then open it to traffic without a new asphalt overlay.
Each CPR technique is designed specifically to repair or prevent the recurrence of a certain distress or a combination of distresses. While each technique can be used individually, they are typically more effective when several are used together.
CPR allows the design engineer to address specific problems or develop system-wide programs that provide
different levels of improvement. Factors such as fiscal constraints, adjacent pavement conditions and future agency programming may make one approach more suitable than another.
CPR also can correct design or construction deficiencies before any distresses develop, or repair a concrete pavement before an overlay is placed. Repairing a design defect or the pavement before an overlay minimizes future distress and maintenance.
Finally, it is feasible to repair a concrete pavement that has been previously overlaid with asphalt. Agencies in the past overlaid many concrete pavements because of roughness. This does not correct the cause of roughness and the asphalt overlays often deteriorate quickly, requiring a second overlay. In many cases, overlays have to be replaced several times within a short period. With CPR, it is possible to remove an asphalt overlay and repair an underlying concrete pavement close to its original condition.
To determine if a project is a candidate for CPR, an agency must look at all relevant information systematically. The information desired includes existing pavement data; initial cost; anticipated maintenance; future rehabilitation requirements; anticipated serviceability; experience; and constructability. Of these, the most important to the pavement engineer is the existing pavement data. The existing pavement data tells the engineer which distresses are present, then helps assess why the distresses developed. The pavement data can be grouped into the following categories:
• Design data: The primary design information for the existing pavement. It includes the pavement type and thickness, layer materials and strengths, joint design, shoulder design and drainage system design.
• Construction data: Details about the conditions during construction can give insight and may highlight some event or problem that may be the cause of the distress.
• Traffic data: Past, current and expected traffic growth figures help determine the pavement’s remaining structural capacity. Knowing the remaining structural capacity may limit the number of options.
• Environmental data: Precipitation, temperature and freeze-thaw cycles will indicate the drainage adequacy and material durability of the pavement system.
• Previous CPR activities: Along with regular maintenance activities, previous CPR activities can help show the rate of deterioration as well as indicate distresses that may develop. They also allude to how well CPR will work.
• Pavement condition: The condition of the pavement and underlying layers are the most important data needed by the pavement engineer. They tell whether or not CPR can be used to repair the pavement.
Once the available pavement data has been collected, the pavement engineer must answer four questions: Which distresses are present? What caused the distresses to develop? What are the viable solutions to correct the distresses in the pavement or prevent their return? Is the timing appropriate for these solutions to be effective and economical?
The best way to identify pavement distresses and determine why they developed is to perform a site-condition survey once a year, preferably in the early spring. The survey should identify the type, severity and quantity of each distress. It also should try to determine whether the pavement design, applied load, water, temperature, pavement materials or construction caused the distress. In addition to the visual survey, destructive and nondestructive testing can be used to determine structural condition and material properties below the pavement surface.
The two goals of the condition survey and the structural assessment testing are to determine the root cause of the pavement’s distress and to track the rate of pavement deterioration. Knowing the root cause of the pavement’s distress helps determine which CPR techniques are appropriate. Knowing the rate of pavement deterioration helps determine whether a pavement is a good candidate for CPR or if another rehabilitation technique is more appropriate.
An agency also can use the condition survey to change design or construction practices for new pavements. Changing poor design or construction practices may prevent similar distresses in newer pavements. Finally, the condition survey can serve as the basis for a CPR-Area Management contract.
All distresses are either structural or functional. Structural distresses primarily affect the pavement’s ability to carry traffic. Examples include: cracks; deteriorated joints and/or cracks; loss of support; punch-outs and durability distress such as D-cracking, alkali-silica reactivity and freeze-thaw damage.
Functional distresses affect the quality and safety of the pavement. They include roughness caused by faulting or the difference in elevation between slabs at joints or cracks; surface polishing or the wearing away of the surface texture to expose the concrete coarse aggregate on pavements carrying heavy traffic; noise; or surface defects such as scaling, pop-outs, crazing and cracking from plastic-shrinkage.
There are two categories of CPR techniques: corrective activities and preventive activities. Corrective activities repair a given distress and improve the serviceability of the pavement.
Full-depth repairs (FDR) are used to repair cracked slabs and joint deterioration by removing at least a portion of the existing slab and replacing it with new concrete. This maintains the structural integrity of the existing slab and pavement. FDRs also are used to repair shattered slabs, corner breaks, punch-outs and some low-severity durability problems.
FDR involves marking the distressed concrete, saw cutting around the perimeter, removing the old concrete, providing load transfer and placing new concrete. Each repair must be large enough to replace all significant distress and resist rocking under traffic, yet small enough to minimize the patching material. Typically, patch areas that are a full-lane wide and at least a half-lane long meet this requirement.
Partial-depth repairs (PDR) correct surface distress and joint/crack deterioration in the upper third of the concrete slab. When the deterioration is greater than one-third the slab depth or contacts embedded steel, an FDR must be used instead. PDR involves removing the deteriorated concrete, cleaning the patch area, placing new concrete and reforming the joint system.
Preventive activities are proactive activities that slow or prevent the occurrence of a distress in order to keep the serviceability high. Examples include:
• Joint and crack resealing: Minimizes the infiltration of surface water and incompressible material into the joint system. Minimizing water infiltration reduces subgrade softening; slows pumping and erosion of subgrade or subbase fines; and may limit dowel-bar corrosion caused by deicing chemicals. Minimizing incompressibles reduces the potential for spalling and blow-ups. Joint sealing also can maintain small sliver spalls that can develop into larger spalls if left alone.
It is especially critical to reseal the joint along the pavement/shoulder edge. Most of the surface water that enters the pavement system does so at the lane/shoulder longitudinal joints.
• Retrofitting concrete shoulders: Adds a tied concrete shoulder to an existing pavement. It is similar to dowel-bar retrofit because it decreases the critical edge stresses and corner deflections and reduces the potential for transverse cracking, pumping and faulting.
• Retrofitting edge drains: Adding a longitudinal drainage system to a pavement aids in the rapid removal of water and may prevent pumping, faulting and durability distress from developing. Despite these potential advantages, the placement of retrofit edge drains must be considered carefully.
There also are several techniques that may be used in both corrective and preventive CPR strategies. These include diamond grinding, dowel-bar retrofit, slab stablization, cross-stitching and grooving.
Diamond grinding improves a pavement’s ride by creating a smooth, uniform profile by removing faulting, slab warping, studded tire wear and patching unevenness. This extends the pavement’s service life by reducing impact loadings, which can accelerate cracking and pumping.
Dowel-bar retrofit increases the load transfer efficiency at transverse cracks and joints in jointed plain concrete pavement (JPCP) and jointed reinforced concrete pavement (JRCP) by linking the slabs together so the load is distributed evenly across the joint. Improving the load transfer increases the pavement’s structural capacity and reduces the potential for faulting by decreasing the stresses and deflections in the pavement. Dowel-bar retrofit consists of cutting slots in the pavement across the joint or crack, removing the concrete, cleaning the slot, placing the dowel bars and backfilling the slots with new concrete.
Slab stabilization restores support to concrete slabs by filling small voids that develop underneath the concrete slab at joints, cracks or the pavement edge. The voids, often not much deeper than 1/8 in., are caused by pumping or consolidation of the subgrade from high corner deflections. Without proper support, the pavement may develop faulting, corner breaks and extensive cracking.
Cross-stitching repairs longitudinal cracks that are in fair or low-severity condition. It increases load transfer at the crack by adding steel reinforcement to hold the crack together tightly. This limits the crack’s horizontal and vertical movement and prevents it from widening.
Grooving restores skid resistance to concrete pavements. It increases the surface friction and surface drainage capabilities of a pavement by creating small longitudinal or transverse channels that drain water from underneath the tire, reducing the hydroplaning potential.
CPR and pavement distress
Properly matching the CPR techniques to the distress or combination of distresses is essential. In some cases, more than one CPR technique may be applicable, but depending upon the condition of the distress one technique may be more suitable than another. For example, transverse cracks that are working cracks can be sealed early, which helps them perform for many years. Later, it may be necessary to restore pavement integrity with dowel-bar retrofit or full-depth repair if the cracks develop severe spalling, pumping or faulting.
The sequence of work also is very important. Slab stabilization and retrofit edge drains should precede full- and partial-depth repairs. Full- and partial-depth repairs, dowel-bar retrofit, retrofit concrete shoulders and cross-stitching must precede diamond grinding. Grooving and resealing joints follow grinding to ensure proper groove and sealant depth.
Timing CPR activities
Determining the appropriate time for CPR is a question of both engineering and economics. Performing CPR too early repairs the distressed areas, but may yield only minor results. Likewise, performing CPR too late may yield a substantial improvement, but at a high cost.
In most cases, CPR is effective and economical when the pavement has only slight deterioration. Repairing the pavement when it shows only slight deterioration slows the rate of deterioration, repairs only those specific areas that are causing problems and decreases the amount of distress to be repaired later. To accurately assess the time frame in which CPR techniques may be applied with optimal effectiveness and economic benefits requires evaluation of the windows of opportunity using trigger and limit values. Essentially, the trigger values signal an open window, while limit values indicate the window of opportunity is closed.
Trigger and limit values are based on the structural and functional condition of the pavement. Structural-based trigger and limit values use the type, severity and quantity of distress to define the opportunity windows. Functional-based trigger and limit values use ride-quality indicators, such as the International Roughness Index (IRI), Present Serviceability Rating (PSR), or Profile Index (California profilograph) to define the window of opportunity. Functional-based trigger and limit values are useful because they tell when the pavement section has or will reach some unacceptable condition because of the combination of distresses, even though each distress trigger and limit value may not have been reached.
Both the structural- and functional-based trigger and limit values should be used to determine the timing for CPR activities. The functional-based values show that a pavement has reached some unacceptable level, but do not tell why it has reached that level. The structural-based values are needed to tell why the pavement has deteriorated and to show what distresses are present, as well as to help identify possible CPR techniques.
Because performance will vary with local traffic and environmental conditions, it is not possible to develop precise trigger and limit values for all pavements in all conditions. Each agency should develop trigger and limit values to reflect local experience. One of the problems of CPR in the past has been that the actual field quantities have been greater than the quantities specified on plans. This is because the pavement continues to deteriorate between the time of the distress survey and the time of repairs. Agencies must realize that the pavement will continue to deteriorate and this must be taken into account when developing plans.
Finally, once a pavement passes its CPR opportunity window and deteriorates into a poorer condition other rehabilitation procedures—such as a bonded overlay, unbonded overlay or reconstruction—are better alternatives.
An effective network program
The key to an effective CPR program is commitment to the program. This first involves developing personnel with an understanding and knowledge of CPR. It also involves designating appropriate CPR funding levels. The consequences of poorly trained personnel or inadequate funding are that expertise or money are not available to apply CPR effectively or appropriately.
Inadequate funding and training also can force an agency to select another rehabilitation strategy based on policies that use the same standard repair for every pavement. Unfortunately, applying the same standard rehabilitation techniques to every situation is neither rational nor economical. The rehabilitation often is under-designed and has poor performance, or over-designed and not cost-effective.
Once an agency commits to a CPR program, balancing proper rehabilitation strategies with available funds becomes a significant challenge. Ideally, each agency will develop a preventive preservation program, both short and long term, with separate designated funding. This strategy will help an agency apply CPR while it is the most viable option and within its CPR window of opportunity.