Repair and maintenance often play a role in the pavement selection process. Unfortunately, concrete is too often perceived in a negative light when it comes to repair and maintenance. The fact that these activities aren't needed for many years doesn't seem to offset the perceived disadvantage in ease of repair and time required to complete the work.
A public works or DOT official can reduce road closure time by considering three basic questions: What strength do I need? How do I get it? How do I measure it? The strength needed to open a road to traffic may be less than most engineers realize. There are also some aspects of a patch repair that need to be considered that would allow for faster opening. Then, once the opening strength is determined, a real-time, nondestructive test of that strength is essential.
What strength is really needed for opening? Most repair specifications are merely derivatives of those used for new pavement construction. They were developed before nondestructive testing was available and, therefore, needed to contain a great deal of conservation to account for the inability to make accurate measurements of the strength of the in-place pavement.
Opening strength is affected by a number of factors, including traffic and depth. With the exception of very high traffic locations with a high percentage of trucks, it will take many hours—if not days—to accumulate a large number of equivalent single-axle loads over the patch. By that time, the patch will have gained significantly more strength than at the time of opening.
Research has shown that 300 psi (flexure third-point loading) is sufficient for most pavement 10 inches or thicker. Others found that at 10 inches thick, 200 psi may be sufficient. The point is that 300 psi, depending on the conditions, may be sufficient to provide the strength needed to open the patch area to traffic.
Patches often fail for reasons other than lack of strength. Excessive shrinkage, cracking due to thermal differentials, or lack of sufficient curing are as common as low initial strength. Iowa State University's James Cable found in a 2004 study that conventional concrete can produce patches that give long life. A slower strength gain with conventional concrete avoids most of the problems mentioned above, thereby limiting the possible failure mechanisms, and actually increasing the life of most patches.
A key to durable patches may be providing extra depth. The in-place cost of replacing unsuitable material in the bottom of the patch is astronomical compared to the cost of a few extra inches of concrete. If a minimum patch depth of 10 inches is used, for example—regardless of the thickness of the existing pavement—the strength needed to open is greatly reduced.
Maturity testing is an accurate and real-time method to determine the strength gain. No beam or cylinder will exactly match the curing conditions in the patch in an efficient way for an along-the-side-of-the-road measurement of multiple patch type projects. Maturity testing measures the in-place strength development and allows you to watch the concrete get hard.
In summary, quick opening, durable concrete patches can be constructed with conventional concrete—using extra depth patches—at a lower-than-conventional opening strength, and through the use of maturity to monitor the strength gain.
–Jim Grove is the Portland cement concrete paving engineer at the National Concrete Pavement Technology Center at Iowa State University.