Question: I have been told that unbonded concrete overlays are the best way to rehabilitate a deteriorated asphalt or concrete pavement. But can’t the overlay be thinner if it’s bonded to the old pavement?
Answer: Let’s start by explaining the difference between bonded and unbonded concrete overlays, whether placed on an existing concrete or asphalt pavement.
Bonded overlays become part of the underlying existing pavement—they work together. They are 2 to 5 inches thick and are usually applied to strengthen an existing pavement or for preventive maintenance. Bonded overlays cannot be used if the existing pavement is in poor condition.
Unbonded overlays, 4 to 11 inches thick depending on the desired life span and anticipated traffic, are a pavement structure on their own—the existing pavement is only used as a supporting base. This means they are thicker than an unbonded overlay, but are the only choice if the pavement is in less than good condition.
As the name implies, bonded overlays must be bonded to the existing pavement since the load-carrying capacity of the pavement is relying on the strength of the existing pavement. This means good surface preparation is necessary. It also means that the joints (and any cracks) in the overlay must match up exactly with the joints in the existing pavement; otherwise the joint or crack will reflect through the overlay. And since the old pavement and the overlay move as one, the thermal expansion properties of the two materials must match. For these reasons, bonded overlay projects are more challenging; although, since they are thinner, they do use less material.
Unbonded overlays are used whenever the condition of the existing pavement is poor. They are basically a new pavement so the strength of the existing pavement is not relied upon to carry the load. The damage can even include that due to alkali-silica reaction. On existing asphalt pavements, nothing special must be done to prevent bonding since concrete is so much stronger it will move independently of the asphalt whether intentionally unbonded or not. In fact, whitetopping overlays (concrete over asphalt) tend to benefit by some bonding to the asphalt. An unbonded overlay on an existing concrete pavement, though, must be completely unbonded. This is sometimes accomplished by installing 1.5 inches of asphalt between the existing pavement and the overlay, or more recently by using a woven fabric bond breaker (see Innovations).
Many public agencies have the erroneous idea that concrete overlays are expensive or difficult to install. In reality, concrete overlays are simple to install and unbonded overlays require very little work in advance. Typically, the damaged surface is milled off and major failures in the existing pavement are repaired. And concrete overlays have the maintenance and life span advantages of new concrete pavements. In Colorado, CDOT budgets annual maintenance costs of $1270/lane-mile for asphalt overlays and only $400/lane-mile for concrete overlays.
Anyone considering concrete overlays should get the “Guide to Concrete Overlays,” which is published by the National Concrete Pavement Technology Center. It is available for free download at www.cptechcenter.org/publications/overlays/.