What About Overlays?

Agencies looking for dollar value balanced with road quality are looking at concrete overlays

By Daniel C. Brown

This 8-inch unbonded, plain-jointed concrete overlay was placed on I-44 in 1999 in Webster County, Mo.

More than ever before, state departments of transportation (DOTs) and other roadbuilding agencies are choosing concrete overlays as a way to rehabilitate and preserve their existing pavements. Concrete overlays allow agencies to save the cost of removing an existing pavement. And significantly, concrete is generally more competitive these days with asphalt on a first-cost basis, as well as a lifecycle basis.

“Concrete overlays allow us to take the substantial equity that we have in our existing pavement system, which generations before us have invested in, and to hold onto that investment,” says Dale Harrington, P.E., principal senior engineer, Snyder & Associates, on behalf of the National Concrete Pavement Technology Center. “We can hold that investment by utilizing the strength of that existing pavement, be it in concrete or asphalt, and enhance it with a concrete overlay. That way, we bring the pavement into a whole new cycle without having to destroy the existing slab.

This bonded concrete overlay was placed 4 inches thick on asphalt and sawed into 4-foot-square panels. It’s on U.S. 60 in Neosho, Mo.

“The other major benefit of concrete overlays is sustainability,” says Harrington. “By leaving the existing pavement in place, and not wasting it, we are making the pavement even more sustainable. That is a huge plus!”

What’s more, concrete overlay technology has matured, which helps spur growth, says Leif Wathne, P.E., vice president of highways and federal affairs, American Concrete Pavement Association (ACPA). “In large parts of the country, there has been phenomenal growth over the past five years in concrete overlays,” says Wathne.

“Concrete overlays give states and other agencies an opportunity to seize a long-term solution,” says Gerald F. Voigt, P.E., ACPA president and CEO. “Most of the DOTs are operating under tight budgets these days, and we don’t have a new highway bill. A concrete overlay does not require an expenditure to tear out the existing pavement. Overlays are quite adaptable to a tighter budget scenario.”

Concrete overlays come in two primary categories: bonded and unbonded. The key difference is that with bonded overlays, the new structure depends on the underlying pavement – asphalt or concrete – to act in unison with the overlay. The new pavement is designed to count heavily on the strength of the underlying structure. Bonded overlays require that the existing pavement be in fair to good structural condition, and they are generally thinner than unbonded overlays.

Unbonded overlays treat the old pavement as a stable base for the new overlay. Unbonded concrete overlays, which are generally thicker than bonded overlays, are often placed to rehabilitate asphalt or concrete pavements that are in poor condition. Unbonded overlays do not require a bond between the new overlay and the existing pavement.

Both bonded and unbonded concrete overlays can be placed on three types of pavements: concrete, asphalt, or composite structures. Composite pavements are generally those where asphalt has been placed over concrete.

“The adoption of concrete overlays by DOTs has enabled them to introduce competition in a market segment where they traditionally have not seen much competition between the two industries,” says Wathne. “That competition enhances the ability of agencies to stretch their highway dollars.”

From 2004 to 2009, concrete overlays of 6 inches or less have grown from about 1.2 million square yards to about 5.4 million square yards, ACPA data shows. And according to bid-letting information, a total of 17 million square yards of overlays were built in 2009 and 2010 – and about half of that was composed of thinner overlays. “Most of it was built on existing asphalt pavement,” says Voigt. “So that is just a huge jump from what we have seen in the past.”

Wathne says that when you introduce the concrete pavement industry into the rehabilitation market segment, that not only lowers prices, it stimulates innovation. “You have another industry in there challenging the existing industry, and that is a good thing for the customer, which in this case is the DOT or the public sector, actually,” says Wathne.