New Construction. Some agencies use asphalt concrete overlays on new decks and protect the deck with a waterproofing membrane below the asphalt overlay. Currently, however, concrete-wearing surfaces are more popular on new bridges than asphalt concrete. Concrete surfaces may be placed as an integral part of the structural bridge deck (monolithic), or placed as bonded overlays of various types including dense concrete, latex-modified concrete, and silica fume concrete (see Art. 4.5.1).
Repair of Existing Bridge Deck Surface. The concrete overlays discussed above can be used, in combination with patching of spalled areas, as a means of repairing deteriorated existing bridge decks. In addition to these overlays, which are at least 1/4 in (32 mm) thick and usually thicker, thin overlays 1/2 in (13 mm) or less thick are available. Binder materials include epoxy, epoxy-urethane blends, and polyester resin. Because of their thinness and light weight, they are advantageous for bridges where weight reduction is desirable, or where thicker overlays would present problems with expansion joint or scupper modification, or where railing height would be reduced more than an acceptable amount by a thicker overlay. In recent years use of methyl methacrylate (MMA)-type material has gained popularity due to its waterproofing properties coupled with extremely high strength and chemical resistance with an abrasion-resistant surface.
The repair of a spalled bridge deck involves removal of the fractured or disintegrated concrete by some means. Mechanical methods include scabblers, scarifiers, and jackhammers. Because these methods all tend to create microfractures in the sound concrete, a better method is hydroblasting, or use of very high pressure water jets. This method is selective in that it automatically removes unsound concrete while leaving the sound concrete undamaged. The operation consumes large quantities of water and is noisy, and passing motorists must be protected from stray jets and flying debris. The muddy effluent must be disposed of properly, and not allowed to flow into catch basins.
After removal of unsound concrete, or concurrently with it, the surface of the remaining good concrete is removed to a depth of about 1/4 in (6.4 mm). The entire surface to be overlaid is dried or wetted to the required moisture condition, and the overlay placed. Where deep removal areas are present, it is generally preferable to patch these areas in a separate operation from the general overlay. After texture is applied, the fresh overlay concrete is then given an appropriate cure of the required duration. In cold weather the overlay must be prevented from freezing. For this reason specifications require placement at temperatures well above freezing.
