Q: Experience shows that repairs have been made at disturbingly frequent intervals on a bridge deck that bears heavy traffic.

Many of these repairs are replacements of patches that have been put in more than once before.

The patches have been made of good quality concrete containing a latex and have been carefully featheredged. Old concrete has always been removed right down to the rebar. Can you suggest a better patching method?

A.: Your trouble may be in getting the right configuration for the patch. Sufficient deteriorated material, whether old patching mix or original concrete, must be removed to give at least 3/4 to 1 inch clearance around all exposed rebar, and the bars should be thoroughly cleaned by sandblasting to remove thick, scaly rust, though not thin, tightly bonded rust. If the bars are deteriorated, a structural engineer should be consulted about how much lap to provide on the new steel that you attach to the existing bars.

In cutting out the concrete, the edges should be made sharp, not featheredged, though corners should be rounded as shown in the figure. It is also a good idea to undercut the edges so the patch will be wider at the bottom than at the top, providing keying action. Before installing the new patch, all the surfaces should be thoroughly cleaned to remove loose particles and dust. This can be done by high pressure water jetting, sandblasting, blowing with compressed air or vacuuming. If the exposed surfaces contain oil, asphalt or other contaminants, those also should be removed.