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Q.All of the foundation specs we build on require rebar to be free of rust and mill scale. For years this hasn’t been an issue, until recently when a project manager called us on it. All we can think to do is wire brush the entire lot of rebar. Is this really necessary?

A.Tim Fisher responds: Fortunately, there are a couple of standards you can cite in your defense. The ASTM standard for deformed steel reinforcement (A706) and the Concrete Reinforcement Steel Institute (CRSI) Manual of Standard Practice both give the same recommendation: Reinforcing bar with rust, mill scale, or a combination of both is satisfactory, provided the minimum dimensions, weight, and height of the deformations (the formed protrusions on the bar) of a hand-wire-brushed test sample are not less than the values specified in the standard. In other words, if the rust or mill scale is light, it will not affect the bond to the concrete. In fact, studies have shown that mill scale and light rust enhance the bond between concrete and steel.

Tim Fisher is the field engineering editor for Aberdeen’s Concrete Construction magazine.