We're involved in a bridge overlay project in which the concrete cover over the reinforcing steel will be milled before placing the overlay. Before full-scale construction, a small section of the bridge deck was milled and an overlay was placed as a test area to evaluate the in-place, direct tensile bond of the repair. Many of the cores failed below the minimum required tensile-bond strength of 200 psi. Of those cores, most of them failed within the substrate concrete, and we suspect that microcracking, or bruising, of the substrate during milling may have been a contributing factor. How deep into the substrate must failure occur for microcracking to be written off as a cause of failure?
We spoke to Michael Sprinkel of the Virginia Transportation Research Council, and he had several observations concerning tensile-bond testing and modes of failure for concrete overlays. Sprinkel said that, in his experience, tensile-bond testing of overlays placed on properly prepared substrates usually results in a break in the substrate concrete near the bottom of the drilled core. If specimens consistently fail at this depth at strengths lower than the minimum specified, this usually indicates weak substrate concrete. In this case, you should consider replacing the deck because placement of an overlay will most likely result in delamination. If, however, specimens fail in the substrate concrete within 1_4 inch of the bond line, microcracking is often the cause. Sprinkel said that microcracking often can be remedied by shotblasting or grit blasting this weakened layer off the concrete surface. Breaks at the bond line are usually caused by oil or other bond-inhibiting substances that have not been adequately removed from the substrate.