Rusting of Metallic Surface Hardeners: Case History

Abstract

Sprinkled surface hardeners are incorporated into freshly placed concrete slabs while high-strength overlays are placed on a previously poured new concrete slab or an existing deteriorated slab, providing improved abrasion and impact resistance. Metal filings are combined with cement corrosion inhibitors and water-reducing admixtures. Depending on the intended application, the metallic particles may either be treated with chemicals or directly sieved and graded. Rusting of metallic surface hardeners can occur when the installation of these products or the conditions during installation are not closely controlled. Visual tests, including microscopy and chemical analysis, were employed to investigate some of the potential factors that contribute to the rusting problem. Observations and possible causes contributing to the rusting and debonding problems are identified: (1) a decrease in the alkalinity of the cement paste due to carbonation; (2) exposure of unprotected metallic particles to air during the brushing procedure; and (3) timing of the finishing operations.