Low Porosity, High Strength Concrete for Bridge Decks

IN TR O D U CTIO N -TH E PROBLEM A few years ago a problem that previously had been a modest highway maintenance concern suddenly became a major source of dif­ ficulty and expense to highway departments in many areas of the coun­ try; including Indiana. In brief, concrete bridge decks, some new, started to spall, then delaminate, then come apart in relatively large chunks. The problem was quickly traced to steel corrosion, specifically to corrosion of the steel in the upper mat of concrete bridge decks; and more specifically to corrosion induced by chloride ions penetrating from the surface of the bridge deck. The chloride is derived from de-icing salt (usually sodium chloride but occasionally calcium chloride) applied to keep the bridge deck clear during winter snows. Researchers established that chloride ions that penetrate to the vicinity of the steel interfere with the protective “passivating” layer of oxygen that ordinarily keeps steel embedded in concrete from rusting. The rate of corrosion of embedded steel depends on several factors, but most pertinently, in this connection, on the pH or alkalinity of the pore solution of the concrete and on the amount of dissolved chloride pre­ sent. Generally speaking, high levels of alkali (high pH levels) protect the steel; however, if the concentration of chloride ions gets too high, the protective action tends to break down regardless of how high the alkalinity may be. The concrete bridge deck corrosion problem is a bit different from most corrosion problems in that the damage is not due to loss of metal of the corroding steel but due rather to the buildup of the hydrated iron oxide corrosion product within the concrete. The hydrated iron oxide is bulky and as it builds up in the restricted pore spaces within the con­ crete it exerts expansive force that tends to lift up the concrete cover above the level of the steel. A layer of separation is thus produced; the loosened concrete above the layer then tends to spall and break up under the action of traffic. There have been a variety of solutions to the problem suggested and worked on by the Federal Highway Administration and by researchers