Results from experiments to determine the effects of microsilica on concrete freeze-thaw resistance and on the permeability to chlorides and subsequent early corrosion rates of steel in the concrete are described. The results to date show that air-entrained microsilica concretes pass laboratory freeze-thaw tests and demonstrate reduced chloride permeability and corrosion rates. ASTM C 666 testing was performed on concretes with no entrained air, below-normal entrained air, and normal air-entrainment levels on numerous mix designs with and without microsilica slurry additions. In all cases, properly air entrained microsilica mixes behaved as well as or better than control mixes. When air entrainment was not added, all mixes failed. Chloride permeabilities were determined using AASHTO 277. Results show that silica fumes significantly reduced chloride permeability. Concrete resistivity measurements were performed using the A. C. impedance method. Microsilica significantly increased concrete resistivity over that of the control concretes, indicating that macrocell corrosion should be reduced. Corrosion-rate measurements show reduced rates (essentially noncorroding) for silica-fume concretes at 0.43 and 0.5 water-cement ratios, whereas some controls have gone into corrosion. Chloride analyses are to be performed to determine whether reduced permeability, increased electrical resistivity, or a combination of the two is responsible for the better corrosion performance.
Abstract