Effect of Silica Fume and Fly Ash Replacement on Stability and Strength of Fluid Concrete Containing Anti-Washout Admixture

Anti-washout admixtures are used to enhance stability of cement-based systems, such as in the case of concrete intended for underwater repair of marine structures, self-leveling and segregation-free concrete for above-water placement, and for shotcrete. Mixtures modified with an anti-washout admixture can also incorporate silica fume or fly ash to enhance fresh and hardened properties. Anti-washout admixtures are relatively new, and there is a lack of systematic data comparing the efficiency, advantages, and limitations of using them in concrete, especially in silica fume or fly ash concrete. A laboratory investigation was undertaken to evaluate the effect of cementitious material combinations on fresh and hardened properties of fluid concrete made with three commonly used anti-washout admixtures. Each anti-washout admixture was used at two concentrations corresponding to relatively low and medium dosages that are typically used in concrete. The anti-washout admixtures included a power-based product, a liquid-based cellulosic material, and a liquid-based microbial polysaccharide. The mixtures were prepared with 100% of a Type 10 cement, a blended silica fume cement containing 8% silica fume, as well as a Class F fly ash used at 20% replacement. All mixtures were made with a 0.41 water-to-cementitious materials ratio and had initial slump values of 230+_ 5 mm (9.1 +_0.2 in.). The influence of fly ash and silica fume incorporation on slump retention, washout resistance, bleeding, segregation, surface settlement, setting time, compressive and splitting-tensile strength developments was evaluated for the fluid mixtures. The paper summarizes the results of the experimental study that involved the comparison of data obtained from 21 concrete mixtures.