International Concrete Abstracts Portal

The term "efficiency factor" for condensed silica fume in concrete can be defined as the number of parts of cement that may be replaced by one part of silica fume without changing the property studied. This factor was first introduced for concrete compressive strength after 28 days curing in water at 20 C. In this situation, factors around 3 to 4 are reported. However, if the concrete is exposed to other temperatures, other curing conditions, or other curing periods, the factor may be as low as zero. This is very important information for the practical application of silica fume, especially when forms have to be stripped very early in winter concreting. The durability of concrete structures is more in focus than ever. When studying durability, the efficiency factors of silica fume in concrete can be calculated at the same time. Durability efficiency factors are also affected by the curing conditions. A comprehensive research program has enabled efficiency factors to be calculated for different conditions. This has covered strength and durability parameters such as permeability, carbonation, and chloride penetration. This work will be a helpful indication of how silica fume can be used in the most efficient way in concrete structures.

The effect of finely ground silica with a high specific surface of about 12 mý/g and 20 mý/g in mortar was compared with that containing silica fume. The dosage requirement of a high-range water-reducing agent to maintain a constant flow of fresh mortar was determined for 1:3 mortar containing 5 to 25 percent finely ground silica or silica fume as cement replacement. The compressive and flexural strengths of the mortar were investigated at different ages. Also, the resistance of mortar specimens to chemical attack and chloride penetration was determined. It was concluded that the use of finely ground silica was as effective as silica fume for improving quality of mortar.

The results of studies on the contributions of alkalies in fly ash, slag, and silica fume to the expansion of concrete due to alkali-silica reaction are presented and discussed. A wide range of concrete mixtures was made. Each mixture contained a different amount of cement and different proportions of one type of fly ash, one type of slag, or one type of silica fume. All mixtures were made with amorphous-fused silica as a synthetic reactive aggregate. The alkalilevel of some mixtures was increased by sodium hydroxide to study the effects of pozzolans or slag mixtures at higher concentrations of alkalies. Concrete prisms were made and stored in water at 38 C. The test results indicate that the effectiveness of these supplementary cementing materials in reducing or increasing expansion due to alkali-silica reaction varies widely. The results also indicate that the supplementary cementing materials can contribute significant quantities of alkalies to the reaction, under particular replacement and test conditions employed.

Paper reports the accelerated carbonation test results to investigate the effect of replacement ratio of fly ash, initial curing period in water, and air content on the carbonation phenomena in concrete. Using these test results, equations for the prediction of carbonation depth of concrete with and without fly ash are proposed, and these effects are also evaluated by these equations. Furthermore, the accelerated carbonation test results are compared with natural exposure test results for 15 years, and a method to predict the carbonation depth of concrete with and without fly ash exposed to natural indoor conditions is proposed. Concrete with fly ash is affected by initial curing period in water rather than concrete without fly ash from the viewpoint of depth of carbonation and compressive strength. The higher the fly ash content of concrete is, the deeper is the depth of carbonation. Depth of carbonation can be evaluated by compressive strength of concrete cured in water for 28 days, irrespective of the fly ash content of concrete. Carbonation depth of concrete with and without fly ash naturally exposed indoors can be predicted by the equation obtained by the accelerated carbonation tests.

Copper slag, a by-product of copper production, contains large amounts of iron oxide and silicate. It is chemically stable and its physical properties are similar to that of natural sand. The physical and chemical properties of copper slag were investigated. Copper slag, in amounts of 20, 40, 60, 80, and 100 percent, was substituted for fine aggregate in cement mortar and concrete. The fineness modulus of the combination of copper slag and fine aggregate was roughly 2.6, the optimum fineness modulus for concrete mix design. At this value, workability was found to be satisfactory with minimal bleeding. Addition of copper slag also improved the strength of the concrete. When the substitutional amounts exceeded 80 percent, lower strengths were obtained, possibly due to the formation of ettringite. It was also found that the effect of copper slag on long term strength development was also dependent on the amount used and its fineness. It was concluded that copper slag could be used as a fine sand substitute for ordinary reinforced concrete.