International Concrete Abstracts Portal

Red mud is a by-product from the aluminum industry. To investigate the possibility of using this waste material as a pozzolan in the cement and concrete industries, tests were carried out to examine the pozzolanic properties of calcined red mud. Red mud was calcined for 5 hr at five different temperatures: 600, 650, 700, 750, and 800 C. Blended portland cements containing 30 or 50 percent of the calcined red mud were studied for hydration products, strength, and durability. The results indicated that the red mud had the maximum reactivity when calcined at 600 C, because on hydration the lime content of the blended cement was considerably reduced. The calcined red mud when used in combination with portland cement contributed to the formation of hydrated alumina-silicates and hydrogarnets. Very good compressive strengths were obtained with the blended cement containing 30 percent calcined red mud. Mortars cast with these blended cements were placed in solutions of seawater and acetic acid. The results indicated good stability of mortars to these environments.

Discusses the relationship between the composition and the mechanical properties (compressive strength, modulus of elasticity, autogenous shrinkage) of high-strength concretes (HSC) in the range of 50 to 100 Mpa. The models proposed for each of these properties are based on an analysis of the hardened concrete as a composite material, making it possible to go from the properties of the concrete to those of its matrix. The properties of the matrix are related to the two main parameters of composition (water-cement and silica-cement ratios) by empirical models obtained by smoothing the experimental data. Eleven concretes were made using the same constituents; the parameters of composition were varied separately to determine their influence on the properties in question. These experimental data, together with other data taken from the literature, were used to evaluate the accuracy of the proposed models. It is finally shown that these models, which sum up the current knowledge of the material, can be useful in designing HSCs according to specifications.

In normal strength concretes, the compressive strength is limited by the strength of the binder and the binder-aggregate bond. In high-strength concretes, however, the binder strength and the bond may be fully comparable to the strength of the aggregate. This fact may lead to the conclusion that the strength of high-strength concretes may be improved by replacing an ordinary aggregate type with a high-strength aggregate. A number of aggregate types have been combined with high-strength binders to evaluate the impact of the aggregate strength on concrete compressive strength. The significance of the aggregate strength has been compared with the effect of the cement type and the use of silica fume. According to the obtained results, the impact of the aggregate strength on the strength of high-strength concrete is limited, compared to the binder type, while the difference in E-moduli between the different aggregate types is fully reflected in the concrete E-moduli. This contradiction is explained by a hypothesis based on stress concentrations due to the difference in rigidity between the binder and aggregate.

Summarizes the work conducted by the Virginia Department of Transportation to evaluate the characteristics of concrete containing silica fume in the overlays as a protective system to prevent the penetration of chlorides into concrete. The first three field installations of silica fume concrete overlays in Virginia are described. The practices of other states in the USA for low-permeability silica fume concretes are also compared. The results indicate that silica fume concretes can be placed successfully in thin overlays on bridge decks. These concretes can provide the low permeabilities required to prevent the penetration of chlorides and other detrimental solutions into the concrete. Adherence to good construction practices is necessary, especially for the prevention of plastic shrinkage cracking.

In the case of high-strength concrete, the problem of temperature rise due to hydration is compounded, where the unit cement content is much higher than that encountered in normal concrete. This study was carried out to determine whether the merits of slag and silica fume addition could be combined to develop a low-heat high-strength concrete, in which the heat generation can be controlled by blending the cementitious constituents, keeping the compressive strength about 80 MPa (at 91 days). The program was divided into two phases, using mortar in the first phase to study the effect of partial replacement of cement by slags of varying fineness and silica fume on the consistency, temperature rise, and strength development. It was found that, from an overall point of view, a blend of cement, slag, and silica fume in proportions of 2:7:1, using a slag with 6000 cm²/g by Blaine, yields the best result. Concrete specimens were then cast in the second phase, using the mix of cement just mentioned, and it was verified that the temperature rise could be brought down by as much as 30 C without adversely affecting the strength at 91 days (about 80 Mpa), though the early age strength was slightly lower.