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Results of an investigation of cement paste structure, and strength, permeability, and frost resistance of concrete with admixtures of silica fume type are given. The admixtures are waste materials from metallic silicon, low-grade ferrosilicon, ferrosilicon chrome production, containing SiO2 in the amount of 92, 70, and 66 percent, and surface area of 25.0, 44.9, and 18.5 mý/g, respectively. The influence of the admixtures on the cement paste microstructure results in an increase of gel porosity volume, decrease of capillarity porosity, and in an increase of strength. Thus, concrete strength increases and its permeability decreases. Physical and chemical properties of the silica fume-type admixtures insignificantly affect gel pore volume, whereas they have significant influence on capillary porosity. An increased dosage of high-range water-reducing admixture (HRWR) is a beneficent factor in increasing hydration degree and gel porosity, decreasing capillary porosity volume, and, consequently, increasing strength. Concrete frost resistance with silica fume dosages up to 10 percent of cement mass is not lower than the reference concrete with the same amount of binder.

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.

Engineered enchancement of or engineered alternatives to shallow land disposal of low-level radioactive (LLW) is likely to be the disposal technique adopted by a majority of states in the U.S. Such disposal techniques involve extensive use of concrete as an engineered barrier to prevent the escape of radionuclides into the environment. The LLW will be contained in concrete vaults or bunkers buried underground or covered with earth. U.S. Regulation 10 CFR 61 establishes the regulatory responsibilities for licensing LLW disposal sites. Implicit in the regulations is the need for the concrete of the LLW disposal system to have a service life of 500 years. Discusses the regulatory responsibilities governing LLW disposal. It also discusses results of a research project at the National Institute of Standards and Technology for the U.S. Nuclear Regulatory Commission to predict the service life of underground concrete for LLW applications. Assuming that disposal will be above the water table, the major degradation mechanisms affecting the concrete would be those due to sulfate attack, chloride ions, alkali-aggregate reaction, and leaching. Mathematical modeling of the degradation mechanisms and the validation of those models with accelerated laboratory tests suggests that service lives of 500 years for concrete structures can be reliably achieved.

The interface between the cement matrix and aggregate is mostly regarded as a weak link in concrete with respect to durability and strength. It is shown that the positive effects of pozzolans on the permeability of concrete are partly related to a decrease in the thickness of the weak, lime-rich, interfacial zone. Results for various mineral admixtures, such as ground granulated blast furnace slag, powder coal fly ash, silica fume, a synthetic colloidal silica, and metakaolinite are presented. It is shown that in the presence of mineral admixtures, the calcium hydroxide content in the interfacial zone is reduced substantially.