International Concrete Abstracts Portal

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.

Presents results of investigations carried out on high fly ash content concrete (HFCC) cores removed from several structures constructed in the U.K. since 1979. Structures investigated included a road pavement, a major road viaduct, water-retaining and industrial structures, and a slipway subjected to marine exposure. Concrete properties measured after 10 years of service include compressive strength, depth of carbonation, permeability, and chloride and sulfate penetration profiles. In addition, petrographic analysis of thin sections was also undertaken. The HFCCs studied were designed considering the fly ash to be just a further ingredient in the concrete rather than as a cement replacement. This led to higher fly ash contents and lower cement contents than is generally normal practice. The structures examined were in excellent condition after 10 years. Results show a durable concrete exhibiting increases in compressive strength beyond 28 days, little evidence of carbonation, low to average permeability, and resistance to chloride penetration. In this respect, it is significant that at the marine exposure sites, the chloride concentrations decreased significantly with depth. No evidence of alkali-silica reaction was detected in spite of reactive aggregates being present in some of the concretes.

The results of a great number of trial mixes for mix design of roller compacted concrete (RCC) are presented. This particular RCC used a local fly ash, rich in lime and sulfates, which does not meet any existing specification. This fly ash's performance in concrete has been studied for some time at the Laboratory of Reinforced Concrete of Aristotle University of Thessaloniki. Recently, this fly ash was used in the construction of a large RCC dam in northern Greece. Measurements of the strength development and the elasticity of RCC mixes showed that the 80 percent (by weight) of the cementitious material could be substituted for this fly ash. Therefore, it was proven that in RCC mixes this fly ash is more effective than in conventional concrete.