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.

Silica fume has an accelerating effect on the early hydration of portland cement. Also, silica fume reduces the retarding effect of lignosulfates. At standard curing conditions, the contribution to strength from the pozzolanic reaction takes place primarily at 5 to 7 days. As a result, existing equations for prediction of strength development based on pure portland cement are no longer valid for concrete with silica fume. Some new equations for concrete with various contents of silica fume are presented.

29Si NMR has been employed as a tool to characterize the reaction mechanism of hydration in several blended cements up to 6 to 9.5 months. The cements investigated were blends with silica fume, fly ash, activated kaolinite, and blast furnace slag. Spectra deconvolution indicated that, in the silica fume as well as in the activated kaolinite blend, the reaction of the anhydrous calcium-silicates is initially accelerated with respect to the ordinary portland cement. In the fly ash blends, this effect is smaller. Both in the silica fume and fly ash blends, an increase in the amount of silica middle groups (Qý-type) at - 84 ppm, relative to the amount of silica end groups (Q1-type) at - 79 ppm, is notable, which indicates an increased tendency to form longer CSH chains. The size distribution and glass content of the fly ashes used seem to influence the hydration reaction, which is reflected by somewhat higher Qý/Qý ratios and an increased initial hydration. In the blends with activated kaolinite, it was not possible to deconvolute the Q1 and Qý chemical shifts at all ages, due to changes in the shift maxima Q1 and/or Qý. This may be due to the formation of amorphous noncrystalline alumina-containing reaction products. The chemical shift of the blast furnace slag appeared too broad for a successful deconvolution. In general, both the total (Q1 + Qý) as well as the Qý/Q1 ratio correlate with compressive strength data, Qý species contributing markedly. Paper contains a general overview of the application of NMR spectroscopy in cement and concrete research.