International Concrete Abstracts Portal

SP-132 Published in two volumes...The first volume contains papers dealing with fly ash and natural pozzolans. The second volume consists of papers dealing with condensed silica fume and ferrous and non-ferrous slags.

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 chloride-ion attack on low water-cement ratio pastes containing silica fume was studied by soaking small paste disks in four different pH-controlled sodium chloride solutions for periods of up to 12 months. The pastes were made using water-cementitious material ratios of 0.30 and 0.25. Three types of cementitious materials were used: an ASTM Type III cement (Canadian CSA Type 30), the same Type III cement with 6 percent silicafume, and a French CPA-HPR cement with 6 percent silica fume. The four solutions in which the paste disks were soaked were the following: 3 percent NaCl (by weight) at a pH of 13.0, 3 percent at 11.5, 0 percent at 13.0, and 0 percent at 11.5. The curing period was fixed at 28 days for all mixtures. Mercury intrusion porosimetry, x-ray diffraction, scanning electron microscopy, and electron microprobe measurements were the techniques used to study the various samples after removal from the solutions. The chloride-ion attack on these low water-cementitious material ratio pastes was always very small, even after 12 months of exposure to 3 percent NaCl solutions at pH values of 13.0 and 11.5. After several months of exposure at a pH of 13, only very small amounts of chloride ions ( 1 percent) were detected and only minor changes to the microstructure were noted. At a pH of 13.0, the penetration of chloride ions was not found to be a function of the paste characteristics [w/(c + sf), type of cement, silica fume content]. The major parameter controlling chloride-ion penetration in low water-cementitious material ratio pastes is the pH of the NaCl solutions. When the pH is 11.5, the penetration of chloride ions into the pastes is easier because of the leaching of calcium ions creating a very fine microporosity. For this relatively low pH, it was found that the use of lower water-cementitious material ratios and silica fume can reduce the amount of chloride ions that can penetrate the cement paste.