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.

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.

Heavy damage due to alkali-aggregate reaction has been observed in concrete structure in and along the sea. An accelerated test is performed on mortar to evaluate effectiveness of fly ash for controlling alkali-aggregate reaction in the marine environment. Mortar bars using Pyrex as aggregate and cements with 0.6 and 1.1% of equivalent sodium oxide are made. The alkali content in the mixture is adjusted by adding NaOH or NaCl. Specimens are stored in distilled water, NaCl solution, and under more than 95% of relative humidity. The controlling effect of fly ash and the effect of internal and intruded chloride ion in mortar on alkali-aggregate reaction is studied by measuring the expansion of mortar. Expansion of mortar depends on the type of cement and chemical reagents used for alkali adjustment, the amount of fly ash used and the exposure condition. Even with the same equivalent sodium oxide in the mixture, mortar using NaCl for alkali adjustment shows higher expansion than mortar using NaOH. The highest expansion is revealed for mortar cured in NaCl solution. The controlling effect of fly ash also depends on the type of cement and the exposure condition.

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.