International Concrete Abstracts Portal

Reports the results from a laboratory investigation of the effect of fly ash on the temperature rise and early-age tensile strain capacity of concrete. Twelve different fly ashes, with a wide range of chemical compositions, were used in various proportions (25, 40, and 56 percent) in the study. The results of conduction calorimeter tests show that the rate of heat development was strongly influenced by the composition of the ash. Generally, the rate and quantity of heat evolved increased with the calcium level of the fly ash. High-calcium ashes (>20 percent CaO) did not significantly reduce the seven-day heat of hydration when used at a replacement level of 25 percent. However, the heat of hydration decreased as the level of replacement was increased for all ashes tested, regardless of composition. Consequently, even high-calcium ashes may be effective in reducing the temperature rise in concrete, provided they are used at a sufficient level of replacement. Flexural tests were carried out on concrete prisms at early ages; the tensile strain capacity was determined as the strain (in the tensile fibers) at 90 percent of the flexural strength. The flexural strength decreased with higher levels of replacement; however, the strain capacity was similar or slightly higher in fly ash concretes (compared with control specimens) at three and seven days. These results imply that the beneficial effect of reduced temperature rise in fly ash concrete is not necessarily offset by a reduced capacity to resist thermal strains.

The increasing construction of high-rise buildings in recent years had led to a demand for lightweight, high-strength concrete. In this study, the compositions of the matrix and the air void structure of aerated mortar containing silica fume were investigated as the basis for manufacturing lightweight, high-strength concrete. Mortars made with cement containing silica fume and fine or ultra-fine silica stone powder, having a particle size between that of cement and silica fume, were tested; the properties of cement paste in fresh and hardened conditions were improved. The compressive strength and the air void structure of prefoamed aerated mortars were determined and their relationship studied. Based on the results, it was confirmed that lightweight, high-strength concrete could be made with an effective combination of aerated mortar containing silica fume and lightweight coarse aggregate.

A rapid-hardening cement was made by blending mixtures of high- alumina cement (HAC) and ground granulated blast furnace slag (GGBS). The addition of slag alters the course of hydration reactions in HAC. A chemical compound 2CaO.Al 2O 3.SiO 2.8H 2O (gehlenite hydrate or stratlingite), only seen in plain HAC in small amounts, readily forms and becomes the main stable hydrate in the blended cement concretes in the temperature range of 5 to 38 C, replacing the metastable hydrates which lead to loss of strength in HAC through the conversion reaction. The properties of mortars and concretes made with this cement were assessed in a series of durability studies carried out by the Building Research Establishment. Mortars made with the blend have shown excellent sulfate resistance. Concrete specimens were compared with those from HAC concretes of similar proportions, following exposure for two years in aggressive sulfate, marine, and soft acid water environments. The findings, at this relatively early stage, are very encouraging. Longer term tests will be carried out at five and 10 years. Concretes made with the blend have shown a greater tolerance of high water-cement ratio mixtures in forming stable products with reduced temperature rises and enhanced durability in terms of their excellent sulfate, seawater, and soft acid water resistance.

The use of blast furnace slag aggregate (BFSA) is not new, but its application in the production of high-performance concrete (HPC) is nonexistent at least, in Australia. This paper presents the results of a preliminary optimization of the high-strength concretes made using BFSA, normal sand, portland cement, ground granulated blast furnace slag (GGBFS), condensed silica fume (CSF), and a proprietary superplasticizer. The paper also describes some additional characteristics of the optimized concretes. In all, 15 types of concretes were made. The properties examined were workability, density, compressive strength, elastic modulus, shrinkage, and water penetration. The maximum strength achieved using the slag aggregate was 107 MPa, which placed the slag aggregate concrete well into the very high strength range of concretes. The workability was found to be unaffected by the use of the slag aggregate. The tensile strength of the concrete was relatively high (5.4 Mpa); the shrinkage was found to be lower than concretes produced with normal aggregates, as was the water penetration and absorption. Of particular importance, the elastic modulus was found to be markedly lower than that of concretes made with normal aggregates. It is concluded that the slag aggregate can be used successfully in the production of high-performance, high-strength concrete.

Describes chemical attack caused by a high concentration CaCl 2 solution and its preventive measures by the addition of a mineral admixture. Changes which occur in mechanical strengths and chemical properties in mortars with and without fly ash, blast furnace slag, and silica fume when immersed in a 30 percent CaCl 2 solution at different temperatures were investigated. Portland cement mortars seriously deteriorated at early ages of exposure to a high concentration CaCl 2 solution, its deterioration being associated with cracking and spalling on the surfaces of specimens. On the other hand, 10 percent silica fume and 50 percent blast furnace slag mortars showed a good resistance to calcium chloride attack, although 30 percent fly ash mortars slightly deteriorated at late ages of exposure. X-ray diffraction and differential thermal analysis indicated that the deterioration of portland cement mortars cause by the chemical attack of a high concentration CaCl 2 solution was attributed primarily to both the dissolution of calcium hydroxide and the simultaneous formation of a complex salt in the mortar. Thus, the combined effect of a decrease in calcium hydroxide content and a reduced chloride ion permeability by the addition of a mineral admixture effectively improved the resistance of mortar to calcium chloride attack.