International Concrete Abstracts Portal

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.

SP-153 In 1995, CANMET, in association with ACI, U.S.A. Electric Power Research Institute, Canadian Electrical Association, and several other organizations in Canada and the United States, sponsered the Fifth International Conference on fly ash, ferrous and nonferrous slags, and silica fume was held. The two volume proceedings of the Fifth CANMET/ACI Conference contains 62 papers from 23 countries.

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 authors have examined the lime-reactivity strength data of 14 samples of fly ash obtained from different thermal power plants of India. The sand-lime-fly ash mortars cured at 50 C and relative humidity of 90 percent were tested in compression at different ages up to 90 days. It was found that lime reactivity is best correlated with combined parameters of fineness and soluble silica content, rather than with each parameter considered individually. Also examined were the strength of concrete mixtures in which part of the cement is replaced by a low reactive fly ash. Fineness of fly ash and testing ages for strength were the variables. It is concluded that the soluble silica content was related to later-age strengths, while the early-age strength correlated better with fineness of fly ash. The mechanism for the latter may not be chemical, but physical, such as dispersion of cement particles or micro-filler effect.

Classified fly ash (CFA) is produced by separating the fine components of fly ash by means of air classification. CFA is made of fine particles of micro-meter size and spherical shape and is expected to improve the consistency of fresh concrete and the durability of hardened concrete. The use of CFA in roller compacted concrete (RCC) pavement has the effect of reducing the water content of RCC mixtures and, therefore, the drying shrinkage and number of joints in pavement. RCC pavements have become popular for roads and streets in Japan. The maximum thickness of RCC slabs that may be placed in one layer is limited to 25 cm, because of limitations in the compactibility of the concrete and control of the pavement surface profile. To increase the slab thickness of RCC placeable in one layer, an improved concrete that requires minimal energy for obtaining a high filled-volume ratio is desirable. In this paper, the effects of CFA additions to cement on compactibility and water content of RCC mixture were studied.