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 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.

The moisture diffusivity is of considerable importance for quantitative assessments of creep and shrinkage, as well as durability of cementitious material. For this reason, the influence of the composition of repair mortars on their effective moisture diffusivity as a function of the relative humidity of the surrounding air was investigated. Silica fume, superplasticizer, and polypropylene fibers were added to reduce permeability and to control cracking induced by drying shrinkage. It was shown that the moisture transport in cementitious materials can be realistically described by a nonlinear diffusion process governed by Fick's law. A computer program based on the finite volume method was used to get the best effective moisture diffusivity by comparing experimental results (moisture losses of drying mortar cylinders) with the numerical solution. The applicability of a combined experimental-numerical approach to characterize repair mortars regarding their moisture diffusivity was demonstrated. The material properties necessary for the characterization and qualification of new materials can be found numerically. Moreover, the diffusivities obtained provide useful input data for further numerical calculations. The positive effect of the addition of silica fume on moisture diffusivity was clearly shown. The positive combined effect of polypropylene fibers and silica fume with increasing entrained air content was observed. Finally, no significant detrimental effect on the addition of fibers (even at relatively high volumes) has been observed for materials cast under shrinkage free conditions.

Sulfate resistance of two types of silica fumes from ferrosilicon (FeSi) and silicoferrochromium (SiFeCr) furnaces has been evaluated using ASTM C452 and ASTM C1012 test procedures. Cubic mortar specimens have also been immersed separately in 10 percent N

Hgh Reactivity Metakaolin (HRM) is produced by controlled thermal activation of purified kaolinite, an aluminosilicate mineral, to a reactive, amorphous state. HRM, being pozzolanic, reacts with free lime (Ca(OH) 2), a byproduct of portland cement hydration. In this investigation, two high- performance concrete mixtures containing HRM were studied. In the first mixture proportion, HRM was formulated as an addition to the cement. In the second mixture, HRM was used as a cement replacement. The compressive strength and rapid chloride permeability of the HRM concretes was compared to nonpozzolanic concrete controls and concretes that contained equal amounts of silica fume. The results of this study show that the strength and impermeability of HRM concrete is significantly higher than nonpozzolanic concrete. The HRM concrete showed properties equivalent to similar silica fume (SF) concretes, while using significantly less superplasticizer to reach an equivalent consistency.