International Concrete Abstracts Portal

Paper presents the results of the tests conducted on reactive andesite produced to determine if Japanese fly ashes produced in Japan have an effect in controlling the alkali-aggregate reactions in concrete. Fourteen fly ashes produced were subjected to Japan Industrial Standard (JIS) alkali-silica reaction (ASR) mortar bar test (40 x 40 x 160 mm, alkali content in cement 1.2 percent, s/a = 2.25) with (c + f) ranging from 5 to 30 percent. With f/(c + f) at 20 percent or higher, all the mortar test bars incorporating fly ash had little expansion even after 6 months, but with f/(c + f) at 10 percent, different expansions were produced depending on the type used. The analysis of the data indicated that the component Na2Oeq of fly ash would accelerate the expansion while the component SiO2 will restrain the expansion. The controlling ability is also related to the alkali content of the cement: the greater the alkali from the cement and fly ash, the greater the quantity of fly ash required for preventing the expansion. An empirical formula expressing such a relationship has been derived. 123-389

Rice husk ash (RHA) was obtained under different burning conditions from 400 to 1200 C. The changes in the properties of RHA were investigated by XRD, DAT, and microanalytical techniques. When RHA obtained at different burning conditions was added to cement paste or mortar, several properties such as hydration, setting time, porosity, and strength revealed changes.

The first silica fume admixture aimed at the ready-mixed market appeared in the United States in 1983. Since then, the use of silica fume has developed slowly. Currently, it is being used as a cement replacement material or as a performance-enhancing admixture. This paper reviews the practical aspects of working with silica fume in ready-mixed concrete, with an emphasis on the use of silica fume in the property enhancement role. The forms of silica fume currently being marketed in the United States are briefly described. Then, the current state of specifications for silica fume, admixtures containing silica fume, and concrete incorporating silica fume is examined. Next, aspects of concrete production are discussed. Finally, transporting, placing, finishing, and curing practices are reviewed.

Zinc coatings, applied mainly by galvanizing, have been widely used to provide supplementary corrosion protection to reinforcing steel in concrete exposed to aggressive media. Their performance, particularly in concretes contaminated with chloride salts, has been variable; this is believed to be due, at least in part, to the effects of differences in cement alkalinity on the rate of zinc dissolution. To investigate this, specimens were made in which well characterized zinc coated steel electrodes were embedded in cement pastes containing various proportions of silica fume and sodium chloride. They were exposed to moist air for several months, during which time the pore electrolyte compositions were analyzed and the corrosion rates of the embedded electrodes were monitored by linear polarization. It was found that the major influence on the corrosion rate of the zinc coatings was the pH of the pore electrolyte phase, so that quite modest levels of silica fume were capable of reducing corrosion rates by orders of magnitude when compared with those sustained in the unblended pastes. The implications regarding the effective service lives of the coatings are believed to be considerable. Analysis of the relationship between corrosion potential and corrosion rate for the embedded electrodes revealed that the rates of corrosion were generally subject to anodic control, except at very high values when oxygen diffusion became the rate-limiting process.

Purpose was to study the influence of the length of moist curing time on weight change behavior, chloride ion content, and chloride ion distribution profile through 10 cm concrete cubes made from concretes containing silica fume. Three concretes containing 2.5, 5, and 10 percent silica fume by weight of cement were prepared and tested. The concrete cubes were moist cured for 1, 3, 7, and 21 days. Then, after 21 days of air drying, all the cubes were immersed in 15 percent NaCl solution for 21 days. Following the 21-day soaking period and a subsequent 21-day final air-drying period, chloride ion contents at four different depths were determined using a potentiometric titration procedure. The test results showed that weight gain rate and chloride ion penetration in all tested concretes decreased when the length of moist curing period increased. All tested concretes showed the best performance for both reductions after the maximum moist curing period of 21 days used in this study.