International Concrete Abstracts Portal

An extensive research program was performed on the rate of carbonation of concrete produced with ordinary portland cement, portland blast furnace slag cement, and portland fly ash cement, both with replacement of cement by fly ash and without. After various periods of wet curing, concrete samples were exposed to various exposure conditions. The wet curing ranged from 1 to 90 days, the exposure concerns outdoor sheltered from rain and at 20 C, 65 percent relative humidity in the laboratory. For the various exposure conditions, a relation has been found with respect to the carbonation rate as a function of the compressive strength at 7 days or at 28 days per type of cement and for all types of cement when the lime content is involved. Results of measurements over a period up to 2 years are presented.

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.

Chemical resistance of concrete containing condensed silica fume (CSF), 5 percent H2SO4, 5 percent HCl, and 10 percent Ha2SO4 was studied. The deterioration of concrete was investigated by measuring the changes in weight and modulus of elasticity. The penetration depth of chloride ion by 3 percent NaCl solution spray and the carbonation depth by 5 percent CO2 gas were also measured. Chloride was measured by colorimetry. Mortar and concrete containing CSF show higher compressive strengths at 28 and 91 days than the control mix. The penetration depth of chloride into CSF concrete was reduced and at 10 percent CSF, the penetration depth was reduced by 65 percent. At 30 percent CSF, carbonation depth is increased with respect to the control concrete.

Longitudinal thermal strains of high-strength, air-entrained, lightweight concrete containing silica fume were monitored during a single frost cycle between 15 and -157 C to measure the coefficients of thermal length changes at different temperatures. The immunity of such concrete to repeated exposures to liquified petroleum and natural gas temperatures was also evaluated. Scanning electron microscopy was employed to observe the deterioration of concrete prior to and after five frost cycles to -73 C. The coefficients of thermal length changes of air-dried and water-saturated concretes were calculated to be 3.2 and 4.1 x 10-6 cm/cm/C, respectively, before freezing, and 2.9 and 3.0 x 10-6 cm/cm/C at post-freezing temperatures. The strength deterioration study indicated that dried concrete cycled to -40 C can experience maximum drops in compressive and splitting tensile strengths of 7 and 3.5 percent, respectively. These reductions can be expected to increase to 15 percent if the extreme cooling temperature is lowered to -73 C. Water-saturated concrete can lose 12 to 17 percent of its initial strength after five freeze-thaw cycles to either temperature range.