International Concrete Abstracts Portal

Penetration of chloride ions from the environment into reinforced concrete is important in relation to corrosion behavior of embedded steel. Blended cements containing ground granulated blast furnace slag (GGBFS) are expected to offer a greater degree of protection compared to that of OPC. The kinetics of chloride ion diffusion through hardened cement pastes made from SRPC, OPC, and OPC/GGBFS blends have been determined by a steady-state (thin disc) method. Concrete slabs containing similar cements and quartzite aggregate have been made and ponded regularly with 5 percent NaCl solution. Material taken from various depths within these slabs has been subjected to pore solution expression and analysis, and the concentration profiles of free and total chloride have been determined. Values of chloride diffusivity obtained by the steady-state method have been used to calculate the chloride concentration profiles expected when penetration is into a semi-infinite medium. Comparison between the two techniques shows the same general trends in relative performance of the various cements, but actual chloride concentrations, at a given depth, are greater in the concrete slabs. Results from total and free chloride measurements indicate that the chloride-binding capacity of slag cements exceeds that of OPC and SRPC.

Reports on study of the chloride distribution profile in hardened cement paste cylinders of 5 cm diameter. The specimens were made from ordinary portland cement and blended cement with 10 percent fly ash. The condensed silica fume was used as cement replacement, with replacement levels of 5, 10, and 15 percent by weight of cement. Other experimental variables were water-to-(cement + silica fume) ratio of 0.5, 0.7, and 0.9. The specimens were immersed in stagnant seawater at 20 C. After 6 months of exposure, the specimens were cut, ground, dried, and the chloride ion content determined by a potentiometric titration procedure. By applying Fick's second law, the effective diffusion coefficient and the effective supply concentration of chloride were calculated by using an approximation method. Results show the effective diffusion coefficient is reduced highly when condensed silica fume is used as cement replacement.

The use of silica fume (microsilica) to improve the compressive strength at a given cement level or as a cement replacement is on the rise. Additional benefits of adding silica fume to improve the corrosion resistance of embedded steel and improve concrete durability in erosive or severe chemical exposure were investigated. Concretes with embedded steel were produced with silica fume levels varying from 0 to 15 percent by mass of cement. Additional variables were water-cement ratio and calcium nitrite content. All concretes were air-entrained and had high-range water-reducers. Plastic properties of the concretes are reported as well as compressive strength, freeze-thaw, and resistivity and rapid chloride data. Corrosion rates and chloride contents are reported and show substantial improvements with silica fume and/or calcium nitrite. An accelerated hydraulic erosion test was conducted, in which ball bearings impact the concrete surface, simulating abrasive action of waterborn particles. Mass loss was measured for concretes with 0 to 15 percent silica fume by mass of cement. Silica fume significantly improved erosion resistance. Chemical testing was performed in 5 percent acetic acid, 1 percent sulfuric acid, 5 percent formic acid, and mixed sulfates. A cyclic method involving drying, weighing, and wire brushing was used. Results show that silica fume concretes had superior chemical resistance that improved as silica fume levels increase.

Purpose was to examine, under uniaxial compression, the monotonic and hysteretic stress-strain properties of normal and lightweight concrete with the addition of silica fume, and to propose a mathematical model of stress-strain curves based on experimental data. The mathematical model has been developed to predict the complete stress-strain curve of concrete in compression, in terms of the addition of silica fume to cement. The parameters of the model have been adjusted using system identification techniques to obtain the best match possible between the experimental data and the model predictions. The experimental data were obtained by testing normal and lightweight concrete cylinders; the amount of the addition of silica fume was varied, respectively, from 0 to 28 and from 0 to 24 percent by weight of cement.

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.