International Concrete Abstracts Portal

The transport of ions related to the penetration of chlorides into concrete has been studied in the field by drilling 100-mm concrete cores from a marine bridge column. A 4-year-old concrete column in Sweden was selected. The concrete was of high quality (i.e., frost- and sulfate-resistant, with a low-heat, low-alkali portland cement with a maximum water-cement ratio of 0.40) according to new Swedish recommendations. Concrete cores were drilled from the submerged, splash, and atmospheric zones. Selective rinding from the concrete surface (profile grinding) revealed concentration profiles of acid-soluble chlorides, carbonates, sulfates, and water-soluble alkalies. Selected parts of the concrete surface were examined by SEM and thin-section microscopy for microstructural studies. Laboratory estimates of chloride diffusivities were carried out on 6-month-old laboratory concrete of similar mix proportions, and also on unexposed parts of drilled concrete cores. Chloride diffusivities obtained from laboratory exposure were then compared with the values obtained from the field concentration profiles, from both the bridge column and a field station, using Fick's second law of diffusion. Maximum chloride diffusivities calculated from the field profiles after 4 years of exposure were more than ten times lower than those obtained from the same concrete in the laboratory. Clearly, there are important mechanistic problems associated with laboratory procedures, resulting in serious misjudgments, if such laboratory tests are used for linear extrapolation of the service life for marine concretes.

The repair and rehabilitation of bridge decks in the western countries and reinforced concrete structures in the countries along the Arabian Gulf is a major challenge to civil engineers. The need for repair of these structures results from concrete deterioration caused mainly by reinforcement corrosion. The use of deicer salts in North America and Europe accelerates reinforcement corrosion in bridge decks. Aggressive environmental conditions in the Arabian Gulf are responsible for deterioration of concrete structures in this area. This investigation was carried out to evaluate the durability performance of various repair materials. The repair materials were exposed to thermal variations to evaluate their performance in arid environments, such as in the Arabian Gulf. Durability performance was evaluated by measuring water and chloride permeability, and resistance to reinforcement corrosion. Results indicate that the water permeability in all the repair materials was less than that in plain concrete. Water permeability was significantly increased in all the specimens that were subjected to thermal variations, compared to those cured in the laboratory temperature. Ordinary cement mortar specimens indicated higher chloride permeability and lower corrosion resistance than other repair materials and plain concrete, which could be attributed to its lower electrical resistivity in saturated condition.

Presents field data of up to 8 years on chloride penetration into concrete and consequent steel corrosion in a test structure exposed to an aggressive environment favoring rapid transportation of chloride ions into concrete. The structure consisted of reinforced concrete beams, slabs, and columns. Two types of concrete, one without salt and the other containing 0.5 percent NaCl by weight of concrete, were used in the construction. Parts of the structure were left exposed and unprotected, while the other half was protected with a highly elastic acrylic rubber coating previously subjected to intensive examination. The chloride contents in the structural members were determined regularly over a period of 8 years. In addition, the influence of the coating and the different salt concentrations on corrosion of the embedded steel were evaluated. It is shown that the acrylic rubber coating can almost totally protect the concrete from chloride penetration and consequent steel corrosion and maintain this protective effect for many years.

Reports on part of a substantial research program on the properties of condensed silica fume (CSF) concretes cured in temperate and hot climates carried out in the Department of Civil Engineering at Loughborough. Three strength grades, C25, C40, and C55, were investigated, with the CSF (10 percent) mixes proportioned to have workability and 28-day strengths equal to ordinary portland cement (OPC) control mixes (when water-cured). The research examined the effects of curing environments (temperate and hot), curing time (1, 2, 4, and 8 days), and curing method (water and polythene) on the near-surface air and water permeabilities and water absorption of the concretes between 14 and 180 days. All specimens were subjected to diurnal temperature/humidity cycles representative of either a temperate or hot arid climate, the latter by storing specimens after casting in an environmental room. The research demonstrated that curing environment and duration greatly influence the permeability of ordinary portland cement and CSF concretes and, therefore, their potential durability in terms of resisting carbonation and ingress of other aggressive mediums such as chlorides. The hot-curing environment was favorable to the early-age absorption and permeability of ordinary portland cement mixes, but was detrimental at later ages compared with concrete cured in a temperate climate. The early- and later-age durability properties of CSF concretes were improved by the hotter curing environment.

The physical properties of concrete with additions of silica fume and activated natural amorphous silica were investigated. Much research has been carried out to this point regarding the effects of silica fume on concrete, and it has been made clear that silica fume additions are effective in improving strength characteristics, resistance to water permeation, etc. In the present study, the effect of addition of a mix of silica fume and activated natural silica on the properties of concrete was investigated. The naturally occurring amorphous silica was activated by first treating it with alkali, after which it was neutralized by acid treatment. It was found that addition of this activated amorphous silica and silica fume is effective in reducing the water permeability of concrete to a significant extent. The amount of silica fume required in this case is less than a third of what is normally used in silica fume concrete. SEM observations suggest that the decrease in water permeability is due to a denser structure being obtained because of the uniform distribution of the activated amorphous silica and silica fume particles within the concrete and the effective filling up of the interparticle spaces by calcium silicate gel.