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.

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.

The effects of steam-curing at atmospheric pressure were investigated on three kinds of concrete: pozzolanic portland cement concrete, high-strength portland cement concrete, and high-performance concrete manufactured with high-strength portland cement containing a superplasticizer and retarder. The early and final compressive and flexural strengths, coefficients of capillary absorption, and apparent porosities were determined on prismatic specimens stored for 140 days in water and ammonium nitrate solution. The dynamic moduli of elasticity were also measured every week during the storage period. Two steam-curing cycles with maximum temperature of 60 and 80 C, respectively, were used. The steam-curing processes were found to be suitable and less detrimental for high-performance concrete compared to pozzolanic portland cement and high-strength portland cement concretes.

The Siberian Metallurgical Institute (SMI) has developed low-cement concrete with acid and basic slags from a Russian foundry. The concrete contains neither natural aggregates (crushed stone, gravel, or sand) nor artificial porous aggregates (claydite, aggloporite, polystyrene, or others). It includes the following materials: 680 to 1140 kg/m 3 of acid slag sand form PTP with a particle size of 0 to 5 mm used as fine aggregate; cementitious materials (340 to 400 kg/m 3 of fine-grained basic slag and 100 to 170 kg/m 3 of M500 portland cement); 260 to 290 kg/m 3 of water; 0.3 percent by weight of cement of plasticizing admixture (technical grade lignosulfonate); and 1 to 2 percent of air-entraining admixture (secondary sodium alkyl sulfate). To make the wide application of this concrete possible in construction (mainly for low-rise cottages), deformation properties, protection of reinforcement from corrosion, frost resistance, and water and gas permeability were studied during a 1-year period. The investigation shows that concrete developed in this way complies with Russian international standards for low-strength concrete and can be used in housing construction. Air-entrained concrete for external walls should be protected by mortar or some other finishing material.