International Concrete Abstracts Portal

Inspection and service reports from concrete platforms up to 20 years old in the hostile environment of the North Sea are positive. The need for remedial work has been minimum and these high-quality concretes demonstrate excellent performance under marine conditions. To meet the demands for durability and development in structural design and construction methods, a continuous effort has been made to advance concrete materials. Improved concrete properties are predominantly a consequence of improved cement qualities, more efficient admixtures, and better controlled processing of aggregates. The soundness of the aggregates has been verified from the start. Examinations of concrete specimens drilled out from different elevations of some of the platforms have revealed rather high concentrations of chlorides close to the concrete surface. For most of the specimens, the chloride content is, however, negligible at the reinforcement. Epoxy coating applied to the concrete surface in the splash zone of some platforms has shown to be efficient in preventing ingress of chlorides.

Despite the excellent resistance of high-performance (HP) concretes in the presence of aggressive agents, instances of application have shown that the microstructure of the concrete surface can be greatly disturbed by the curing method, thereby compromising durability on the part covering the reinforcement. Paper presents results of a study on three concrete design mixes (one reference concrete and two HP concretes with and without silica fume), each subjected to three curing methods and three durability tests. Results on carbonation, variation in free lime, and microcracking indicate that HP concrete with silica fume is more sensitive to the curing method than the reference concrete or concrete without silica fume, as evidenced by increased carbonation and a larger reduction in alkalinity. The study of microcracking in the various concretes showed that desiccation causes more microcracking in the HP concrete with silica fume than in the HP concrete without silica fume. Results of microstructural inspection and physical and chemical tests explain these variations in mechanical properties and carbonation behavior of various concretes, depending on the curing method.

In this study, results are reported on classified fly ash with a maximum particle diameter of 10 m. Durability of concrete with this fly ash has been verified experimentally. Silica fume, which is well known for its improvement of concrete durability, was used as the basis of comparison. Durability tests included water permeability, freeze-thaw resistance, air-void spacing factor, pore size distribution, and pozzolanic activity. Based on the experimental results, remarkable reduction of the water diffusion coefficient, enhancement of freeze-thaw resistance, densification of air-void structure, increase of noncrystalline products, and other features were noted, and it was determined that classified fly ash is an effective admixture that contributes to the improvement of concrete durability.

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.