International Concrete Abstracts Portal

Corrosion of steel reinforcement within concrete structural elements is a major problem in both research and practice. Laboratory studies have been conducted on fundamental mechanisms of corrosion within concrete in the presence of high chloride and others under conditions of reduced alkalinity. However, little has been published on the interactive effects of these two conditions and the ways in which corrosion rates of steel in concrete are thereby influenced. These two conditions occur concurrently under many practical environmental exposures. This paper presents data on methodology used to determine corrosion rates of steel in concrete. Information on corrosion activities in both carbonated and high-chloride environments is presented with reference to mechanisms involved in breakdown of steel passivation. Interactive effects of the two conditions are examined for a range of concrete types and grades. The data suggest that for normal reinforced concrete structural elements, the interactive effects of carbonation and chloride ion ingress lead to much more rapid corrosion than where the two phenomena occur independently. The interactive effects of carbonation and chloride ions as they influence concretes under service conditions are discussed. In particular, the reduction of carbonation rate in the presence of high-chloride ion concentrations is noted.

The sulfate resistance of concretes with compressive strengths between 60 and 110 MPa was evaluated. The test comprises several soaking/drying cycles of samples in a Na2SO4ù10H20 solution, followed by measurement of mass variation and residual compressive strength. Visual inspection and sulfate recovery by distilled water immersion increased the accuracy of test results. Results reveal significant differences compared to those tests normally used, involving prolonged immersion. The resistance to sulfate attack depends on concrete porosity and capillary absorption and not on permeability, because pozzolanic reactions seem to interrupt pore continuity. The reduced water-cement ratio obtained with the aid of the superplasticizer was much more effective than the chemical characteristics related to the presence of mineral admixtures in concrete regarding its resistance to sulfates.

The durability of different types of cement to ammonium nitrate attack was studied. The tests were conducted on mortar prisms kept in ammonium nitrate solutions at different concentrations, ranging from 0.0 to 50.0 percent. Five types of cement were used: ordinary portland, pozzolanic, blast furnace, aluminous, and supersiliceous. Changes in length and weight were registered during 14 years; also determined were the mechanical strength and changes in composition. Initially, the most concentrated solution was the most aggressive, but due to crystallization of the ammonium nitrate in the pores of the mortar, its relative aggressivity diminished with time, so that it was the 0.5 and 5.0 percent solutions that showed the greatest aggressivity. The aluminous cement showed the best resistance to ammonium nitrate attack. Of the other cements, blast furnace cement was the one which best resisted attack.

Long-term durability of structural lightweight concrete used in bridges, ships, and buildings is reviewed. Particular attention is given to mature structures located throughout the world that have been subjected to severe weather conditions. Ongoing testing programs carried out on structures subjected to several decades of exposure are reported. The nature of both the vesicular lightweight aggregate itself as well as the interfacial contact zone between aggregate and cement paste matrix are analyzed, as the microstructure of lightweight concrete reveals factors that contribute to long-term durability. The information gained on the microlevel is used to explain observed performance, and provides a basis for predicting behavior. To facilitate the practical design of durable structures, long-term field exposure studies of normal weight and lightweight concretes are being conducted to assess their relative performance in a severe environment. The results obtained from ongoing testing programs conducted by the Canadian Concrete Technology Section of CANMET at the U.S. Corps of Engineers Treat Island Severe Weather Exposure Station are discussed in relation to the design process.

Concretes having strengths ranging from 54 to 73 MPA and densities ranging from 1920 to 2080 kg/m3 were produced from two lightweight aggregate sources. Supplementary cementitious materials (including silica fume, fly ash, and ground granulated blast furnace slag) were used in the mixtures. Test specimens were subjected to a variety of freezing and thawing test procedures and conditioning methods. These included standard ASTM test procedures as well as procedures designed to simulate service in arctic offshore environments. The high-strength lightweight concretes exhibited outstanding performance, with virtually no degradation during standard freezing and thawing testing. Prolonged exposure was needed to cause significant damage under simulated arctic offshore conditions. Durability was found to be a strong function of cumulative freezing and thawing cycles and moisture content, with saturation of aggregates prior to test leading to premature failure.