International Concrete Abstracts Portal

Corrosion of reinforcing steel is now recognized as the most critical limit state affecting the durability and long-term stability of concrete structures. Although concrete is inherently alkaline and provides a natural protection to any embedded steel in it, thereby insuring its chemical passivity, concrete structures deteriorate for a variety of reasons. Steel corrosion is the most insidious and destructive form of damage, and once it starts, it is almost impossible to stop the process (unless such remedial measures as cathodic protection systems are applied), until eventually the safety, stability, and design life of the structure are all drastically reduced in time. The primary causes of steel corrosion are inadequate cover to steel, carbonation, neutralization due to atmospheric pollutants, and/or chloride penetration. The quality of concrete, its pore structure, and permeability characteristics are thus major factors controlling steel corrosion. However, it is inherent in the nature of concrete construction that there is no single method of corrosion protection that may be presumed to insure satisfactory serviceability throughout the life of a structure. This paper briefly reviews the factors that influence concrete deterioration and loss of alkalinity. It is shown that long-term durability of concrete structures can only be insured by a dual approach--developing a durable concrete, and providing a protective coating to the steel. Such methods of steel protection as cement-slurry coating, epoxy coating, and galvanizing are discussed. Extensive test results are presented on the corrosion resistance of plain, epoxy-coated, and galvanized bars when exposed to a marine environment, and chloride intrusion in cracked members. Cracking, cover, and concrete quality are identified as the major parameters influencing steel corrosion, but cover to steel is the most critical factor in preserving the electrochemical stability of steel. The paper shows that both epoxy-coated reinforcing bars and galvanized reinforcing bars can provide excellent resistance to chloride-induced corrosion.

Experiments were conducted to determine the influence of immersion vibration on the air-void system parameters of air-entrained concrete, as a function of both radial distance and depth from the point of vibrator insertion. For a 1½ in. (40 mm) diameter immersion vibrator, one could conclude that vibration has little or no effect on air-void systems at distances of 5, 8, or 10 in. (125, 200, or 250 mm) from the point of insertion. The same vibrator in the same concrete can reduce the total air content by 50 percent, and increase specific surface by as much as 100 percent directly at the point of vibrator insertion. Which particular effect one may observe in hardened concrete, therefore, depends on the selection of core location relative to point of vibrator insertion. These observations have implications for specifying, casting, and testing air-entrained concrete.

Good curing is now recognized as essential to achieving good durability of concrete and other cementitious material surfaces. However, it has not been easy to judge whether or not it has been achieved on site, so surface failures continue to occur. The Department of Civil Engineering at the Queen's University of Belfast is developing a number of test techniques to allow the measurement of surface strength, surface absorption and permeability, and surface abrasion-resistance of structures on site. These have been used to assess the performance of various curing regimes for concrete and mortar, first to see if the test methods can extract meaningful measurements of durability-related properties, and secondly to get an indication of the magnitude of the changes in these properties for different curing regimes and water-cement ratios. It is hoped that they may eventually provide a means to assess a surface in terms that could allow an objective judgment of its durability.

The principal mechanism for the deterioration of concrete is transport of fluids either into or out of the pore structure of hardened body. The fluid transport occurs via a complex network of interconnected porosity incorporating both the cementitious matrix and matrix/aggregate interfacial regions. Paper describes the development of an experimental method and a mathematical background for a rapid water-permeability measurement method and a mathematical model relating porosity, described in terms of a log-normal distribution, to permeability.

Among the important stages of the overall SR process as it occurs in concrete is the conversion of alkali sulfate dissolved from the cement to alkali hydroxide. The results of laboratory studies are presented to provide an understanding of this process and its effects. This conversion process depends on continued formation of ettringite and does not take place until the cement gypsum is exhausted. Provision of a large excess of gypsum or interference with early ettringite production by high-temperature exposure may postpone or prevent it, thus reducing the OH - ion concentration and the possibility of ASR reaction.