International Concrete Abstracts Portal

The mention of alkali-aggregate reactivity (AAR) often conjures up visions of an intolerable and unremediable cancerous disease to which all concrete is subjected. It generates a lack of precise and thorough understanding of the phenomenon and strikes fear in the minds of concrete technologists, engineers, and the public alike. The aim of this paper is to put the phenomenon of AAR in a proper perspective in relation to its mechanisms, effects, and influences. An attempt is made to unravel the mysteries of AAR and the myths and mythologies associated with it. Paper describes the behavior and method of operation of the phenomenon, and assesses its impact on the engineering properties of concrete and structural performance of load-bearing members. It is shown that it is possible to check, and indeed contain, the effects of the attack both in new construction and in existing structures.

It is now commonly accepted that there are two basic frost durability problems: internal cracking due to freezing and thawing cycles, and surface scaling, generally due to freezing in the presence of deicer salts. Although there are still parts of the problem that are not well understood and warrant further investigation, particularly with respect to the differences between laboratory tests and field exposure, the way to make concrete resistant to freezing and thawing cycles is very well known. It is simply to insure that the hardened concrete has an adequate system of entrained air voids. Field experience as well as laboratory data have shown conclusively that internal cracking due to frost in properly air-entrained concrete is almost nonexistent. In the years to come, it will be necessary to increase our knowledge of some of the parameters that influence air entrainment, particularly in the new types of concrete that are being used, such as, for instance, high-strength concrete and roller compacted concrete. Simple methods to determine the characteristics of the air-void system in fresh concrete will also be required. Scaling due to freezing in the presence of deicer salts is a much more complex problem than internal cracking for many reasons, but probably mainly because it is related to the microstructure of the surface layer or "skin" of concrete. Laboratory as well as field data are often contradictory. But if scaling is a complex problem, this does not mean that it is a monumental one. It is only one aspect of the complex question of the durability of concrete structures.

Discusses the severe deterioration of concrete that can result from an unusual form of sulfate attack, in which C-S-H and calcium hydroxide in the cement paste are converted to gypsum and thaumasite. This deleterious reaction only occurs at low temperatures in the presence of continuous moisture and where sources of sulfate and carbonate ions are available. Severe deterioration of this type occurred in concrete columns and slab-on-grade at facilities in the Canadian Arctic within 2 years of casting, to the point where some replacement was necessary. Evidence of continuing deterioration led to an extensive investigation of the structures when the concrete was 4 years old. Although it has been relatively well documented in Europe, a few cases of concrete deterioration due to thaumasite formation have been reported in North America. Currently, North American codes and standards do not include any guidance for the avoidance of this type of sulfate attack. Paper contains data that should be of interest to agencies responsible for the development of codes and standard specifications for concrete construction practices in cold areas.

Presents the results of an experimental investigation conducted to determine the flexural fatigue strength of high-strength lightweight concrete under water. This concrete was produced using expanded shale aggregate and high-performance concrete admixtures such as silica fume and superplasticizer. Properties of fresh concrete and elastic and mechanical properties of hardened concretes are presented. The fresh concrete was tested for slump, air content, unit weight, and temperature. The hardened concrete was tested for moist-cured dry weight, compressive strength, modulus of elasticity, and flexural fatigue strength. The investigation indicates that a highly workable high-strength lightweight concrete can be produced successfully. The high-strength lightweight concrete had a higher endurance limit (10 to 16 percent) than normal weight concrete of equal compressive strength. In general, there was no reduction in the flexural fatigue strength for the lightweight concretes when tested under water. The static flexural strength determined from specimens that had successfully resisted 2 million cycles was always greater than that of specimens which had not undergone fatigue loading.

This research was conducted to present state-of-the-art information on fatigue behavior of plain concrete with and without mineral admixtures and to evaluate fatigue characteristics of Class C fly ash concrete under flexural stress. A number of studies have shown that concrete fatigue strength is significantly influenced by a large number of variables, including stress range, loading rate, load history, stress reversal, rest period, stress gradient, material properties, etc. Effects of these parameters on fatigue characteristics of concrete are addressed. In general, endurance of fatigue flexural limit of plain concrete was found to vary between approximately 50 and 70 percent of its static flexural strength. But it can be lower than 50 percent when concrete is tested in water. Experimental investigations conducted in this research revealed that a fly ash concrete mixture with 15 percent cement replacement showed superior performance relative to high-volume fly ash mixtures with 50 percent cement replacement with respect to compressive strength and flexural fatigue strength. However, fly ash concrete mixtures showed essentially the same results when the flexural fatigue strength was expressed as a percentage of the flexural static strength.