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.

The fact that a stable air-void system in concrete enhances its durability is well documented today. In Denmark, for example, all concrete to be used for exterior components is air-entrained. The main problem is to insure the stability of the air-void system in the concrete, independent of mixing time, transportation, consistency, fines content, and type of plasticizer or superplasticizer. A new type of air-entrainer has been developed that provides excellent performance in concrete containing a melamine sulfonate-type of superplasticizer. The concrete has a slump of more than 150 mm, and even after 60 min of transportation and pumping, the air-void content and air-void structure are still satisfactory. The air-entrainer is a combination of special surfactants chosen after a long series of tests. Carrying out such a range of tests within the short time available was possible only because of newly developed testing equipment, which made it possible to measure air content and air-void system parameters within 30 min while the concrete was still fresh. Results of the testing, in accordance with ASTM C 457, are presented.

The environmental condition in the coastal areas of the Arabian Gulf are considered to be very aggressive with regard to concrete durability. The reduction in the useful service life of concrete structures in this region is attributed to an interplay of geomorphic and environmental factors characterized by high concentrations of chloride and sulfate, high ambient temperature and humidity, daily and seasonal variations in temperature and humidity, contaminated groundwater at shallow depths, and contaminated and absorptive aggregates. While the major cause of deterioration is reinforcement corrosion, degradation of concrete due to sulfate attack and salt weathering are not uncommon. Paper presents an overall view of concrete deterioration phenomena in aggressive service conditions, such as the Arabian Gulf, highlighting the role of each degradation phenomenon and the need for further research to produce durable concrete that is economical and has a long service life.