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.

Novel cements used in the making of very strong or otherwise high-performance cementitious materials are discussed. These include chemically bonded ceramics (CBC), a generic term describing ceramic-like materials formed by chemical reactions at ordinary temperatures. Innovations in chemistry, as well as processing, are responsible for the major improvements. MDF and DSP cements are discussed briefly, as are rapid-setting belite-sulfoaluminate and related cements. Applications of certain cements in waste management are also discussed. These include alkali-activated cements, which generate very high early strength, develop a working strength in about 4 hr, have relatively low porosity, and consequently have low permeability. A summary is included of the current status of the materials and their applications, limitations, and potential for further innovations, with a view to applications in the 21st century.

New superplasticizers are needed to produce concretes with less variability in the slump of the mix at the time of placement, and therefore with less variability in the water-cement ratio. In particular, there are two factors that affect slump at the time of placement: the procedure of superplasticizer addition (immediate or delayed) and time of transportation from the batching plant to the worksite. A new method to produce frost-resistant concrete in structures is needed that does not depend on the numerous factors affecting air volume and, therefore, the performance of air-entrained concrete. The preceding characteristics should allow the concrete to be manufactured under better quality control, which is already in place in the industrial process of other construction materials, such as steel, polymers, and ceramics.

In current construction practice, discontinuous fibers are added to cementitious matrices at relatively low volume fractions (usually < 1 .O%), mostly in order to improve the toughness, or the post-cracking ductility, of the composite. At these addition rates, there is relatively little improvement in strength. Moreover, there are no generally accepted methods of characterizing the improvements in other mechanical properties which the fibers may impart to the concrete. As a result, the various national structural design codes do not recognize fiber reinforced concrete (FRC) as a distinct material, and this inhibits its use in structural applications. However, there is continued research on the use of fibers in conjunction with conventional continuous steel reinforcement to improve the structural behaviour of concrete. In addition, a new generation of micro-fibers is being developed, which can be used at higher addition rates to bring about major improvements in the mechanical properties of FRC. In this review, current FRC technology is described. Likely future developments of FRC are also considered, such as applications in the design of concrete structures subjected to dynamic (blast, impact or earthquake) loading. The next generation of FRC materials will have the capacity of being tailored for a wide range of specific applications, and should be able to compete with other structural materials in a variety of applications.