International Concrete Abstracts Portal

SP-144 Eighteen review papers and twelve research papers are included in the Proceedings of the Mohan Malhotra Symposium on Concrete Technology: Past, Present, and Future. The purpose of the Symposium was to serve as a forum for discussion on the current state of the concrete industry and technology, and to identify important issues that need to be addressed in the future. The proceedings of the Mohan Malhotra Symposium, which excel for the variety and richness of information contained in the 30 papers, reflect the respect and admiration of the authors for the honoree. As a researcher, scholar, and technology-transfer crusader, Mohan has undoubtedly made unique contributions to the concrete technology. It would indeed be a fitting tribute to him if the deliberations of the Mohan Malhotra Symposium are able to make a significant impact in preparing the concrete industry for the 21st century.

Highlights the importance of insuring uniformity of in-place concrete for quality assurance in construction. The functional requirements of strength, serviceability, and durability can be assured by quality assurance in design, specification, choice of materials, and workmanship. Steps to insure uniformity of cement received in an irrigation project from different sources are described. Tests to establish uniformity of concrete in dam construction and shotcrete for tunnel lining are highlighted with the help of case studies.

Roller compacted concrete (RCC) became an accepted method for dam construction during the latter half of the 1980s and during the early 1990s there has been a very rapid growth in this form of dam. High dams up to 200 m in height and containing several million m 3 of concrete are now being designed. Paper describes the development of RCC dams over the last decade and shows how there has been a move away from the early RCC dams that contained a relatively low cementitious content toward a dam that contains higher cementitious contents, usually with a high proportion of pozzolan. Although a number of different pozzolans have been used, the significant majority of RCC dams have contained low-lime fly ash. The in-situ properties that have been achieved in RCC dams already in service for a number of years have been found to be suitable for gravity dams more than 200 m high. Arch-gravity RCC dams have been completed with heights approaching 80 m, together with a thick-arch RCC dam of a comparable height. It is considered that the traditional concrete dam will be superseded within the not-too-distant future by the RCC method of construction, and on a number of sites where rock-fill dams were originally planned, RCC dams will eventually be constructed.

Recent literature suggests that there is considerable potential for reduction in the emission of CO 2 to the environment through the manufacture of new types of cement that do not rely on the calcination of limestone (and accompanying release of CO 2). The 1988 1-billion metric t worldwide production of cement accounted for 1 billion metric t of CO 2 release, i.e., 5 percent of the 1988 world CO 2 emission (human activity only). This is equivalent to the CO 2 emission of all Japanese activity. The use of lesser amounts of calcium-based cements could be achieved through their partial replacement by alkali-activated alumino-silicate materials, which do not release large quantities of CO 2 in their manufacture. The fostering of low-CO 2 high-alkali-based cements will mean a dramatic change in the research and development presently carried out in the USA and other countries. Alkalies are generally thought of as the cause of deleterious alkali-aggregate reaction. As a result, the tendency has been to avoid any addition of alkali portland cement products, often requiring cement manufacturers to supply low-alkali cements. The use of MANSMR spectrography for the determination of composition of alkali-activated cements, in combination with ASTM C 227 bar expansion, allows the prediction of potential for alkali-aggregate reaction. A preliminary study involving Al and Si MANSMR spectroscopy revealed that the alkali-activated alumino-silicate cements are the synthetic analogues of natural pozzolans that are known to effectively suppress alkali-aggregate reaction. These cements, even with alkali contents as high as 9.2 percent, do not generate any deleterious alkali-aggregate reaction, according to the ASTM C 227 bar expansion test. Industrial experience based on the use of alkali-activated slags in Eastern Europe since 1964, associated with the commercially produced alkali-activated cements in the US since 1988, suggest that high-alkali cements will ultimately improve the concrete used in buildings and highways, and also serve global need by reducing emission of CO 2 and reducing energy consumption during cement manufacturing. In terms of a 5 percent growth scenario, the predicted business as usual (BaU) world cement production for the year 2015 equals 3500 million metric t. Based on an amount of blended portland cement production on the order of 1850 million metric t (1000 portland + 560 slag + 290 fly ash) in the 21st century, the need for novel alkali-activated cementitious materials could be in the range of 1650 million metric t.

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.