International Concrete Abstracts Portal

Concrete for the twenty first century can be much stronger, more durable and at the same time cost and energy efficient. However, this will not be possible unless we understand this material better. In spite of its wide spread use, compared to other structural materials there is very little well organized expenditure on research and development of concrete. One critical gap in our understanding is relating microstructure with macroscopic properties, and relating what happens at the ionic level to the response of concrete structures. Integrating the understanding of microstructure with processing and engineering properties is one of the major goals of our Science and Technology Center for Advanced Cement-Based Materials in the United States, established in 1989. Interactions with Industry have flourished due to the coordinated multidisciplinary and multi-institutional approach of the center. The Industrial Affiliates Program has eighteen members representing a wide range of internationally active corporations who provide invaluable input regarding the commercial significance of the Center's research. An overview of some of our new research results will be presented. The center has made significant progress in (1) characterizing pore structure, (2) developing experimental tools and computer models to relate evolving pore structure with permeability and conductivity, (3) understanding rheology, (4) designing a new class of organo-silicate composites, (5) untlerstanding fracture process zone, and (6) high performance fiber reinforced composites.

Fly ash, a by-product from the coal-burning power generation process, is often used, for its pozzolanic properties and its fineness, to enhance the strength and durability of concrete and high-strength concrete. The quality assurance of fly ash is frequently questioned since its properties tend to vary depending on the source of coal, type of boiler, pulverizing equipment, and the removal efficiency of the air pollution control devices. Since fly ash is cornmonly used as one of the main components in the development of high-performance concrete, a closer look at the effects of fly ash on the properties of high-performance concrete is critical. In this study, two types of fly ash, dry and wet bottom ashes of different particle size distributions, were used. Physical and chemical properties of these fly ashes were tested and compared with the original feed fly ashes received directly from the utility. The effects of these fly ashes on the strength of concrete were studied when used as 15, 25, 35, and 50 percent cement replacement by weight of cement. The results show that fly ash, when proportioned properly, can enhance the properties of concrete. The chemical composition of fly ash of different particle size distributions varies slightly. For the same type of fly ash, the finer the particle, the higher the specific gravity. The smaller fly ash particle has a faster reactivity rate than the coarser one. The compressive strengths of several selected mixes of fly ash concrete are equal to, or higher than, the control concrete before the age of 28 days. For fly ash with large particle size distribution, the fly ash concrete reaches only 85 percent of the control concrete strength at the age of 180 days. It was also found that fly ash concrete exhibits excellent acid resistance when compared to conventional concrete.

This paper discusses recent research on high performance concrete with a focus on cemenentitious materials designed for durability. A major key to suchp erformance originates with the concrete microstructure. Recent advances in optimizing cement and concrete materials by using calculated packing diagrams offer the promise of superior products achieved by increased packing efficiency. A high packing density coupled with adequate processing and cement binder characteristics makes possible the formation of a fine microstructure. In turn, this fine microstructure results in a low permeability and therefore provides a resistance to aggressive forces from the environment, which together enhance its long term durability. The favorable interaction among physical and chemical phenomena gives rise to better long term performance, whether the application is structural, or chemical, such as in waste management.

Technology transfer within the construction industry is difficult due to (1) organization of the industry, (2) lack of construction industry participation in research, (3) societal impediments. These barriers are discussed in relation to transferring innovation to practice. Differing activities that can mitigate the barriers are discussed and recommendations are offered to enhance the transfer of technology from research results to application in the construction industry.

Properties of high strength concrete under uniaxial states of stress were studied. Main emphasis was given to tensile, compressive, and fracture properties of concrete with compressive strengths ranging from 6000 to 12000 PSI. Complete stress-deformation curves under uniaxial tension were obtained using a closed-loop servo-hydraulic testing system. Important mechanical and fracture properties such as moduli of elasticity, fracture energy, and the critical crack tip opening displacements, were evaluated from the experimental results. Fracture energies were evaluated from the descending branch of stress-crack separation curves using the direct tension test results. For the range of high-strength concretes studied, experimental results indicate that the relationship between tensile and compressive strengths are different from those of normal strength concretes. Comparison of stress-deformation curves in tension reveals a significant decrease in post peak compliance of the higher strength concretes.