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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.

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.

This paper provides a summary of part of the results of SHRP project C-205 on the Fresh and Hardened Properties of High Early Strength Fiber Reinforced Concrete (HESFRC). HESFRC was defined as achieving a minimum target compressive strength of 5 ksi (35 MPa) in 24 hours. Fresh HESFRC properties included air content, inverted slump test, temperature, and plastic unit weight. Tests on the mechanical properties included compressive strength, elastic modulus, flexural strength, splitting tensile strength, and fatigue life. Seventeen different combinations of parameters were investigated for each type of test. The main parameters included: (1) three different matrix mixtures (one control, one with silica fume, and one with latex), (2) two different volume fractions of fibers (1 percent) and (2 percent), (3) two fiber materials (steel and polypropylene), (4) two steel fiber lengths corresponding to aspect ratios of 60 and 100 respectively, and (5) hybrid mixes containing either an equal amount of steel and polypropylene fibers, or an equal amount of steel fibers of different lengths. The compression and the bending tests also included a time variable; the compressive properties were measured at ages 1, 3, 7, and 28 days, and the bending properties at ages 1, 7, and 28 days respectively. Information from the compression tests comprised the compressive strength, the elastic modulus, and the strain capacity. Information from the bending tests included the modulus of rupture and the toughness indices as per ASTM standards. Optimum mixtures that satisfied the minimum compressive strength criterion, and showed excellent values of modulus of rupture, toughness indices in bending, and fatigue life in the cracked state are identified. Potential applications in repair, rehabilitation, or construction of transportation structures are suggested. In this paper a description is given of key results of the bending tests only.

Flowable concrete has been used for a long time in Thailand and has been known as tremy concrete for bored pile construction. For this type of concrete, strength may be less important in early development, but workability must satisfy the performance in construction. In the second stage of development, research has concentrated on high strength concrete. Even though higher strengths have been achieved, workability of the concrete may be slightly poor. Several studies have emphasized utilization of concrete here high strength can be an advantage. The structural members were both reinforced concrete and prestressed concrete; and the emphasis was placed on flexure, shear, and compression. In recent development, both workability in the fresh state and strengths in the hardened states have been considered so as to satisfy the performance in workability, strengths, serviceability, and durability. Efforts have been concentrated on constituent materials for high performance concrete, mix proportions, and concrete properties. Satisfactory structural behaviors for compression, flexure, and shear in reinforced and prestressed concrete members have been reported. Various developments to suit particular applications such as bored pile, mass concrete, structural concrete, and durable concrete have been made for the construction industry. Demands of HPC are growing, further development and utilization are expected.