International Concrete Abstracts Portal

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.

Practical use of high-strength concrete in Norway has shown that it is susceptible to cracking at early ages and that it normally is subjected to high curing temperatures due to high cement contents. The paper summarizes the results of a number of investigations at The Norwegian Institute of Technology and SINTEF research institute, both in Trondheim, Norway. The topics are: (1) early volume changes in the binder phase and early cracking sensitivity of high-performance concrete; and (2) the consequences of elevated curing temperatures on pore structure characteristics, permeability to chloride, and frost resistance of high- performance concrete.

Approximately 30 direct tensile tests have been performed on so-called Compresit matrix. This matrix is based on micro silica particles compacted in between the cement particles. The dense matrix, which shows high brittleness, is provided with ductility by means of steel fibers mixed randomly with respect to both position and orientation. Compressive strengths reaching 200 MPa are experienced with this particular matrix. The fiber reinforcement index is varied throughout the test series by means of three different fiber geometries and contents. Besides a plain mix without any fibers, the fiber reinforcement index is varied from 0.9 to 3.6, which is a wide range compared to other fiber reinforced concrete investigations. The test results consist of measured bridging stresses versus crack widths after the initiation of the first crack. A micro-mechanical model developed by V.C. Li, et al., is evaluated and compared to the results. This model agrees with low and moderate contents of both steel fibers and synthetic fibers. It is concluded that the micro-mechanical prediction does not seem to be sufficient to model the post-crack behavior of high-strength matrix reinforced with high amounts of steel fibers. However, the post-crack strength provided by the fibers crossing a crack plane is modeled satisfactorily.

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.