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In general, the structural member using high-strength concrete is accompanied by high brittleness, which may result in the unexpected dangerous failure. For economy and safety, high-strength concrete may be used for compressive members (vertical members) and low-strength concrete for flexural members (horizontal members). ACI 318-89 recommends that when the specified compressive strength of concrete in the column is greater than 1.4 times that specified for the floor system, the column concrete shall extend 600 mm into the slab from column face to avoid unexpected failure. The structural behavior of beam-column joints with two different compressive strengths of concrete for the beams and the columns has not been investigated adequately. ACI-ASCE Committee 352 recommends that for joints that are part of the primary system for resisting seismic lateral loads, the sum of nominal moment strengths of the column sections above and below the joint ( M c), calculated using the axial load, which gives the minimum column moment strength, should not be less than 1.4 times the sum of the nominal strengths of the beam sections at the joint ( M b). Thus, those recommended values should be examined before high-strength concrete can be used with confidence and convenience in structural members. The results showed that the ACI 318-89 extension distance of 600 mm is safe at least for members up to 300 mm in total depth, and the 2h (h is overall depth of the beam) extension distance was found to be safe also for members under flexural loading with a column-to-beam flexural strength ratio of 1.8.

The time-dependent properties of high-strength concrete subjected to tensile and compressive loading have been studied experimentally and have been compared with those of normal concrete. Two kinds of load application were used during this investigation: loading with a constant strain rate and sustained loading. The range of strain rates is chosen between the so-called static and the creep strain rate limits. The ratio of adopted stress to 28-day prismatic strength in the sustained loading tests was chosen at 0.15, 0.35, 0.50, 0.75, 0.85, and 0.95. The research program mainly focused on the influence of the type of load application on the behavior of high-strength concrete in compression and tension. The phenomena observed in the experiments are interpreted by referring to a basic mechanism of rate sensitivity of concrete. The differences of the material structure between high-strength concrete and normal strength concrete are emphasized. In general, it is found that some properties of high-strength concrete in compression, such as strength and deformation characteristics, are more sensitive to the strain rate than those of normal strength concrete, whereas in tension, this tendency is less pronounced. On the basis of the test results, the long-term strength of high-strength concrete is defined.

The shear friction analogy is a valuable and simple tool that can be used to estimate the maximum shear force transmitted across a cracked plane in a concrete member. The expressions to determine the shear friction capacity up to now have been based on experiments on concretes with cylinder strengths of at most f' c = 60 N/mm 2. In such concretes, the aggregate particles normally do not break at the formation of cracks through the concrete. In high-strength concrete, however, the cement matrix is strong enough to cause fracture of the aggregate particles. As a result, the crack faces are relatively smooth, so that the shear friction capacity is expected to be reduced. In this paper, shear friction tests are described on concrete with a cylinder strength of f' c = 100 N/mm 2. The experiments are carried out on cracks in plain concrete and on reinforced cracks. It is shown that the reduction in shear friction capacity due to aggregate fracture is considerable.

The Strategic Highway Research Program (SHRP) awarded a contract to North Carolina State University (NCSU) to investigate the use of high-performance concrete (HPC) in highway pavements and bridge structures. The goals of the project were threefold. First, a number of HPC mixtures were developed for highway applications. Second, laboratory testing of the HPC mixtures was conducted. Finally, a number of field test sites were constructed and monitored. Three different classes of HPC were established for this research. These are very early-strength (VES), high-early-strength (HES), and very high-strength (VHS) concrete. Two types of VES and VHS concrete were developed. The VES mixture was developed for use primarily as a rapid repair material where time is critical and cost is a lesser concern. The HES mixture was developed for bridge deck construction where deterioration due to freezing and thawing and steel corrosion is a major problem. The HES mixture can also be used for repair where cost is important and time is a lesser concern. The VHS mixture was developed for use in bridge structures where high-long-term strength is needed rather than rapid strength gain characteristics. Paper summarizes the development of the mixture proportions for the three classes of HPC. Included in the paper are the strength and serviceability requirements for the mixtures. Recommendations are made for adapting the HPC mixtures for local conditions.

A significant proportion of Australian infrastructure is located in a zone that is close to or in direct contact with seawater. At most of these locations, the coastal environment is coupled with high ambient temperatures and large diurnal temperature ranges, conditions that are conducive to promoting corrosion of steel reinforcement in concrete structural elements. Users of concrete are thus always looking for ways to maximize concrete performance for long-term use under these aggressive conditions. The options available in terms of binder systems for concretes in a marine environment have increased in recent years. There are currently available a range of cements and blended cements that include fly ash, slag, and silica fume, which have a place in specifications for marine concrete applications. To provide technical data for potential specifiers and users of such concrete types, a collaborative CSIRO-CSR research and development project was initiated to consider the performance of a range of concretes for marine environments. Concretes considered had a water-binder ratio of 0.35 and included both portland and blended cements. Paper reviews current standards on specifications of concrete for marine environments and goes on to present some recently produced Australian data for different concretes reflecting potential performance. Techniques considered include chloride-ion penetration of concrete based on charge transfer measurements, chloride-ion penetration through concrete, and some mechanical properties of concrete. Conclusions are drawn as to the suitability of certain concrete types under marine conditions.