International Concrete Abstracts Portal

Presents the results of an investigation undertaken at the Technical University of Denmark to determine the parameters that affect the ultimate load capacity of a concrete structure subjected to concentrated loads originating from reinforcement bars bent 90 deg. The following parameters have been found to have a decisive influence on the ultimate load capacity of the concrete bar: bar diameter, internal height of the specimen, side concrete cover, and concrete compressive strength. The results show that the relative load-carrying capacity of the concrete åc / fc decreases for increasing concrete compressive strength. However, the use of high-strength concrete (HSC) still results in an increase in the absolute load-carrying capacity of the concrete whencompared to normal strength concrete (NSC).

A 5-year National Research Project on advanced concrete buildings with high-strength and high-quality materials has been in progress in Japan since 1988. A High-Strength Concrete Committee was organized to establish guidelines to be used in applying the high-strength concrete of 30 to 120 MPa to reinforced concrete buildings; it has started to investigate the following items: development of cements, aggregates, chemical admixtures, mineral admixtures of high-strength concrete and establishing of the quality standards of these materials and the design method of mix proportion; establishing the evaluation method for properties of fresh concrete required in construction; establishing of evaluation methods for compressive strength and other properties of hardened concrete; and establishing of the quality control procedure and evaluation method for concrete strength in structures. Paper describes the problems of production, transportation, and placement when high-strength concrete is applied to reinforced concrete buildings standing in seismic zones and urban areas such as Tokyo. The results obtained from the preliminary studies and experiments by the high-strength concrete committee will also be briefly described.

Compressive strength and water penetration of three grades of high-strength concretes with cement contents ranging from 400 to 500 kg/m3 and a proprietary superplasticizer are reported. The control mixes were redesigned by adding a Class F-type fly ash at fly ash/cementitious ratios of 0.15 and 0.35. All concretes were designed for a similar workability. The strength development was monitored in three curing regimes. It is concluded that the superplasticized concrete developed a higher strength than that predicted from a reduction in the water/cement ratio. The curing conditions significantly influenced the strength development and the water penetration of the concretes. An optimum fly ash/cementitious ratio of 0.15 was found to be appropriate for the concretes; larger amounts of fly ash were found undesirable for higher strength development.

High-performance concrete has been made using different cementitious combinations: portland cement and fly ash; portland cement and silica fume, and portland cement, ground granulated slag, and silica fume. The use of a supplementary cementitious material like fly ash or ground granulated slag is not only interesting from an economical point of view but also from a rheological point of view. Replacing in some cases up to 20 percent of cement by a less reactive cementitious material like fly ash or up to 50 percent by ground granulated slag can solve the slump loss problem observed with some very reactive cements when used at water/cement ratios ranging from 0.25 to 0.30. Moreover, the use of a supplementary cementitious material results in a significant decrease in the superplasticizer dosage needed to achieve a given workability. In terms of rheology, compressive strength, and cost, one of the most promising combinations of cementitious materials for high-performance concrete is a mixture of ground granulated slag, silica fume, and portland cement, when ground granulated slag is available at a reasonable price.

Three earthquake-type loading tests of reinforced concrete (RC) columns, short beams, and beam-column joints using high-strength concrete were carried out. The main objectives of this program were to investigate the seismic behavior of RC members using high-strength concrete, and to obtain guidelines for their design in high-rise buildings. Concretes having three levels of compressive strength, 400, 600, and 800 kg/cmý (39, 59, and 78 MPa), were used. High-strength reinforcing bars with nominal yield strengths of 8500 and 14,000 kg/cmý (834 and 1370 MPa) were provided for lateral reinforcement. Longitudinal reinforcement with a yield strength of 6000 kg/cmý (588 MPa) was also used for beam-column joint test. Emphasis was put on the combination of high-strength concrete and high-strength reinforcing bars. The seismic behavior of columns, short beams, and beam-column joints under high axial load, high beam shear, and high joint shear, respectively, were observed. The relationship between ductility and amount of lateral reinforcement were particularly discussed in the column and short beam tests. In the beam-column joint test, several joint details were considered, and their behavior was investigated. The design guidelines for these high-strength concrete members were also presented in this paper. The results of this experimental program show that the combination of high-strength concrete and high-strength steel bars can be quite effective in improving strength and ductility of RC members of high-rise buildings.