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.

During the 1980s, the use of high-strength concrete gained wide acceptance. The materials and mix proportions for making high-strength concrete are selected empirically by extensive laboratory testing since there are no accepted procedures, such as the ACI method of proportioning normal concrete mixtures. For someone who, for the first time, would like to make high-strength concrete from local materials, the problem is complicated by the fact that a variety of newly developed chemical and mineral admixtures may have to be incorporated simultaneously into the concrete mixture. The published literature has enough information on the new admixtures, but is essentially of little help in selecting the type and optimum dosage of these admixtures. In this paper, the authors have attempted to address the problem of selection of materials and mix proportions for high strength from a microstructural standpoint. Principles underlying the strength of brittle solids are discussed and important features of concrete microstructure, which influence the strength, are described. Microstructural considerations are used as a basis for the selection of materials and for establishing guidelines that are helpful in the development of a simple procedure for concrete mix proportioning.

Discusses the need and requirements for a method of proportioning high-strength concrete mixes. The development of the method, which is based on a well-established method used for conventional concrete, is described. Design charts are given for various stone sizes, and an example of such a chart is illustrated. Because the method is based on easily determined aggregate properties, it is suitable for any type of aggregate: crushed or naturally occurring stone and sand, and graded or single-sized stone.

Results of studies on the application of a high-strength concrete, with compressive strength of 42 to 60 MPa, to a high-rise reinforced concrete residence are presented. First, experiments were performed in accordance with the construction procedure, applying full-scale test structure modeling on part of the actual building. As a result, workable high-strength concrete was achieved by using a high-range water-reducing agent at the plant where concrete is being manufactured, and by adding a superplasticizer and placing the concrete carefully on site. In addition, for the quality control method of a ready-mixed concrete, water-cement ratio measurement before placement was useful. It is desirable to control the structure strength of high-strength concrete by not only using a test specimen cured in water on site, but also by taking out core specimens. Secondly, requirements for a construction method were set, by reference to the test results, and construction of the actual building was undertaken. Results of all the tests satisfied the requirements necessary to demonstrate the stable manufacturing control of ready-mixed concrete.