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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.

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.

Concrete repair materials applied in thin layers often fail under severe weathering conditions and high loading due to sensitivity in the bonding area to water, alkalinity, and mechanical strain. High-strength concrete, with its dense cement matrix, makes it even more difficult to connect repair materials to the old concrete. More than 15 years of experience in development and use of different systems for repair of high-strength concrete has shown that cementitious mortars with modification by high amounts of superplasticizers perform best. Practical aspects of application are shown on a large project carried out on a high-strength concrete floor in an airplane hangar. Cementitious repair systems are suitable for any kind of high-strength concrete repair where adequate surface preparation and the application of a special cementitious bridging agent is provided, but have to be adopted to the individual job site conditions. Shrinkage compensation techniques and sophisticated curing methods have to be used to achieve improved results with respect to drying shrinkage cracking. The durability of high-strength floor repairs with new technologies, used on a large scale in Europe, has proved to be reliable even under severe service conditions.

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.

SP-121 The Second International Symposium on the Utilization of High Strength Concrete was held in Berkeley, CA, May 1990. A substantial amount of research work and project construction with high strength concrete was completed since the last Symposium. Recent findings were presented and discussed.