International Concrete Abstracts Portal

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.

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.

High-strength concrete tends to mean small water-cement rations, implying poor workability. This tendency becomes more pronounced when much higher strength is required, and conventional concreting processes cannot sufficiently guarantee high-quality work. In current construction work, therefore, maximum use has been made of precast concrete (guaranteeing quality and minimizing the need for concrete cast in situ) and a new high-performance, air-entraining, and plasticizing admixture has been used for the necessary in situ concrete. The concrete prepared in this way exhibited a mix strength of 55 MPa at best. This value, in itself, is by no means high, but meaningful efforts to establish methods of concreting that insure still greater strength have been made. This construction work has demonstrated that combining the reinforced concrete (RC) layer method (which uses a large proportion of precast members) with high-strength concrete obtained from mixing with the new high-performance, air-entraining, plasticizing admixture is an extremely effective way to secure quality structures. Since this admixture is a novel product, the physical properties of the resulting concrete have been thoroughly checked to supplement the results of laboratory experiments and preliminary field tests.

Briefly reviews five joint industry-research programs pertaining to offshore concrete structures. These programs were sponsored by the oil and gas industry and related construction industries. These studies, conducted in both North America and Norway, included the use of high-strength, lightweight aggregate concretes in both material and structural evaluations. Selected characteristics of the high-strength, lightweight aggregate concretes used in these studies (such as ductility in reinforced concrete elements, punching shear behavior, and fatigue characteristics) are summarized. Future research needs are discussed.