International Concrete Abstracts Portal

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.

Deals with the design of a concrete capable of increasing the airtightness of the primary containment of nuclear power stations. The general context of structures of this type and the types of damage commonly found in them (thermal cracking) are introduced. Then an ideal concrete is described and an attempt is made to approximate it by applying a rigorous formulation process. The result is a high-strength concrete having a low cement content (270 kg/m3), a 28-day strength of about 70 MPa, and a high workability through the use of silica fume and calcareous fillers. This concrete and a more conventional concrete are put through a series of characterization tests which makes it possible to conduct numerical simulations of the temperatures and restrained deformations in the containment. The reduction of the risk of thermal cracking is clearly demonstrated. Finally, all of these laboratory investigations are verified on a full-scale containment element, in which all the benefits of using this new type of high-performance concrete appear (temperature rise cut by 25 percent, near disappearance of cracking, tenfold reduction of airleaks). The advantages of such a concrete are not restricted to the nuclear context, but cover all applications for which a dense, crack-free concrete is desired.

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.

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.