International Concrete Abstracts Portal

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.

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.

The objective of this paper is to show that the use of high-strength concrete (HSC) (especially concrete with silica fume) can notably reduce the long-term deformations of reinforced concrete (RC) slabs. This may be achieved by reducing creep deformation, increasing the elastic modulus, the tensile strength, and the steel-concrete bond properties. Moreover, this paper shows that the CEB (Comite Euro-International du Beton) moment-curvature relationship established for ordinary concrete is still valid for HSC. A procedure for the nonlinear finite element analysis of RC beams and slabs is briefly described. The proposed procedure is based on the nonlinear CEB moment-curvature relationship incorporated into an iterative secant stiffness algorithm. Predicted deflections from the proposed procedure are compared with experimental results from slabs made with ordinary or HSC.