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.

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.

When high-strength concretes are used in high-rise buildings, long-span bridges, and offshore structures, special attention must be given to the dimensional changes that occur in the concrete members. For design purposes, the length changes are usually considered to consist of instantaneous shortening, shrinkage, and creep. Instantaneous shortening depends on stress level, cross-sectional dimensions of the member, and modulus of elasticity of steel and concrete at the age when the load is applied. Shrinkage deformations generally depend on type and proportions of concrete materials, quantity of water in the mix, size of member, amount of reinforcement, and environmental conditions. Creep deformations depend on concrete stress, size of member, amount of reinforcement, creep properties of concrete at different ages, and environmental conditions. In recent years, questions have been raised about the validity of methods for calculating deformations in high-strength concrete members and the in-place properties of high-strength concrete members. These properties include compressive strength, modulus of elasticity, shrinkage, and creep. This paper reviews existing state-of-the-art technology concerning instantaneous shortening, shrinkage, and creep of high-strength concrete members.

A total of 21 test specimens with lapped reinforcing bar splices were tested using concretes with compressive strengths in the range of 21 to 99 MPa. For each test specimen, the concrete compressive strength, splitting strength, fracture energy Gf was determined. It was found that fracture energy of concrete appears to have a strong influence on the strength of lapped tensile splices. A comparison of the experimental results and computed values using the regression analysis equation of Orangun et al. based on a large number of tests from USA showed that the equation may be unconservative in cases of lapped splices in high-strength concrete.

The results of 18 tests on slender composite columns consisting of rectangular hollow steel sections filled with concrete are presented. The columns had a length of 3 m and a cross section of 120 x 120 mm. They were simply supported and the load was normally applied with an eccentricity of 20 mm. As a reference, the squash load was evaluated with tests on short columns (stub tests). The purpose of this study was to evaluate the possible advantages of high-strength concrete, confining effects of composite sections, and the shear transfer at the interface. Basic parameters that varied between the tests were: concrete compressive strength, steel yield stress, and thickness of the steel tube. In additional tests, the effect of load eccentricity, additional reinforcement in the column, debonded interface, and the way of load application were examined. These tests showed that the load-bearing capacity, as well as the ductility in the ultimate state, increased for these eccentrically loaded columns.