International Concrete Abstracts Portal

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.

Evaluates cracking resistance for concretes with compressive strengths between 60 and 110 MPa, including superplasticizers and/or condensed silica fume. Two types of concrete ring with 81 cm external diameter are tested and their shrinkage is measured over time. The first ring is cast around an aluminum ring, shrinkage-induced strain is measured, and the strains are subsequently transformed into stresses based on the theory of elasticity and knowledge of the elastic constants of aluminum. After some days, the ring breaks and the rupture stress by restrained deformation of the concrete is determined. A second concrete ring is cast, but without the internal metal ring. For this ring, measurement is made of the free shrinkage of the concrete. The value of the stresses and strains, in conjunction with the compressive and flexural strength, creep, and coefficient of hygrometric permeability (measured in other test specimens) are measured. Based upon available test data, the superplasticizer raised the mechanical strength but reduced the cracking strength of the concrete. The joint introduction of the superplasticizer, together with condensed silica fume, raised the mechanical strength of the concrete even further, but also increased its cracking resistance. To explain the test results, it is necessary to resort to the coefficients of hygrometric permeability and stress gradients, responsible for a reduction in the rupture stress of the concrete, which is higher in the first case than in the second.

Fatigue properties of high-strength concrete in compression were studied. Two types of normal-density concrete and one type of lightweight aggregate concrete have been tested. The numbers indicate the planned mean strength in MPa of 100 x 100 x 100 mm cubes. The influence of different moisture conditions was studied in an introductory investigation. Three different sizes of cylinder were tested for each of the three curing and testing conditions: in air, sealed, and in water. The tests showed that the fatigue properties of both the air and water conditions were scale-dependent, while the sealed condition was hardly influenced by the sizes of the specimens. The main investigation dealt with the influence of the variation in stress levels on the fatigue life. Test conditions with constant maximum stress levels showed significantly longer lives when the stress range was reduced. If the load levels were defined relative to the static strength, there was no obvious difference between the fatigue properties of the concrete qualities included in these tests. An additional investigation was performed on ND95 cylinders exposed to different combinations of cyclic load levels. It was found that initial cycling at lower load levels was beneficial for the fatigue life at the higher load levels. Based on the results of the experimental work, a design proposal for fatigue of concrete in compression was established.

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.