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.

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.

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.

The object of this study is to investigate the possibility of improving ductility of high-strength concrete columns with the lateral reinforcement. Eight column specimens confined by lateral reinforcements having 328.4 and 792.3 MPa in yield strength were tested under reversed cyclic lateral loads with constant axial compressive load levels from 0.254 to 0.629. The concrete compressive strengths were 85.7 and 115.8 MPa, respectively. Volumetric ratio of lateral reinforcement was 1.6 percent in all specimens. Test results indicated that the very large ductility could be achieved by using high yield strength lateral reinforcement, even for such high-strength concrete columns. Modifications of previously proposed stress-strain models on confined concrete were also made for applying them extensively into the calculation of moment-section curvature relationships of high-strength concrete columns with lateral confining reinforcement.

Three earthquake-type loading tests of reinforced concrete (RC) columns, short beams, and beam-column joints using high-strength concrete were carried out. The main objectives of this program were to investigate the seismic behavior of RC members using high-strength concrete, and to obtain guidelines for their design in high-rise buildings. Concretes having three levels of compressive strength, 400, 600, and 800 kg/cmý (39, 59, and 78 MPa), were used. High-strength reinforcing bars with nominal yield strengths of 8500 and 14,000 kg/cmý (834 and 1370 MPa) were provided for lateral reinforcement. Longitudinal reinforcement with a yield strength of 6000 kg/cmý (588 MPa) was also used for beam-column joint test. Emphasis was put on the combination of high-strength concrete and high-strength reinforcing bars. The seismic behavior of columns, short beams, and beam-column joints under high axial load, high beam shear, and high joint shear, respectively, were observed. The relationship between ductility and amount of lateral reinforcement were particularly discussed in the column and short beam tests. In the beam-column joint test, several joint details were considered, and their behavior was investigated. The design guidelines for these high-strength concrete members were also presented in this paper. The results of this experimental program show that the combination of high-strength concrete and high-strength steel bars can be quite effective in improving strength and ductility of RC members of high-rise buildings.