International Concrete Abstracts Portal

A theoretical study was carried out to investigate the effects of increasing concrete strength on the depth of rectangular beams. Two series of beams were investigated. The first series comprised reinforced concrete beams with spans from 6 to 15 m, and the second comprised prestressed concrete beams with spans from 12 to 30 m. The concrete strength ranged from 20 to 120 MPa and from 30 to 120 MPa for the reinforced and prestressed concrete beams, respectively. The results show that for rectangular concrete beams, an increase in concrete strength results in a rather significant reduction in the beam depth, whereas for rectangular prestressed concrete beams no significant reduction in the beam depth is gained from increasing the concrete strength because the deflection governs the design.

Strength and deformability of High-Strength Concrete (HSC) columns are presented based on recent experimental and analytical research. HSC columns under concentric compression and under combined axial compression and lateral load reversals are discussed. Experimentally observed column strengths are compared with those computed based on the provisions of AC1 3 18 (1) building code and analytical models proposed for HSC columns. The results indicate that the rectangular stress block currently used for normal-strength concrete is not applicable to HSC, especially for columns under high compression where the overall response is dominated by concrete. A triangular and a modified rectangular stress block is presented. Column capacity under concentric compression is illustrated with due considerations given to early spalling of cover concrete. Axial and lateral deformabilities of HSC columns are discussed with emphasis placed on the parameters of confinement. It is shown that HSC columns conforming to the current building code requirements may exhibit ductile behavior under moderate and low levels of axial compression. Higher grade lateral reinforcement can be utilized effectively to confine HSC columns to produce improved inelastic deformability under high levels of axial compression.

High strength concrete with a specified compressive cylinder strength fi of up to 70 MPa for ductile elements in seismic design and of up to 100 MPa for other elements is now permitted by the recently revised New Zealand concrete design standard NZS 3101:1995. Also, longitudinal reinforcement with a characteristic yield strength of up to 500 MPa is allowed, and for transverse reinforcement in strength calculations a useable steel stress of up to 500 MPa for shear strength and 800 MPa for confinement is permitted. For concrete with f' f' c greater than 55 MPa the parameters for the equivalent rectangular compressive stress block have been modified to take into account the stress-strain characteristics of high strength concrete. Also, new design equations for confining reinforcement have been included to better account for the affect of the variation of axial load level. Simulated seismic load tests have been conducted in New Zealand to investigate the behaviour of high strength concrete columns confined with normal and very high strength transverse reinforcement. The tests demonstrated that the yield strength of very high strength confining reinforcement may not be attained at the stage when the column reaches the peak flexural strength and that the thickness of concrete cover has an important influence on the behaviour of the columns.

Use of high-strength reinforced concrete walls in regions of high seismic risk is evaluated using current U.S. code provisions, an example building, parametric studies, and experimental results. The format of current U.S. code provisions for structural walls promotes the use of high-strength concrete; however, the use of these provisions has not been evaluated for high-strength concrete. Analytical studies of building systems utilizing slender walls indicate that there is not a significant advantage associated with the use of high-strength concrete waUs and that this advantage tends to diminish with increasing concrete strength. Evaluation of test results conducted in Japan for low-aspect ratio walls indicates that ACI 318-95 requirements do not represent the observed shear strength well. Based on the limited database considered in this study, a value of 1.0 f' c MPa (126) was found to provide a good estimate of wall shear strength.

A total of twenty one high-strength reinforced concrete shear wails were tested as a part of a five-year national research project in Japan. Concrete with compressive strength ranging from 60 MPa to 120 MPa, and reinforcing steel with grades ranging 700 MPa to 1200 MPa were used for one-quarter scale specimens. The loading conditions and the reinforcement ratios were systematically varied to observe the strength and deformation capacities attained in various failure modes, such as flexural failure and shear failure before or after yielding. This paper summarizes the test results as well as the results of other tests on high-strength reinforced concrete shear walls conducted in Japan. Design equations for flexural and shear strengths based on the resistance mechanisms are verified through evaluation of experimental data. Methods of estimating the yielding deformations and the ultimate deformation capacities at web-crushing are also discussed.