International Concrete Abstracts Portal

Sixteen very high strength concrete beams (3600 x 350 x 200 mm) with and without steel fibers were tested under different combinations of shear force and bending moment. The beams were singly reinforced and without shear (web) reinforcement. The cylinder compressive strength of concrete was about 110 MPa. The main variables in this program were: shear span/depth ratio (a/d), the steel fiber content (V f), and the percentage of the longitudinal flexural reinforcement ({rho}). The test results showed that, adding steel fibers to high-strength concrete increased the ultimate shear strength, increased the stiffness, reduced the deflection, and transformed the failure mode into a more ductile one. Based on the test results, two empirical expressions have been proposed to predict the shear strength of steel fiber high-strength concrete.

Presents the results obtained from tests on high-performance concrete columns. Twenty-four columns were tested. The test parameters included shape and size of cross section, longitudinal reinforcement ratio, load eccentricity, and concrete compressive strength. Analytical methods to predict the behavior and strength of columns are also presented. Good correlation between test and analytical results is found.

In recent years, moment-resisting frames built using high-strength concrete have been used for high-rise buildings, primarily for economic reasons. When such high-rise buildings are subjected to severe earthquakes, cyclic horizontal and axial loading can be imposed on the exterior columns. The ductile behavior of such columns needs to be insured. In this study, improvement of the flexural ductility of high-strength concrete columns under high axial compressive load is attempted by arranging longitudinal bars with mixed steel grades. The basic concept of this method is to achieve the gradual attainment of yield of longitudinal bars, from low- to high-strength steel, as the column deflection increases, and thus to delay the column reaching the maximum moment capacity until the column deflection attains the required level. To verify the adequacy of the preceding design concept, six cantilever columns with 400-mm-square cross section have been constructed and tested under simulated severe seismic lateral loading with axial compressive load of either 0.3 f' c or 0.6 f' cA g. The compressive strength of concrete f' c was 65.7 MPa on average, and steels with yield strengths of 442 and 1033 MPa were used for longitudinal reinforcing bars. The adequacy of the preceding design concept was verified from the test results, and it was found that the New Zealand concrete design code could provide a good guideline for its application to design.

Ductility of high-strength concrete columns is very important in the aseismic design. There are many factors affecting the ductility of compression-bending members. The axial load ratio and volume stirrups ratio are main factors. Based on the experimental research of reinforced concrete columns with high-strength and normal strength concretes under monotonic and cyclic loading, it can be observed that under different axial load ratio and stirrup volume ratio the damage pattern of members is different, and there is also obvious difference in the ductility. To control ductility of members, we must control the damage pattern. If the axial load ratio is high, the shear-compressive damage of the column should be avoided to provide the required ductility. On the basis of experiments, the mechanism of the effect of axial load ratio on the ductility of column is also discussed. The axial load ratio limits are proposed under the condition of limited ductility. The experiments show that the relationship between stirrup ratio and axial load ratio is not linear if the axial load ratio is high, which is different from previous research. In the design, a simplified bilinear relationship can be adopted that agrees well with the experimental results.

Experimental results on the bond behavior of high-strength concrete are presented. A total of 36 specimens was tested. The variables were the concrete compressive strength, the bar diameter, and the embedded length. The concrete compressive strength varied from 42 to 78 MPa (6000 to 11,000 psi). The bar diameters were 14, 16, 18, and 20 mm. The bond tests were conducted using a modified version of the Danish Standard DS 2082 pullout test in which the concrete surrounding the bar was in uniform tension. The test results indicate that the average bond stress at failure increases with the increase in the concrete compressive strength and decreases with the increase in the embedded length. The embedded length calculated using the ACI Building Code 318-89 equation caused a steel yielding failure. The predominant type of failure was the splitting of concrete; however, yielding of the embedded steel preceded the splitting failure in more than half of the specimens. It was observed that the ACI Building Code equation underestimates average bond stress for high-strength concrete. A model is developed to predict the bond strength of high-strength concrete in terms of the concrete cover, bar diameter, embedded length, and concrete compressive strength as variables. The proposed equation gave good prediction to the bond stress at failure of the pullout specimens tested in this investigation. 260-594