Effects of Confinement and Small Axial Load on Flexural Ductility of High-Strength Reinforced Concrete Beams

This dissertation, "Effects of Confinement and Small Axial Load on Flexural Ductility of High-strength Reinforced Concrete Beams" by Siu-lee, Chau, 周小梨, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled EFFECTS OF CONFINEMENT AND SMALL AXIAL LOAD ON FLEXURAL DUCTILITY OF HIGH-STRENGTH REINFORCED CONCRETE BEAMS Submitted by CHAU Siu Lee for the Degree of Master of Philosophy at The University of Hong Kong in August 2005 Compared with normal-strength concrete, high-strength concrete has higher strength but is generally more brittle. Its use in a reinforced concrete structure could lead to an undesirable reduction in ductility if not properly controlled. In this thesis, the effects of confinement and small axial load on the flexural ductility of reinforced concrete beams cast of both normal- and high-strength concrete have been evaluated by analyzing the complete moment-curvature behaviour of the beam sections. The results reveal that the use of high-strength concrete would at a constant tension steel ratio increase the flexural ductility, while at a constant tension to balanced steel ratio decrease the ductility. On the other hand, provision of confinement enhances the ductility of both normal- and high-strength concrete sections at both a constant tension steel ratio and at a constant tension to balanced steel ratio. It does this in two ways. Firstly, it increases the balanced steel ratio of the section. So, for a constant steel ratio, the section with higher confinement is more under-reinforced. Secondly, it increases the residual strength and ductility of the concrete such that at the same tension to balanced steel ratio, the ductility of the section increases. From the results of the analysis, it can be concluded that providing confinement to a section is an effective way of improving the ductility of reinforced concrete beam sections, especially those cast of high-strength concrete. However, most codes of practice do not specify a suitable design method for reinforced concrete beams that takes into account the effect of confinement. Therefore, design formulas for the flexural strength and ductility design of high-strength concrete beams with the effects of confinement taken into account have been developed. On the other hand, it is proposed to compensate for the reduction in flexural ductility due to the use of high-strength concrete by adding compression and/or confining reinforcement. A simple design method that correlates the amount of addition reinforcement needed to maintain a constant level of minimum ductility and the concrete strength is developed. Conversely, the presence of compressive axial load, even at a low level, has an adverse effect on flexural ductility. As a portion of concrete is used to resist the axial load, the section becomes less under-reinforced. Therefore, the flexural ductility decreases with the level of axial load applied. From the results obtained, it is found that the presence of axial load mainly affects the degree of the section being under- or over-reinforced. Measures should therefore be taken to maintain the ductility level of sections with applied axial load at an acceptable level. The study recommends the provisions of additional compression reinforcement to resist the applied axial load, and proposes a design method for restoring the ductility of a section with applied axial load to a ductility level attained by an identical section without axial load. DOI: 10.5353/th_b3199766 Subjects: