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Concrete In Australia : March 2013
Concrete in Australia Vol 39 No 1 37 Anchorage of reinforcement in concrete structures subjected to cyclic loading* Raymond Ian Gilbert -- Emeritus Professor Zhen-Tian Chang -- Research Associate Maruful Hasan Mazumder -- PhD student Centre for Infrastructure Engineering and Safety, School of Civil and Environmental Engineering The University of New South Wales When designing a reinforced concrete member for strength, ductility and robustness, it is essential that the tensile reinforcement at the critical section can not only develop the yield stress of the steel, but that it can sustain this level of stress as deformation increases. is paper describes a current experimental research program to assess the impact of dynamic and cyclic loading on the anchorage requirements of modern high strength steel reinforcing bars, including the case of lapped splices. e results of the first stage of a four stage testing program are presented. e aim of the project is to develop procedures for anchoring reinforcement in concrete structures that provide reliable and consistent factors of safety and that allow structures to be ductile and robust throughout their design life. 1.0 INTRODUCTION An extensive experimental program is under way at the University of New South Wales to assess the impact of cyclic loading on the anchorage requirements of modern high strength steel reinforcing bars, including the case of lapped splices. e effects of prolonged periods under sustained loads, including restraint to drying shrinkage, is also to be considered experimentally. e aim is to develop procedures for anchoring reinforcement in concrete structures that provide reliable and consistent factors of safety and that allow structures to be ductile and robust throughout their design life, without an increase in risk of premature collapse through bond and anchorage failure. is paper describes the first stages of the experimental research program aimed at assessing the impact of cyclic loading on the development length and lapped splice length of Grade N Australian deformed bars. 2.0 DEVELOPMENT LENGTH When designing a reinforced concrete member for strength, ductility and robustness, it is essential that the tensile reinforcement at the critical section can not only develop the yield stress of the steel, fsy , but that it can sustain that level of stress as deformation increases. If the yield stress is to be reached and maintained, a minimum length of reinforcing bar (the development length) is required on either side of the critical section (or point of peak stress). AS3600-2009 1 specifies a minimum development length, Lsy.t, over which a straight bar must be embedded in the concrete in order to develop the yield stress. Specified values for Lsy.t in the major concrete Standards 1, 2, 3, 4 differ widely, and have been developed independently as empirical fits to experimental data obtained from load tests involving monotonically increasing static loads. e influence of dynamic and cyclic loads on the anchorage requirements of reinforcement has received relatively little research attention. In real structures, where loads may be repetitive and dynamic in nature, where shrinkage and temperature induced cracking may reduce the bond between the concrete and the steel with time and where deterioration may occur due to extensive periods of exposure to the elements, the anchorage requirements of reinforcement may not be the same. When such structures are subjected to extreme events, failure of the anchorage of the reinforcement is often the trigger that initiates collapse. Adequate anchorage is essential for strength and robustness. In the derivation of expressions for the development length, an average ultimate bond stress fb is usually assumed at the interface between the concrete and the reinforcing bar, even though extreme variations in local bond stresses exist along the development length, particularly in the vicinity of flexural cracks. e average ultimate bond stress is affected by numerous factors including: the type of reinforcing bar (ribbed or deformed bars have a much higher value of fb than plain round bars); the condition of the steel surface (a slightly rusted surface is better than a bright surface); the degree of compaction of the concrete surrounding the bar (bottom bars usually have better bond than top bars); the concrete strength (the bond strength increases with the concrete strength); the magnitude and spacing of lateral reinforcement (the more lateral reinforcement within the development length the higher the bond strength); the magnitude of pressure normal to the developing bar (compressive pressure improves bond strength); the concrete cover and the distance to the next parallel bar (increases in the * is paper was first presented at Concrete 2011, the conference of the Concrete Institute of Australia. It is republished with permission.