STUDIES ON STEEL FIBRE REINFORCED CONCRETE JOINTS UNDER OPENING BENDING MOMENTS

Abstract

Interest in the behaviour of opening corners was fueled by the observed failure of connections between the wing and front walls in bridge abutments in the late 60's. It was soon recognized that the efficiency of the corner reinforcement in practice those days was unacceptably low and vigorous efforts were directed at exploring detailing arrangements which had the potential of offering superior ultimate load behaviour and ductility coupled with better serviceability characteristics. The examination of the mechanics and mode of failure of joints soon made it clear that the unsatisfactory performance of opening corners primarily stemmed from the poor tensile strength of concrete, resulting in its premature failure. Consequently, the detailing of the reinforcement in opening corners, which was expected to share a major burden of the tensile loads acting at the corner was more critical as compared to that of closing corners, wherein the compressive strength of concrete played a critical role in influencing the behaviour of the joint. Hence, not surprisingly, the research and development efforts in the early years were primarily directed at evolving an optimum arrangement of the reinforcement bars in the corner, due to the fact that constraints were imposed by the inadequate performance of the concrete matrix under tensile loads. Out of a number of detailing arrangements examined, a few were proposed for practical application, often with conflicting results and consensus has generally eluded researchers and practicing engineers alike, on the reinforcement layout which has the capability of delivering the best performance in terms of serviceability and ultimate load behaviour. Till date, the determination of the most desirable reinforcement arrangement for opening corners is one long saga of continuous research and development and the last word is yet to be heard on the subject. The efforts in the present investigation, unlike many of the past investigations, have been directed more towards attacking the problem from the 'material' point of view. Emphasis has been laid on overcoming the basic weakness of opening corners having its roots in the poor tensile performance of the plain concrete matrix. This was achieved by harnessing the potential improvements in the mechanical properties of the matrix afforded by addition of steel fibres. This though, has not been done at the cost of ignoring or belittling the contribution of a rational detailing arrangement in influencing the behaviour of opening corners. Various options for the detailing of reinforcement in opening corners which have been in vogue from time-to-time have also been critically examined. Therefore, the present investigation was planned to study the behaviour of steel fibre reinforced concrete planar opening corners, primarily under flexural loads. The corner performance was proposed to be evaluated in terms of cracking characteristics, ultimate loads, mode of failure, ductility and corner efficiency. Governing equations, empirical or otherwise, were proposed to be developed so as to provide guide lines for the design ofopening corners corresponding to the given detailing and fibre parameters. In the absence ofwell defined behavioural models, as existing for typical reinforced concrete members like beams, columns and frames, it was also proposed to evaluate the deformation characteristics of opening corners and suggest, wherever possible, an idealized load-deformation response for opening corner specimens against which performance could be benchmarked. An extensive experimental investigation consisting ofthirty-six full sized portal-type opening corner specimens divided into nine groups in all, was planned in which the specimens of each group were dedicated to observing the corner behaviour in terms of avariable related either to the corner detailing arrangement or to any one of the fibre parameters, namely, the fibre aspect ratio, the fibre volume fraction or blending of fibres (mixed aspect ratio of fibres). All the corner specimens were tested under monotonically increasing static loads. Five detailing systems viz. L-type detailing, L-type detailing with corner stirrups, Loop detailing, detailing with over-lapping U-bars and detailing with overlapping U-bars in tandem with splay (diagonal) steel, were investigated in all. The other parameters related to the detailing system which were investigated were, percentage tensile steel, percentage splay steel and the reinforcement bar diameter. In all the fibrous concrete specimens, the fibrous concrete was used exclusively in the corner region. Either of the following two types of crimped-type flat steel fibres viz. 25 x 2.0 x 0.6-mm or 50 x 2.0 x 0.6-mm, corresponding to fibre aspect ratios of 20 and 40 respectively, were used either in stand alone application or in combinations of various mix proportions by weight of each (mixed aspect ratio of fibres). Four volume fractions of the 50-mm long fibres viz. 0.75%, 1.25%, 1.50% and 1.75% and five volume fractions of the 25-mm long fibres viz. 0.75%, 1.25%, 1.50%, 1.75% and 2.00% were used in addition to the following combinations of the 50 and 25-mm long fibres in mix proportions by weight of 35% : 65%, 50% : 50% and 65% : 35% respectively, at each of the volume fractions of 1.25%, 1.50% and 1.75%. The physical properties of the basic constituent materials of concrete viz. steel, cement, fine aggregates and coarse aggregates were determined so as to conform to the relevant Indian Standard Specifications. The mix proportion of C: F.A: C.A