PERFORMANCE OF FIBER BASED CONCRETE IN TRANSFER BEAMS

Abstract

To cater against the lateral drift in high-rise buildings, closely spaced columns are placed at the periphery ofbuilt-up plan area and are connected through rigid links known as spandrel beams. However, these closely spaced columns at the periphery create hindrance to the moment ofmen and goods at the entrance (ground floor) level. To fulfill this requirement, the columns at this level have to be placed at larger spacing. As a result an interface has to be provided between the closely spaced columns ofupper floor and the widely spaced columns at the ground floor. This interface has to be a horizontal RC element and hence is referred to as beam. Conventionally, a beam is taken to be a flexural member of the structural system. The above mentioned interface beam, however, does not behave as a common flexural member since it gets sandwiched between closely spaced upper columns and widely spaced supporting columns below it. To transfer heavy magnitude ofloads, the depth ofthis interface beam is usually kept much higher than normal beams. It ranges often from one to few meters. As a result of this the load transfer mechanism through this beam becomes altogether different than the usual mechanism of the flexure. Such an interface beam is in general referred to as Transfer Beam and from design point of view it is called as deep beam. These beams also find their applications in pile caps and off-shore structures. The loads get transmitted through the body of the beam mainly in shear mode and nominally in flexure mode. As a result, the failure of transfer beams takes place in shear. Unlike flexure, shear failure is sudden, violent and hence treacherous. This mode of failure has to be, therefore, transformed, more or less, into a ductilemode. Usually vertical steel known as stirrups are provided in beams to build-up shear resisting capacity. These stirrups are discrete elements and hence do not provide acontinuous medium in the body of concrete. Therefore, the Failure of concrete due to shear induced tension occurs in brittle manner. This mode of failure has to be suppressed through appropriate measures. Induction of ductility into the body of concrete, thus, becomes a primary requirement. Ductility can be induced by incorporating randomly distributed steel fibres, embedded into the body ofconcrete. The concrete so produced is referred to as Fibre-Based- Concrete (FBC). Ductility can also be induced in concrete by introducing steel bars in a continuous mode, distributed all along the depth of the beam. It is, therefore, considered imperative to study the growth ofshear resistance and ductility in concrete members through an appropriate combination of Fibres, longitudinal and vertical steel. This has framed the primary objective of this research to study the shear behavior of beams with high depth/s, -made with steel fibre based concrete, inhabiting suitable amount of horizontal and vertical steel. Extensive experiments incorporating volume fraction and aspect ratio of fibres, strength of concrete, percent longitudinal steel, percent vertical steel and varying shear arm-to-depth ratio has been carried out in this study. This has resulted into testing of more than 350 beams yielding a large set of relevant and reliable data. The span of the beams has been maintained constant at 1 m with 0.1 m overhang on either side of the supports. While-as the depth of the beams has been varied at 150, 250, 300, 400, 500 and 600 mm respectively. The spacing between the top two point loads has been kept at 200 mm. The study on the beams related to varying shear arm-to-depth ratio (a/d) has been carrying out by varying the depth (d) and not by varying the shear arm (a). This has been consciously done to achieve the flow of applied load through the entire depth of the body of concrete. This allows the concrete to develop the stresses over the full depth of the beam which varies non-linearly across the depth. The results from the experiments have been processed suitably to comC out with empirical expressions for estimating the shear capacity of the beams incorporating all the variables discussed above. The proposed expressions for shear design have been discussed in the light of Strut-and-Tie model, a theoretical approach for the design of deep beams. Further the comparison of predicted expressions have been made with shear design provisions as available in national codes such as ACI 318, with a view to seeing their goodness of fit. Moreover a comprehensive study on the stress-strain behavior of FBC in compression has been performed here through testing on cylindrical specimens (150 mm diameter x 300 mm long). On the instance of regression analysis on experimental data, an analytical expression for the stress strain curve in compression of FBC has been proposed, which describes the significance of both fibre factor and strength of concrete in stress-strain behavior of FBC. Fibre factor is expressed as the product of volume fraction and aspect ratio of fibres