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A modified technique for short-term laboratory strain measurements in concrete reinforcement using electrical-resistance strain gauges Khanna, Jitendra

Abstract

Studies in Reinforced Concrete involving transference of stresses by bond from steel to concrete require a method for strain measurement in concrete reinforcement which is reliable and free from local effects. A very simple and economical method based on a technique first used by Brice⁶, has been developed using ordinary etched foil electrical resistance strain gauges, applied to the interior of a split pipe, which is joined together by a room-temperature epoxy adhesive, before embedment in concrete as reinforcement. Experiments performed to prove the reliability of the Technique have given indications that stress transference by bond is affected by the volume of concrete surrounding the reinforcement. Some supporting evidence from the results obtained by previous investigators⁴'¹² is presented. Also observed during the "proving experiments" is the pronounced effect of cracks on strain measurements, and the regular spacing of cracks in tensile specimens. A cast-in-situ joint between precast and prestressed members has been studied with the help of the modified technique, and the results point to the formation of micro-cracks detected by local variations in strain measurements which precede visible cracking. These variations, however, tend to disappear on higher or cyclic loading. Also observed during the joint test is the composite action even after complete horizontal slip between the cast-in-situ and precast concrete, provided the two portions are prevented from further slipping, and at high loads when slip is allowed to occur, it is seen that a moment mechanism other than in the composite section governs vertical forces in the stirrups.

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