Due to the relatively low tensile strength of concrete, cracks are inevitable in reinforced concrete structures. Therefore, studying the crackingbehaviour of reinforced concrete elements and controlling the width of cracks are necessary objectives both in research and in design. The introduction of higher strength reinforcing steel has exacerbated the problem of crack control. Using higher strength steel, means less steel isrequired for a given structure to satisfy the strength requirements. The stiffness after cracking is reduced and wider crack widths will occurunder normal service loads. Unserviceable cracking may encourage corrosion in the reinforcement and surface deterioration, and may lead tolong term problems with durability. Indeed excessive cracking results in a huge annual cost to the construction industry because it is the mostcommon cause of damage in concrete structures. In this study cracking caused by both shrinkage and external loads in reinforced concretemembers is examined experimentally and analytically. The mechanisms associated with cracking and the factors affecting the time-varyingwidth and spacing of both direct tension cracks due to restrained shrinkage deformation and flexural cracks due to the combined effects ofconstant sustained service loads and shrinkage are examined. Laboratory tests on eight fully restrained slab specimens were conducted for upto 150 days to measure the effects of drying shrinkage on the time-dependent development of direct tension cracks due to restraineddeformation. The effect of varying the quantity, diameter, and spacing of reinforcing steel bars was studied. In addition, an analytical modelpreviously developed without experimental verification by Gilbert (1992) to study shrinkage cracking was modified and recalibrated. A secondseries of tests on twenty four prismatic, singly reinforced concrete beams and slabs subjected to monotonically increasing loads or to constantsustained service loads for up to 400 days, were also conducted. The effects of steel area, steel stress, bar diameter, bar spacing, concretecover and shrinkage were measured and quantified. An analytical model is presented to simulate instantaneous and time-dependent flexuralcracking. The tension chord model (Marti et al, 1998) is modified and used in the proposed model to simulate the tension zone of a flexuralmember and the time-dependent effects of creep and shrinkage are included. The analytical predictions of crack width and crack spacing are inreasonably good agreement with the experimental observations.
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