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Time-Dependent Effect on Shear Strength of RC Beam: A review

February 20, 2022
halwanalfisa

Time-Dependent effects on concrete

A. Effects on concrete in compression and tension

Initial investigations on the effects of sustained loads and slower loading rates were carried out under uniaxial compressive and tensile stresses. Depending on the duration of loading and load intensity, nonlinear creep strain and micro-cracks can be potentially developed causing damage to the internal structure and reduction of concrete uniaxial compressive strength [17-20]. The origin of creep is due primarily to the loss of absorbed water and the development of micro-cracks under sustained loads and slower loading rates. Creep become less significant with the improvement of the quality of concrete but increase with the rise of surface area-to-volume ratio and the lowering of the environment relative humidity. In the period immediately after first loading, creep occurs rapidly and slows appreciably with time [21-26]. The amount of creep in tension is much lower than that of creating in compression [27]. If the increases in the aggregate content can reduce the amount of creep in compression, it will conversely increase the amount of creep in tension [28, 29]. The terms of a creep coefficient, which is the ratio of creep strain to elastic strain, is usually used to measure the capacity of concrete to creep. The creep coefficient is determined by extrapolation from relatively short-term test results and its accuracy relies on the type of concrete and period of measurement in creep tests [23, 30]. In case of concrete fails under a sustained load, the proportionality between linear (for load intensity below 40%) and nonlinear creep strains creep coefficient (for load intensity above 40%) was no longer valid based on recent research due to the unstable growth of cracking [19]. Furthermore, it was revealed that time dependency did not only occurred before reaching the peak (ultimate) load but also in the post-peak region [31].

B. Effects on concrete in flexure

Experimental works on concrete structures under a three-year sustained load conducted by Washa and Fluck [32] and Gilbert and Guo [33] reported that the measured long-term deflection was several times as great as the initial short-term deflection. The deflection increased rapidly at the beginning of loading test (one until three months of loading) and increased slowly afterwards. The existing cracks extended and widened under long-term loading. It may cause structures more vulnerable to aggressive environmental attacks [34]. Besides, Bazant [30] also collected data of 56 large bridge spans around the world and reported that 43 among them being experienced excessive deflection within less than 100 years, which is usually required minimum lifetime. The excessive deformation problems due to sustained load appear to be more dominant than a direct reduction in bending capacity. Neville [35, 36] mentioned that large non-elastic movements which occur in the concrete and the steel at high stresses lead directly to readjustments in the stresses in the tension steel and compression concrete. These readjustments compensate or eliminate the effects of the stresses induced by the previous loading. Besides, the increase in the steel stress is accompanied by a continuous decrease in the maximum concrete stress.

B. Effects on concrete in shear

Time-dependent shear transfer models were generally proposed based on a theoretical assumption that the contribution of all shear-transfer action decrease for increasing openings of the critical shear cracks due to sustained load [37]. A well-known shear design equation adopting this theory is the modified compression field theory (MCFT) proposed by Vecchio and Collins [3]. The MCFT relates the shear stress transferred across the inclined shear plane of a beam to the flexural crack widths corresponding to the tensile strain of reinforcement bar. The shear resistance of RC beams decreases as the tensile strain of longitudinal reinforcement increases due to creep. However, an experimental work conducted by Sarkhosh et al. [38, 39] indicated that there is no significant effect of sustained load on shear performance of RC beams. They investigated the behaviour of RC slender beam (with a/d 2.9) under high load intensity within duration of time up to 1113 days. The beams deformed due to creep and some cracks opened over a period. Eventually, no reduction of the shear strength was reported at the end of long-term loading. In contrast, a case of RC box culverts constructed several decades ago exhibited excessive deflection at the top slabs of the structure accompanied by out-of-plane shear failures was reported by Maekawa et al. [40].  The findings indicated that the coupling of three factors (which are 1. subsidence of the backfill soil foundation; 2. differential shrinkage; and 3. sustained higher stress that caused creep and transient crack propagation) led to excessive deflection with delayed shear failure. The time-dependent effect on the shear capacity of RC beam seemed to be more pronounced with decreasing ratio of shear span to effective depth of the beams (a/d < 2.5), increases in the bearing size, and increases of the longitudinal restraint at the supports [41,42].

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