A New Concept for the Early Age Shrinkage Effect on Diagonal Cracking Strength of Reinforced HSC Beams

Abstract

The effect of early age shrinkage on the diagonal cracking strength of reinforced high-strength concrete beams is investigated, using thirty two reinforced concrete beams with a distance from the compressive fiber to the centroid of the reinforcing bars (effective depth) of 250 mm, 500 mm and 1000 mm, made of conventional high-shrinkage and low-shrinkage high-strength concretes with a water-to-binder ratio of 0.23. The loading test results show that the shear strength at diagonal cracking of reinforced conventional high-shrinkage high-strength concrete beams is 5% to 18% lower depending on the effective depth compared with that of reinforced low-shrinkage high-strength concrete beams, and the dependence of diagonal cracking strength on the effective depth is apparently different between both. The ultimate shear strength of the former is also 23% to 45% smaller than that of the latter on average. Moreover, a new concept based on the equivalent tension reinforcement ratio for the evaluation of the shrinkage effect on the shear strength at diagonal cracking, consisting of a tension reinforcement ratio modified taking into consideration the effect of the tension reinforcement strain produced by deformation of concrete at early ages, is proposed. This concept shows succesfully the linear relationship between the shear strength at diagonal cracking and the effective depth to -2/5 power independent of the magnitude of the early age deformation of concrete, and a design equation for the shear strength at diagonal cracking applicable to concrete compressive strengths from 90 to 130 N/mm 2 is proposed.

Shear Strength of Reinforced Concrete Beams: Concrete Volumetric Change Effects

“…When the average modified shear strength of high-shrinkage concrete beams was compared with that of low-shrinkage concrete beams, the reduction ratios were 22% and 10% for the small and large beams, respectively. These reduction ratios of beams were larger or almost similar values as the results by Sato and Kawakane (2008), which showed 5% and 12% reductions by autogenous shrinkage in their high-strength beams with the effective depth of 250mm and 500mm. Figure 9 shows the relationship between ultimate shear strength and average tensile strain of the longitudinal reinforcements.…”

“…To consider the effect of autogenous shrinkage of high-strength concrete on the shear strength of RC beams, Sato and Kawakane (2008) proposed a new evaluation method based on the concept of strain change in tension reinforcement caused by concrete shrinkage. Here, they used the following equivalent tensile reinforcement ratio: tension reinforcement strain at section 1.5d distant from the loading section in the shear span at the diagonal cracking and ɛ s0,def : tension reinforcement strain when the concrete stress at a depth of reinforcement is zero, which is positive in tension and negative in compression.…”

“…The shear strength of the RC beam was calculated from a significant diagonal cracking load, which is referred to the discussion by Sato and Kawakane (2008). The significant diagonal cracking load was determined from sudden increased points of measured vertical deformation, the stirrup strain in the shear span of the beam, and visual crack observation during the loading test.…”

“…The effects of internal tensile stress and initial cracking caused by drying shrinkage and the applicability of the concept proposed by Sato and Kawakane (2008) to the evaluation of these effects have been investigated previously Nakarai 2009, 2010;Matsushita and Nakarai 2013). Mitani et al (2011) also performed experimental studies on the influence of drying shrinkage on the shear performance of RC beams with different effective depths.…”

“…Furthermore, some studies on structural strength have been published. Sato and Kawakane (2008) revealed that the autogenous shrinkage of high-strength concrete reduced the shear strength (diagonal cracking strength) of reinforced concrete (RC) beams without stirrups that were formed using high-strength concrete based on experimental comparisons between high-and low-shrinkage concretes. They also showed that the reduction in shear strength caused by autogenous shrinkage could be reproduced by their proposed method based on the concept of strain change in tension reinforcement.…”