Application Of Ultra-high Toughness Cementitious Composite In Deep Flexural Member

Ultra high toughness cement composites (UHTCC) is a new kind of high performance cementitious composite, which possesses significant strain hardening properties with the crack width below 100μm. Because UHTCC shows the great deformation capacity, harmless crack dispersive capacity and excellent energy dissipation capacity, it can be expected that the use of UHTCC will provide a great potential to solve the difficulties caused due to the low ductility and brittle cracking in concrete members. Such the outstanding tension properties and nonlinear deformation of UHTCC made it wide applications in some structure members where the high ductility and seismic resistance are needed. So far, a great deal of researches focused on the basic mechanical behavior of UHTCC, but little on reinforced ultra high toughness cement composites (RUHTCC) deep flexural members. Supported by the national science fund project "Seismic performance of ultrahigh toughness cementitious composite coupling beam with diagonally-reinforced short span-depth ratio " (51478078) and the central university basic scientific research special fund "Improving shear performances of RC deep beam with use of ultrahigh toughness cementitious composite" (DUT15LK39), this paper carried out a study on the use of UHTCC in RC deep flexural beam, main contents and the conclusions obtained are as follows:(1) The shear tests have been conducted on both RUHTCC and RC deep beams to compare their shear behavior. Results proved that, all tested RUHTCC deep beams presented a stable cracking pattern with the appearance of multiple-cracks, therefore resulting in a significant improvement in shear performance of the RC deep deam with the use of UHTCC. For the RUHTCC deep beams, their averaged ultimate shear strength and mid-span deflection at peak load are 1.2 times and 1.22 times of RC ones, respectively. And, the averaged ductility index μ1,μ2 and μ3 in the RC deep beams can be increased by 213%、70% and 94% respectively. It was also observed that the RUHTCC members had about 0.168mm～0.181mm maximum diagonal crack width at the service ability limit state, which is far smaller than the RC deep beams where the maximum diagonal crack width was up to 0.3mm.(2) The influences of two shear span ratios (0.7,1.05) and the reinforcement ratios of distributed reinforcementss (0,0.14%,0.2%,0.3% and 0.4%) on the shear performance of deep beams were investigated. It was founded that, a decrease of about 20% in ultimate shear capacity and an increase of about 40% in midspan deflection corresponding to peak load was showed for both RUHTCC and RC deep beams with the increase of the shear-span ratio from 0.7 to 1.05. With the increase of distribution steel reinforcement ratio from 0 to 0.3%, in the case of shear-span of 0.7, ultimate shear load and corresponding midspan delfection can be increased by 9% and 51% for the RUHTCC beams while for the RC deep beams the increase were 14% and 39% respectively; in the case of shear-span of 1.05, the ultimate shear load can be increased by 33% and 20% for RUHTCC and RC deep beams, respectively, and midspan deflection had an increase of about 30%. In addition, it was also observed that both RUHTCC and RC deep beams with larger shear span- ratios and more amount of distribution reinforcement ratios had comparatively high shear ductility and the impact degree of the distribution reinforcement ratios was more significant in RUHTCC deep beams compared to RC deep beams.(3) The local compressive behavior of RUHTCC and RC deep beam was investigated, both experimentally and theoretically. The tested shear span ratio was 0.5. It can be observed that the member with UHTCC presented the slight damage in the top node zone while the serious concrete spalling was noiced in the RC deep beams. And, the utilization of UHTCC can obtain an improvement of roughly 43% for the ultimate local compressive strength. Besides, the failure mode of tested beams was predicted based on strut-and-tie model. The good agreement was showed between the experimental observations and theoretical predictions. Finally, the smallest bearing length of 0.45h corresponding to the no need of local compressive capacity calculation is given.(4) According to the shear calculation equations given in the design codes such as GB50010-2010, ACI318-05 and several shear prediction equations proposed based on the strut-and-tie model in the relevant references, the shear capacity of RUHTCC and RC deep beams were predicted analytically. Results showed that, the strut-and-tie model is effective in predicting the shear capacity of RC deep beams, but underestimated the shear capacity of RUHTCC deep beams. Based on this, a new equation for predicting RUHTCC shear capacity was developed with the consideration of the tensile contribution of PVA fibers after cracking. The good agreement between the experimental resluts and the calculated ones was seen.