Papers by Author: Xiu Fang Zhang

Abstract: Ultra-high toughness cementitious composite (UHTCC) exhibits the pseudo-hardening feature when subjected to tensile load and has high tensile strain capacity of normally up to 3%. Also, UHTCC has a unique cracking behavior. From cracking up to ultimate tensile strain capacity, the crack width in UHTCC could be still kept below 100m. This paper presents the utilization of UHTCC to replace a layer of concrete surrounding the main flexural reinforcement in ordinary RC beam to improve flexural performance especially beam durability as UHTCC displays high toughness and shows multiple fine cracks. Analytical closed-form formulae for flexural capacity, curvature and deformation of UHTCC/RC composite beam derived based on the elastic beam theory is presented first. Subsequently, experimental results of two groups of different reinforcement ratios of UHTCC/RC beams and control RC beams tested under flexural loading to verify the feasibility of analytical formulae as well as to examine the performance improvement of UHTCC/RC composite beam over the control beam is presented. Moment-curvature curves and load-mid span displacement curves for the tested beams are compared with the theoretical analysis. A good agreement between experimental and analytical results is found. The experimental results show that the use of a layer of UHTCC in RC beams can enhance both flexural capacity and ductility. The improvement is not significant with the increase in reinforcement ratio; however, the maximum crack width under service load even in the case of lightly reinforced beams can be limited within 0.1mm.

Abstract: Using the double-edge notched geometry proposed by Xu and Reinhardt recently, the dimension of 200 mm×200 mm×100mm concrete cube specimens, of which the crack length are 10 mm, 20 mm, 30mm, 40mm, 50mm respectively, are designed to experimentally measure mode II fracture toughness KIIC of concrete. For almost all specimens, typical shear fracture features i.e. approximately 0º initial cracking angle as well the following crack forwards propagation along the direction of ligament is phenomenally observed. This fact strongly confirms that this double-edge notched geometry is validly and capable of being utilized as a mode II fracture geometry to evaluate mode II fracture behavior. Then, from the discontinuity point of the measured load-displacement plot, the critical shear fracture load Pc is determined and the corresponding mode II fracture toughness KIIC is also calculated using the formula developed by Xu and Reinhardt. The computed results show that KIIC has no dependency on initial crack length, about 3.36MPa·m1/2 for the tested specimens.

Abstract: The high pseudo-ductility of Engineered Cementitious Composites (ECC) makes it a particularly effective material to resist the propagation of cracks. In applications where failure is due to cracking initiated by localized stresses, the application of ECC around the stress concentrated region should result in significant improvement in the ultimate failure load. In this investigation, we will study the use of ECC in (i) the anchorage zone of post-tensioned concrete members, and (ii) the region around embedded anchor bolts in concrete blocks. For the first application, the replacement of concrete with ECC at the anchorage zone is found to be very effective in increasing the load capacity under concentrated compression. Based on our test results, ECC can actually replace all or part of the conventional steel stirrups in resisting splitting failure at the anchorage zone. For embedded anchor bolts, the placement of a small ECC disc above the steel bolt can effectively delay the propagation of cone-shape failure and increase the pull-out force. Through the present experimental program, we have illustrated the advantage of strategically applying ECC in critical region of structural components, to improve performance without significant increasing the material cost.