Influence of Interface and Type of Strain Hardening Cementitious Composite (SHCC) on Crack Control in SHCC-Concrete Hybrid Beams

Abstract

A combination of conventional concrete members with innovative material such as Strain Hardening Cementitious Composite (SHCC) in the tensile zone, can exhibit better crack-width control. This is attributed to the bridging effect by the Polyvinyl Alcohol (PVA) fibers used in SHCC in combination with the special material composition containing fine particles. This hybrid combination is able to satisfy the Serviceability Limit State (SLS) criteria for crack-width control by eliminating the requirement of additional reinforcement. Optimisation of micro-cracking of SHCC requires an investigation to study the influence of the interface in SHCC-Concrete hybrid beam and the type of fiber used in SHCC. This master thesis research is a continuation of the previous Msc. thesis study performed by Zhekang Huang [1] on the experimental flexural behaviour of reinforced concrete beams with a layer of SHCC in the tension zone. Variation in the bond interface property such as smooth interface profile, partially debonded, completely debonded and grooved interface profile are investigated to study their influence on the bearing capacity and crack-width control. To assess the impact of the type of fibers on crack-width control, commonly used PVA fibers are replaced with High Modulus Polyethylene Fibers (HMPE) based on pre-study. All hybrid beams are tested experimentally in a four-point bending set-up to generate cracks within the constant moment region and Digital Image Correlation (DIC) is performed to evaluate the crack patterns and the crack-widths. Numerical analyses of the beams are performed using Finite Element Analysis software Diana {version 10.3} with the modelling of similar interfaces and material properties used in the experiments. From the obtained results, it is observed that for all the beam specimens tested, varying the interface property did not have a significant impact on their bearing capacity as the interface within the constant moment region is only varied and the surface outside this region is bonded. For beam specimens with partial and completely debonded interface in the constant moment region, it is observed that the initial stiffness is reduced but the overall bearing capacity remains the same. Within the constant moment region, varying the interface property resulted in different crack patterns. The profiled and smooth interfaces, despite exhibiting different crack patterns, reached the maximum allowable crack-width of 0.3 mm at similar load steps of 69 KN and 71 KN respectively. This is because a sufficient bond at the interface is developed and as a result, due to local adhesion, the surface roughness plays a minor role. The beam specimens with completely and partially debonded interfaces localised at a much earlier load step of 45 KN and 50 KN respectively. This is because the stress generated in the SHCC layer is higher due to the severely cracked concrete layer on top. The beam specimens with smooth interface and HMPE fibers in SHCC perform similar to the beams with smooth interface and PVA fibers in SHCC. This is because of the additional constraint created due to the conventional concrete layer at the top in the SHCC-concrete hybrid beam. Despite the higher flexural capacity of the SHCC layer containing HMPE fibers, the concrete layer crushes much before the SHCC layer fails, thus, compromising the performance of the hybrid beam. By reducing the interface stiffness properties, an effort is made to reproduce the behaviour of specimen beams in four-point bending test through numerical modelling on Diana FEA. However, the prediction of the number of cracks formed and the crack-widths is inaccurate for all the beam specimens modelled. This is because the material models and interface properties available on Diana FEA to the authors knowledge, are insufficient in replicating the experimental cracking behaviour.