Interfacial behavior of hybrid SHCC-Concrete beams with a joint at midspan

Abstract

Strain Hardening Cementitious Composite (SHCC) is a new type of concrete that is able to control the crack-width in concrete structures. The application of this material in the tension zone of hybrid concrete structures is thus of interest in practice. The interfacial behavior for these hybrid structures is something that is of importance for the composite action. Therefore, a study is performed to investigate the behavior of the interface in hybrid SHCC-concrete beams. This is done by performing an experimental study on beams with a joint at midspan. Several parameters such as the interface roughness, coupling reinforcement cover, curing method and protruding reinforcement have been investigated. The hybrid SHCC-concrete beams are tested in a four-point bending configuration to obtain a constant-bending moment region along the interface. The crack propagation along the interface and through the SHCC/concrete has been evaluated with the use of Digital Image Correlation (DIC). Furthermore, also a numerical analysis has been performed using DIANA FEA based. The numerical analysis was used to help to determine the experimental campaign. Furthermore, also several experimental samples have been modelled to study the behavior of the beam in more detail and to make recommendations for future modelling of the interface. Based on the experimental results, the influence of the interface roughness resulted in an increased bond at the interface. This is both seen in an increased bearing capacity and a reduction of the interface and joint opening at equal loads. The profiled interface and holed interface resulted in an increased capacity due to the mechanical interlocking at the interface. The bearing capacity of these samples increased by 78.4% and 54.7% respectively compared to the reference sample (13.9 kN). In case of the profiled sample, no delamination of the interface occurred as the sample failed due to a horizontal crack at the level of the coupling reinforcement. This can be attributed to the localized tensile stresses at the reinforcement level. The last sample with a different interface roughness consisted of epoxy and sand (1-2 mm). For this sample, the bearing capacity increased by 51.8%. The failure of this sample was a combination of interface delamination (initially at the joint) and a horizontal crack through the coupling reinforcement. For all the samples with a different interface roughness, no yielding of the coupling reinforcement occurred. Several beams with a smooth interface have also been tested by adjusting the reinforcement cover, curing method and applying protruding reinforcement. The influence of the reinforcement cover didn’t have an effect on the bearing capacity. However, the interface and joint opening reduced significantly as the eccentricity of the reinforcement bars also reduced. Furthermore, also the effective height increased resulting in lower reinforcement stresses. The influence of a different curing method was investigated by placing the sample in a humidity-controlled (50 %) room to investigate the effect of shrinkage. The results showed that the bearing capacity remained similar to the reference samples. This can be attributed to the fact that the bond internally was still good, caused by the restraint of the reinforcement bars. However, the deflection of the beam increased as a result of the reduced stiffness by the shrinkage induced cracks. Finally, one sample consisted of stirrups at a distance of 50 mm from the joint to take up the tensile stresses at the interface (e.g. due to reinforcement eccentricity). The results showed an increase in bearing capacity by 102.7% compared to the reference sample. However, also in this case, no yielding of the coupling reinforcement occurred. The failure of the sample is also caused by the delamination of the interface. Furthermore, as a result of this delamination, fracture of the top part of the SHCC occurred at the location of the protruding reinforcement due to the rotational restraint of the stirrup. The 2nd part of the study consisted of a numerical analysis. A Coulomb-friction model with an interface tensile strength cut-off is used to model the interface. Based on this, a good correspondence between the experimental results and FE results is found in terms of the bearing capacity and failure mode for the sample with a smooth interface(delamination). However, the crack propagation of the flexural cracks through the concrete and SHCC was substantially different.
The sample with a profiled interface has also been modelled in Diana FEA. This is done by implementing the profiled interface manually in the FE model. The model showed a good correspondence with the experimental results in terms of bearing capacity and interface and joint opening. The crack propagation in the concrete is also similar to the experimental results. The model however doesn’t show any flexural cracking in the SHCC layer as a result of the limitation of DIANA FEA. Also, an additional study is done to replicate the behavior of this beam using a smooth interface. Based on this model, it is recommended to use a perfect bond at the interface. For both these models, the FE model wasn’t able to replicate the strain hardening behavior of SHCC. This is due to the limiting material models in DIANA.