245 / 2022-06-15 14:22:52

Shear Failure Mechanism and Loading-Capacity Model of Reinforced ECC Beams

Engineered Cementitious Composite (ECC), Shear strength components, Stirrup

To enhance the structural and seismic resistance, as well as durability of concrete structures, an ultra ductile fiber reinforced cementitious composites called Engineered Cementitious Composite (ECC), also known as Strain Hardening Cementitious Composite (SHCC), was developed. ECC has a similar compressive and tensile strength to conventional concrete, but it exhibits a pseudo-strain-hardening behaviour under uniaxial tension with excellent crack control ability. The ultimate tensile strain of ECC can reach 3-12%, which is 300-1200 times higher than that of concrete. It is reported that ECC can also exhibit at least twice as high shear carrying capacity compared to traditional concrete, signifying a potential to use ECC material in shear-resistance elements. However, the shear resisting mechanism of reinforced ECC (R/ECC) members is still not clear.

In most existing codes and models, the shear strength of reinforced structural members (Vu) is divided into two parts, i.e., shear resistance coming from the matrix (Vc) and from the transverse reinforcement (Vs). To quantify accurately Vc and Vs and also their development throughout the loading, a well-designed testing method consisting of continuous strain quantification along the stirrups, was used in this research. That is, cutting the stirrup leg into two halves, and installing strain gauges continuously inside a small cavity at the center of the bar. With such a full record of strain, the Vs can be determined by the stirrup strain where shear crack crossed, and Vc can be calculated by subtracting Vs from the total shear force. Six steel reinforced beams incorporating different matrix (ECC, concrete and mortar) were tested under four-point bending.

Shear compression failure was found for all tested beam specimens with crushing of matrix in compression zone. Unlike sudden and rapid shear crack propagation in reinforced concrete and mortar beams, steady and multi-cracking behavior was observed in ECC beams. For all samples, R/ECC, RC or R/mortar, Vc was found not constant after first shear cracking. Besides, stirrups crossed by shear crack did not always yield when ultimate shear strength was reached. Therefore, assuming yielding for all the stirrups along shear cracking path would lead to overestimation on Vs. The shear strength of R/ECC without stirrups was found to be 170% that of the reference concrete beam. However, when reinforced with stirrups (ρt=0.38%), the shear strength of R/ECC beam was found only 13% higher than that of concrete ones, which resulted from incomplete yielding of stirrups at ultimate shear failure. Finally, a simplified truss-strut model for predicting shear carrying capacity of reinforced ECC beams is proposed, and a good agreement is achieved with the experimental results.