Strengthening of RC flat slab-column connections against punching shear using engineered cementitious composites (ECC) with PE and hybrid steel/PE fibres: An experimental and theoretical study

Engineered cementitious composite (ECC) exhibits excellent tensile properties and strain-hardening behaviour, which has proven to be a potential material for addressing the brittle failure mode occurred in reinforced concrete (RC) flat systems. This study employs a systematic experimental and theoretical approach to investigate the improvement of punching shear behaviour in slab-column connections through the incorporation of ECC. Eight connections were designed accounting for the effects of ECC zone area, steel reinforcement ratio, and fibre type and dosage in ECC, which were subjected to concentric loading to investigate the load-bearing capacity, crack patterns, deformation characteristics and failure mechanisms. Experimental results indicate that all specimens ultimately failed through punching shear, while ECC-enhanced connections exhibited significantly improved ductility compared to conventional RC counterparts. ECC increased the first crack load and load-bearing capacity of connections by 18.58–33.31 % and 34.15–37.76 %, respectively. Yielding of the reinforcing bars was observed in all specimens, with higher reinforcement ratios and fibre dosages enhancing the yield load. Increasing the polyethylene (PE) fibre dosage in ECC from 1.0 % to 2.0 % led to a rise of the load capacity, deformation resistance and yield load. ECC with hybrid steel (0.5 %) and PE (1.5 %) fibres enhanced the structural stiffness but had a negligible effect on punching shear capacity when compared to ECC containing 2 % PE fibres. Punching shear failure surfaces consistently formed outside the ECC zone, effectively expanding the base perimeter of the shear cone and increasing the critical punching shear crack angle, which was considered to determine the critical section coefficient k . Applying the modified k to GB50010–2010 and DIN 1045–1 provisions gave accurate predictions of the punching shear strength for ECC-enhanced connections featuring critical sections of the punching shear cones outside the ECC zone.