Effects of vertical reinforcement on the shear performance of UHPC deep beams with synthetic and steel fibres

Abstract

This study examines the shear behaviour of ultra-high-performance concrete deep beams (UHPC-DBs) reinforced with three fibre types : 2 % hooked-end (HE) steel fibres, 0.76 % 5D steel fibres, and 0.11 % Forta-Ferro (FF) synthetic fibres. It evaluates the effects of fibre type, fibre content, vertical web reinforcement ( ρ sv ), and shear span-to-depth ratio ( λ ) on shear capacity. Load-deflection curves, crack patterns, and ρ sv strain were analysed, and the optimal ρ sv was determined using the particle swarm optimization (PSO) algorithm to maximise shear capacity while minimising costs. Findings reveal that, compared to specimens without ρ sv , those with ρ sv = 0.38 % exhibit shear capacity increases of 12.42 %, 15 %, and 16.46 % for specimens reinforced with HE, 5D steel fibres, and FF synthetic fibres, respectively. Additionally, ρ sv strains increase by 19.99 %, 26.45 %, and 24.03 % for specimens reinforced with HE, 5D steel fibres, and FF synthetic fibres, respectively, when ρ sv rises from 0.075 % to 0.38 %. Higher ρ sv enhances stiffness and ductility, underscoring its role in improving these properties. Additionally, specimens reinforced with 2 % HE fibres exhibit more ductile behaviour than those reinforced with 0.76 % 5D steel fibres. Beams reinforced with 0.11 % FF achieve 80.35 % of the shear capacity of those reinforced with 0.76 % 5D, indicating FF as a viable alternative. Shear-compression failure occurs in all UHPC-DBs at λ = 0.923 and λ = 0.739, except for those reinforced with 2 % HE steel fibres and ρ sv values of 0.19 %, 0.30 %, or 0.38 %, which fail in diagonal tension . An increase in ρ sv reduces crack width in UHPC-DBs reinforced with all three fibre types. Additionally, UHPC-DBs reinforced with 5D fibres exhibit narrower cracks than those reinforced with FF fibres. Three 3D graphs were developed to illustrate the combined effect of λ and ρ sv on the shear capacity of UHPC-DBs. The results show that increasing ρ sv and decreasing λ enhance shear capacity. These findings highlight a gap in ACI 318–19, which does not explicitly address the interaction between λ and ρ sv in fibre-reinforced UHPC-DBs, suggesting that these graphs could help refine existing standards. When the optimal ρ sv of 0.19 % is incorporated, the resulting shear capacity reaches 90.47 %, 98.42 %, and 89.23 % of the capacities achieved with ρ sv = 0.38 % for UHPC-DBs reinforced with HE, 5D steel fibres, and FF synthetic fibres, respectively, while reducing steel reinforcement costs by 46 %.

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