Shear behaviour of uhpc beams with hybrid fibres

The enhanced workability and the long-term durability of UHPC have raised the interest of researchers to fully understand its structural performance. Typically, UHPC is reinforced with a 1-3% volume fraction of one type of fibre, which can be steel fibres or synthetic fibres. The effects on the shear behaviour when hybrid fibres reinforce the UHPC matrix of beams without stirrups was studied in this paper. The objective of the experimental campaign was to evaluate the effect of hybrid fibres for deep and slender beams using different percentages of volume fraction as well as a different type of combinations of fibres. For comparison, UHPC beams without fibres or reinforced with monofibres were also cast to understand the synergetic effects of hybrid fibres. The experimental results indicate that the structural system performance improved because the hybrid fibres could increase the cracking strength at the microcracking and macrocracking level simultaneously. The shear response of the 24 fibre-reinforced beams shows evidence that superior strain hardening behaviour was obtained from the beams with hybrid fibres, which developed extensive multicracking before crack localization. Furthermore, the efficiency of the long steel fibre was enhanced when combined with the short steel fibre leading to superior performance that achieved flexural failure even without the use of stirrups. Finally, a statistical assessment of a shear strength model proposed in previous studies shows that with simple interactions of the main parameters that influence the shear capacity, it is possible to obtain a reasonable prediction that allows a broader number of engineers to use it in structural applications. The shear strength model is suitable for UHPC deep and slender beams reinforced with monofibres or hybrid fibres.