Modified yield-line theory approach to determine spayed concrete flexural capacity

Yield-Line Theory (YLT) is an upper bound method for determining the bending capacity of a thin ductile slab. YLT has been shown to overestimate the flexural capacity of sprayed concrete slabs in some cases (e.g. Uotinen et al. 2013). In this paper, the EFNARC three point bending test on square panel with notch is evaluated using the original YLT, modified YLT and a fracture mechanics approach, and compared to a finite element solution. A small, 20 sample, comparative testing campaign is planned to be carried out in Aalto University in fall 2014. The emphasis is on the cracking capacity of the sprayed concrete, as cracked structures are not accepted and already require renovation. The solutions for each approach are shown and precomputed for the selected C35/45 comparison concrete strength class. The notched slab test was published in 2011 and not many cases are yet available. Comparison of the predictions to two known cases is shown. For future research, we suggest further modifications to the YLT method to represent the structural behaviour more accurately. One approach is to apply the load in steps and calculate the corresponding moment line pattern separately for each load stage. Second approach is to modify the line moment equation to reflect the semi-ductile behaviour, and to accommodate the effect of steel or polymer fibres. The third approach is to incorporate crack growth energy directly into the virtual work equations.