Corner spalling and tension stiffening in heat-damaged R/C members: a preliminary investigation

Corner spalling in fire and tension stiffening past a fire are addressed in this paper by investigating reinforced and unreinforced prismatic specimens in the former case, and by testing reinforced tension members in the latter case. An experimental procedure to asses concrete sensitivity to spalling due to pore pressure is proposed as well. The focus is on the material as such, in order to allow different cementitious mixes to be compared in terms of spalling sensitivity under realistic conditions regarding the thermal gradients, the pore pressure and the moisture transfer (thermal and load-induced stresses are not a primary concern). Concrete spalling and tension stiffening are investigated within the same project, because (a) a bar—whose cover has been subjected to the risk of spalling during a fire—exhibits reduced tension stiffening, and (b) the same prismatic specimens were used (firstly thermal shock at 750 °C to provoke spalling and later rest at high temperature to investigate tension stiffening past cooling). Since only 10–20 % of the twenty-one specimens suffered medium/light spalling, nearly all the reinforced specimens were later tested to investigate tension stiffening in residual conditions. The prismatic specimens were made of SCC (target strength fc = 50, 80 and 90 MPa). The results show that (a) during a thermal shock the spalling tendency is rather weak if no load-induced stresses and/or thermal self-stresses occur, (b) tension stiffening appears to be still effective in 50 % of the reinforced specimens and (c) bond stiffness is a roughly linear function of the actual compressive strength of the concrete, be it heat damaged or not.