Preliminary results on tension stiffening in heat-exposed R/C tension members

Tension stiffening and - more generally – bond has attracted scanty attention in the recent past with reference to fire and high temperature, probably because of scholars’ and designers’ opinion that sectional collapse due to the strength loss in the rebars be more critical than the overall collapse due to bond loss. (As a matter of fact, bond losses involve the length of the reinforcement and not a single section, as bending). However, since many R/C structures survive a fire and require their residual safety level to be assessed, knowing bond properties and having information on tension stiffening in residual conditions is a necessary step to reliably analyze the structural behavior past a fire. In this research project, tension stiffening is investigated by testing a number of reinforced tension members initially exposed to a rather severe fire (750°C) to investigate concrete spalling. Later, the specimens were kept at 750°C for a time length sufficient to have a uniform thermal field in excess of 600°C, in and around the bar. An electric furnace pre-heated to 750°C was used. After the removal from the furnace, each specimen was tested by applying opposite forces at the end sections, to check the effectiveness of tension stiffening in residual conditions. The total number of specimens in this preliminary experimental campaign was 11. Three self-compacting mixes were used (fc = 50, 80 and 90 MPa). Only 5 specimens showed a non-negligible tension stiffening, and the justification of such weak effect was found in the marked bond sensitivity to high temperature, something that was also checked by means of a rather simple slip-based model.