Effetto irrigidente dell'aderenza armatura-calcestruzzo in presenza di alta temperatura

The two aspects of bond in R/C structures – anchoring ability and tension stiffening - have attracted scanty attention in the recent past with reference to fire and high temperature. 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 preliminary research project, tension stiffening is investigated by testing a number of R/C tension members exposed to a severe fire (temperature-time curve comprised between the hydrocarbon and the standard fires in the first 20 minutes, and exposure to 750°C for further 100 minutes). After two hours, the average temperature of the specimens was close to 650°C. An electric furnace was used for heating the specimens, once at a time, and natural cooling followed inside the furnace. After the removal from the furnace, each specimen was loaded by applying opposite forces to the rebar at the end sections, to check the effectiveness of tension stiffening in residual conditions and specifically to evaluate the residual bond stiffness. Ten specimens made of three self-compacting mixes (fc = 50, 80 and 90 MPa) were tested. Tension stiffening turned out to be rather weak, as it occurred only in 5 specimens, where bond stiffness exhibited a marked decrease compared to the virgin situation, as confirmed by modelling bar-concrete interaction via a simple axisymmetric slip-based model.