A good bond between reinforcement and concrete is essential for the integrity of reinforced concrete structures. For a concrete structure subjected to fire, failure of bond might lead to a premature and brittle failure of the structural members. Owing to high thermal gradients and the associated cracking in the concrete cover, there is a relatively high chance that the bond failure would occur due to concrete splitting prior to the attainment of full bond strength. In order to rehabilitate a reinforced concrete structure after fire, the assessment of residual bond strength is essential.
Bond between reinforcement and concrete under fire goes much beyond the traditionally investigated aspect of bond failure in case of pull-out specimen under elevated temperature. The pull-out test is generally performed in confined conditions, i.e. the concrete surrounding the rebar is supported in the direction of loading, which prevents concrete breakout and delays concrete splitting. This is primarily done to identify the bond characteristics unadulterated by other possible failure mechanisms. Furthermore, the majority of studies were conducted under low heating rate (typically 1-10°C/min). Upon reaching the target temperature the specimen were allowed to reach a steady state. Such heating conditions provided data on bond strength as a function of the interface temperature and these were practical for constitutive modeling of bond.
Even though useful for the development of constitutive bond law during and after exposure to high temperatures, this approach does not consider several important aspects. The boundary conditions of a bar embedded in an actual beam are generally quite different. Under realistic fire scenario, the reinforcing bar is protected by a relatively small cover, the heating regime is fast and non-uniform and the confinement is only partial, provided by either stirrups (in beams) or concrete continuity in the transverse direction (in slabs). Due to these boundary conditions, more often than not, splitting of the concrete cover limits the usable bond strength of the bar.
This project deals with investigating the bond between reinforcement and concrete under realistic boundary conditions and fire scenario considering bond degradation and concrete splitting. The project utilizes beam-end specimens subjected to ISO 834 standard fire to simulate the realistic conditions of dimensions, reinforcement detailing and fire. The project is funded by the German Research Foundation and is undertaken jointly along with Dr. Josipa Bosnjak.