Behavior of bond between reinforcement and concrete under fire

• Applicants: Dr.-Ing. Josipa Bosnjak; Professor Dr.-Ing. Akanshu Sharma
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: from 2016 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 317099919

Understanding the bond behavior under fire is essential for both design and assessment of reinforced concrete structures. One of the challenges in case of reinstatement is the assessment of the residual capacity of the existing members. Currently, there are no clear guidelines on the evaluation of the residual bond capacity after exposure to fire. The simplified guidelines given by codes are insufficient for a realistic estimation of the residual bond capacity. In order to realistically model the bond behavior, it is necessary to differentiate between the possible failure modes (pure bond failure and/or concrete splitting failure), which is recognized by the fib Model Code 2010 for the bond at room temperature. This code recommends an analytical bond stress-slip relationship for pull-out failure or splitting failure based on different parameters such as concrete cover, concrete strength, rebar diameter etc. Even though a lot of research work on bond behavior under elevated temperatures has been performed so far, an appropriate model for the bond behavior under heating (real fire scenarios) and realistic boundary conditions (unconfined specimens) is still missing. The majority of the existing literature data on the bond behavior at high temperatures was obtained for steady state conditions and by preventing concrete splitting. Furthermore, the majority of the experiments were performed at relatively low heating rates. These data lack the information about the effect of thermally induced stresses and consequent concrete damage developed in real fire scenarios on bond behavior. Increased concrete damage can contribute to concrete splitting before the theoretical bond strength is achieved, resulting in a possible overestimation of the theoretically obtained residual bond capacity. The main objectives of the present research project are to investigate the bond behavior and to develop a practical model for the estimation of residual bond capacity after exposure to standard fire considering realistic boundary conditions. The proposed research work includes an experimental campaign augmented by numerical simulations. The investigations are to be performed using beam-end test specimens exposed to standard fire curve (ISO 834). The following parameters will be investigated: heating duration, heating type (one- or three-sided exposure), thickness of the concrete cover, transverse reinforcement and reinforcement diameter. The two heating types, one- and three- sided heating, represent the realistic fire exposure in slab and beam elements, respectively. Experimental results will be further extended by numerical simulations. The results of the experimental and numerical work will be used to propose a practical model for the estimation of residual bond capacity after exposure to standard fire. The model will be developed as an extension of the existing bond model (fib Model Code 2010).

• DFG Programme: Research Grants
• Co-Investigators: Professor Dr.-Ing. Jan Hofmann; Professor Dr.-Ing. Josko Ozbolt