Fire resistance tests on polypropylene-fiber-reinforced prestressed concrete box bridge girders

Abstract

This paper presents an experimental study on fire behavior of polypropylene fiber prestressed concrete (PFPC) bridge girders subjected to localized hydrocarbon-fuel fire (mixed diesel oil and natural gas) and applied structural loading. The thermal responses, including temperature inside chamber, temperature within concrete and prestressing strands, were measured and analyzed throughout entire fire exposure duration. Further, structural responses, embracing mid-span deflection, effective prestress, polypropylene (PP) fiber concrete spalling, fire resistance, damage evolution and failure modes, attained from test were investigated in detail. The experimental results show that PP fiber can significantly release pressure of water vapor generated within high-strength concrete used in PFPC bridge girders, thus effectively enhancing spalling resistance of concrete in PFPC girders at elevated temperatures. The diaphragm can keep temperature inside chamber and also within top flange of PFPC box bridge girders steady for a period of time when temperature reaches 100 °C. However, the diaphragm has little effect on structural response of PFPC box bridge girders. The mid-span deflection increases sharply at final stage of fire exposure. Meanwhile, fracture of prestressing strands occurs and the effective prestress decreases suddenly towards final stage of fire exposure. Thereafter, PFPC bridge girders rapidly get to limit state dependent on rate of deflection, thus exhibiting a brittle failure characteristic. This failure of PFPC bridge girders can be prevented using reserved prestressing strands. PFPC bridge girders, designed as per under-reinforced girder at room temperature, vulnerably present failure characteristics of rare-reinforced girder under fuel fire exposure conditions.