A STUDY ON STRUCTURAL FIRE BEHAVIOR OF REINFORCED CONCRETE WALL WITH SHORT FIBER REINFORCEMENT AFTER LOW VELOCITY IMPACT

Abstract

 Collisions between vehicles and structures can occur as a result of accidents. Occasionally, fire may break out if the vehicle payload includes oil, fuel or other combustible materials. Low-velocity impacts on reinforced concrete (RC) walls can lead to overall failure in the form of bending fracture or shear fracture, but in order to protect life and property near or inside the structure, collapse isn't acceptable. Moreover, if fire does break out after impact, the fire resistance of the structure must be retained to protect life and property. This means a fire inside or outside the structure mustn't spread to other compartments and spaces nor pass through the walls, and the structure mustn't collapse as a result of fire. Therefore it is necessary to investigate the influence of overall failure on the fire resistance of structures.
 The work of the authors focuses on combined impact and fire. They have carried out an experimental and analytical study on temperatures in concrete plates exposed to fire after high-velocity impact as well as an experimental study on the structural fire behavior of RC walls after high-velocity impact by a hard projectile. The experimental and analytical study reported here is the third step in this research program on impact and fire.
 The study focuses on low-velocity impacts and the structural fire behavior of load-bearing RC walls with short fiber reinforcement exposed to a hydrocarbon fire after such impacts. The study comprises low-velocity impact tests, fire tests on specimens under centrally loaded conditions after low-velocity impacts and numerical analysis of the structural fire behavior of these RC walls.
 The small-scale RC wall specimen was designed not to fail by shear but by bending against the load perpendicular to the wall axis, under the assumption the wall is hit horizontally by a rigid body. Six specimens measuring 300mm in width, 800mm in height and 80mm in thick were made with normal-strength concrete of design strength 24MPa. Deformed steel bars (SD295A) 6mm in diameter were used. Polypropylene short fibers of dimensions 0.05mm in diameter and 10mm in length were mixed into the concrete used for three of the six specimens at a ratio of one percent to the concrete volume.
 Four specimens were subjected to a low-velocity impact. ‘Overall failure’ in this study was defined as the maximum tension strain and residual tension strain of the main reinforcing bars exceeding the yield strain. The impact load was provided using a 150kg weight. In order to find in advance the specific velocity at which overall failure of a specimen occurs, a plain concrete specimen was continuously impacted by the weight at speeds of 0.5, 1.0, 1.5, and 2.0m/s. Based on this, one normal specimen and one short fiber reinforced concrete (FRC) specimen were struck by the weight at 1.5m/s, while one of the FRC specimens was struck by the weight at 2m/s. These low-velocity impact tests clearly demonstrate the advantage of short fiber reinforcement against low-velocity impact.
 Fire tests under long allowable centrally loaded conditions were carried out on five specimens. The two non-damaged specimens, one of plain concrete and the other FRC, exhibited the same fire resistance. Spalling occurred on the plain concrete specimen struck at 1.5m/s, but there was no spalling in the case of the FRC specimen. The fire resistance of the FRC specimen struck at 2.0 m/s was lower than that of 1.5 m/s. These differences in fire resistance among specimens confirmed through the numerical analyses without main re-bars on the heating side of a RC wall. The next stage of the project is to introduce a buckling phenomenon of main re-bar into numerical analysis.