Experimental study on spalling of concrete of prestressed slabs in fire

If the concrete of a prestressed slab spalls in fire, the reinforcement will be exposed to flame, and the slab would soon lose its bearing capacity due to breaking of reinforcement caused by burning. Fire experiments are conducted on fourteen simply supported unbonded prestressed concrete slabs, one simply supported pretensioned prestressed concrete slab and nine two-span prestressed concrete continuous slabs. The spalling of concrete, breaking of tension reinforcements, distribution and development of cracks for 8 simple slabs and 3 continuous slabs are described. Preliminary conclusions are: concrete spalls more easily or more seriously when the compressive stress is higher or the tensile stress is lower in the surface exposed to fire, and when the strength of concrete and water content in concrete at normal temperature are higher. Based on the test results, two expressions of top envelope curve for concrete spalling are presented by taking the average standard concrete cubic strength at normal temperature as the x-axis and the normal stress level of concrete extreme fiber in pre-compression zone of slab in fire as the y-axis, and taking the water content in concrete at normal temperature as the x-axis and the normal stress level of concrete extreme fiber in pre-compression zone of slab in fire as the y-axis; to facilitate design and application, two expressions of top envelope curve of concrete spalling are presented by taking the average standard concrete cubic strength at normal temperature as the x-axis and normal stress level of concrete extreme fiber in pre-compression zone of slab at normal temperature as the y-axis, and taking the water content in concrete at normal temperature as the x-axis and normal stress level of concrete extreme fiber in pre-compression zone of slab at normal temperature as the y-axis.