Experimental fire tests of two-way concrete slabs

A series of full-scale fire tests conducted at the Cardington steel framed test building in the United Kingdom have shown that unprotected composite floor slabs do not collapse after a compartment burnout, despite suffering considerable deformations and very high measured steel temperatures. When the unprotected steel beams weakened, the load resistance progressively transferred from the beams to the slabs which resisted the loads by tensile membrane action. These tests have prompted extensive computer modelling by many researchers to simulate the behaviour of steel framed buildings and tensile membrane action of the slabs under fire conditions. However, there are no published fire tests of two-way concrete slabs in a controlled furnace environment This report describes the fire tests of two-way concrete slabs conducted at the BRANZ fire resistance furnace. The tests were conducted to investigate the behaviour of unrestrained simply suppmted slabs in a controlled furnace environment. The test data can be used to verify current simple design methods and sophisticated computer models. Six slabs were tested, comprising three reinforced concrete flat slabs and three composite steel-concrete slabs. The slabs measured 3.3m by 4.3m and had thicknesses ranging from 90mm to 130mm. The three flat slabs had different quantities of reinforcing steel to investigate their effect on controlling crack widths to prevent integrity failure. The slabs were simply supported on all four sides over the furnace with no horizontal restraint. The slabs were subjected to a live load of 3.0kPa and were heated on the underside with the gas time temperature curve following the ISO 834 standard fire for three hours. The slabs petformed very well in the fire tests, supporting the loads for the full duration of three hours without collapse. By three hours, the gas temperatures had reached 1100°C and high temperatures were measured across the depth of the slab. The temperatures of the reinforcing steel exceeded 700°C. All the slabs suffered extensive surface cracking and loss of moisture. Some of the slabs suffered large midspan deflections (up to 270mm) and full depth cracks which were associated with the yield line crack pattern. The slabs with higher steel contents and closer bar spacings suffered only surface cracking, while the slabs with the lower steel content suffered full-depth cracks. The cold-drawn mesh used in the tests performed well and did not fracture as might have been expected if such large strains had been imposed at ambient temperature; this is due to the increased ductility of the steel caused by the elevated temperatures. The slabs resisted collapse even though the calculated ultimate load capacities had dropped significantly below the level of the applied loads. The structural fire resistance of the slabs in the tests exceeded the predictions of code recommendations. The tests illustrated the significant effect of tensile membrane action on the structural fire resistance of two-way slabs which resisted collapse despite significant loss of flexural strength