Load-defl ection behaviour of hybrid concrete flat slab

Due to the important role of high strength concrete in the structural systems, present work focuses on the use of this material as a strengthening technique incorporating with the normal strength concrete in flat slab system. Eight simply supported flat slab models with (1,000 × 1,000 × 120 mm) dimensions are investigated based on three groups including normal strength concrete and high strength concrete. The first group represents models containing of two flat slabs fully with one type of concrete; NSC and HSC as control flat slab. The second and third groups consist of six flat slabs as hybrid flat slabs of two layer of concrete with different thicknesses. Concrete mixture HSC was used in tension zone in three hybrid flat slabs (second group) with three thicknesses (30, 60 and 90 mm), while the remaining three hybrid flat slabs (third group) was used the HSC in compression zone with the same previous thicknesses. The experimental results shown that the ultimate load increased about (19.4%) when HSC was used fully (hH / h = 1) instead of using NSC in the control flat slab (NSC slab). The hybrid flat slabs with use HSC in compression zone showed higher in cracking and ultimate flexural loads compared with those of the hybrid flat slabs with use HSC in tension zone and also were stiffer in load-defl ection curve with the hybrid flat slabs with HSC in tension zone, also the hybrid flat slabs showed an improvement in the cracking load and ultimate flexural load when increasing the thickness of the HSC layer (hH / h) in both tension and compression zone as compared to control flat slab (NSC slab).