Flexural Behavior of Two-layers Reinforced Concrete Beams

Abstract

High-strength concrete is used to reduce the size of the beams in addition to enhancing the strength, this leads to overestimated cost in comparison with normal-strength concrete while using normal-strength concrete leads to the overestimated amount of concrete (layer size) of the beam section. For balancing the condition between the cost and size of beams, the benefit of both materials is used, by using beams in two layers, high-strength concrete in the compression zone (top layer), which is more beneficial for beam strength, and normal strength concrete in tension zone (bottom layer), which is no need using high strength concrete the in-tension zone. This study investigates the flexural and shear behavior of reinforced concrete beams consisting of two layers with different concrete strengths (grades), for beams with and without shear reinforcement (stirrups), considering the effect of shear-span ratio, layer thickness, layer compressive strength, and the overlap time casting of the two layers. The experimental program consists of a total of nineteen reinforced concrete beams of dimension (125 mm x 250 mm) with a total length of 1200 mm, the beams are reinforced with longitudinal reinforcement (4Ø12mm) and using (Ø8mm) bar as transverse reinforcement (stirrups). The experimental results show that the crack pattern of the two-layer reinforced concrete is closer to the crack of the control beam with one layer. Increasing the compressive strength of the concrete of the top layer, the ultimate failure load increased by (8.35%, 15.6%, and 18.85%), with respect to the (control beam) with the full depth of normal concrete. By increasing the high-strength layer thickness, the value of shear strength (Vc) and ultimate shear strength (Vu) increased linearly. The casting overlap time of up to (60 min) can be used for casting two-layered reinforced concrete beams, which is recommended, beyond this time the strength of the shear strength (Vc) and, ultimate shear strength (Vu) decreases. With increasing the shear span ratio (a/d) from (1 to 1.5 and 2) the ultimate load failure decreased by (33% and 50%). The shear strength capacity decreases with increasing stirrup spacing.