Bending and Shear Behavior of Two-Layers Casting 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.
Key Words: Two-layer RC beams, HSC beams, NSC beams, Shear strength, Ultimate shear
strength.