Structural Behavior of Voided Reinforced Concrete Beams Under Combined Moments

Abstract

The ability of reducing the reinforced concrete beams weights without significantly affecting its structural adequacy especially when combined loads are more eventual, is the aim of present paper. Therefore, the structural behavior of reinforced concrete beams containing air voids and subjected to combined moments of equal bending moment and torsional moment was observed.Five identical beams in external dimensions and internal reinforcement were the specimens of the study. The five beams were divided into two solid, two contain series of plastic balls and one contains a single plastic pipe. The plastic balls and pipe were used to create air voids inside the beams to reduce its concrete amount for economic and environmental considerations. All the beams were tested until failure, but two from its (one solid and one contains plastic balls) were tested under pure bending moment while the rest three beams were tested under combined moments. Cracking load, ultimate load, deflection and twisting angle were measured during tests as well as angle and width of cracks at failure.The experimental results revealed the similarity in structural behavior between voided and solid tested specimens is clear even when the values are different. For the section strength, when the beam containing voids, it may be lost about (13 – 23) % of its solid ultimate strength under combined load of equal fractions of bending and torsion moments. Also, the cracking load of the beam may be unaffected by voids presence under this combined load. When deformability is focused, the voided beams stiffness against bending moment is better than the torsional one; therefore, the twisting angle can be more noticeable than the deflection. Besides, the torsional moment governs over the bending moment, so that inclined cracks caused in failure. Finally, creating the voids by small size plastic pipe can make the beam stronger but more deformable than counter beam containing big size plastic balls under the combined moments.

Keywords: Structural behavior, Combined moments, Concrete amount, Voids presence, Spherical voids, Tubular void