The reduction in compressive strength with increasing in the amount of polypropylene grains can be attributed to the tiny voids created within the concrete matrix by the presence of the fluffy polypropylene grains, these voids were filled with water when the concrete was wet, and this also contributed to the high water cement ration of 0.75 that was used and the eventual low
cube strength of 19.07 N/ mm2 obtained for the control sample.
The average cube strength of the four different batches were determined as 32.6 N[/[mm.sup.2], 32.5 N/[mm.sup.2], 21.63 N/[mm.sup.2] and 0.88 N/[mm.sup.2] respectively, as shown in Table 4.
Notations [A.sub.c] Cross-sectional area of concrete, [A.sub.s] Cross-sectional area of steel, b Internal breadth of the section, [b.sub.f] External breadth of the section, D External depth of the section, h Internal depth of the section, [f.sub.ck] Characteristic strength of concrete, [f.sub.cu] Characteristic 28-day cube strength of concrete, [f.sub.y] Yield strength of structural steel, [M.sub.u(the)] Ultimate theoretical moment of resistance, [M.sub.u(exp)] Ultimate experimental moment resistance, [M.sub.u(pla)] Ultimate plastic moment of hollow steel section, t Thickness of steel section, y Depth of neutral axis, [Z.sub.p] Plastic section modulus of the steel beam.