Concrete is generally known as a two-phase composite material containing cement matrix and aggregate particles. The effect of transition zone between aggregates and cement paste on compressive strength is well known for traditional concrete but its effect on modulus of elasticity is not yet obvious. In this study, it is aimed to apply composite material rules on estimating modulus of elasticity of rubberized concrete. In experimental study, some cylindrical specimens are produced by using waste tire rubber aggregate (RA). RAs replace traditional sand at 15, 30, and 45% ratios by volume. Modulus of elasticity of concrete is determined by conducting experimental test. Hashin-Shtrikman (H-S) bounds are applied and it is attempted to determine whether rubberized concrete match the interval between H-S bounds or not. Consequently, it is indicated that analysis of rubberized concrete as a two-phase material is not corrected by using H-S bounds and it should be considered as a three-phase composite material. It is also pointed out that transition zone must be considered as the third phase.