DOES LOWER ELASTIC MODULUS LEAD TO LOWER FLEXURAL STIFFNESS IN REINFORCED ENGINEERED CEMENTITIOUS COMPOSITES?

Engineered Cementitious Composites (ECC) are a special class of fibre-reinforced cementitious materials exhibiting strain-hardening and multiple-cracking under tension; therefore, ECC have clear advantages over normal concrete for many construction applications. However, ECC are also well known to have a lower elastic modulus compared to plain concrete of similar compressive strength due to lack of coarse aggregates. This gives rise to concern for its use in structural applications due to the possibility of higher deflection for a given load. The experimental results presented in this paper clearly showed that such misgivings are unwarranted. Compared to the Reinforced Concrete (R/C) beam with plain concrete showing not only higher elastic modulus (68% higher) but also higher compressive strength (23% higher), the Reinforced ECC (R/ECC) beam with identical steel reinforcement showed comparable post-crack flexural stiffness under four-point bending. This is due to the tensile stress carried by ECC in the cracking regime below the neutral axis where plain concrete carries very little stress. The experimental results were validated by analytical calculations of the load-deflection response of both R/C and R/ECC beams and moment-curvature behaviour of the section using the Euler's beam theory. The contrasting failure modes and cracking patterns of R/C and R/ECC beams were also illustrated and discussed. The findings from this study provide new insights on the influence of lower elastic modulus of ECC on the structural stiffness, and encourage wider adoptions of this innovative material in various applications.