Long-term mechanical properties of hybrid fiber-reinforced engineered cementitious composite

Hybrid fiber-reinforced engineered cementitious composites (hybrid ECC) employing short straight polyethylene (PE) and steel fibers have attracted growing interest in engineering applications due to their superior strength and ductility. In most studies, the mechanical properties of hybrid ECC were determined based on samples cured for 28 days, while their long-term mechanical properties are seldom studied. Since hybrid ECCs often incorporate supplementary cementitious materials that alter the hydration process over time, relying solely on the 28-day strength of samples may lead to inaccurate structural designs. To better understand the long-term mechanical properties of hybrid ECC, this work presents an experimental investigation of hybrid PE-steel fiber-reinforced ECC samples cured under standard conditions for up to 3 years. Uniaxial compressive tests, direct tensile tests, and four-point bending tests were conducted with samples cured at standard conditions for 28 days, 1 year, and 3 years. It was found that the compressive and tensile strengths of hybrid ECC increased with age. However, as the age increased to 3 years, the ultimate tensile strain and flexural ductility decreased significantly by 54% and 35%, respectively, compared to their 28-day values. Furthermore, most changes occurred within 1 year. It was also found that the main damage pattern of the PE fibers was transformed from pull-out to rupture failure as the curing age increased. Thermal-gravity analysis revealed that the hydration process of hybrid ECC may last for up to 3 years, which explains their changes in mechanical properties and PE fiber failure mode.