Four-point bending test of high-early-strength high ductility concrete (HES-HDC): early mechanical behavior and toughness evaluation

To investigate the early mechanical response of high-early-strength high ductility concrete (HES-HDC), a four-point bending test was conducted. The effects of fiber type, fiber content, curing age, sand-binder ratio (s/b), and hydroxypropyl methylcellulose (HPMC) content on the crack pattern, load–deflection curve, flexural strength, and deflection of HES-HDC were studied. Based on the test results, a flexural toughness evaluation method suitable for the characteristics of HDC was proposed. The results showed that under flexural load, the load–deflection curve of HES-HDC exhibited a deflection-hardening response, with multiple cracks developed during the failure process. The flexural strength of HES-HDC reaches more than 5 MPa in 2 h, meeting the requirements for open traffic. Fiber type, fiber content, and curing age had a significant impact on the cracking behavior, deflection-hardening response, and flexural strength of HES-HDC. The higher the fiber pullout proportion and bridging stress during the crack-bridging was, the higher the deflection and energy absorption of HES-HDC exhibited. The existing flexural toughness evaluation methods were not applicable for HDC assessment due to their inability to consider the influence of different fiber types on the deflection-hardening response of HDC. A flexural toughness index in the form of energy ratio and a flexural strength coefficient in the form of strength ratio was proposed, which can effectively evaluate the flexural toughness of the HES-HDC beam during its whole loading process. This method took into account the large deformation of HDC and eliminated the influence of specimen size by using dimensionless forms.