Damage evolution and failure warning in early-age flexible-formwork concrete for underground support

The evolution law of mechanical properties and damage characteristics of early-age flexible formwork filling concrete have a decisive influence on the stability control of surrounding rock of large deformation roadway. This study obtained the mechanical evolution characteristics of flexible formwork concrete filling body by using the standard ratio of engineering site and laboratory system test, clarified the time-space coupling mechanism of acoustic emission characteristic parameters and stress field evolution in the process of damage accumulation, and established a multi-parameter damage constitutive model of early-age concrete considering aging characteristics in combining with the theory of damage mechanics. The results show that: (1) Under the same curing age, the compressive strength of the filling body is significantly negatively correlated with the water-cement ratio, and the correlation decreases with the increase of the curing age, showing obvious strain softening behavior in the post-peak stage; (2) During the loading process, the concrete filling body presents a typical’ three-stage’ acoustic emission response characteristics, that is, the rising period of the initial micro-fracture accumulation, the active period of the main fracture development and the attenuation period after the failure; (3) At a certain curing age, with the increase of water-cement ratio, the total number of acoustic emission b -value signal points generated by the specimen during the test gradually decreases, and the b -value curve changes, and the minimum value appears near the peak stress point; and (4) The pre-peak and post-peak complete damage constitutive equations are established, which can accurately predict the mechanical response of concrete backfill under different curing times and water-cement ratios. The research results provide a basis for selecting the support time and support parameters for large deformation roadway. • The mechanical behavior of early-age concrete filling body is revealed. • A damage constitutive model and failure prediction method for early-age concrete backfill were established. • The correlation between its acoustic emission signals and failure modes was revealed.