Fractal characteristics of fragmentation and evolution law of dissipated energy in early-age concrete–surrounding rock composite under impact loading

To investigate the damage mechanisms of blasting loads on early-age concrete lining structures, this study combines split Hopkinson pressure bar (SHPB) experiments and mesoscale simulations to investigate the dynamic response and damage mechanisms of early-age concrete–granite composite specimens under blast-like loading. Specimens cured for 2–4 days were tested to evaluate mechanical properties, energy evolution, and fragmentation characteristics. The results indicate that with increasing curing age, the composite transitions from a more deformable state with higher energy dissipation capacity to a stiffer state characterized by enhanced brittle behavior . Dissipated energy increased significantly with impact velocity—by 96.07% (2-day), 172.63% (3-day), and 207.11% (4-day). Both energy absorption efficiency and fragment fractal dimension decreased with curing age under identical impact conditions. Mesoscale simulations revealed that the interfacial transition zone material, serving as a weak interface, dominates the damage initiation and propagation processes in the specimens, thereby validating the dynamic fragmentation mechanism of the composite specimens. Through comparative analysis with experimental results and based on conventional wave impedance matching theory, a wave impedance correction model was established, reducing the prediction error from the original 14% to 4%. These findings confirm the significant influence of shotcrete age on the failure characteristics of tunnel lining structures and provide theoretical support for the precision design of tunnel blasting parameters. • Fractal dimension D & dissipation energy show opposite trends with velocity/age • Fractal dimension D & absorption rate decrease with age under same impacts • ANSYS/LS-DYNA sims show ITZ as meso weak zone controlling concrete crack init/prop • Wave-impedance modified model reduces theory-simulation error from 14% to 4%.