There are a wide variety of epoxies and polyurethanes used for crack injection. Smaller crack widths would require a lower-viscosity material to penetrate the crack. You should contact an engineer or injection specialist experienced in shotcrete and cracking issues to evaluate the cracking and make a specific recommendation for repair. Proper concrete mixture design, placement techniques, and early water fogging and curing can help to reduce plastic shrinkage and drying shrinkage cracking in the future.