A very thin epoxy resin is shot into concrete cracks. The resin is much stronger than the original concrete. Because it is so thin and injected under pressure the resin can penetrate to the full depth of a crack and weld concrete like you would weld steel. It is also very effective in stopping leaks through concrete cracks. Slab on grade cracks can be typically injected by gravity, requiring several passes with the resin to finally fill the crack.