When it comes to filling voids under concrete slabs, the technology has evolved from simply plugging the gaps with more of the same materials. Nowadays, there are several methods to fill the gaps and bring the slab to similar conditions. The most common procedure involves drilling penny-sized holes in the slab and injecting gap filling foam through them. As the foam expands and spreads, it not only fills existing voids, but also lifts and brings the slab to similar conditions.
Another method is to lift the tile and replace the missing substrate with expandable polymer foam, inflatables, or jacks. This is done by drilling small holes through the slab and directing the liquid foam into the vacuum. As the foam cures, it also expands and applies upward pressure to the tile, lifting it up. This method also uses foam as a substrate, so often no other support or filler is required.
Alternatively, you can repair discrete sections of the damaged concrete slab by breaking it with jackhammers. A vacuum under your concrete when not repaired immediately will cause extensive damage to the slab that lifting the slab or repairing the sectional slab can repair the concrete. Finally, you can buy a specialized concrete gap filler, although these tend to be quite expensive. Voids under concrete slabs are very dangerous, especially if there are walls and more floors above, so you should make sure to correct any problems as soon as possible. With these methods in mind, you can now fill any voids under your concrete slabs with ease.