What is a honeycombed slab?
A honeycomb slab is a slab that has honeycombs in the concrete. Usually, the honeycombs are visible only on the outside surface of the perimeter grade beams. A honeycomb is a void area in the concrete.
Why honeycombs are usually not important
Very few honeycombs are structurally significant. They are shallow and not very large. The ones that are visible are usually repaired by the builder.
The honeycombs you cannot see are the problem
The honeycombs that are structurally significant are the ones you cannot see. They are in the stiffening beams where they cannot be seen.
What causes honeycombing?
Honeycombing results when air is trapped between a form board or a trench wall and the concrete.
How builders normally control honeycombing
The only thing I have ever seen a concrete finishing crew do to minimize honeycombing is to take a sledgehammer and use it to tap the form boards for the perimeter grade beam. When the hammer impacts the outside form board, the form board vibrates and air can escape.
Unfortunately, this is not the best way to control honeycombing.
How builders could minimize honeycombing
The most effective way to minimize honeycombing is to use a vibrator. This is a special power tool that is very effective in minimizing honeycombing. I see these used occasionally, but only on the exterior grade beam. Every grade beam and stiffening beam should be vibrated with a concrete vibrator.