How to fill void under concrete slab?

Question

How to fill the void between the concrete slab and the ground? The concrete slab is 50 years old and was made for a carport. However, a shed was made on this section of the slab with no heavy thing on it, except perhaps the carport roof. I was thinking filling the void with concrete and using a reinforcing mesh to hold the concrete vertically. But, I'm not sure I should do it that way. I'd appreciate some advice. Thank you.

JacobZ · Answer

Hi @FXMV,

Quick set is likely the best choice, however standard concrete will work just fine, it will just take longer to set. As long as it is mixed relatively dry, either will work fine. If it is too runny, it will just slump out. A drier mix, when packed into the void, will stay in place long enough that you can add the formwork to hold it there while it cures.

If you use timber for your formwork that finishes just shy of the top of the void, you can vibrate it by tapping the form with a hammer so the concrete settles, then top it up by pressing more concrete in if needed.

Once the concrete has started to go off, and is still what is referred to as "green", meaning firm, but still soft enough that the surface can be smoothed and finished off, you can remove the formwork and tidy up the face using a trowel.

Let me know if you have further questions, I am more than happy to help.

Jacob

FXMV · Answer

Thank you Jacob, I'll take the advice onboard. Would any of the concrete on the list do? Or, should I use quick set concrete? Best

JacobZ · Answer

Hi @FXMV,

Thanks for sharing the photo. It looks like you should be able to fill that void, but the key will be getting the slab edge and surrounding area properly cleaned and prepared first so the new concrete has something solid to bond to.

I’d start by removing any loose soil, crumbling concrete, dust and debris from the void and from the exposed edge of the slab. A stiff nylon brush or wire brush would be useful here, and if the area is particularly dirty, you could also use a pressure washer, just make sure not to undercut the concrete as it still needs to sit on the soil beneath it.

Once it’s clean, apply a bonding agent such as BondCrete to the edge of the existing slab. To give the new concrete something to key into mechanically, you could drive some masonry screws into the slab edge, leaving them partly proud. You can also use tie wire to attach some light rebar to the screws for extra support before packing in the concrete.

I’d use a fairly stiff, standard concrete mix rather than a wet, runny one, then push it firmly into the void so it packs tightly under and against the slab. Once relatively full, a piece of timber fixed or braced against the face would work well as temporary formwork to hold the concrete in place while it cures.

Once the concrete has set, you can remove the timber and tidy up the face if needed.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob