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How to fill a hole in a concrete slab?

Hi all,

After dismantling an old kitchen and removing water pipes, I'm left with a hole in the floor and the concrete slab underneath.

The hole goes right through under the house and is approximately 10cm x 8cm and 20cm deep.

I was thinking of filing it with non-shrink grout, but I'm not sure if I need to apply a sealant first or not, and how to prepare the surface under the slab ( should I remove the old sealant first? )

Anyone has any recommendation on the method and product to use?

Thanks for your help!

Julien

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JacobZ

Hi @julienf,

Filling the hole with a non-shrink grout such as this Lanko 702 Durabed Structural Grout would be the way to go. You would have to do it in two layers, as it can only be applied in layers up to 100mm thick.

The main focus will be on creating a backing for the hole that can support the structural grout. I would start by removing the old sealant using a 100mm Sharp Edge Paint Scraper and a utility knife, ensuring you wear gloves and long protective clothing to help reduce the risk of lacerations.

Once the sealant is removed, I would use a piece of steel plate, such as this Australian Handyman Supplies 600 x 600 x 1.55mm Galvabond Mini Sheet to cover the bottom of the hole. You can cut it to size using aviation snips.

Once cut to size, predrill a series of pilot holes around the perimeter of the sheet for fixings to go through. I would recommend using at least 8 of these fixings. One in each corner and one halfway along each side.

You'll have to predrill a hole in the concrete for each fixing, then tap a Green Wall Plug into each hole. You can then use 12G x 50mm Galvanised Hex Head Screws to attach the steel plate to the concrete.

Once the backing plate is in place, mix your structural grout as per the instructions and fill the hole with a layer 100mm thick. Allow it to dry for 4.5 hours before adding the second layer.

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

Jacob

julienf

Thanks you so much for your response @Jacob

I have a couple of questions:

- Given the slab surface is irregular, do I need to make sure the grout fill the narrow spaces between the plate and the slab?

- Do i remove the plate in the end or I can just leave it there?

Julien

EricL

Hello @julienf

I recommend using a timber stick or a paint mixing stick to agitate the grout when you tip it in so that it settles to the bottom properly and any air pockets are filled in. It would be best to leave the metal plate in place.

If you need further assistance, please let us know.

Eric

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