How to fix hole in the concrete slab around plumbing?

[charlie0123]

After removing old steel shower base, I discovered there is a large hole in the concrete slab around the drain pipe (see photo). The size is 300 x 200 mm, and deep 150 mm (The bottom is filled with sand).

I am going to use ceramic floor tiles in the shower this time (not shower base). A tiler said he could filled the hole with concrete. My concern is that I'm certain there will be cracks in the concrete sooner or later.

I was wondering if I should also use Ceramic Tile Underlay cement board to cover the shower floor, then waterproofing and tiling. Or, are there other options?

[MitchellMc]

Hi @charlie0123,

Filling the hole with concrete is a standard and effective solution. The jagged edges of the hole will help lock the concrete in place, making it unlikely to crack any more than the surrounding slab. If you're concerned about cracking, you could reinforce the area with reinforcing steel bars (reo bar) or mesh, but this is usually not necessary unless there's structural movement. Also, you could paint Bondcrete on the sides of the hole before filling and mix some into your concrete mix.

Using ceramic tile underlay or cement sheet isn't required in this case and could overcomplicate things. Simply fill the hole with concrete, level it, apply your waterproofing, and then tile as planned. This will give you a solid, durable base for your shower floor. 

Please let me know if you have any questions.

Mitchell

[charlie0123]

Thanks, Mitchell. I will use the steel bars to reinforce it for peace of mind.

Just for extra information - In USA, underlayment is recommended if tiling on a concrete slab. Here is an example: https://www.thisoldhouse.com/foundations/reviews/underlayment-for-tile-on-concrete-slab.

But in Australia, we tend not to.