ᐅ Casting a watertight concrete slab with an opening sealed against groundwater pressure

Hello,

I need your collective expertise again. In my basement, the roof drainage passes through the concrete slab into the sewer system. About a year ago, I replaced the 90-degree elbow because it was damaged. Now, groundwater is seeping between the old concrete and the new concrete into the basement.

How can I make the concrete connection to the old slab waterproof against water pressure? I was thinking of first filling 25% of the hole with fast-setting concrete, then pouring in bitumen and letting it harden, and finally filling the rest with regular concrete.

Idea 2:
- Fill 25% with fast-setting concrete
- Apply waterproofing slurry / mortar into the hole
- Fill the rest with screed concrete

Are there any better ideas or options?
Thank you in advance for your help.

If there is hydrostatic pressure, I would recommend hiring a professional company. They will seal the leaking spot using epoxy.

I would also recommend hiring a professional company, but if you definitely want to do it yourself:

I would start by applying a waterproof slurry. Otherwise, moisture could penetrate the concrete slab through the fast-setting concrete. So first the waterproof slurry, mixed quite thickly. Once that has cured, I would apply an acrylic sealant around the pipe. I would then fill the remaining space with waterproof concrete (WU concrete), although honestly, I’m not sure what it is made of. Will the space remain a usable basement?

I wouldn’t recommend installing tiles or anything similar over this construction.

The room is actually intended to be a bathroom. Therefore, it needs to be completely waterproof. Thanks in advance for your information and tips.

In that case, I would definitely hire a professional company. If moisture still rises at the end, your tiles will come loose. That would be a false economy.

Maybe @Jann St can contribute some expertise?