ᐅ Mistakes When Pouring Concrete for a Prefabricated Basement – Consequences and Experiences?

Hello dear forum,

Our house construction has gotten off to a great start (sarcasm): The construction crew simply forgot to install the formwork for the precast elements at the floor junction, and the concrete flowed out at the bottom. As a result, we could no longer be sure that there are no voids within the wall structure and that the 25/30 waterproof concrete wall is actually fulfilling its purpose and is watertight. For the joint sealing tape, they didn’t use Pentaflex (or a similar product) but instead just pressed a sheet metal strip into the freshly poured floor slab. When we asked about this, we received the following explanation: "These joint strips are purely structural, designed to counteract negative water pressure from the waterproofing. This means water cannot flow from the inside to the outside beneath the walls. This has nothing to do with the primary waterproofing on the exterior."

The basement construction company admitted the construction team’s mistake and therefore executed the basement as a black tank waterproofing system. (However, what we ordered was not a “white tank” system but a basement built according to the soil report specifications with waterproof concrete and sealed joints.)

My question: Is this construction method now worse than what we ordered or possibly even better? (Aside from the fact that, of course, we don’t know how many voids may have formed between the formwork.)

Can anyone estimate whether this changed execution as a black tank might lead to increased follow-up costs, for example, during backfilling of the excavation?

In your opinion, is there any reason why the 5-year warranty might no longer be sufficient?

Thank you very much for your attention

Clarissa

I'm not surprised that people are asking questions here – this has become a huge mess.

You even want to have an opinion without any pictures.

Better get a building inspector to check the site in person.

By the way, just because C25/30 concrete is used, the basement is still not watertight, even if it was properly stripped.

HalloClarissa schrieb:

My question: Is this execution now worse than what was ordered, or perhaps even better? (Aside from the fact that we, of course, don’t know how many cavities may have formed between the formwork.)

I think we can absolutely and confidently rule out that it is "even better." How could this have happened?

HalloClarissa schrieb:

Since the home builder does not offer basements, but we need one due to the sloped site, we ordered it from a reputable precast basement manufacturer

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11ant schrieb:

I believe we can confidently deny the idea that it was even "better." How could this happen?

How could this happen?
The site supervisor was on vacation, so the crew (none of whom spoke German) built the basement walls on their own. Unfortunately, they assumed the concrete was thick enough and probably wanted to save time by skipping the formwork removal. The joints were stuffed with expanding foam (which, according to my research, is not even allowed under waterproofing standards). It was lucky that the inspector happened to be on site... he and the concrete supplier approached the three workers and warned them that the foam would likely fail... but they apparently didn’t understand. When it eventually leaked, they panicked and hurriedly shoved wooden battens underneath. If our inspector hadn’t been there by chance, we would never have found out.
I have to give the basement builder credit for not trying to downplay the mess but instead immediately offering the application of a waterproof membrane.

Offtopic schrieb:

It’s no surprise people are asking here; this turned into a huge mess.

You even want an opinion without any pictures.

Better get a building inspector to check on-site.

By the way, just because C25/30 concrete is used, the basement is still not watertight, even if the formwork was done correctly.

We had a building inspector on-site, thank goodness, as they happened to be there when the problem occurred.
I don’t understand how anyone could assess the wall’s waterproofing just from pictures.

There is absolutely nothing sealed. A complete disaster for you. I’m sorry about that.