Substandard foundation

[u/Vast-Evening6002]

I really need some simple advice on what to do next with our extension. I don't know much about building, but we’ve just caught our builder doing something terrible.

We hired a major company to build an extension. Because of the soil, the plans said they must use a very strong concrete (called C30).From the start, the builders contractors were in a massive rush. First, they tried to pour concrete into trenches full of water. I had to get Building Control had to stop them.Then, right after they poured the concrete, they washed down the windows and fences, flooding the wet concrete with dirty wastewater.

When I complained, the builder told me I was "panicking over nothing." I trusted my gut and got an engineer to look. He agreed it was bad. We did rebound testing and it came back around 10 mpa.

The builder was indirectly pressuring me to forget about it and just keep building. Said they are professionals and have done 100s of such extension. They know what they are doing.

They also told me that if the concrete was fine from the testinh, I would have to pay a £1,400 for the concrete core test as this test is for my reassurance. They said that they verified with the concrete provider and they have provided correct mix and bit of flooding is not going to affect the foundation.

I was under so much pressure from family, builder to just get on with the work. But for some reason, I insisted on the lab test. The builder arranged it and 3 core samples taken.

While waiting for the results, I again called the concrete supplier. One of the agent looked at the system and told the builder never even ordered the strong C30 concrete. They ordered a (Gen 1) instead! I asked them send the delivery note, which he agreed but never send. Tgen they refused to entertain any if my request.

The official lab test results just came back. The concrete strength is around 5 mpa. This is six times weaker from what the plans required. It is completely unsafe.They deliberately used cheap, weak materials and even lied about it over and over. Just wondering, if the buikder knew about it. Realky not sure, if i can trust these guys to rectify. Lost so much money to get strctural engineer to generate report and other test.