The strip foundation has developed a crack

Oh, that's what is meant, the floor areas should not be filled with topsoil. How many centimeters should be removed?

Everything. How are we supposed to know how powerful this layer is :-)

Everything! And also outside the stripes. At least 100cm, if not 150. Foundations should have NO contact with earth or topsoil.

So, I'm checking in again, I have now replaced the layer with gravel about 100cm and plan to concrete it soon.

Now I have concerns about the temperatures, during the day we have up to 10°C here but at night it already gets colder down to -3°C.

I have now spoken with the company supplying me with the concrete and they said it is no problem at all as they also have a location in northern Russia and there they also concrete at temperatures down to -20°C.

I guess antifreeze agent is added, but how reliable is that?

What would you say about it, to concrete or better wait until it gets warmer?

With the right mixture, it works. To use the "Russia" argument: elsewhere the weather is often colder, and it works there too - with the right mixture.

Your biggest risk is going it alone, not the recommendation of the professionals.

-20 I don't know, but for me it was up to -5 and no problem