As the clay dries under the greenhouse it will shrink and you want the concrete to be able.to survive a few small voids. The 40kn stuff has never let me.down.
Also, If at all possible. You may want to drive some stakes (even rebar would help) into the ground before you pour over.
The more work you put in now the better. A solid base will pay for itself in the long run! DM me if you have other questions.
So advice from someone who has erected a few garden buildings. Greenhouses included. Particularly on a clay rich soil.
Use the pre mixed high strength stuff:
this stuff or equivalent
As the clay dries under the greenhouse it will shrink and you want the concrete to be able.to survive a few small voids. The 40kn stuff has never let me.down.
Also, If at all possible. You may want to drive some stakes (even rebar would help) into the ground before you pour over.
The more work you put in now the better. A solid base will pay for itself in the long run! DM me if you have other questions.