Builder used the wrong concrete mix

Hi all, I'm currently building in Melbourne. We specified a raft slab and the builders plans and engineering specified 25 mpa concrete, however they have poured the slab using 20 mpa concrete (i.e. a weaker mix).

I'm still waiting for a response but where do I stand on this matter and what would be an appropriate course of action?

Is the 20 mpa likely to be a problem with the same slab design (i.e. pier spacings, 120mm thickness, etc)?

1. how do you know that its 20mpa?
2. what did the engineering specify? If the engineering specifies 25 and you have proof it was poured at 20, the slab needs an engineer to recalc the performance to determine if its still fit for purpose. My guess is it isn't. So even if they give you updated calcs , get a second opinion IMO

HOWEVER if you have no undeniable proof it was a 20mpa mix, you're not going to get very far.

The proof will be in the concrete delivery dockets.
The question should be addressed to the structural engineer that desgned the slab.

if that is the case and you have dockets to prove it I suspect you may struggle to get your occupation certificate unless the engineer has signed off on it

Get some water on the slab if you optimise the curing you will get a stronger slab, also what the others said…

surely if signed contract for 25mpa (both parties), then builder cannot just simply say I have re-engineered slab and everything okay

there is only 20% difference in concrete strength between 20 mpa and 25 mpa, so check with your engineer first if it fits his calculation tolerances.

There are ways to increase concrete strength from the top as well, such as https://duracore.com.au/product/densi-c ... densifier/