And I have had to tear out concrete that didn't break at the spec'd pressure after 28 days. I didn't pay for it though, the cement plant did. They paid for the new concrete, the labor, the disposal of the old concrete, everything.
When you do this to them a couple of times, they start paying close attention to the mix and their quality controls at the plant. Whenever I order concrete now, I always tell them that the testing lab will be there to take samples and do a slump test (whether they are or not). They are minding their Ps and Qs, then.
We have had the odd batch not break at 3500 or 3000, but it is usually in an area where it doesn't matter, so it doesn't have to be torn out. They sometimes break at 2900 or 3400, but for a slab on grade, that is fine. When you get into retaining walls and other critical systems, the heat is on, and if you have all of your paperwork in order, it is on the supplier.
Whenever I have to call the plant and tell them that their concrete is subpar, the first question I always get is How much water did you add? For that reason, I always have the drivers indicate on the delivery ticket if water was added and who requested it. Sometimes the drivers will add water on the way to the job to keep the mix turning. If it is a spec'd job with tests in place, that can come back to haunt them.
Moral: Always keep your delivery tickets, indicate water added, and test, test, test.