GC said "it's not an exact science" 5.5-6.5 inches is fine.
I said that sounds reasonable, but Im measuring 4.5 inches several broad areas. I don't think that's reasonable. What do you think?
He said if Im not happy with it they will do it over.
I don't think it's an issue of being happy. Sounds like they want to charge me to correct it.
The lift manufacturer has already determined the minimum specs for the slab and anchors. Check with them. That said, I agree with what your contractor says. If you're upset on principle, that's one thing, but I wouldn't worry about the pour if the strength is adequate. People get all wound up about adding their own just-in-case factors, as if the engineers crunching the numbers at the lift manufacturer have all missed some critical failure case that will some day rear its ugly head after all these decades, or that the concrete vendors just might screw up the batch this one time.
So you would accept 4.5 inches over broad areas of the slab when you contracted for 6 inches?
The tendons/cables of the post tension slab come to 4 inches above grade, so there will only be 1/2 inch concrete over the cables in these large areas (they are supposed to be at the middle of the slab per the engineers drawing that cost me $1885).
BTW, I don't think it's an issue of the contractor trying to cheat me on concrete. They charge labor per sq ft and I pay the cost of the concrete directly to the redi-mix company. I think it is just an issue of lack of attention to detail.
I suspect 99% of GCs and homeowners don't get out string levels and check to confirm the concrete thickness before the pour.
You stated you had a concern about a lift installed on concrete thinner than what you wanted. I said I thought it would be OK, but that the requirements are set by the lift manufacturer. Obviously you have other, valid concerns regarding engineered post-tensioning. That is a much bigger concern. Where is the inspector in all this?
The ACI standard for minimum concrete floor thickness is an average discrepancy no greater than 3/8" with no individual measurement greater then 3/4". In other words, your floor would average no less than 5 5/8" and be at least 5 1/4" in any spot. With that said, I'd agree with your GC that 5 1/2 to 6 1/2 is fine.
I'd ask them to knock down the high spots and give you a minimum of 5 1/2". You're paying for a 'six inch' slab and should get one without any additional cost.
This soil is heavy clay with "clods", and I suspect very difficult to level accurately. It's mostly under the plastic vapor barrier so I can only the soil on the perimeter in some areas, but that's what it looks like.
BTW, I don't think it's an issue of the contractor trying to cheat me on concrete. They charge labor per sq ft and I pay the cost of the concrete directly to the redi-mix company. I think it is just an issue of lack of attention to detail.
I suspect 99% of GCs and homeowners don't get out string levels and check to confirm the concrete thickness before the pour.
How thick does a slab need to be to support a 2 post lift, 6 in?
What tolerance is reasonable?
My slab is supposed to be 6 in but I pulled a string level across the forms and the measurements vary from 4 to 6.5 with most falling in the 4.5 to 5 range.
Hopefully the concrete guy won't get angry and quit at this. No one likes to have to redo work. I think I should pay for the new fill and maybe some bobcat time but the concrete contractor should absorb some labor cost to fix his mistake, we will see. For sure I want this done right.
Why should he get mad ? If he was given specs to work from, he's the professional. He should have followed the specs, done it right the first time and he wouldn't have to redo it.
@bczygan: Why do you prefer limestone? Sand is used on all the slabs here. I suppose because it is plentiful and cheap. I had talked to a local engineer who said sand or gravel would be essentially equivalent.
Fred