The questions to ask a concrete contractor are the seven that force every bid onto one page: slab thickness, mix strength (3,500 PSI is the floor for a slab you intend to finish), reinforcement type and position, vapor barrier thickness and placement, joint layout, cure method, and finish. Answers in writing turn three incomparable documents into a real comparison.
This list works on every bidder, including us. That is the point of it.
Why do two honest bids on the same slab differ by thousands?
Because they are two different slabs. A 40x60 shop slab in Oklahoma runs $5.32 to $15.61 per square foot installed, which is $12,750 to $37,100 before the drive cost across the same 2,400 square feet of ground. The 40x60 slab cost page walks the full range. The low end covers a standard broom-finish utility slab at 4 inches thick. The high end covers a 6-inch pour built for polishing, epoxy, or heavy equipment.
Picture the cheap version honestly. Four inches. Fibermesh in the mix instead of steel. No vapor barrier. Joints cut whenever somebody swings back through. A sprayed cure-and-seal because it is fast. Broom finish. Every one of those is a legitimate choice on a slab that will hold a tractor and never get a finished floor.
Now the expensive version. Five or six inches. Number 4 rebar sitting on chairs in the upper third of the pour. A 15-mil Class A barrier laid directly against the concrete. Joints on a drawn grid, cut inside the green window. A wet cure. A troweled surface specified for whatever goes on top of it two years from now.
Neither bidder lied to you. They wrote down two different slabs and priced each one correctly. The trouble is that a total at the bottom of a page hides all of it, and the man reading three totals has no way to see which slab he is looking at.
What are the seven scope lines every bid should name?
These seven cover the decisions that cannot be changed after the truck leaves. Ask for each one in writing.
| Scope line | A competent answer sounds like | An answer that should worry you |
|---|---|---|
| Finished thickness | “5 inches through the field, 6 at the drive-in apron, over 4 inches of compacted base” | “Standard” or “code minimum” |
| Mix strength | “4,000 PSI, air entrained on exterior work, slump held at 4 to 5” | “Whatever the plant sends that morning” |
| Reinforcement type and placement | “Number 4 rebar at 18 inches on center, chaired into the upper third, chairs checked after the first truck” | “Rebar” with no spacing, no depth, and no mention of chairs |
| Vapor barrier | “15-mil Class A per ASTM E1745, poly directly against the slab, seams lapped 6 inches and taped per ASTM E1643” | “We will roll some poly out” or “you do not need it under a shop” |
| Joint layout and timing | “Cut on a drawn grid, sawed inside the green window, diagonals at every inside corner” | “We cut them when we come back through” |
| Cure method | “Wet cure under plastic and blankets for 7 days” or a dissipating compound named on the submittal | “It cures on its own” |
| Surface finish | “Broom finish” or “hard trowel to a stated flatness number if the floor gets polished later” | “Smooth” |
None of those answers costs a contractor anything to give. A crew that pours to a spec already carries the numbers in its head, and the ones who do will rattle them off faster than you can write. The bidder who cannot produce a thickness and a PSI has not priced a slab. He has priced a rectangle.
How do you turn three bids into one comparison?
Write your own scope sheet. One page, seven lines, your numbers in the blanks, and send the identical sheet to all three bidders with a request to re-price against it. This is the single highest-value afternoon on the whole project and it costs nothing but the afternoon.
- Fill in the seven lines yourself first, using what the building actually has to do. A shop that parks a loaded gooseneck is not a patio.
- Send the same sheet to every bidder. Same paper, same words, no verbal variations, no “I also mentioned to him that…”
- Ask each one to re-bid against the sheet and to flag anything on it they would change and why. That last clause matters. A bidder who tells you your 6 inches is overkill for a mower shed is doing you a favor.
- Read the three re-bids side by side.
At that point a price difference means something. Before that point it means nothing at all, and choosing the low number is a coin flip wearing a necktie.
Two items that go missing and come back as change orders
Two items go missing more than everything else combined.
A pump truck. A pump is an option chosen for reach, not a default and not a requirement. It shows up when a mixer physically cannot get close enough to discharge into the pour: a back corner, a tight drive, a septic field a loaded truck cannot cross, a power line over the only approach. When a site needs one, pump service runs $1,780 per pour day. Ask every bidder whether his number includes a pump and, if not, what he expects the truck to be able to reach.
Imported subgrade material. If the pad sits low, or the native soil will not hold compaction, somebody hauls material in and rolls it. That volume is measurable with a laser level before anyone writes a bid, and a bidder who has walked the pad can tell you roughly how many loads he is carrying. A bid that assumes the dirt is already right, on a pad nobody graded yet, is a number waiting to move.
The fence is unwritten scope, not do-it-yourself
Nothing here argues against running your own build. Owner-builders write better scope sheets than most general contractors because they care more. The failure mode worth fencing against is different: a slab priced verbally by one crew and finished months later by a second crew, with no written spec connecting them. The finishing crew inherits every decision the pour crew made and has no way to change any of it. That gap is where the arguments live, and a one-page scope sheet closes most of it before either crew shows up.
Three lines get stricter when the slab gets polished or coated
Three of the seven lines tighten up, and they tighten at bid time or not at all.
Flatness stops being a feeling and becomes a number. Standard commercial pours land around 25 to 30 on the FF scale, which passes every general inspection and will not take a high-gloss polish. Write the target into the scope sheet and require the measurement per ASTM E1155 before any finishing crew mobilizes. The mechanism, the finish levels, and the cost of correcting a low score afterward all live on the FF flatness score page.
The vapor barrier stops being optional. Poly goes directly against the underside of the slab with nothing porous between them, and the reason that one detail decides whether a coating survives is worked through on the vapor barrier placement page.
The cure compound becomes a specification instead of an afterthought. A sprayed acrylic cure-and-seal leaves a film that blocks densifier penetration and coating adhesion, so a slab headed for a finish wants a wet cure or a dissipating product named on paper. All four pour-day disciplines and their thresholds are laid out in the Pour-4-Polish SPEC framework, and ACI 332 is the residential concrete document a bidder can be pointed at if he wants a source that is not you.
How do you ask these questions without insulting the crew?
Ask about the slab. Do not ask about the man. Concrete crews get accused of cutting corners constantly, most of them by people who have never poured anything, and a question that lands as an accusation shuts down the exact conversation you need. A 10-minute phone call handles all seven lines if the questions stay pointed at the work.
- “What thickness and PSI are you carrying on this?” is a question about the work.
- “Are you going to skip the barrier like the last guy?” is a question about his character, and you will get a short answer.
- “I am asking all three bidders the same seven things, so nobody gets compared unfairly” turns the list into procedure. Most crews relax when they hear it, because it means the low bidder does not automatically win on a technicality.
- “What would you change about my scope sheet?” invites the one thing you actually want, which is his judgment.
Get the answers in writing regardless of how the conversation goes. Not because anyone is dishonest, but because the man you talked to in April will be on a different job in July, and paper does not move.
What do owner-builders ask most about concrete bids?
These 4 questions about concrete bids come up on nearly every acreage build we walk, and owner-builders ask them in roughly this order.
Do I need all seven answers before I choose?
Yes, and you need them from all three. Six of seven from one bidder and four from another puts you right back to guessing. The seven lines exist because each one changes what the slab is capable of, and a missing line is a decision somebody else makes for you later.
Is the low bid always the wrong bid?
No. Sometimes the low bidder simply priced the slab you actually need while the other two priced a slab you do not. That is exactly what the scope sheet is designed to reveal. On a mower shed, 4 inches with fiber and a broom finish is the right answer and paying for a polish-ready pour is money set on fire.
Should I have a contractor with me when I ask?
No. Ask them yourself, one bidder at a time, and keep your own notes. Bringing one contractor to evaluate another’s bid buys you a biased read and costs you the honest conversation. The seven lines are plain enough that anyone can hear a real answer.
What if a bidder refuses to put the spec in writing?
That is information. A crew that will not commit to a thickness and a mix strength on paper is telling you those numbers are still negotiable after the check clears. There is no ambiguity to interpret; you got your answer.
Put a fourth number on the same sheet
Our quote tool prices against the same scope lines above, so you can hold our answer up against the three you already have.
One page beats three totals
Three bids at three prices are three opinions until somebody writes down what the slab is. Seven lines on one page does that, and it takes maybe 20 minutes of writing against an afternoon you will otherwise spend staring at totals that do not mean anything yet.
If a builder or general contractor is running your slab as a subcontracted scope, the same seven lines belong in that subcontract; our builder and GC page covers how the pour and finish scopes get written as one specification instead of two.
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