If you have ever looked at two concrete bids and wondered why one number is much lower than the other, this page is for you.
Most customers are asked to buy a slab or foundation without ever being shown what the specs actually mean. They hear terms like 3000 PSI, 6 mil vapor barrier, wire mesh, rebar, compaction, or 24-inch footing, but they are rarely told which of those items matter most, how they affect cost, or how to compare one contractor's scope to another.
That creates a problem.
A concrete foundation is one of the few parts of a project that becomes hard and expensive to correct after it is poured. Once the slab is in the ground, most of the important details are buried, covered, or locked in place. That means the time to understand the specs is before the pour, not after.
This guide is meant to help normal buyers understand what they are actually being sold.
The big idea: concrete specs are not just about the concrete mix
When people compare concrete jobs, they often focus only on the PSI number. That is understandable, but incomplete.
A slab or foundation is not just a truckload of concrete. It is a system that includes:
- site prep
- subgrade condition
- base material
- compaction
- slab thickness
- footing size
- reinforcement
- vapor barrier or moisture control
- concrete mix design
- finishing
- joint layout
- curing
If one of those items is weak, the entire slab can suffer.
In other words, a higher PSI number does not automatically mean you are getting a better slab. A lower price does not automatically mean you are getting a worse slab either. What matters is whether the scope actually fits the use of the project and whether the contractor is following through on what was promised.
The 8 specs buyers should look at first
If you only compare eight things when reviewing concrete bids, compare these. Click any item to jump to its explanation.
- Slab Thickness
- Footing Width and Depth
- Concrete PSI
- Reinforcement Type and Placement
- Vapor Barrier Spec
- Base Material and Compaction
- Finish and Joint Details
- Included vs Excluded
Those eight items usually explain most of the price difference between bids.
1. Slab thickness
Slab thickness is exactly what it sounds like: how thick the concrete will be.
This matters because thickness affects how well the slab handles bending, point loads, impact, and long-term service. A slab that is too thin for the intended use may crack more, settle unevenly, or fail under concentrated loads.
Why it matters
Thickness often matters more than customers realize. A stronger concrete mix does not magically make up for a slab that is too thin for the load it will carry.
How it affects cost
A thicker slab usually means more concrete, potentially more excavation or base work, and sometimes more reinforcement. That increases cost, but it can also dramatically change performance.
Ask your contractor
- What slab thickness is included?
- Is that thickness uniform everywhere?
- Are there thickened edges or interior pads where loads are concentrated?
What to verify before the pour
- Check the form height
- Ask where the finished top of slab elevation is
- Confirm whether footing or thickened-edge dimensions match the written scope
2. Footings and thickened edges
A footing is the widened or deepened portion of concrete that carries loads into the ground. On many slab-on-grade projects, the perimeter may be thickened or include a dedicated footing.
Why it matters
Footings carry loads from walls, columns, building frames, and other structural points. If footing dimensions are undersized, omitted, or not formed as promised, the slab may not perform the way the customer believes it will.
How it affects cost
Larger or deeper footings usually mean more excavation, more concrete, more labor, and sometimes more reinforcing steel.
Ask your contractor
- What are the footing dimensions?
- Are they perimeter-only, or are there interior footings too?
- Are all door openings, load-bearing walls, or frame lines accounted for?
What to verify before the pour
- Measure visible footing width and depth where possible
- Photograph footing trenches and rebar before concrete covers them
- Confirm that the footing layout matches the plan
3. PSI: concrete strength
PSI means pounds per square inch, and in this context it refers to compressive strength.
That is the pressure the hardened concrete can withstand in compression when it is tested. In most common discussions, the specified strength refers to the 28-day strength.
Why it matters
PSI helps describe how strong the concrete is supposed to be when cured and tested properly. It matters, but it is not the whole story.
What buyers get wrong
Many buyers assume higher PSI automatically equals a better slab. That is too simple. A slab with poor base prep, weak compaction, poor curing, or bad reinforcement placement can still perform poorly even if the mix itself has a higher strength rating.
How it affects cost
Higher specified strength can increase cost, but often the biggest cost drivers are still thickness, reinforcement, footing size, and site conditions.
Ask your contractor
- What PSI mix is included?
- Is that the specified 28-day strength?
- Is there anything about the project that requires a stronger mix?
When to be cautious: If a contractor talks about PSI but says very little about base prep, thickness, reinforcement, or curing, you are not hearing the whole story.
4. Reinforcement: rebar, wire mesh, and fibers
Reinforcement is used to help concrete perform better when cracking or bending forces occur. It is one of the most misunderstood parts of concrete work.
Reinforcement does not mean the slab will not crack. Concrete cracks. The real question is how the slab is designed, how much cracking is controlled, and whether reinforcement is properly placed so cracks stay tighter and performance stays acceptable.
Common options
- welded wire mesh
- rebar
- fibers
- combinations of the above
Why it matters
The type of reinforcement matters, but placement matters too. Reinforcement that is not supported and ends up lying at the bottom of the slab may not do what the customer thinks it is doing.
How it affects cost
Rebar typically costs more than light mesh. Heavier schedules cost more than lighter schedules. But on many projects, the bigger issue is not just what is specified — it is whether the reinforcement will actually be installed and held in the intended position.
Ask your contractor
- What reinforcement is included?
- What spacing is included?
- Will it be chaired or otherwise supported?
- Is reinforcement for shrinkage control, structural loading, or both?
5. Vapor barrier or vapor retarder
A vapor barrier or vapor retarder is the sheet material installed under many slabs to reduce moisture migration from the ground into the slab and building.
Why it matters
Moisture coming up through a slab can affect flooring, adhesives, indoor conditions, and long-term performance. Even when a slab is not receiving finished flooring today, future use can change.
What buyers miss
Some bids say "plastic included" without saying what type, thickness, or installation method. That is not enough. The quality of the material and the way it is installed both matter.
How it affects cost
A better vapor retarder costs more than thin generic poly, but it is usually a small percentage of the total slab cost compared with the pain of moisture-related problems later.
Ask your contractor
- Is a vapor barrier included?
- What thickness is it?
- Is it a true under-slab product or just generic plastic?
- How are seams, penetrations, and edges handled?
6. Base prep and compaction
This is one of the most important parts of the whole job and one of the least visible once the project is done.
What it means
The slab sits on prepared ground. That prepared support may include native soil, select fill, granular base, or a combination. The key idea is uniform, stable support.
Why it matters
A slab can crack, settle, or perform poorly if the support beneath it is soft, wet, uneven, or poorly compacted.
Why it affects price
Good prep takes time, equipment, and sometimes imported material. Cheap bids often hide poor prep by keeping the description vague.
Ask your contractor
- What prep is included?
- Is grading included?
- Is imported base included?
- Will fill be compacted in lifts if needed?
- What happens if unsuitable soil is found?
What to verify before the pour
- Walk the site before concrete placement
- Look for soft spots, pumping mud, standing water, or trench settlement
- Ask whether the base is truly ready or just "good enough to pour"
7. Finish, joints, and curing
These are often treated like afterthoughts, but they matter.
Finish
The finish affects appearance, wear, slip characteristics, and intended use.
Joints
Joints are not "mistakes." They are part of the plan to help control where concrete cracks.
Curing
Curing helps concrete develop its intended properties. Good curing is not just waiting; it is protecting the slab while hydration continues.
Why this matters in a bid
A scope that says "finish slab" tells you almost nothing. A better scope says what finish is expected, how joints will be handled, and what curing approach will be used.
Ask your contractor
- What finish is included?
- When will saw cuts happen if saw-cut joints are used?
- How is curing handled?
- Is sealing included or not included?
8. Inclusions, exclusions, and allowances
Many disputes happen because the customer and contractor thought they were talking about the same job when they were not.
Common missing items
- excavation beyond a basic allowance
- haul-off
- imported fill
- thickened areas
- reinforcement upgrades
- anchor bolts
- plumbing stub-outs
- saw cutting
- sealing
- cleanup
Ask your contractor
- What is specifically excluded?
- What could change the price later?
- What site conditions are assumed in the bid?
Why two bids can look similar but be very different
Here is a simplified example.
Bid A — Cheaper
- 4-inch slab
- 3000 PSI concrete
- "wire included"
- "plastic included"
- no footing details listed
- vague site prep language
Bid B — Detailed
- 5-inch slab
- 3000 PSI concrete
- specified reinforcement and spacing
- 10 mil vapor retarder called out
- perimeter footing dimensions listed
- compacted base included
- finish, joints, and exclusions spelled out
At first glance, both look like concrete bids. In reality, they may not be close at all.
That is why you should compare scope, not just total price.
What matters most depends on the use of the slab
A shop slab, house slab, patio, equipment pad, and metal building foundation may all use concrete, but they do not all have the same needs.
Questions that change the right spec include:
- Will vehicles or equipment be on it?
- Are there concentrated point loads?
- Will finished flooring be installed later?
- Is the building enclosed and conditioned?
- Are there interior walls or frame loads?
- Is the site flat and stable or does it need major prep?
The right answer is not always "make everything bigger." The right answer is making sure the slab is specified for the intended use.
The smartest way to compare concrete bids
Do not ask only:
Which bid is cheapest?
Ask:
Are these bids describing the same job?
Then compare thickness, footing dimensions, reinforcement, vapor barrier, base prep, finish, joints, curing, and exclusions.
How to protect yourself before the pour
Even if you do not hire us, you should know how to confirm that your contractor is building what they promised. That means checking dimensions, reinforcement, vapor barrier, base condition, footing layout, penetrations and embeds, and cleanup and readiness before placement.
Explore the full concrete hub
What Does 3000 PSI Mean?
Plain-English explainer on PSI, compressive strength, and why the number alone is not the whole story.
Rebar vs. Wire Mesh
The real differences between reinforcement types, what matters most, and how to spot vague bids.
Vapor Barrier Under Slabs
What it does, why it matters even on shop floors, and what to ask about thickness and installation.
Why Subgrade Matters
A slab is only as good as what it sits on. What unsuitable soil means and when soil replacement is needed.
How to Compare Concrete Bids
A practical, line-by-line comparison method so you can tell real differences from sales language.
Verify Before the Pour
Step-by-step guide to confirming your contractor is building what they promised, before it is covered forever.
Pre-Pour Inspection Checklist
A printable checklist you can take to the jobsite to verify every critical item before concrete arrives.
Concrete Terms Glossary
Plain-English definitions of the terms you will see on every bid and spec sheet.
Need a Slab or Foundation Quote?
We provide detailed, clearly-scoped concrete quotes so you can understand exactly what you are paying for. If you already have other bids, we can help you review them line by line.