Concrete Slab Thickness, Rebar and Base Prep: What a Good Contractor Specifies

This is what belongs in a proper specification, and what to ask when it is missing.

Concrete slab thickness for a driveway and other residential work

Thickness follows load, and load varies more than people think.

Walks, patios and light foot traffic areas are typically placed thinner than anything carrying a vehicle. Residential driveways carrying cars and light trucks are commonly specified thicker than a patio, and a driveway that will see an RV, a loaded trailer, a delivery truck or a dumpster wants more still. The apron where the driveway meets the road takes the heaviest loading on the property, including plough blades and municipal traffic, and it is often specified heavier than the rest of the run or given a thickened edge.

Garage floors sit in between, depending on whether anything heavier than a passenger vehicle will sit on them. If you are planning a lift, that changes the specification entirely, and it has to be said before the pour.

Two cautions. Permitting and code requirements vary by municipality across mid-Michigan, so your contractor should confirm rather than assume. And thickness alone saves nothing: a thick slab on a poor base still fails.

What a stated thickness has to actually mean

Here is where bids quietly differ. A slab specified at a nominal thickness is only that thick if the base under it is at a consistent grade. If the base is high in places, the slab is thinner there, and thin spots are where cracks start.

Ask two questions. Is the stated thickness a minimum or a nominal average? And how is base grade controlled, by eye or by instrument? A contractor working to a laser or a string line grade across the whole area is delivering the thickness they quoted. One eyeballing it is delivering an average, and the thin spots are yours.

Reinforcement: fibre, mesh and rebar are not interchangeable

Fibre reinforcement is mixed into the concrete and helps control plastic shrinkage cracking in the first hours. It is useful and it is cheap. It is not structural reinforcement and it does not replace steel where steel is required. Anyone presenting fibre as an alternative to rebar is misrepresenting what it does.

Welded wire mesh works only if it sits in the correct position within the slab depth. Rolled out on the subgrade and poured over, which happens constantly, it does nothing at all because it ends up at the very bottom of the slab. Mesh has to be supported on chairs or lifted into position during placement, and lifting it by hooking it with a rake while the concrete goes in is not a reliable method.

Rebar on chairs is the specification that holds up. Bars in a grid at a stated spacing, supported off the base so they sit within the slab depth, with proper laps where bars join and adequate cover from every face. Cover matters here because of chlorides: steel too close to the surface of a salted exterior slab will corrode, expand and spall the concrete off above it.

Whatever the reinforcement, ask how it will be supported and go look at it before the pour. It is the easiest thing in the world to verify and the easiest thing to skip.

The base is the part nobody sees and everybody pays for

More slabs fail from what is underneath than from anything in the concrete itself.

A proper base sequence starts with evaluating the subgrade. Topsoil, organics, roots and soft spots come out rather than getting compacted over, because organic material decomposes and leaves voids. Around here the subgrade you find varies sharply: heavier clay across much of Genesee and Saginaw counties, sandier ground in places along the Cass River corridor. Clay holds water, is frost susceptible and needs more attention and usually more stone. Sand drains and behaves better.

Aggregate base then goes in as a dense graded crushed material, placed in lifts rather than in one dump, with each lift compacted before the next goes on. Compacting a deep layer in one pass only compacts the top of it. Moisture content matters too, because aggregate compacts best within a range and a bone dry or saturated base will not reach density.

Grade is established at this stage, which means drainage is decided at this stage. Slope away from the house, no low points, and enough fall to move water without being uncomfortable to walk on. Once concrete is on it, that grade is permanent. Our approach to base and subgrade work is described on the concrete flatwork page.

Vapour retarder, and when it belongs

Under interior slabs, particularly anything that will be coated or covered with flooring, a vapour retarder below the slab is standard and important. It blocks moisture migrating up from the ground, which is the same mechanism that ruins basement floor coatings.

Under exterior slabs it is generally not used, because you want an exterior slab to be able to dry in both directions and because trapping water against the underside in a freeze climate creates problems rather than solving them. If a proposal for an exterior patio includes plastic under the slab, ask why.

Joints: layout, depth and timing

Control joints are how you decide where the slab cracks. Three things about them get specified and any one of them being wrong causes visible problems.

Spacing. A common rule of thumb puts joint spacing at roughly twenty four to thirty times the slab thickness measured in inches, giving a spacing in feet. Panels should also be kept reasonably square rather than long and narrow, because a long thin panel cracks across its middle regardless of joints.

Depth. A joint has to be cut deep enough to create a genuine weakened plane, commonly around a quarter of the slab depth. Cut too shallow and the slab cracks somewhere else, usually right beside the joint you paid for.

Timing. Saw cut joints go in during a window after the concrete is hard enough not to ravel and before shrinkage stresses have already cracked it. That window is measured in hours and it moves with the weather. A crew that leaves site without cutting joints and comes back the next afternoon has missed it.

Isolation joints are separate and equally important. Wherever a slab meets a fixed object, a foundation wall, a column, a post, a step, a full depth isolation joint lets the two move independently. Without it, the slab cracks at that restraint.

The mix, in three numbers

A specification should name the compressive strength, the air content and the maximum water to cement ratio. For exterior flatwork in Michigan, an air-entrained mix is required, because entrained air is the only defence against freeze-thaw pressure inside the paste. Higher strength than an interior mix is normal for exterior exposure, and a low water to cement ratio produces the dense, durable surface that resists salt.

Then one behaviour: no water added on site. Adding water to make placement easier raises the water to cement ratio, weakens the paste, increases porosity and can knock out entrained air. It is the single most damaging thing that happens on ordinary residential pours, and it happens because it makes a hard job easier for an hour.

Ask whether you will be given the delivery ticket. It states the mix, and a contractor who is happy to hand it over is a contractor who ordered what they specified.

Curing is part of the specification

Concrete needs to retain moisture to keep gaining strength. A slab left to dry out in sun or wind ends up with a weak, dusty, permeable surface that scales in its second or third winter. Curing compound, wet curing or covering all work. Doing nothing does not, and it is invisible on a bid.

What a good proposal shows

Nine lines. Slab thickness, and whether it is minimum or nominal. Reinforcement type, spacing and how it will be supported. Base material and compacted depth, placed in lifts. Subgrade treatment and what happens if soft material is found. Drainage and finished grade. Joint layout, depth and cutting method. Mix strength, air content and water to cement ratio. Curing method. Return to service dates for foot and vehicle traffic.

You are not being difficult by asking for those. You are asking a contractor to write down what they are already planning to do. If you want a proposal written that way, reach us through the contact page, read how we work on the about page, or see what customers said on the reviews page. The same specification discipline applies to stamped concrete, where a decorative surface still sits on a structural slab, and to heavier work on our industrial concrete page.

Frequently asked questions

How thick should a concrete driveway be?

Thicker than a patio, and thicker again if anything heavier than a passenger vehicle will use it. The right answer depends on the vehicles, the subgrade and any local requirements, which is why a contractor should specify it after seeing the site rather than quoting a standard number. Ask whether the figure they give is a minimum or a nominal average, because that difference is real.

Do I need rebar in a residential concrete slab?

It depends on the loading, the soil and the size of the pour, and the honest answer requires site knowledge. What is not a matter of opinion is that if reinforcement is specified, it has to be positioned correctly within the slab depth on chairs. Mesh lying on the base does nothing. Fibre in the mix controls early shrinkage cracking but is not a substitute for structural steel.

How deep should the gravel base be under a concrete slab?

It depends on the subgrade. Clay soils, which are common across much of mid-Michigan, generally need more compacted aggregate than sandy ground, and any soft or organic material has to be removed rather than compacted over. What matters as much as depth is that the base goes in as compacted lifts rather than as one deep layer, because a single deep layer only compacts near the top.

Why does my new concrete already have cracks in it?

Concrete shrinks as it cures, and it cracks when it is restrained. That is normal. The question is whether the cracks are at the control joints, which means the jointing worked, or somewhere else, which usually points to joint spacing, joint depth, joint timing, a restraint that was not isolated, or an inconsistent base. A crack pattern is diagnostic and worth having looked at.

Can I see the concrete delivery ticket?

Yes, and you should ask for it. The ticket records the mix that was actually delivered, including strength and air content, and any water added at the site. Requesting it is normal, it costs nothing, and a contractor who resists handing it over has told you something useful.