The #1 Reason Concrete Fails: Base & Subgrade Prep (Not the Concrete)

For most residential slabs, put down at least 4 inches of compacted crushed stone, spread and compacted in lifts of 3 to 4 inches, over a subgrade that has been shaped, compacted and proof-rolled. Patios sit at the low end, 4 to 6 inches. Garages and shop floors run 4 to 8 inches. In freeze-thaw country, or under driveways that see trucks, go thicker and ask a local engineer or experienced finisher.

The question in the r/Concrete thread that started this page was about rebar and a 4,000 psi fiber-reinforced mix. That is the question most owners ask. The better question is what the slab is sitting on. Rebar and mix strength decide how well the slab bends. The base decides whether it gets bent in the first place.

Why support matters more than strength

A slab on the ground is like a flat tray sitting on a bed of sand. If the sand is firm and even everywhere, the tray barely flexes. If there is a soft spot in the middle, the tray bridges it and bends.

Portland Cement Association guidance on floors on ground, as quoted in slab design material, makes the same point in engineering terms. It says uniform support, not high strength, is the most important job of the subgrade and subbase, so slab strength is built most economically into the concrete itself, with low-cost material underneath. The same material says a granular subbase is not mandatory. Where grading does not produce a uniform subgrade, though, a granular layer evens out minor defects and acts as a capillary break against moisture.

Concrete Network adds the failure mechanics. Uneven support causes bending. A soft spot under the middle of a slab can bend it upward and put the bottom in tension, and free edges and joints can bend it downward and put the top in tension. Concrete is about 10% to 15% as strong in tension as in compression, roughly 400 to 500 psi, so it cracks long before the 4,000 psi number matters.

That is the reason a perfect mix on a bad base still cracks.

The spec in one place

Here is what the sources I read agree on, in a table you can hand to a contractor.

Item	Typical spec	Source
Base thickness, patio	4 to 6 inches	Everything About Concrete
Base thickness, shed, garage, shop	4 to 8 inches (12+ for heavy loads)	Everything About Concrete
Minimum subbase plus base	At least 4 inches	Concrete Network
Frost-prone driveways	8 to 24+ inches	Everything About Concrete
Material	Angular crushed stone, 3/4 inch to 1-1/2 inch, or dense-graded aggregate (crusher run)	Everything About Concrete
Fines in the base course	10% to 30% passing the No. 100 sieve, no clay, silt or organics (ACI 302)	Concrete Network
Lift thickness	3 to 4 inches in practice; 6 inches max in cohesive soil, up to 12 inches in gravel	Both
Compaction	95% of maximum modified Proctor density	Concrete Network
Proof roll	Loaded truck, tire sink under 1/2 inch	Concrete Network
Grade tolerance	Plus 0, minus 1 inch for ordinary floors	Concrete Network

Everything About Concrete warns against pea gravel and other rounded stone, which is harder to compact, and against sand, which drains poorly and can wash out.

If you hear “3/4 minus” at the yard, that is crushed stone from 3/4 inch down to dust. The dust is what makes it lock up under a compactor, and it matches the dense-graded aggregate described above.

Start at the dirt, not the gravel

The gravel only helps if what it sits on is firm. Everything About Concrete says to compact the exposed subsoil first, and if it is soft, dig deeper and add larger aggregate.

Concrete Network gives a few field tests for what the soil can carry:

• Clay where you can push a thumb in a few inches: about 1,000 to 2,500 pounds per square foot.
• Loose sand where a #4 rebar pushes in by hand: about 1,000 to 3,000 pounds per square foot.
• Sand where a #4 rebar drives about 1 foot with a 5-pound hammer: over 2,000 pounds per square foot.
• Poor soil, such as silt or soft clay: about 400 pounds per square foot.

For a sense of load, a 6-inch slab weighs about 75 pounds per square foot, and a garage can add 50 pounds per square foot of live load, per the same page. That is 125 on soil that may carry 400. It can work, but there is little margin, and a wet spring can erase it.

Frozen or rained-on subbase can soften and become unusable, Concrete Network says. Do not pour on a base that is mush.

Expansive clays are a separate problem. They shrink and swell with moisture and cause settlement, and Concrete Network says post-tensioned or structural slabs are often recommended there. That is engineering work, not a gravel question.

Compaction is the part people skip

A plate compactor is the tool for granular base. Concrete Network lists vibratory plates at 100 to 250 pounds with plates about 1 to 1.5 feet by 2 feet, and rammers at 130 to 185 pounds for clay.

The sequence that works:

1. Strip topsoil and organics. Grade the subgrade to the right slope for drainage.
2. Compact the subgrade. Add fabric if fines are likely to migrate into the stone, which Everything About Concrete lists as an option.
3. Spread the first 3 to 4 inch lift. Lightly moisten dry gravel, which Everything About Concrete says improves compaction.
4. Run the plate over it in overlapping passes.
5. Add the next lift and repeat until you reach the full depth.
6. Check grade with a straightedge, fix high and low spots, and proof-roll if the area is big enough to allow it.

Concrete Network gives an optimal moisture of roughly 10% to 20% for compaction. Too dry and the stone will not settle together, too wet and it pumps under the plate.

Vapor barrier and moisture

For most interior slabs, Concrete Network says the vapor barrier goes in just before the concrete is placed. The same page describes its position inconsistently, one place on top of the subbase and another between the base course and the concrete, so check the project spec and your local code.

Outdoors, a vapor barrier is not normally needed for a patio, but drainage still is. Water pooling under a slab, erosion of soil beneath it and frost heave are the problems Everything About Concrete ties to a missing or weak base. Shape the base so water can leave, and do not let the stone become a bathtub.

Quantity worked out

Say you are pouring a patio that is 12 feet by 20 feet on a 4-inch base.

• Area = 12 × 20 = 240 square feet.
• Depth = 4 / 12 = 0.333 feet.
• Volume = 240 × 0.333 = 80 cubic feet.
• Cubic yards = 80 / 27 = 2.96.
• Weight at about 1.4 tons per cubic yard = 2.96 × 1.4 = 4.15 tons.
• Add about 15% for compaction and spreading, since loose stone settles 10% to 20% = 4.15 × 1.15 = 4.8 tons.

The concrete itself, at 4 inches, is the same volume, 2.96 cubic yards. The Gravel Base Calculator does the base side, the Crusher Run Calculator handles dense-graded stone by the ton, and the Concrete Slab Calculator gives the concrete. To price the job, try the Concrete Slab Cost Calculator.

How base failure shows up

Years later, a slab with a bad base tends to show:

• Cracks that run full depth, which Concrete Network says is typical of poor support.
• A lip or bump where one side of a crack has settled more than the other.
• Sections that rock, sink or feel hollow.
• Water pooling on or beneath the slab.
• Joint cracking from poor compaction or excess load.
• In cold climates, heaved sections after a freeze.

Rebar will keep a cracked slab together. It will not stop the cracking from a soft spot underneath.

What to do next

If you are about to pour, walk the site first. Press your thumb in the subgrade. Look for soft spots, buried roots and low corners where water sits.

Then get the base spec in writing from whoever pours: stone type, depth, number of lifts, compaction method. If a contractor says the base is “just gravel,” ask what kind, how deep and how it will be compacted.

If the slab is already poured and has cracked, a base problem will not be fixed by patching. Look at where it cracked and where water goes, then decide whether to lift, replace or live with it.

Sources

• Concrete slab subgrade and subbase. Concrete Network
• Gravel Under Concrete for Patios, Slabs, and Driveways. Everything About Concrete
• Slab-on-ground design guidance quoting Portland Cement Association material on subgrade and subbase. NFBA