Concrete Flatwork

The honest sentence nobody leads with

Concrete shrinks as it cures, and it moves with temperature. It will crack. A good concrete contractor doesn't promise you a crack-free slab — a good concrete contractor controls where the crack goes, and builds a slab that stays flat and intact around it.

Everything below is in service of that.

Base — yes, concrete needs one too

There's a myth that a slab is stiff enough to bridge a bad subgrade. It isn't. An unsupported slab is a slab in bending, and concrete is weak in bending. That's how you get a crack that opens up and gets a lip in it — one side settles, the other doesn't.

• Excavate to below the topsoil and any organics. Organics rot, then the slab has nothing under it.
• On expansive clay, get the slab up on a granular cushion: 4 to 6 inches of compacted CDOT Class 6 aggregate base, placed in lifts of 3 to 4 inches and compacted between each lift. Same rule as pavers. Same reason.
• Uniform support matters more than stiff support. A slab that's supported hard in one spot and soft in another will crack at the transition every time.

The mix

Two numbers matter for exterior flatwork in Colorado, and both of them get skipped by people racing to the bottom of a bid.

• Air entrainment. Exterior concrete in a freeze-thaw climate needs entrained air — typically around 6 percent for the aggregate sizes used in flatwork. Those microscopic air bubbles give freezing water somewhere to expand into. Without them, the water expands into the paste and blows the surface apart. That's scaling: the pitted, flaking, spalled surface you see on cheap driveways all over the metro after five winters.
• Strength. Exterior flatwork here should be a 4,000 psi class mix, not a 2,500 psi interior mix.

And a rule that costs nothing: don't over-water the truck. Adding water at the site to make the pour easier to place raises the water-cement ratio, drops the strength, and brings bleed water and fines to the surface — which is exactly the weak surface layer that scales off first.

Control joints — the whole ballgame

A control joint is a deliberate line of weakness. You cut a groove in the slab so that when the slab shrinks, it cracks at the bottom of your groove, out of sight, in a straight line, instead of wandering across the field.

The specs that matter:

• Depth: at least one quarter of the slab thickness. A 4-inch slab needs a joint cut a full inch deep. A joint scratched a half-inch deep into a 4-inch slab is decoration — the slab will crack right past it.
• Spacing: a standard rule of thumb is joint spacing in feet of roughly 2 to 3 times the slab thickness in inches. For a 4-inch slab that's 8 to 12 feet. We build to the tighter end of that on expansive soils.
• Panels stay squarish. Keep panel length-to-width under about 1.5-to-1. A long, skinny panel cracks across its middle regardless of what your joint plan says.
• Timing. Saw-cut joints go in early — as soon as the slab will support the saw without raveling, typically within the first day. Cut them late and the slab has already picked its own crack.
• Re-entrant corners. Every inside corner — around a step, a column, a bump-out — is a stress riser. Joints get planned into them, or a crack starts there.

Isolation joints are different and just as important: full-depth compressible material anywhere the slab meets something rigid and independent — a foundation wall, a column footing, a step. The slab has to be free to move relative to the house. Pouring a patio slab tight against the foundation is how you crack the patio, and occasionally how you push on things you don't want to push on.

Finishing, curing, and the first winter

• Broom finish for exterior. A hard steel-troweled exterior slab is a skating rink and it densifies the surface in a way that traps bleed water — bad news for freeze-thaw durability.
• Never finish bleed water into the surface. Working water back into the top is the fastest way to a weak, scaling slab.
• Cure it. Concrete gains strength by hydrating, not by drying. Colorado's low humidity, wind, and sun pull water out of a fresh slab fast. Cure with a curing compound, wet burlap, or plastic — a slab that dries out in the first days will never reach its strength and will craze and scale.
• No de-icing salt the first winter — and go easy after that. Chloride de-icers are hard on the surface of any concrete and brutal on young concrete. Sand is your friend.

Slope

Same as every other surface we build: about ¼ inch per foot away from the house, no ponding. Water standing on a slab in December is water freezing on a slab in December.

Weighing your options? Read pavers vs. flagstone vs. concrete and what a patio costs in Colorado.