How much concrete for a 10×10 slab

How thick

Four inches is the standard for a shed, patio, walkway or hot tub pad on compacted ground. A driveway, a garage floor or anywhere a vehicle parks wants 5–6 in and steel in it. Thickness is the dimension people shave to save bags, and it is the one that decides whether the slab cracks: concrete is strong in compression and weak in bending, and a thin slab bends.

Forms, base and reinforcement

Dig to the slab thickness plus 4 in for the base. Build the forms from 2×4 or 2×6 lumber staked every 2–3 ft on the outside, top edge at the finished height. Check square with the 3-4-5 rule: measure 3 ft along one side and 4 ft along the other from a corner; the diagonal between the marks is 5 ft when the corner is square. The two diagonals of the whole form match on a rectangle.

Spread 4 in of crushed stone inside the forms and compact it. The base drains water away and gives even support; pouring straight on clay is the usual cause of a slab that heaves and cracks. Lay a vapor barrier on top if the slab will be a floor. Set wire mesh or #3 rebar on chairs so it sits in the middle of the slab, not on the base where it does nothing.

Bags or ready-mix

Under about half a cubic yard, bags and a rented mixer are practical for two people. A 10 × 10 at 4 in is more than a yard, and the sheet counts the bags at the manufacturer’s yield (an 80 lb bag makes 0.60 cu ft). Mixing that many by hand takes most of a day, the first bags are setting before the last are poured, and the slab ends up with cold joints.

A ready-mix truck delivers the whole slab in one pour. Plants sell by the cubic yard, rounded up to the quarter yard, and most charge a short-load fee under their minimum, often 5–10 yards; the sheet rounds the same way and tells you when bags would be cheaper. The truck needs to get within a chute’s reach of the forms, about 15 ft, and it is heavy enough to crack a thin existing driveway. If it can’t get close, you wheelbarrow: a 6 cu ft barrow carries 4–5 cu ft of wet concrete, so a yard is six or seven loads.

Measuring and ordering

1. Measure the inside of the forms, not the outside; the form lumber takes 1½ in on each side.
2. Type length, width and thickness, and keep the 10 % allowance: the base is never perfectly flat.
3. Pick the bag size you can lift; the sheet counts bags for whichever you choose.
4. Compare the bagged total with the ready-mix line, then edit the ready-mix price to your plant’s quote, including any short-load fee.
5. Order ready-mix in quarter-yard steps, rounded up, for the morning, with forms, base, mesh and two people ready before the truck arrives.

Pouring and curing

Pour, spread with a rake, and screed with a straight 2×4 across the forms. Float it, wait for the bleed water to leave, then trowel or broom. Cut a control joint down the middle within a day, about a quarter of the slab depth, so the crack that will come goes where you put it. Keep the slab damp for a week: mist it, cover it with plastic, or spray curing compound; concrete that dries in the sun never reaches its strength. Keep foot traffic off for a day and vehicles off for a week.

Common mistakes

• No compacted base. The slab settles unevenly and cracks along the soft spot.
• Thin edges. They take the most load; keep the full thickness to the form.
• Under-ordering. A truck that comes back with a quarter yard costs another delivery fee and gives you a cold joint.
• Mesh on the ground. Reinforcement lifted into the slab works; lying on the gravel it is decoration.
• Letting it dry fast. A slab that goes pale in an afternoon is losing water it needed for curing.