How to pour a concrete slab: mix, finish and cure
Set braced forms on a compacted base, mix each bag with the water its label gives, fill and level the forms, finish once the surface water has gone, then keep the slab damp while it cures.
Your plan or local code sets the thickness, steel and drainage; most patios and walks are 4 inches thick. Wet concrete can burn skin, so wear waterproof gloves, long sleeves and eye protection.
Worked example
Example: a 12 × 10 ft slab, 4 inches thick, with 5% extra and 80-lb QUIKRETE No. 1101 bags.
This example gives 1.56 cubic yards of concrete.
- 12 × 10 × 0.3333 = 40 cu ft per rectangle
- 40 × 1 identical shapes = 40 cu ft
- 40 × (1 + 5/100) ÷ 27 = 1.5556 cu yd
- Edge forms: 2 × (12 + 10) = 44 ft
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Build the forms and the base
Set the forms at the slab’s finished height, with a slight fall away from the house so rain runs off, and stake them so they cannot bow. Compact the soil or gravel base until it is firm: a soft spot lets the slab settle and crack. The QUIKRETE Concrete Mix No. 1101 data sheet asks for the same, secure forms on a firm, compacted sub-base. Mark where the control joints will go before the pour.
Have everything ready before the first bag
For bags you need a mixer or mixing tub, a bucket marked for the water, a shovel or hoe, a wheelbarrow, a straight board long enough to span the forms (the screed), a float, an edger, a groover and a broom or steel trowel. Have the full bag count from the calculator on site, extra included, because a pour that stops halfway leaves a weak seam. For ready-mix, clear the route from the truck to the forms and line up helpers: a cubic yard weighs about two tons and has to be placed before it stiffens.
Mix to the label
The No. 1101 data sheet starts with about 6 pints (3 quarts) of water for each 80-lb bag, then adds a little more only until the mix is workable. Extra water makes weaker concrete, so add it slowly. Mix until there are no dry pockets and no puddles. This mix is blended before it goes in; the pour-it-dry method printed on post-setting mixes does not apply to it.
Fill, strike off and float
Start at the far corner and fill the forms to full depth, working the concrete into the corners and along the edges with a shovel. Pull the screed across the forms with a sawing motion to strike the surface level, fill any low spots and strike again. Then float the surface to push the stones down and bring a smooth layer up.
Wait for the surface water, then finish
Water rises to the top after floating. The data sheet says to wait until it has gone before the final finish; working it back in leaves a weak, dusty surface. Then run the edger along the forms and cut control joints with the groover, a quarter of the slab’s depth (1 inch in a 4-inch slab) and about 8 to 12 feet apart. Finish outdoor slabs with a broom for grip and indoor floors with a steel trowel.
Keep it damp while it cures
Concrete hardens by curing, not by drying out. Keep the slab damp for the first week: mist it and cover it with plastic sheeting, or spray on a curing compound. The data sheet lists moist-curing methods and weather limits, so check the forecast and do not pour when frost is expected. In mild weather a slab usually takes foot traffic after a day or two and a car after about a week; the label gives the times for its mix, and cold weather slows all of them.