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This article explains how to order concrete. We'll use a 10 x 10 ft. slab as an example. Here's a brief rundown of what you need to know.
Amount. Calculate the volume you need in cubic yards. Multiply the length (10 ft.) by the width (10 ft.) by the depth (.35 ft., or 4 in.) and divide it by 27 (the number of cubic feet in a cubic yard). You get 1.3 cu. yds. Then add 10 percent to allow for spillage and slab depth variations.
Strength. Call a local ready-mix company, tell the supplier what the concrete is for, and ask about the best mix (proportions of cement, gravel and sand). For a shed, the supplier will probably suggest a mix with a capacity of about 4,000 psi (pounds per square inch). If you live in a region with freeze/thaw cycles in winter, ask for 5 percent air entrainment to help the concrete withstand freeze/thaw damage.
Cost. Use $90 per cubic yard as a ballpark figure, but this will vary by region. Also, expect a fee of about $60 per load for delivery. There could be other fees for such things as Saturday delivery and small loads. Ask about these fees so you know the total bill before the truck arrives.
Unload time. Ask about the normal unload time (usually 7 to 10 minutes per yard) and if there is a fee for overtime. If the truck can't reach the site, make sure you have two or three people with wheelbarrows ready to go.