The Illinois Ready Mixed Concrete Association's driveway guide says cold-weather concreting practices must be used for concrete placed after November 1. [IRMCA] For many Iowa City homeowners, that makes this a good time to plan a spring driveway and know what to ask your contractor for.
The basics to ask for
Base: a prepared subgrade that's uniform and well compacted. If the ready-mix truck leaves ruts as it backs in, more compaction is needed. [IRMCA][VRMCA]
Thickness: at least 4 inches. The Virginia Ready Mixed Concrete Association says 4 inches works for cars and pickups, and 5 to 6 inches for light trucks. [IRMCA][VRMCA]
Mix: 4,000 psi minimum at 28 days (check your local code), with air entrainment for freeze-thaw durability. IRMCA lists an air content of 6.5 percent, plus or minus 1.5. [IRMCA][VRMCA]
Slope: at least 1/8 inch per foot away from the house for drainage. [IRMCA]
Joints control the cracks
Joints are pre-planned cracks. The National Ready Mixed Concrete Association says a 4-inch slab should have joints about every 10 feet, and never more than 15 feet apart. [NRMCA]
Panels should be square or nearly so, cut at least one quarter of the slab's thickness deep. [NRMCA]
IRMCA calls for a centerline joint on driveways wider than 12 feet and a full-depth isolation joint where the new slab meets the garage floor or other structures. [IRMCA]
Finishing and curing
No extra water on the surface while finishing, no steel trowels, a broom finish for traction, and a curing compound or other curing method afterward. [IRMCA] VRMCA suggests keeping cars off a new slab for 3 days and trucks for 7 days, unless tests show it has gained enough strength. [VRMCA]
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