Cure times for posts set in concrete vary, depending on the composition of the concrete and the weather. However, setting posts is a relatively simple project that doesn't usually require in-depth knowledge of concrete chemistry. The average homeowner needs to understand only the basics of how concrete works and how weather conditions might affect results.
By volume, concrete is 60 to 75 per cent aggregate (sand, gravel and other rock). Cement makes up 15 to 20 per cent of concrete volume, and water the last 15 to 20 per cent. The cement and water react chemically and harden, binding the aggregate into a single solid form.
A concrete mix made with fast curing cement is best for setting posts. For home and small industrial applications, prepackaged concrete mixes, such as Quikrete, Quickset or Rapid Set, are readily available and easy to use. These come in 50 or 60 pound bags that yield about 1/2 of a cubic foot of concrete each. Standard concrete mix can also be used, but cure times are significantly longer.
When setting posts in concrete, always read and follow the concrete manufacturer's instructions. Typically, you'll be instructed to set the post in a hole three times the diameter of the post and deep enough to bury one third of the post. If the soil is sandy or loose, a wider hole is necessary. Fill the hole with dry concrete mix. Add water to moisten the mix at the rate of 1 gallon per 50 pound bag of concrete. Consult the package for exact water amounts.
When fast setting concrete mix is used, setting takes place in 20 to 40 minutes. 400 psi compressive strength is achieved after 2 to 4 hours. 98% of 4000 psi compressive strength is achieved after 28 days. Posts set in fast setting concrete usually require no bracing and can be loaded after 4 hours. Standard concrete mixes may take up to two hours to set; at least two days curing should be allowed before the post is loaded.
Curing time is depends on temperature. Temperatures above 32.2 degrees Celsius may cause excess evaporation and drying of the concrete. This will stop the curing process, leading to poor concrete strength and cracking. Temperatures lower than 4.44 degrees Celsius slow curing significantly. If the temperature of the concrete itself dips to 0-3.889 degrees Celsius, it freezes. Freezing results in the loss of at least 50% of the concrete's potential strength.
In extreme weather, the curing temperature of the concrete can be controlled somewhat by the temperature of the water used to wet the concrete mix. In hot weather, ice water can be used to wet the concrete mix. Keeping the surface of the concrete wet will prevent excess evaporation. In cold weather, hot water can be used for wetting the concrete mix. Also, you can cover the concrete around the post with a tarp or thick blanket as it cures. This helps retain the heat generated by the exothermic reaction of hydration.