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 percent aggregate (sand, gravel and other rock). Cement makes up 15 to 20 percent of concrete volume, and water the last 15 to 20 percent. The cement and water react chemically and harden, binding the aggregate into a single solid form.
A concrete mix made with fast-setting cement is convenient for setting posts for fences, mailboxes and similar non-loadbearing applications. (Burying posts in concrete is not suitable for deck construction.) Prepackaged concrete mixes are readily available and easy to use. These come in 50 or 60 pound bags that yield about 1/3 to 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 that's three times the diameter of the post and deep enough to bury one third of the post, plus an extra 6 inches of depth for a gravel base. The gravel helps promote drainage to keep water away from the bottom of the post. If the soil is sandy or loose, a wider hole is necessary. With the post in place and braced plumb, you fill the hole around the post with dry fast-setting concrete mix, then add water to moisten the mix and begin the curing process. Consult the package for exact water amounts. Alternatively, you can mix fast-setting or standard concrete with water before adding it to the hole. Premixing is recommended for standard concrete.
When fast-setting concrete mix is used, setting takes place in 20 to 40 minutes. Typically, you can apply some weight to the posts after 4 hours, but it's a good idea to wait at least 24 hours before resuming fence construction. Standard concrete mixes may take up to two hours to set and should cure for 24 to 48 hours before any forces are applied to the posts.
Curing time depends on temperature. Temperatures above 90 degrees Fahrenheit 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 40 degrees Fahrenheit slow curing significantly. If the temperature of the concrete itself dips to 25 degrees Fahrenheit, it freezes. Freezing results in the loss of at least 50% of the concrete's potential strength.