The Hot, dry, windy weather of our area this time of year is not the most favorable environment to place concrete. It causes rapid drying and shrinkage of the surface causing early surface cracking, crazing and a false set that induces more substantial cracks. This is all caused by the surface drying out sooner, sometimes much sooner than the rest of the slab. So, what is done to help this unfavorable situation? Here are a few suggestions:
• Do not over wet the subgrade. If we are attempting to even out the evaporation of the entire depth of the slab, it doesn’t make sense to give the bottom more water while the surface is fighting to keep its water. Give the free water a chance to escape down ward as well as up. If it is a dry hot day, you may not want to apply any water to that subgrade.
• Consider adding micro fiber to your order. Each dose per yard include over a million polypropylene fibers that are distributed throughout the mix. These fibers act as a micro crack inhibitor. Surface cracking can be drastically reduced by adding fiber to your concrete.
• Don’t add excess water to the mix. This only exasperates the problem. That water will eventually leave the concrete and what is left behind are voids that enable shrinkage thus cracking to happen. Have you ever seen the bottom of a dried up lake bed? As the water leaves the surface It has cracked and shrinks making these squares that are all curled up. This is a drastic picture of the same thing that happens to the surface of the concrete. The slump, or wetness or workability of the concrete, is increased with water and is very tempting if you have ever had to work low slump or stiff concrete, but this can also be achieved by using mid and high range water reducers. These water reducers have become the best friends of the professionals. They get the workability without jeopardizing the integrity of the mix and increasing shrinkage and cracking. Every gallon of water added to a yard of concrete over the design amount reduces the compressive strength by up to 500 psi. Ask a Duke City representative to guide you on which water reducer is best for your application.
• That free water must be able to leave the entire depth of the slab, therefore do not seal the surface too early, i.e., put a steel trowel on it. Air entrained mixes, used for exterior concrete, will blister even more if the surface is closed too early. Use a fiber or magnesium float to flatten the surface. This rough application leaves the surface open to allow free water to escape. Sealing too early causes blistering and scaling by trapping water under the surface. I have seen it to many times, people want to get to work and start troweling way too early. Patience is the best approach. This sealing also produces a false set on the surface (the surface is hard but the concrete under that surface is still not at final set). The concrete seems spongy and will eventually shrink enough to pull the immovable surface apart. Most exterior concrete requires a broom finish; this will also keep open the surface for escape of that free moisture.
• Curing the concrete after the finish is so important. After the “FREE” water is gone and we have final set, we must allow the water intended for the cement particles to stick around. That surface is shrinking and cracking because it is starving for water. Any way you can provide that water is a plus. A membrane curing compound is a start. It seals the surface and doesn’t allow moisture to leave the concrete. The cement will use that water to hydrate, but this membrane can be torn or rubbed off by foot traffic, making it ineffective. The best way to cure is to keep water on the surface by either diking and flooding or using a felt backed vapor barrier on the surface. This felt backing provides a reservoir of water for that surface. Another approach is applying water to the concrete for the first 4 to 7 days will help.
Plan your placement for the most favorable conditions. Early cool mornings are much better than HOT windy afternoons.
Duke City Redi-Mix has developed and continue to refine our mixes to help meet our southwest sometimes harsh environment, but there are safeguards that must be used to place that concrete to complete the job of the concrete.
We have used our many years of combined experience in this Valley to develop a line of mixes that are designed for the hot, dry summers and the cold freezing winters. We call them DC Zia Everlast.
The combination of a double dose of fiber, replacing welded wire mesh, water reducers to give you the workability you desire for flat surfaces, our high quality aggregates and using the most technological advanced cement in our area gives you the customer a concrete you can trust all in one mix. Ask for more information when you call.