What is concrete?
Ever wonder what concrete is really made of? Read this and find out.
Most people say “cement” and “concrete” interchangeably. But they’re not the same thing. Cement is just one of the ingredients in concrete, and if you ask for cement at the lumberyard, you might get a bag of Portland cement.
Aside from cement, concrete contains sand and stones. Those stones, or “aggregate,” are a carefully measured mix of various sizes. Small stones fill in the gaps between larger ones, and sand fills in between them. Cement is the glue that holds it all together.
Water makes concrete “work”
Cement is mostly limestone that’s been ground up and superheated. Adding water causes a chemical reaction; microscopic crystals develop, grow and interlock, binding the aggregate together and forming a rock-hard mass.
Don’t overwork it
To finish the surface, concrete is first “floated” with a float, sometimes called a “mag”. This pushes the aggregate down and pulls fine sand and cement to the surface—just what you need to form a smooth, troweled finish or a rough-broomed finish later. But limit float work to two or three passes. Too much floating leaves a topping of watery cement. And that means a weak, porous surface. Too much troweling causes the same trouble.
Cement is caustic. So it can cause anything from dry skin to nasty burns that require medical attention. Wear gloves and protect your eyes. Concrete dust is bad for your lungs. So strap on a respirator while mixing or cutting it.
Keep it wet longer to make it stronger
Concrete will continue to harden until it dries out completely. That’s why pros often use blankets, plastic sheets or spray-on coatings to retain moisture. Since most of the strength gain takes place in the first few days, experts often recommend a “wet cure” of three to seven days. Typically, concrete is considered fully cured after 28 days.
Allow for cracks
Concrete shrinks as it cures, which causes shrinkage cracks. Settling and frost heaves also lead to cracks. Instead of risking random, meandering cracks, provide “control joints” for straight, invisible cracks to occur. You can plow control joints into fresh concrete with a groover (as shown here) or use a saw when the concrete is partially cured. On sidewalks, control joints are needed every 4 to 5 ft. On wide slabs like driveways, place them 10 to 12 ft. apart.
By adding an “accelerator” to the mix, manufacturers produce bagged concrete that hardens in an hour or less. That’s great for small jobs but risky for big jobs that require ample finishing time. Fun fact: in the 1930s, bagged concrete mix hit the market. Before that, folks had to mix up cement, sand and aggregate themselves.
Find out how to prep for a slab here.
Find out how to pour a slab here.
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