It may be a personality flaw, but I love concrete. I will take the snarl of a loaded ready-mix truck turning into the driveway over the whine of a 737 flying me away on vacation any day of the year. But as much as I relish the prospect of 20 tons of wet concrete being delivered for me to shape into a slab or a building footing, the fact is that most of my concrete work has been too small for a ready-mix order.
Consequently, I’ve mixed up a fair number of bags of concrete mix for deck footings, for stair landings, and to repair a variety of slab cuts and miscellaneous damage. It used to be that lumberyards and home centers would have three choices: concrete mix, sand mix, and mortar mix. Now they have at least three choices of concrete mix alone.
What’s changed? Like most things in our lives, technology has affected bagged concrete. Frank Owens, Quikrete’s VP of marketing, says, “Bagged concrete mixes today are designed for specific purposes.” This is largely down to the development of admixtures—natural or manufactured chemicals that improve certain properties of fresh or hardened concrete, such as work ability or strength. “All the components are carefully weighed and computer-batched. This allows admixes to be added correctly.” Which, in turn, allows for mixes aimed at specific purposes.
Stronger together
Concrete choice is largely about strength. While other additives are often used to achieve specific ends, all concrete contains five chief ingredients: coarse aggregate (gravel) and fine aggregate (sand), which compose 60% to 75% by volume of the mix; 7% to 15% Portland cement, 14% to 21% water, and up to 8% air. The ratios of these components affect strength and hardening time, as well as workability and longevity. The most expensive ingredient is the Portland cement, and it’s also the one that has the most obvious effects: The more Portland, the stronger the concrete and the faster it sets.
Concrete sets because of a chemical reaction between the Portland cement (there are other compounds that act similarly, but they’re outside the scope of this article) and water. That reaction takes time, and concrete’s ultimate strength is measured at 28 days. That said, the initial hardening of concrete depends on the temperature: The warmer the weather, the faster the reaction. The reaction is exothermic, meaning it gives off heat. That’s what keeps concrete from freezing when poured in below-freezing temperatures, though you can’t go too far below freezing without insulating the pour (bagged mixes typically include temperature recommendations). If the water in the mix freezes, the reaction stops, and the ice crystals wreak havoc on the structure of the weak, new concrete.
The strength of a concrete mix is determined by measuring the pressure required to crush a sampling of test cylinders cast from that mix and allowed to harden for 28 days. Obviously, each bag of concrete mix isn’t tested in this way, but samples have been evaluated. The minimum strength used in residential concrete work is a 2500-psi mix. All the bag mixes I know of are claimed to be at least 4000 psi, and some specialty mixes yield more than double that strength.
Conditions can affect product choice
The two most important conditions to consider when working with concrete are weather and time. These factors become especially notable when you’re pouring a slab, because the concrete has to set partially before it can be finished. In hot weather, concrete sets more quickly, and you want to be sure there’s time to finish placing the last of the mix before you have to finish the first sections that you placed. Conversely, in cold weather, a slow-setting mix can have you finishing a slab by headlight when you’d really like to be home having dinner.
I’m also including with conditions the speed with which the concrete needs to enter service. For slabs that will likely see traffic in a short period of time—sidewalks and driveways, mainly—it’s worthwhile to spring for high-early-strength concrete.
Finally, there’s budget. Concrete mixes containing more Portland cement and other additives will cost more per bag. In my area, standard concrete mix costs $4.90 for 80 lb., while the high-early-strength mix costs $6.80 for the same amount. There are times when you’ll want to spend the extra for it, but it’s not always worthwhile.
Stay hydrated, but not too hydrated
Cory Olson, Senior Vice President of Sakrete, says that, “Overwatering is a common error.” Theoretically, only enough water to fully react with the amount of Portland cement in concrete mix is needed. Any water added beyond this results in weaker concrete. That excess water expands the volume of the wet concrete, and some of it remains in place for a time after the concrete sets. But eventually the excess water will evaporate, leaving concrete that’s less dense and not as strong.
However, adding only the chemically necessary amount of water yields a mix that’s too stiff to mix and work by hand. More water is almost always needed to create a workable mix. That said, modern bagged concrete mixes usually contain some form of plasticizer, a chemical that makes the mix more workable with less water. The directions on the bag will tell you how much water to use. Start there, and only add more water if absolutely necessary. If you find you still need to add water, Olson suggests using a high-strength mix where some loss of strength may be acceptable.
Additionally, adding too much water can create a workability problem, particularly in slabs. As the concrete sets, much of the excess water makes its way to the surface, and you can’t finish the concrete until that water goes away. That can be a particular problem in hot, humid weather, because the underlying concrete can become too hard to work before the surface water evaporates. In these conditions, I have literally mopped water off the top of a slab with my wife’s ShamWow. (Yes, we’re still married.)
All of that said, it’s important to keep concrete wet as it sets. Concrete should be kept wet for 5 to 7 days after it’s poured. Common approaches are to cover the concrete with plastic and flood the space between or cover it with burlap and keep that wet. This isn’t done very often in residential work, but keeping freshly finished concrete wet for the first week makes a difference in its long-term strength.
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