Everybody uses concrete, and nearly everybody gets it wrong sometimes. I sure have, and thinking about that led me to spend some time researching the material and how it’s best used for deck footings.
According to the PCA (Portland Cement Association; cement.org), concrete is a blend of aggregate, water, and Portland cement. Aggregate makes up the largest portion and consists of gravel and coarse sand. Portland cement is mostly lime and silica, plus a bunch of minerals and chemicals with very long names. It was patented in 1824 by a British mason named Joseph Aspdin, who called it Portland cement because he thought its color resembled that of limestone found on the British island of Portland.
Portland cement reacts with water to form a paste that hardens over time. When mixed with aggregate, this paste coats the surfaces of the gravel and sand, essentially gluing it together. A blend containing larger aggregate makes concrete that’s less likely to crack. Smaller aggregate blends can be easier to work, though. Most truck-mixed concrete used residentially contains 3⁄4-inch and smaller aggregate, while sack mixes usually contain aggregate from about 1⁄2 inch on down.
The ultimate strength of concrete is expressed in psi (pounds per square inch) — for example, 3,500 psi. Properly mixed concrete of a particular designation will support that pressure — or more — 28 days after placement. Its ultimate strength depends chiefly on the ratios of aggregate, cement, and water, but proper handling also plays a role. Whether you buy truck-mixed concrete or sack mix, the ratios of aggregate and cement have been figured out for you. In the case of truck-mixed concrete, so has the proper amount of water, although more is commonly added at the job to make the concrete easier to handle.
How Much Water?
Most of us have heard that adding too much water weakens concrete. I never quite understood why until the PCA’s Terry Collins explained it to me. It’s all about cement/water ratios. For example, because 5,000 psi concrete has more cement added to the batch, it has a higher cement/water ratio than, say, 3,500 psi concrete. If you keep adding water to the 5,000 psi concrete, eventually you would reduce the cement/water ratio to that of 3,500 psi concrete, and that’s what you’d be left with.
Ideally, concrete should be just wet enough to be workable. Collins suggests this field test so you know when to stop adding water. Wearing gloves (concrete is alkaline enough to produce chemical burns), toss a baseball-sized lump of wet concrete from hand to hand. It should hold together in a sort of football shape. If it crumbles, it’s too dry. If it won’t hold the football shape, it’s too wet.
We know the downside of placing concrete that’s too wet, but what about concrete that’s too dry? I’ve known contractors who simply dumped dry mix into a hole in the ground figuring “it’ll harden up in time.” It will, but it won’t be as strong. Concrete that’s placed too dry traps a lot of air. By volume, each 1 percent of trapped air reduces ultimate strength by 5 percent to 6 percent.
Truck-Mixed or Sack Mix?
Either can work. Truck mix is usually cheaper per cubic yard, but you may have to pay a short-load charge, as concrete companies figure costs and profit based on full truck loads, typically 10 cubic yards. They sell smaller quantities with a surcharge. You’ll have to compare the truck vs. sack economics based on the size of each job.
The quality of truck-mixed concrete is reliable, and it’s a lot less labor. But you’re at the mercy of the producer’s delivery schedule, and your 2-yard load won’t get the priority that a 40-yard foundation job does. And if you’re working in the backyard of a house, you’ll have to wheelbarrow the concrete from the street. Each yard is about a dozen wheelbarrow loads, depending on your brawn.
Perhaps the chief downside of truck-mixed concrete is having a limited amount of time to unload the truck. My local supplier allows six minutes per yard before it starts whacking me with a $100/hour holding charge. And on a hot day, the concrete can start to set up as you unload it.
Site mixing concrete gives you more control of the time, but it’s hard work. You have to haul it to the backyard. Then, you’re either mixing it with a hoe in a wheelbarrow or hefting 80-pound sacks up to the mouth of a mixer — and a mixer is the way you want to go if you’re in a northern, severe-weathering climate, where concrete should be “air entraining.” That means the concrete mix has additives that encourage the formation of small air bubbles that provide cushioning to reduce the spalling effects of freeze-thaw cycles. There is some strength loss, but that’s figured in to the mix ratios. Drum mixing is required for air entrainment.
Almost all cast-in-place deck footings are placed in a cardboard tube form. That offers several advantages, including a nice finish above grade, a set size that allows you to accurately figure concrete quantities, and easy connection to spread-footing forms.
If you aren’t required to use spread-footing forms, the PCA’s Collins suggests that you don’t need to take the tube forms to the bottom of the hole. The hole should go to below frost line, or to a mineral soil capable of load bearing, whichever is deeper. The form, however, can stop at the bottom of the topsoil layer. His reasoning is that when the sides of the concrete are in contact with bearing soil, friction adds considerable bearing capacity to the footing. Calculating the amount takes engineering, so you should base the footing sizes and numbers strictly on bottom bearing and take the frictional bearing capacity as a bonus. Skipping the lower end of the form also reduces the amount of backfill and its attendant settling.
When placing concrete, it’s important to consolidate it, by moving it around with a vibrator or a 2x4 thrust repeatedly into the wet concrete to get out the excess air. Tapping the top of the form with a hammer will minimize bubbles and provide a smoother surface above grade. If using a mechanical vibrator, don’t overdo it. Just 5 to 15 seconds gets out the air.
The best practice is to bring the footing out of the ground by 6 inches or so. That will elevate the base of the post it will support, away from ground moisture and splash-back. Trowel-finish the top of the footing, for looks and to make the surface smooth so less water soaks in. Slope the edge of the top for drainage while leaving the center flat for a post base. Use a post base that raises the post above the concrete and that anchors to the footing with a bolt. You can use a cast-in-place bolt, a wedge bolt, or an epoxied anchor, as long as it meets the hardware manufacturer’s specs.
Center the post on the footing. An off-center post loads a footing unevenly and can cause it to rotate, which drops the post — particularly on shallower footings because they have less lateral support from surrounding soil.
Setting posts in concrete is not recommended. As the post shrinks, it allows room for water and dirt to enter the footing. Eventually, swelling wood or freeze-thaw cycles can crack the concrete, admitting additional water and dirt. Trapped water and dirt creates a ripe environment for rot; even pressure-treated posts have been known to succumb.
Concrete requires certain conditions to set up properly. It should be kept damp for the first 28 days. Simply wrapping the top of the footing with plastic will do this in most cases. You may have to wet the concrete as well in very dry conditions.
Temperature is crucial. Once it’s below 40°F, concrete should be made using warm water. Concrete can be placed in temperatures as low as 10°F, but you have to keep it from freezing for at least the first couple of days. The chemical reaction that causes cement to set gives off heat, which works to your advantage in the cold. Once you’ve placed and tooled the footing, simply insulating it with hay and plastic is usually enough. Don’t be tempted to skip this step — if the concrete freezes before most of the excess water has escaped, it can be ruined.