Buying Concrete – 12 Tips to Determining Project Success
Whether a project is small—like steps—or large like a patio, figuring how much concrete you need is calculated the same way. How the concrete makes its way to your home is another story.
Although most projects are expressed in square feet, when buying concrete, you measure in cubic yards—a project’s length, width and depth determine the amount needed. Multiply length by width (to find square feet), multiply that by depth or thickness (for cubic feet) and divide by 27 (the number of cubic feet in a cubic yard) to determine how much concrete (in cubic yards) is required. Building material suppliers offer free conversion charts for the math-challenged. Or you can buy inexpensive “enter your dimensions” hand-held project calculators; and, there are project estimators on the Internet.
Once you’ve figured how much concrete you’ll need, determine how to get it to the job. Concrete is a mixture of sand, gravel, water and Portland cement. It can be obtained in three ways: separate dry ingredients that you mix on site, premixed in the sack where you just add water, and ready-mixed from a batch plant and delivered by truck. There is a variation to the latter where you can haul a small amount of ready-mix material using a trailer provided by the batch plant or a rental yard. Of the three methods, raw materials mixed on site are used the least frequently due to the availability of premixed product.
Small projects such as pouring a step or stoop, setting a few fence posts or making small repairs can best be accomplished using the premixed sack product. Premixed bags are reasonably priced. They’re available in two sizes: 60-pound (average $1.35-$1.80) and 90-pound ($2-$2.30). There are also a number of special mixes for setting fence posts and mailboxes. They’re sold in 40- and 50-pound bags in two “no-mix” formats: 1) you dump the concrete into the hole, then add water, and 2) the reverse—first the water, then the concrete.
Taking on a path, patio or other sizable project using anything other than ready-mix can be a disaster. There is a point of diminishing returns where the number of bags needed for larger projects simply overwhelms the economy of mixing it yourself. For example, a 10-foot X 10-foot X 6-inch patio needs 1.85 cubic yards of concrete or about two yards, with waste. Using 60-pound bags that yield 1-half cubic foot per would require 100 bags. The concrete will set faster than you can mix and pour it, and you’ll end up with a poor pour and a nasty finish (unless you are willing to work in phases over time).
Ready-mix is a no-brainer for mid- to large projects, but what about cost? If you use 60-pound bags, concrete for a patio this size ranges from $135 to $180. But you’ll need to rent a mixer, which adds another $40 to $60 per day. While ready-mix reduces work, it also boosts the cost. Each cubic yard costs about $65. However, a fully loaded cement truck will hold 10 cubic yards—and partial “short” loads cost $15 to $20 extra for every cubic yard less than a full load.
So, our 1.85 cubic yard patio first requires purchasing two cubic yards (including waste) of ready-mix (2 X $65 = $130) plus another $135 for the “short load” (10 yards full load minus 2 yards = 8 yards “short” X $17 per yard average)—and now totals around $265. With assorted fees added (environmental impact, fuel surcharge, sales tax, etc.), the final cost is about $295. For this project, ordering ready-mix would cost from $55 to $120 more than premixed—depending on where you buy your bags and rent your mixer. It’s a small price to pay for a job that can be finished in a day, and to avoid a concrete project gone bad.
Beyond shopping price, there are things to know about ordering ready-mix. While some think a cement truck goes from site to site dumping concrete until it is empty, this is not how it works. Rather, each batch is individually formulated and mixed for a specific use. The number of “sacks” of cement used per yard of concrete influences the strength of the fully cured product. For example, “five-sack” mix is stronger than “four-sack,” and so on. Also, ready-mix concrete must be off-loaded within 90-minutes of being mixed or within 300 revolutions of the truck’s tank—whichever comes first. Thus, a supplier’s location is a key consideration. Too far away creates problems with “set” times and delivery cost increases. Narrow your search to those companies closest to your home or job site, then go to work comparing costs and services. Also determine lead time needed.
The success of a project depends on knowing how much concrete you need and determining at what point a ready-mixed delivery makes more sense than premixed bags. Knowing there are other ready-mix options helps, too, such as:
- “Short-load” services will mix from 1 to 9 cubic yard orders right on your job site. Average cost: $85 service charge plus $70 per cubic yard. Most charge only for what you use (in 1/4 yard increments).
- Many building material suppliers and tool rental companies offer ready-mix concrete that can be hauled using a small trailer with about 1 cubic yard capacity that they will provide as part of the price. However, you’ll need to have a vehicle that is powerful enough to haul it and a project that won’t be foiled by the time it takes to go back and forth to the supplier, when more than one yard is needed. Average cost: 1 cubic yard, $75; 1/2, $65; 1/3, $55. Tip: When self-hauling ready-mix cement, regardless of distance, try to rent a “spin-tank” rig vs. “tub-type” trailer if possible—to avoid water separation.
The following tips can help determine the success (or failure) of a project:
- Don’t get caught short when ordering material. Always add 10 percent to compensate for grade variations and spills. Trying to save a little can end up costing you a lot.
- Concrete “cures” chemically over days. To prevent cracking, cover with plastic or damp fabric.
- Additives strengthen, add color and speed or slow cure time. They are available for both premixed bags and ready-mix.
- Concrete usually will require added support for strength—steel rebar, wire mesh, etc. Discuss project needs with a pro.
- Cement is caustic. Wear eye protection and gloves, and wash off skin immediately. Do not breathe dust.
- Concrete strength is measured in PSI (pounds per square inch) indicating the weight it can support or bear.
- Strength is increased by adding more cement in the mix.
- Weekend ready-mix delivery costs 5 percent to 10 percent more.
- Ready-mix companies allow four to five minutes for each yard delivered. After that, the clock ticks at $2 per minute.
- If concrete has to be pumped from truck to site, there are additional costs and charges. _Rather than ready-mix truck returning-disposing of order overages, plan ahead for additional use: fence posts, flag pole, stepping stones, etc. Why waste it?
- One cubic yard of ready-mix yields nine contractor-size wheelbarrows of concrete.
- Smooth concrete is not always best. Leave top of footings and foundations rough for better bond with mortars. Use a broom to texture walks and steps for safety.