Whether a project is small — like steps — or larger in scope like a patio, figuring how much concrete you need is done the same way. However, as you will soon discover, how the concrete makes its way to your home is another story.
Figuring how much material you need isn’t nearly as complicated as some would have you believe. Although a project is typically expressed in square feet, when buying concrete, it is measured in cubic yards – and a project’s length, width and depth determine the amount needed. Regardless of the size of the project, the formula is the same: 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. Confused? Building material suppliers offer free conversion charts for the math-challenged. Or you can purchase inexpensive “enter your dimensions” hand-held project calculators and there are numerous project estimators on the Internet.
Once you’ve figured how much concrete you will need you must next determine how to get it to the job. Concrete is a mixture of sand, gravel, water and Portland cement and can be obtained in three ways: separate dry ingredients that you mix on site, pre-mixed 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 pre-mixed product.
What’s the best means of obtaining concrete for your project? Small projects such as pouring a step or stoop, setting a few fence posts or making small repairs can best be done using the pre-mixed sack product. Pre-mixed bags are great cost-wise too. Available in two sizes: 60-pound (average $1.35 – $1.80) and 90-pound ($2.00 – $2.30). There are also a number of special mixes for setting fence posts and mailboxes – 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 recipe for 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’ X 10’ X 6” patio needs 1.85 cubic yards of concrete or about two yard with waste. Using 60-pound bags that yield one-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 a period of time.
Work-wise, ready-mix is a no-brainer for mid to large projects, but what about cost? Using 60-pound bags, concrete for a patio this size ranges from $135 to $180. Keep in mind, however, that you’ll need to rent a mixer, which adds another $40 to $60 per day. While calling in 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 yard 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 pre-mixed – depending on where you bought your bags and rented 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 and so forth. 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 centers on knowing how much concrete you need and determining at what point a ready-mixed delivery makes more sense than dealing with pre-mixed bags. Knowing there are other ready-mix options helps too, such as:
• “Short-load” services will mix from one to nine 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 one 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: One cubic yard – $75.00, 1/2 – 65.00, 1/3 – 55.00. Tip: When self-hauling ready-mix cement, regardless of distance, try to rent a “spin-tank” rig versus “tub-type” trailer if possible to avoid water separation.
Aside from project size, cost and how concrete is obtained, the following tips can help determine the success (or failure) of a project.
- Don’t get caught short when ordering material. Always add 10% to compensate for grade variations and spills. Trying to save a little can end up costing you a lot in the long run.
- Concrete “cures” chemically over days versus drying. To prevent cracking, cover with plastic or damp fabric.
- Additives strengthen, add color and speed or slow cure time. They are available for both pre-mixed bags and ready-mix.
- Concrete will usually require added support for strength – steel rebar, wire mesh, etc. Discuss project needs with a pro.
- Cement is caustic. Wear eye protection, gloves and wash off skin immediately. Do not breathe dust.
- Concrete strength is measured in PSI (pounds per square inch) indicating weight it can support or bear.
- Strength is increased by adding more cement in the mix.
- Weekend ready-mix delivery costs 5% to 10% more.
- Ready-mix companies allow four to five minutes for each yard delivered. After that, 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 broom to texture walks/steps for safety.
Concrete was used as a building material as far back as the Roman Empire, and – as many of these projects are still around. It’s why we continue using it today. Done right, yours will last for ages too.