Different terms used to sell and measure concrete

Q: I am getting estimates on a driveway from a home in Northern California and wonder what to look for in terms of the type of cement used. Previously you mentioned that one should use a 3,500- to 4,000-pound test for the driveway. How many sacks per yard does that translate to? One contractor said he uses only 3/4 aggregates. Is that better or worse than Portland cement?

A: The strength of concrete is rated by the number of bags used in a mixture of sand and aggregate to form a cubic yard of concrete (3 by 3 by 3 feet). Some companies prefer to sell their mixture by the test of the concrete, as in 3,500 psi (pounds per square inch) -- sometimes referred to as "pound test" -- while other companies sell concrete by counting the bags of cement used per cubic yard.

A typical 3,000-pound test concrete is equal to a five-bag mix. The more cement used per cubic yard of concrete, the higher the test rating will be. A 5-1/2-bag mix is equal to a 3,500-pound test.

It's all marketing or what is custom for your area. In Indiana, we order by the pound test and just 30 minutes up the road they sell concrete by the bags mixed. The aggregate, sand and gravel or stone make up the majority of the concrete mix other than the water added. As the concrete mixture dries, the water evaporates and leaves behind the hardened cement and aggregate.

Bags and pound test are just different terms used to market a product and to get your business; in the end it's just concrete. I would suggest you consider at least a 4,000 psi or a six-bag mixture for a residential driveway to be poured in a cooler-climate region of the country.

The mixture should also contain fiberglass fibers to strengthen the concrete, use wire mesh or steel reinforcing bars set midway between the bottom and top of the 4-inch-thick slab, or both.

The other important questions you need answered are: How will the areas under and around the driveway drain off rain or storm water? What finish will be applied to the driveway to make it a safe, nonskid surface? If the driveway is on a steep slope, what will be incorporated to prevent the concrete from creeping down the slope? Do you require a coloring agent or an exposed aggregate? What type of and how many expansion joints will be used to prevent cracking?

Concrete can be stamped to take on the form of bricks or cobblestone. Do you want a pattern for the finished concrete?

Ask for references and take a good look at the contractor's finished products. Ask previous customers if they are satisfied with the work, and check your local Better Business Bureau for updates on the contractors you choose.

(Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at C. Dwight Barnett, Evansville Courier & Press, P.O. Box 286, Evansville, Ind. 47702 or e-mail him at d.Barnett(at)insightbb.com.)