Reinforcing Steel in Concrete
DEAR TIM: I am having a large amount of concrete work done at my house. As you know, it can be expensive. I want the concrete to last and last. What are the secrets to long lasting concrete that does not crack, scale, or fall apart? The concrete walks and driveway in my childhood home are in very good shape and my guess is that these surfaces are nearly 40 to 50 years old! Is it still possible to install concrete that will last longer than I will? Beverly D., Schaumburg, IL
DEAR BEVERLY: Concrete is one of the most abused building materials that I am aware of. All too often the people who work with it have absolutely no formal training with respect to how the material needs to be specified, placed, finished, and cured. The components that make up standard concrete are very basic - gravel, sand, water, cement, and certain additives. But the science surrounding concrete is very, very complex.
Installing concrete that will last a lifetime is absolutely possible. The process begins by ordering concrete correctly. Always keep in mind that the cement powder in the mixture is the glue that holds the sand and gravel together. If you have small amounts of glue the concrete will be weak. Concrete suppliers and concrete trade associations have developed tables that tell you and your builder how strong concrete needs to be for different applications. For example, concrete used in standard residential foundation footings can be lower strength than concrete used for outdoor patios, sidewalks and driveways. The average person might think the opposite is true.
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I know you don't want to hear this but concrete is supposed to crack. As concrete hardens it begins to shrink ever so slightly. In fact, concrete shrinks 1/16th of an inch for every ten feet of length that is poured. Wise concrete finishers know this and install contraction or crack control joints that encourage the concrete to crack at these locations. For the joints to work correctly they need to be a minimum depth. Always make sure that these joints are cut 1/4 or more of the thickness of the slab.
Concrete is incredibly strong when you try to squeeze or compress it. But it has only about 10 percent of this strength if you try to stretch or bend it. You can prevent small hairline cracks from offsetting or widening by installing reinforcing steel. Wire mesh and one half inch diameter steel bars are readily available and very inexpensive. This steel needs to be in the middle of the concrete or slightly above the middle of the slab to provide the necessary reinforcement.
Water is needed to mix and cure concrete but if excess water is added to concrete after it leaves the central mix plant or is mixed into the surface of concrete during the pour or finishing activities, it can severely harm the concrete. Water added to already mixed concrete can dilute the amount of cement in the mixture. This added water weakens concrete just as added iced tea to your half empty glass at a restaurant dilutes the sweetness of the remaining tea.
Once the concrete is finished, it needs to be cured. The water within the concrete fuels a chemical reaction called hydration that continues to happen within the concrete for weeks and months. If this water evaporates from the concrete too rapidly, then the concrete might never achieve its design strength. Liquid spray curing compounds can be added to the concrete as soon as the concrete finisher completes his/her final trowel stroke. You can also cover concrete with sheets of plastic. Curing is a simple step that is often neglected.
I can assure you that concrete that is specified, mixed, placed, finished, and cured in accordance with industry accepted guidelines will last and last. The biggest problem I feel you are going to have is finding a concrete contractor that is familiar with these guidelines. If you mention Portland Cement Association, American Concrete Institute, or World of Concrete to a contractor and his eyes glaze over, then keep looking!