Concrete Foundation

DEAR TIM: I’m going to be building a new house, even in this tough economy. I’m leaning towards a concrete foundation, but I’ve seen all sorts of alternative foundations such as concrete block, wood, and even rock. What can you tell me that will help me decide? I’m looking for strength and energy efficiency. What would you install in your own home? Connie W., Athens, GA

DEAR CONNIE: Oh boy, your question is more dangerous than walking through a minefield. All of the foundation types you mentioned have the characteristics that are important to you, so each contractor or trade group that makes the products that goes into each foundation could argue a very strong case. You can build any of these foundations and make them very energy efficient.

This concrete foundation should last for hundreds of years, if not longer. PHOTO CREDIT: Tim Carter

I tend to look at history when faced with questions like yours. It’s important to me to see what materials last when put to the test against Mother Nature. Look at things that have lasted thousands of years and you’ll always come back to rock or masonry. Wood, even treated lumber, doesn’t fair as well as rock. Granted, you can find wood that has lasted for thousands of years, but there are almost always special conditions that have allowed it to resist decay.

While on the subject of wood, I’d personally never used treated lumber as a foundation material. About 18 years ago, I built a playset using treated lumber. Five years ago, I took it down to build a storage shed. Much to my surprise the posts that were in the ground were attacked successfully by termites. These posts were treated for below-ground burial. Perhaps some error happened at the factory such that the lumber didn’t get the correct chemical or it was mislabeled. Whatever, I don’t want to hope my foundation will last.

A residential concrete foundation is actually rock. There is an actual rock type that looks just like concrete called conglomerate. Man-made concrete is artificial rock. Mixed, poured, finished and cured correctly it can last for thousands of years. Add steel to the concrete and you get additional strength.

You may not care that the foundation lasts for even a hundred years, as you may move or otherwise vacate the house. But you want strength so that while you occupy it you’re able to live in the house never having to worry one moment about the foundation moving, cracking or failing.

You can achieve the energy efficiency you want by building an insulated concrete foundation. For over a decade you’ve been able to use foam forms that are filled with concrete. Some allow you to create ledges that allow you to install a natural stone veneer wherever the foundation is exposed above ground. I’m fairly certain these are the exact concrete foundation forms I’ll be using on my own new home.

Concrete block is technically a poured concrete foundation, as each block is made from concrete that contains small pieces of rock. If you decide to use concrete block, be sure the inner cores are filled with concrete after the block are laid. You also need vertical and horizontal steel in the concrete block to resist stresses that Mother Nature will exert against the block.

If you decide to go with concrete, you’ll need some great concrete foundation specifications. Don’t hope that your contractor will do it correctly. Remember that hope is the emotion of last resort. Never ever make a decision based on hope.

The written specifications will address the thickness of the foundation walls, the need for horizontal and vertical steel bars, the size, spacing and placement of the steel, the strength of the concrete, etc. You live where it rarely gets cold, but if you were in a cold climate, there are additional specifications that must be followed if the foundation is poured in cold weather.

The concrete foundation mix will usually contain stones, sand, Portland Cement and water. To allow the concrete to flow in the forms, it needs to be fairly plastic. This almost always means that more cement is added to the mix at the ready-mix concrete plant.

If you blend the concrete as you might for a driveway or sidewalk and then add water at the job site to get it to the proper consistency, the cured concrete may not be as strong as it should be because the Portland cement in the mix was diluted too much.

Be careful about placing dirt against the freshly poured concrete foundation. It can take at least a week under ideal curing conditions for the concrete to achieve 75 percent of its design strength. You can easily crack a new foundation if you backfill too quickly.

Don’t forget concrete foundation waterproofing if there is any chance that the space on the other side of the foundation might become finished living space. I would apply the best waterproofing compound even if the interior space was going to be just used for storage. Who wants wet and damp belongings?

Column 849