QUESTION:

I am presently studying the Purdue course on "Termites and other wood destroying pests". I have read that expansion joints are an entry way through which termites can attack a building. What is an expansion joint? How can I find them in the structure? How does one treat an expansion joint?

ANSWER:

Expansion joints are those seams you see in concrete as you walk along a sidewalk, and appropriately that seam is a cut all the way through the concrete. Like so many other materials concrete expands and contracts with changes in temperature, and if the concrete cannot expand slightly it might just crack and buckle, giving you a bigger problem. This joint may have only dirt in it or it may be filled with some kind of pliable sealant that fills it but still allows the sideways expansion to occur. Because the termites are able to burrow through this soil or the soft filler it does provide them with a way to pass through what otherwise would be impervious concrete.

Exterior slabs of concrete show the expansion joints pretty obviously. Another line you may see on concrete walkways is only a depression that goes an eighth of an inch or so into the concrete, and not all the way through. This is not a legitimate expansion joint. Instead, according to one account I read, this line is there only to allow normal cracking of the concrete upon drying to occur at this weakest, thinnest point. The crack then, hopefully, occurs as a straight line across the walkway at an intended location rather than haphazardly anywhere.

In the slab of a structure you might normally have a solid pour, and there are various kinds of slabs under structures - floating, monolithic, etc. Some may have expansion joints, particularly around the perimeters over the foundation. A Floating Earth slab is a slab poured on top of a gravel fill, with the expansion joint around the perimeter. A Supported Earth-filled Slab is similar, except it is poured on top of the foundation, but after the foundation already is in place and hardened. A Monolithic slab is one solid pour creating the slab and the foundation all at once, so no expansion joints exist around the perimeter.

Even if there is a solid slab with no expansion joints there are usually many other invading objects that go through the slab, such as pipes, drains, etc., and where these enter they may offer that break in the concrete around them that the termites can find a way through. Over time it also is normal for slabs to settle and crack, and even a thin crack could eventually become wide enough to admit the tiny worker termites. The only way to identify the presence of these expansion joints and stress cracks would be to pull back any coverings over them to see the concrete itself. This usually is carpeting, but the tough one for you would be hard flooring such as tile or wood, which you are not going to be able to destroy just to expose the slab. A basement may already have the slab exposed, easing the problem for you.

Treating expansion joints generally is done by drilling holes next to each joint and treating under the slab through those holes, just as you would treat any other location under the concrete slab. The goal would be to ensure that the soil under and around that expansion joint has been treated so that termites are forced to contact treated soil if they are attempting to access that joint.

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