Q.: I attended a seminar at which a speaker said that concrete doesn't expand. I told him I've always read that concrete expands when the temperature rises or when the moisture content increases. The speaker held his ground, however, and said that concrete doesn't expand. Is my literature outdated?

A.: When it first dries, concrete shrinks and undergoes structural alterations that make some of the shrinkage irreversible. Thus, even if it is later resaturated, the initial drying shrinkage isn't fully recovered. Because of this, some concrete-industry people say that once a concrete slab has dried, it never gets larger than its initial volume unless there's an abnormal expansion such as that caused by reactive aggregates. That may have been the point the speaker was trying to make.

However, concrete does indeed expand when it gets hot or when the moisture content changes. That's why you need expansion joints in bridges, buildings, and other structures. In exterior concrete, joints widen during cold weather because of cooling contraction and get narrower during hot weather as the concrete expands. If the joints fill with incompressible material during the winter, concrete expansion during the summer can cause pavement blowups.