Question: I was told that what we used to call expansion joints I should now call contraction joints. Does that mean that concrete doesn't expand?
Answer: It may be that part of the confusion is between joints for interior slabs and joints for exterior slabs or pavement. When concrete first dries, it shrinks but at the same time it is changing its internal structure so that some of that shrinkage is irreversible. So, even if the concrete is later resaturated, the initial drying shrinkage isn't fully reversed. 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. But, concrete does indeed expand when it gets hot or when the moisture content changes. That's why with pavements or bridges—and even buildings—you do indeed need expansion joints. In exterior concrete, joints widen during cold weather because of cooling contraction and get narrower during hot weather as the concrete expands. During the cold period, if the joints get filled with incompressible material (such as dirt or gravel) when it gets hot and the concrete expands, you can get pavement blowups. This is why an engineer should calculate the largest possible expansion and design the joint accordingly, and also why the joint needs to be filled with a compressible material.