Q: Are joints in concrete meant to accommodate expansion or contraction? Someone told me that what I've always called expansion joints I should now call contraction joints. Does this mean that concrete doesn't expand? What are these joints actually supposed to do?

A: Some of the confusion may stem from the difference between joints for interior slabs and joints for exterior slabs or pavement. Concrete shrinks when it first dries, and some of that initial shrinkage is irreversible because of changes in the concrete's internal structure that occur at the same time. Even if the concrete is saturated again later, the initial drying shrinkage isn't fully reversed. For this reason, some people in the concrete industry say that after a concrete slab has dried, it only gets larger than its initial volume when there's an abnormal expansion, such as expansion caused by reactive aggregates.

On the other hand, concrete does expand when it gets hot or when the moisture content changes. That's why pavements, bridges, and even buildings need expansion joints. Joints in exterior concrete widen during cold weather because of cooling contraction, but they get narrower during hot weather as the concrete expands. If the joints get filled with incompressible material, such as dirt or gravel during cold weather, pavement blowups can occur later when the weather gets hot and the concrete expands. To prevent that from happening, an engineer should always calculate the largest possible expansion and design the joint accordingly. This is also the reason why joints need to be filled with compressible material.