The expansion joint was a gap left on purpose. A line cut into the concrete or a strip of flexible material set between two slabs. The joint let the concrete move. Concrete expanded in the heat and contracted in the cold and the expansion joint gave the concrete room to grow and shrink without cracking. The joint was planned failure. The builder decided where the concrete would break instead of letting the concrete decide. The concrete was going to move. The joint said move here.

The control joint was a groove cut into the top of a fresh slab with a jointing tool. The groove was a quarter of the slab thickness deep. The groove created a weak line. When the concrete shrank the crack formed at the groove instead of wandering across the surface in a jagged line. The crack was hidden inside the groove. The crack was still there but nobody could see it. The control joint did not prevent cracking. The control joint controlled cracking. The difference was everything. You could not stop concrete from cracking any more than you could stop wood from warping. You could only decide where.

The isolation joint was different. A strip of compressible material placed between the slab and the wall or the slab and a column. The strip kept the slab from touching the structure. The slab and the structure moved at different rates. The slab moved with the ground. The structure moved with the load. If the slab touched the structure the two movements fought and the weaker one broke. The isolation joint was the divorce. The slab and the structure stayed married to the building but they lived in separate rooms and the rooms had a gap between them and the gap kept the peace.

Every sidewalk square has expansion joints. The lines between the squares. Children step over the lines. Step on a crack break your mother's back. The children knew the joints were important even if they did not know why. The joints were the plan for failure built into the surface of every walkable thing. The city is a grid of planned failure. Every block. Every sidewalk. Every bridge deck. The joints are everywhere and nobody sees them until the joint fails and the slab buckles and the sidewalk heaves and then everybody sees the joint and wonders why it was not wider.

See also: Screed, Caulking