Wear work gloves and eye protection before beginning the repair because you must remove damaged particles that could fly up and cause injury.
Kneel down on the pool deck and clean out the damaged expansion joint by scraping it out using a slotted screwdriver. To remove material that does not detach easily, tap the head of the screwdriver with a hammer and employ a chiseling action.
Sweep the expansion joint with a broom to remove all particles from it that could interfere with the patch. Sweep the debris away from the pool so it doesn't get in the water.
Open a tub of concrete expansion joint repair and scoop up a trowel full. Place the material into the expansion joint. Continue filling the expansion joint with patching material until filling the entire gap and it mounds up by half an inch.
Turn a 2-by-4 inch board on its side and place it on top of the patch material in the location of the expansion joint. Hit the top of the board firmly with a hammer to pack the material tightly into the joint.