Slabs at isolation joints must be free to move horizontally but not vertically. True or false.

• A. True
• B. False
• C. Sometimes
• D. Not specified

Answer: (B)

Slab isolation joints are used to relieve restraint between concrete elements so they can move independently in response to temperature, shrinkage, and loading. That movement isn’t limited to sliding side-to-side; the joint must be able to accommodate vertical movement as well, such as differential settlement or deflection under load. If vertical movement is restrained, stresses build up and cracking, spalling, or joint damage can occur. That’s why a properly designed isolation joint uses a flexible filler or backer and sealant that allows movement in multiple directions, not just horizontally. Consequently, the statement is false.