Q: Do isolation joints need to be cut completely through a slab? For instance, will a 2-inch-deep cut allow vertical movement in a 5-inch-thick slab?

solation joints are used to permit both horizontal and vertical movement at adjoining parts of a structure. To permit vertical movement, they should pass all the way through the concrete cross section. Otherwise, aggregate interlock would restrict vertical movement.Rather than being sawed, isolation joints are often formed with isolation-joint material such as premolded fiber strips that extend the full depth of the slab. Columns are often isolated from floor slabs with circular or diamond-shaped box outs.

Reader Response:

Your reply is for the most part correct. But it contributes to the general confusion caused by the fuzzy nomenclature related to joints. Isolation joints must be formed or possibly cast around a full-thickness blockout. They can't be cut or tooled to achieve a reliable and complete separation between concrete elements. The response should have been more firm on this point.Grant T. Halvorsen

Structural Engineering and Concrete Materials

Wheaton, Ill.