This article is primarily concerned with the joints that are needed to allow the concrete structure to function properly. It also gives some instruction about installing various embedded items that may be required: sleeves, anchors and other inserts, waterstops, heating ducts, radiant heating systems and snow melting systems.


Residential concrete may have any or all of three types of joints: isolation joints, contraction joints and construction joints. Isolation joints are used to prevent or minimize cracking. Contraction joints are used to control the locations of the inevitable cracks. Construction joints are used to facilitate the construction process.


Isolation joints should extend completely through the slab. Isolation joints should be made to follow the lines shown on the plans and should be carefully installed so that the joint is a close to vertical as possible. Isolation joints are normally filled with preformed joint fillers made of wood, cork, bituminous-impregnated vegetable or mineral fiberboards, solid or cellular rubber, and expanded plastic foams. In a continuous floor slab on ground the contraction joints should be spaced not more than 15 feet apart in both directions in order to prevent intermediate cracks.


Wherever a wall meets a slab, an isolation joint is needed. An isolation joint is also needed wherever a partition wall meets a loadbearing wall. ACI Committee 332 recommends using contraction joints in walls to eliminate random shrinkage cracking. Contraction joints in walls are made by attaching strips of wood, metal or plastic to the inside faces of the formwork. It's rarely necessary to use vertical construction joints in one- or two-family houses.