Cement & Concrete Basics FAQs


Joints: What type are used and where?

Isolation/Expansion Joints: Isolation joints are used to relieve flexural stresses due to vertical movement of slab-on-grade applications that adjoin fixed foundation elements such as columns, building or machinery foundations, etc. Expansion joints are used primarily to relieve stress due to confinement of a slab. If the slab is placed adjacent to structures on more than one face of the slab an expansion joint should be placed to relieve stress. For example, if a slab were placed between two buildings, an expansion joint should be placed adjacent to the face of at least one of the buildings. Confinement on three faces would normally be handled by placing expansion joint on all three faces, and confinement on four faces should be isolated on all faces. This allows for thermal expansion and contraction without inducing stress into the system.

Contraction (control) joints are placed to control random cracking. Contraction joints should be placed at two times the slab thickness in feet for a maximum aggregate size of less than ¾ inch.

For example for a 5 inch slab with a ¾ inch coarse aggregate the maximum joint spacing would be 10 feet. When the maximum coarse aggregate size is greater than ¾ inch the spacing could be increased to 2 ½ inches times the thickness. For the prior example this would increase to 13 feet.

Applications that require thick slabs of eight inches or more and good load transfer across joints, due to heavy loading, should be limited to 15 foot contraction joint spacing to ensure aggregate interlock.

Construction joints are stopping places in the process of construction. Construction-joint types (a) and (b) are also used as contraction joints.

For more information see Concrete Floors on Ground (EB075).