Frequently Asked Questions
Cement & Concrete
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Q: Joints: What Type are Used and Where?
 A:
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
2 times the slab thickness in feet for a maximum aggregate size
of less than ¾”.
For example for a 5” slab with a ¾” coarse aggregate the maximum
joint spacing would be 10’. When the maximum coarse
aggregate size is greater than ¾” the spacing could be increased
to 2 ½” times the thickness. For the prior example this would
increase to 13’.
Applications that require thick slabs of 8" or more and good
load transfer across joints, due to heavy loading, should be limited
to 15' 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).
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