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Contraction Joints in Elevated Slabs

ASCC Position Statement #23

ACI 318 “Building Code Requirements for Structural Concrete,” ANSI/ASCE 3 “Standard for the Structural Design of Composite Slabs,” and SDI #30 “Design Manual for Composite Decks, Form Decks, and Roof Decks” do not require the use of contraction joints for elevated structural slabs. Occasionally, however,
project specifications require contractors to saw cut contraction joints in elevated slabs. 

Contraction joints are used in slabs-on-ground to create weakened planes that limit the frequency and width of random cracks caused by volume changes due to restrained drying shrinkage, thermal contraction, or both. Elevated slabs usually consist of a structurally reinforced concrete slab, a concrete slab composite with a steel deck, or a concrete slab on a steel deck. All three types of slabs may also be exposed to restrained drying shrinkage and thermal contraction that can cause cracks. Saw cutting an elevated slab, however, doesn’t create the same weakened plane that it does in a slab-on-ground. The reinforcing steel and steel deck, either composite or noncomposite, provide more restraint than the subgrade for a slab-on-ground. Therefore, cracks typically occur between the joints cut in the elevated slab.

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