Page 76 - Lecture Notes DCC3113
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Contraction joints
Figure 5-2: Contraction joint detail
Contraction occurs when water is lost or temperatures drop. Contraction joints allow induced
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stresses to be released by permitting the adjacent slab to contract, thereby causing a reduction in tensile
stresses within the slab. The joint reduces the thickness of the concrete slab, inducing a concentration of
stress and subsequent cracking at the chosen appropriate location. The reduction in thickness is usually
achieved by cutting a groove in the surface of the slab, causing a reduction in depth of approximately
30%. A dowel bar placed in the middle of the joint delivers the requisite vertical shear strength across it
and provides load-transfer capabilities. It also keeps adjacent concrete surfaces level during temperature
induced movements. In order to ensure full longitudinal movement, the bar is debonded on one side of
the contraction joint.
Expansion joints
Figure 5-3: Expansion joint detail
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Expansion occurs when water is absorbed or the temperature rises. Expansion joints differ in
that a full discontinuity exists between the two sides, with a compressible filler material included to
permit the adjacent concrete to expand.
Warping joints
Figure 5-4: Warping joint detail
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