Tuesday, March 19, 2013

Expansion and contraction joints in Rigid Pavements.


I have studied about the expansion and contraction joints. In the rigid pavements, like the cement concrete pavements, the cement slab is laid on the sub-grade. The concrete changes its volume with the change in the temperature.

If the natural temperature of the concrete is low i.e. concrete is laid down in the season of winter, then the chances are there that concrete will undergo expansion in the summer season.
Expansion joint

Similarly, if the concrete is laid in the summer season then natural temperature of the concrete is already high, so the concrete will contract in the season of the winter.

Whether the concrete is going to expand or shrink, one has to provide the expansion or the contraction joints respectively.

  • Spacing of the expansion joints: The spacing of the expansion joint can be determined as below:
            (expansion) e = co-efficient of thermal expansion * Ls * (t2-t1)

we provide width of the joints double then 'e', 50% allowance is provided for the filler material. 

  • Spacing of the contraction joints: The contraction of the concrete is opposed by the frictional force between the sub-grade and the concrete.

This frictional force should always be less than the ultimate strength of the concrete which resists the tensile forces developed in it due the contraction force and the frictional force.
Contraction joints on rigid pavements

Frictional force is dependent on the surface area of the concrete(L*B), and the resisting force depends upon the (h*B) of the concrete, so we can get the minimum spacing required if we equate the two forces.

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