Two contraction joints and one expansion joints are usually adopted for drainage channels. Why?

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In the life cycle of a concrete structure (not prestressed concrete), it will generally undergo the following process of contraction and expansion:

Contraction:
(a) Early thermal movement
(b) Seasonal contraction owing to drop in temperature
(c) Shrinkage

Expansion:
(a) Seasonal expansion owing to drop in temperature

The order of magnitude for items (a) to (c) is more or less the same. Hence, qualitatively speaking, for a given length of concrete structure, the number of contraction joints should be more than the number of expansion joints and they are roughly in the order of 3:1 to 2:1 based on the number of expansion and contraction process above. Of course, the actual spacing and number of contraction joints and expansion joints should be determined case by case.

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