Concrete is not an elastic substance, and therefore it does not bend or stretch
without failure. However, concrete moves during expansion and shrinkage, due to
which the structural elements shift slightly.
Bridge expansion joints are structural components designed to provide smooth
passage over the gap between adjacent sides of a deck joint, while also permitting
relative translation and/or rotation of the two sides of a bridge deck
(deck movement joint).
A deck movement joint is generally defined as a structural discontinuity between
two elements, at least one of which is a deck element, and is designed to allow
relative translations and rotations due to: thermal expansion and contraction of
the superstructure, concrete shrinkage or creep, elastic shortening due to pre-stressing,
displacement of the structure under load, or any other action the desk is subjected to.