Expansion joint

An expansion joint, or movement joint, is an assembly designed to hold parts together while safely absorbing temperature-induced expansion and contraction of building materials. They are commonly found between sections of buildings, bridges, sidewalks, railway tracks, piping systems, ships, and other structures.

Building faces, concrete slabs, and pipelines expand and contract due to warming and cooling from diurnal and seasonal variation, or due to other heat sources. Before expansion joint gaps were built into these structures, they would crack under the stress induced.

Bridge expansion joints are designed to allow for continuous traffic between structures while accommodating movement, shrinkage, and temperature variations on reinforced and prestressed concrete, composite, and steel structures. They stop the bridge from bending out of place in extreme conditions, and also allow enough vertical movement to permit bearing replacement without the need to dismantle the bridge expansion joint. There are various types, which can accommodate movement from 30 to 1,000 millimetres (1.2 to 39.4 in), including joints for small movement, medium movement, and large movement.

Modular expansion joints are used when the movements of a bridge exceed the capacity of a single gap joint or a finger type joint. Modular multiple-gap expansion joints can accommodate movements in all directions and rotations about every axis. They can be used for longitudinal movements of as little as 160 mm, or for very large movements of over 3000 mm. The total movement of the bridge deck is divided among a number of individual gaps which are created by horizontal surface beams. The individual gaps are sealed by watertight elastomeric profiles, and surface beam movements are regulated by an elastic control system. The drainage of the joint is via the drainage system of the bridge deck. Certain joints feature so-called sinus plates on their surface, which reduce noise from over-passing traffic by up to 80%.

Masonry control joints are also sometimes used in bridge slabs.

Clay bricks expand as they absorb heat and moisture. This places compression stress on the bricks and mortar, encouraging bulging or flaking. A joint replacing mortar with elastomeric sealant will absorb the compressive forces without damage. Concrete decking (most typically in sidewalks) can suffer similar horizontal issues, which is usually relieved by adding a wooden spacer between the slabs. The wooden expansion joint compresses as the concrete expands. Dry, rot-resistant cedar is typically used, with a row of nails protruding out that will embed into the concrete and hold the spacer in place.

Control joints, or contraction joints, are sometimes confused with expansion joints, but have a different purpose and function. Concrete and asphalt have relatively weak tensile strength, and typically form random cracks as they age, shrink, and are exposed to environmental stresses (including stresses of thermal expansion and contraction). Control joints attempt to attenuate cracking by designating lines for stress relief. They are cut into pavement at regular intervals. Cracks tend to form along the cuts, rather than in random fashion elsewhere. This is primarily an aesthetic issue; the appearance of even, regular cracking, which may be hidden in the joint's crevice, is often preferred over random cracking.

Thus, expansion joints reduce cracks, including in the overall structure, while control joints manage cracks, primarily along the visual surface.