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
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, 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.
Masonry
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 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.Comparison to control joints
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. 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.
Roadway control joints may be sealed with hot tar, cold sealant, or compression sealant. Mortar with a breakaway bond may be used to fill some control joints.
Control joints must have adequate depth and not exceed maximum spacing for them to be effective. Typical specifications for a four-inch-thick slab are:
- 25% depth of material
- Spacing at 24× to 36× of slab depth
- Special care for inside corners
Tile and stone flooring movement joints
Railway expansion joints
If a railway track runs over a bridge which has expansion joints that move more than a few millimeters, the track must be able to compensate this longer expansion or contraction. On the other hand, the track must always provide a continuous surface for the wheels traveling over it. These conflicting requirements are served by special expansion joints, where two rails glide along with each other at a very acute angle during expansion or contraction. They are typically seen near one or both ends of large steel bridges. Such an expansion joint looks somewhat like the tongue of a railroad switch, but with a different purpose and operation.Ducted air systems
Expansion joints are required in large ducted air systems to allow fixed pieces of piping to be largely free of stress as thermal expansion occurs. Bends in elbows can also accommodate this. Expansion joints also isolate pieces of equipment such as fans from the rigid ductwork, thereby reducing vibration to the ductwork as well as allowing the fan to grow as it comes up to the operating air system temperature without placing stress on the fan or the fixed portions of ductwork.An expansion joint is designed to allow deflection in the axial, lateral, or angular deflections. Expansion joints can be non-metallic or metallic. Non-metallic can be a single ply of rubberized material or a composite made of multiple layers of heat and erosion resistant flexible material. Typical layers are: outer cover to act a gas seal, a corrosion-resistant material such as Teflon, a layer of fiberglass to act as an insulator and to add durability, several layers of insulation to ensure that the heat transfer from the flue gas is reduced to the required temperature and an inside layer.
A bellows is made up of a series of one or more convolutions of metal to allow the axial, lateral, or angular deflection.
Pipe expansion joints
Pipe expansion joints are necessary in systems that convey high temperature substances such as steam or exhaust gases, or to absorb movement and vibration. A typical joint is a metal bellows, plastic, fabric or an elastomer such as rubber. A bellows is made up of a series of convolutions, with the shape of the convolution designed to withstand the internal pressures of the pipe, but flexible enough to accept axial, lateral, and angular deflections. Expansion joints are also designed for other criteria, such as noise absorption, anti-vibration, earthquake movement, and building settlement. Metal expansion joints have to be designed according to rules laid out by the Expansion Joint Manufacturers Association, for fabric expansion joints there are guidelines and a state-of-the-art description by the Quality Association for Fabric Expansion Joints. Pipe expansion joints are also known as compensators, as they compensate for the thermal movement.Pressure balanced expansion joints
Expansion joints are often included in industrial piping systems to accommodate movement due to thermal and mechanical changes in the system. When the process requires large changes in temperature, metal components change size. Expansion joints with metal bellows are designed to accommodate certain movements while minimizing the transfer of forces to sensitive components in the system.Pressure created by pumps or gravity is used to move fluids through the piping system. Fluids under pressure occupy the volume of their container. The unique concept of pressure balanced expansion joints is they are designed to maintain a constant volume by having balancing bellows compensate for volume changes in the bellows which is moved by the pipe. An early name for these devices was pressure-volumetric compensator.