The expansion joint may have any material capable of taking compression like rubber or any other material. The bridge girders (for example) are built in sections and joined with these joints in between. So if due to high temperature the reinforced concrete tends to expand, the expansion is allowed and the forces due to these expansion and contraction is made less. once the temperature comes back to the normal the material comes back to the original position as it was earlier.
Hope you understand the concept 😀
expansion joint or movement joint is an assembly designed to safely absorb the heat-induced expansion and contraction of various construction materials
and this can cause cracking at an early age. Irregular
cracks are unsightly and difficult to maintain but generally do not affect the integrity of concrete. Joints
are simply pre-planned cracks. Joints in concrete slabs
can be created by forming, tooling, sawing, and placement of joint formers.
Some forms of joints are:
a. Contraction joints – are intended to create weakened planes in the concrete and regulate the location where cracks, resulting from dimensional
changes, will occur.
b. Isolation or expansion joints – separate or isolate
slabs from other parts of the structure, such as walls,
footings, or columns; and driveways and patios
from sidewalks, garage slabs, stairs, lightpoles and
other points of restraint. They permit independent
vertical and horizontal movement between adjoining parts of the structure and help minimize cracking when such movements are restrained.
c. Construction joints – are surfaces where two successive placements of concrete meet. They are typically placed at the end of a day’s work but may be
required when concrete placement is stopped for
longer than the initial setting time of concrete. In
slabs they may be designed to permit movement
and/or to transfer load. The location of construction joints should be planned. It may be desirable to achieve bond and continue reinforcement
through a construction joint.
WHY THIS JOINTS CONSTRUCTED?
Cracks in concrete cannot be prevented entirely, but
they can be controlled and minimized by properly designed joints. Concrete cracks because:
a. Concrete is weak in tension and, therefore, if its
natural tendency to shrink is restrained, tensile
stresses that exceed its tensile strength can develop,
resulting in cracking.
b. At early ages, before the concrete dries out, most
cracking is caused by temperature changes or by
the slight contraction that takes place as the concrete sets and hardens. Later, as the concrete dries,
it will shrink further and either additional cracks
may form or preexisting cracks may become wider.
Joints provide relief from the tensile stresses, are easy
to maintain and are less objectionable than uncontrolled or irregular cracks.
A construction joint occurs when there are multiple concrete placements. A control joint or contraction joint is a joint that is put in the concrete to control cracking. For example, when they sawcut joints into the concrete pavement, these are control joints. These are necessary, because we know the concrete will crack. We just need to try to control where it cracks. These are called contraction joints, because concrete tends to contract when it is curing. Some people refer to these as expansion joints, but that is not technically correct.