Various Joints in Concrete Construction
Cracks in concrete can’t be prevented completely, but they can be prohibited or minimized by properly designed joints
Why are Joints Constructed?
Cracks in concrete can’t be prevented completely, but they can be prohibited or minimized by properly designed joints. Concrete cracks are caused because:
- The Concrete is weak in tension, if its natural tendency to shrink is restrained, in such a way that tensile stresses exceed the tensile strength it can develop, it will result in cracking.
- At early stages, cracking is caused by temperature changes or by slight contractions that take place as the concrete sets and hardens. As the concrete dries, it shrinks further developing additional cracks or widening preexisting cracks.
HENCE, Joints provide relief from the tensile stresses. Joints are easy to maintain and are less detrimental than uncontrolled or uneven cracks.
Concrete expands & shrinks with variations in moisture and temp. The overall affinity is to shrink and this can cause cracking at an early age. Uneven cracks are unpleasant and difficult to maintain but usually do not affect the integrity of concrete. The Joints are pre-planned cracks. Joints in concrete slabs can be created by forming, sawing, tooling, & placement of joint formers. Some forms of joints are:
Contraction joints are intended to create weak planes in the concrete and to regulate the locations where cracks, as a resulting of dimensional changes, can occur.
Isolation or expansion joints are provided to separate or isolate slabs from other parts of the building, such as walls & footings or
columns; also driveways and patios from sidewalk, garage slabs, light poles or other points of restraint. Joints permit free vertical and horizontal movement between adjacent parts of the structure and help reduce cracking when these movements are restrained.
Construction joints are provided at surfaces where two successive placements of concrete occur. They are typically placed at the end of a day’s work or when concrete placement is stopped for longer time than the initial setting time of concrete. For slabs they may be designed to allow movement and/or to transfer load. The locations of construction joints should be predetermined. This is desirable to achieve bond and continue reinforcement through joint.