Concrete is a good material for construction of structures such as buildings, roads and bridges. However for a number of reasons, it is necessary to create spaces called joints within the concrete surface. so as to protect it. Joints are a special measure of protecting concrete against cracks. There a number of joints that can be created in concrete as discussed below.
Types of concrete joints and their applications
1. Contraction joints
Contraction joints help to provide planes of weakness where cracks induced by drying shrinkage and temperature can occur. They are commonly used in walls and on-grade slabs. The contraction joints can either be full or partial contraction joints.
Full contractions have fully discontinuous reinforcement, and the reinforcement is normally stopped at about 50 mm from the joint. Full contraction joints are ideal for most buildings.
Partial contraction joints are normally used for liquid containment structures , and are constructed with a maximum of 50% of the reinforcement passing through them.
Contraction joints also need to have waterstops to provide for watertightness. The recommended spacing of the joints is in a range of 5 to 9 m.
2. Expansion joints
Once a structure is restrained which is the case for most buildings, temperature induces volume changes which cause stresses in the structure and eventually cracks. To limit on these stresses, expansion joints are introduced.
Expansion joints need to be sufficiently wide to avoid parts of the building on either side of the joint from making contact. The width of the joints ranges from 25 to 150 mm. The joints can also be wider incase of need to account for differential movement of the building due to either earthquake or settlement.
Expansion joints can be either filled or empty. For fire rated structures, joint fillers are needed. The spacing of the joints also depends on the amount of movement and stresses allowable as well as the capacity of the structural members. The spacing of expansion joints can range from 10 to 120 mm.
3. Construction joints
Construction joints are created as a result of the concreting process. This is because it is not practical to cast concrete in a continuous process except for very small structures.
To form a construction joint, the percentage of reinforcement through the joint can be limited to create a plane of weakness. It is ideal for construction joints to coincide with either expansion or contraction joints.
Since construction joints are critical, they need to be located by the structural engineer and placed on the design drawings with considerations for adequate load transfer and aesthetics.
4. Isolation joints
Isolation joints simply isolate one part of a structure from another. They are commonly used to isolate concrete slabs from either walls or columns. They are required because different structural elements have varying displacements as a result of shrinkage or expansion.
In general, concrete joints can be made either as formed or sawed. Formed joints to created using joint formers which can be made of rigid or flexible materials such as plastic T-shape strips. On the other hand, sawed joints are cut as grooves into a structural member.
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