The joints create a plane of weakness in the concrete, directing the concrete where to crack; there are several types of joints, i.e. contraction joints, construction joints and isolation joints, the most common is contraction joints that control cracks caused by restrained shrinkage, loads, and other stresses.
Concrete is not a tensile material and it does not stretch or bend without breaking.
The concrete shrinks and spreads in different parts of a building in different ways, it causes concrete cracks hence joints are rendered concrete.
Reasons for providing concrete joints:
- The concrete road requires joints to allow expansion, contraction, and warp of the road slabs due to temperature changes.
- They are sometimes required if there is a sudden stop in construction.
- The day work is not finished in the joints specified to create a proper bond between the old and new construction work of the road slab.
Types of Concrete joints:
1.Construction joint in concrete:
These are temporary joints left between two concreting operations, each stage of raised concrete is called a lift, and construction joints are joined between two consecutive elevators.
The joints ensure a strong connection between the two elevators of concrete, these joints perpendicular to the flat slab.
As slab panels are inserted at different times, they are joined by construction joints to ensure bonding and prevent leakage.
The same joints that are used vertically in slab panels can be used in a horizontal position between the layers of a gravity dam.
2.Expansion joints in concrete:
Concrete expands and contracts for several reasons, tolerance must be provided for the expansion of concrete or it will develop internal stresses.
In small buildings the expansion of slabs is negligible, but for slabs longer than 45m, expansion joints have to be provided.
Expansion joints are provided by keeping a gap between the concrete panels and subsequent sealing, i.e. dowel bars may or may not be used.
These joints are dummy joints, they are provided to avoid cracks due to plastic shrinkage and drying shrinkage.
In cold weather, contraction of concrete can cause large internal stresses to get rid of them, it is necessary to have contraction joints.
These are provided at a distance of 5 to 10 meters apart from each other.
This type of joint must be provided whenever differential movement occurs in parts of the slab, i.e. machine foundations or joints of beams and columns with slabs.
Methods of Concrete joints:
Cleaning: The old concrete surface is first cleaned thoroughly with a wire brush.
Chiseling: The old concrete surface is made rough by denting it with a chisel, for a strong, bond with the new concrete.
Use of the rich cement slurry or paste: Using rich cement paste will give additional strength to the joint as this will make the concrete surrounding the joint richer and stronger.
Splitting of reinforcement provides overlap for reinforcement: To give homogeneity to the reinforcing bars, the overlap is provided and the overlapped portion inbound tightly with high tensile steel wire.
Also read: Concrete spalling & Concrete forms
various requirement of a good Concrete joints:
- The joint should not allow the infiltration of rainwater and the entry of stone pits or protruding.
- Joints should not disturb or spread the normal level of the pavement.
- Joints should not move independently.
- Due to atmospheric variation in temperature, there should be sufficient gaps to allow free expansion or contraction.
Also read: Concrete Joint filler & Concrete finishing tools