A contraction joint is one in which the two concrete surfaces are free to move away from one another as a result of shrinkage or thermal movement. Relative movement in the plane of the joint is prevented.
Why to Provide Contraction Joint?
As concrete hardens and dries out, it shrinks. Unless this shrinkage is unrestrained, it creates tensile stresses in the concrete which may cause it to crack.
Whilst reinforcement will resist these tensile stresses and help prevent the formation of large cracks, it does not completely prevent cracking. It merely ensures that the cracks, as they occur, are more closely spaced and of smaller width. In properly designed reinforced concrete, they will not be obvious or of concern when seen from normal viewing distances.
Unreinforced concrete, on the other hand, will tend to develop somewhat larger cracks at more irregular intervals; wherever the tensile strength of the concrete is exceeded by the shrinkage stresses.
To prevent such cracks, contraction joints must be installed at appropriate intervals. It may also be advisable to install contraction joints in reinforced concrete rather than relying solely on reinforcement to control shrinkage stresses.
Contraction joints may also be required in mass concrete or very large members, to allow for the shrinkage or reduction in volume which occurs as concrete cools or loses temperature after it has been placed.
How to Locate Contraction Joint?
The location of contraction joints is a matter for the designer or supervising engineer to decide. For example, their location will often be defined on the drawings for pavements, industrial floors and similar applications, while in other cases they will be in a regular pattern or be an integral part of the architectural features.
Generally they will be situated where the greatest concentration of tensile stresses resulting from shrinkage are to be expected:
At abrupt changes of cross-section; and
In long walls, slabs.
Contraction joints are most common in large areas of concrete pavement where they are used to divide the concrete into bays. Ideally, these should be approximately square. They may also be necessary in long walls, particularly where an unplanned crack would be undesirable.
Contraction joints form a convenient point at which to stop concrete work at the end of the day.
Construction joints should never be formed in the middle of a bay.
Contraction joints are formed by creating a vertical plane of weakness in the slab or wall. Movement is allowed at this point to accommodate that due to shrinkage. On the other hand, it is usually necessary to prevent movement in other directions, i.e. in directions parallel to the plane of the joint Fig-1. These twin requirements have the following consequences:
The bond between abutting concrete surfaces in the joint must be broken.
Reinforcement is terminated on both sides of the joint.
Dowel bars if used must be unbonded on one side of the joint.
A control joint is a form of contraction joint which is formed by building a plane of weakness into either a vertical or horizontal member. As the concrete shrinks, tensile stress is concentrated on this plane causing the concrete to crack there rather than elsewhere.
Normally, mechanical interlock across the two faces of the joint is expected to prevent other movement in the joint.
Control joints are, therefore, a relatively simple alternative to a fully formed contraction joint. They are placed wherever a formed joint would have been placed and are most widely used in unreinforced floors and pavements. Joint spacing in these applications, range from 1 m for thin pedestrian pathways and driveways to, say, 5 m for road pavements.
Control joints can be made at any one of three stages during construction, viz:
A premoulded strip may be inserted into the concrete, as it is being placed, to create a plane of weakness. Metal strips inserted into terrazzo or preformed plastic strips inserted into concrete pavements to form the centre line of the pavement are examples.
A joint can be formed in the surface of the concrete with a suitable jointing or grooving tool. Upon hardening, the concrete cracks at this point, creating a joint.
After the concrete has hardened sufficiently to prevent ravelling of the edges, a sawn joint may be formed. The joint should be made as early as possible and prior to drying shrinkage starting to occur. Delay can result in unplanned cracking of the pavement. The sawn joint is then filled with a joint sealant to prevent dirt and other debris entering it Fig-2 as unsealed joints tend to fill with dirt and become ineffective.