A contractile joint is one in which two solid surfaces are free to move away from each other as a result of shrinkage or thermal movement. Relative motion is prevented in the plane of the joint.
As concrete hardens and dries, it shrinks. Unless this shrinkage is uncontrolled, it creates tensile stresses in the concrete that can cause it to crack.
While reinforcement will resist these tensile stresses and help prevent the formation of large cracks, it does not completely prevent cracking. This only 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 a concern when viewed from a normal viewing distance.
On the other hand, non-reinforced concrete tends to develop somewhat larger cracks at more irregular intervals; Wherever the tensile strength of concrete exceeds the shrinkage stress.
To prevent such cracks, contraction joints should be installed at appropriate intervals. It may be advisable to install contraction joints in reinforced concrete rather than relying solely on reinforcement to control the shrinkage stresses.
Massive concrete or very large members may also require contraction joints, to allow for shrinkage or volume reduction that occurs when concrete cools or loses temperature after it has been laid.
The location of contraction joints is to be decided by the designer or the supervising engineer. For example, their location will often be defined on drawings for sidewalks, industrial floors and similar applications, while in other cases they will be in a regular pattern or be an integral part of architectural features.
Typically they will be located where the greatest concentration of tensile stress should be expected to result in shrinkage:
- at a sudden change 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 concrete into bays. Ideally, these should be roughly square. They may also be necessary in tall walls, especially where an unplanned crack would be undesirable.
Contraction joints create a vantage point at which to stop solid work at the end of the day.
Construction joints should never be built in the middle of a bay.
Contraction joints are constructed by creating a vertical plane of weakness in the slab or wall. Due to shrinkage at this point movement is allowed to adjust it. On the other hand, it is usually necessary to prevent motion in other directions, i.e. in directions parallel to the plane of the joint. Figure 1, These twin requirements have the following consequences:
- The bond between adjacent solid surfaces at the joint must be broken.
- Reinforcement is finished on both sides of the joint.
- Dowel bars if used should be unfastened on one side of the joint.
a The control joint is a form of contraction joint. Which is formed by creating a plane of weakness in a vertical or horizontal member. As the concrete shrinks, the tensile stress is concentrated on this plane, causing the concrete to crack there instead of elsewhere.
Generally, mechanical interlocks in the two faces of the joint are expected to prevent other motion in the joint.
Therefore, control joints are a relatively simple alternative to a fully formed contractile joint. They are placed wherever a formed joint would have been placed and are widely used in non-reinforced floors and sidewalks. The combined distance in these applications is 1 m for narrow walkways and driveways, up to 5 m for street pavements.
Control joints can be created at any of three stages during construction, namely:
- A premolded strip may be inserted into the concrete as it is being laid, to create a plane of weakness. Metal strips inserted into terrazzo or plastic strips inserted into concrete pavements to form the centerline of the pavement are examples.
- A joint can be made in the concrete surface with a suitable jointer or grooving tool. When hardening, the concrete cracks at this point, forming a joint.
- After the edges of the concrete have hardened enough to prevent crumbling, a jigsaw joint can be made. The joint should be made as soon as possible and shrinkage should begin before it dries. Delay may result in unplanned cracking of pavement. The saw joint is filled with a joint sealant to prevent dirt and other debris from entering line drawing number 2 Because unsealed joints get clogged with dirt and become ineffective.