Concrete joints are used to compensate when concrete expands or shrinks with changes in temperature. Concrete joints are normally used to prevent cracks when the concrete shrinks, by creating forming, tooling, sawing and placing joint formers. The pre-planned cracks will provide a better finish concrete product and will be formed in specific locations where those cracks could be monitored.
1. Expansion Joints
These are structural separation between building elements that allow independent movement without damage to the assembly. Expansion joints are designed to safely absorb the thermal expansion and contraction of various construction materials, to absorb vibration. They are commonly provided in bridges, railway tracks, piping systems, and other structures.
2. Contraction Joints or Control Joints
Control Joints (often confused with expansion joints) are cuts or grooves made in concrete or asphalt at regular intervals. These joints are made at locations where there are chances of cracks or where the concentration of stresses are expected, so that when a concrete does crack, the location will be known to you. In such a way a concrete will not crack randomly but in a straight line (i.e. control joint). In other words Contraction or Control Joints are Pre-Planned Cracks. The cracks may be due to temperature variations or drying shrinkage or other reasons.
Joints depth should be 25% of the depth of the slab. For instance a 4″ thick slab should have 1″ deep cut. Joints Interval (taken in feet) should not be more than 2 – 3 times the slab thickness (in inches). Let say a 6″ slab should have joints 2 x 6=12 to 3 x 6 = 18 feet apart. For fresh concrete grooving tools are used while saw is used for hardened concrete.
3. Construction Joints
In mega projects there is a starting and stopping points, the entire concrete work may not be done at once, hence concrete pouring need to be stopped causing a joint in element known as Construction Joint.Construction joints are placed at points of ending and beginning of construction for provision of a smooth transition between pours. These joints are formed between successive building element parts during construction work, in which one part is allowed to harden before the next is placed. These joints may be intentional or unintentional. Reasons for intentionally providing construction joints are;
Certain time of a day i.e. Labour Hours ( e.g. 8:00 am to 6:00 pm)
Certain day of a week (e.g. Sunday, or Friday)
Certain Months of an year (e.g. extreme weather in Winter or Summer)
Religious Holidays etc (e.g. Eid or Christmas etc)
Unintentional provision may occur due to
Unexpected shortage of material
Concrete Joint Tips
The following recommended tips should be observed:
- Maximum Joint Spacing should be 24 to 36 times the thickness of the slab
- Joints should be spaced about 10 feet and a maximum of 15 feet
- When using joint groove for contraction joints, the joint should be a minimum depth of ¼ thickness of the slab
- Saw-cut joints should be done within 4 to 12 hours after the concrete has been finished
- Keyed joints are not recommended for industrial floors
- Dry-cut joints should be made between 1 to 4 hours after completion of finishing
- Premolded joint filler should be used to separate slabs from building walls or footings. Place 2 inches of sand over the top of a footing to prevent bond to the footing.
- If the slab contains wire mesh it is recommended to discontinue the mesh across contraction joints
- Metal dowels should be used in slabs that carry heavy loads
- Plan exact locations of all joints including timing of saw cutting
- Use isolation joints between slabs and columns, walls and footings and where curbs or sidewalks meet other concrete structures