Why Joints are Provided in Construction?
Before explaining the Control joint vs Expansion joint, let us know why different joints are provided. Expansion joints in the concrete structures are provided to prevent crack and deformation due to (thermal) expansion (and contraction) of structural members over a long period of time.
The allows free movement of structural on expansion or loading avoiding deformation of structures, thus are also known as Movement joint.
Expansion Joints
The expansion joint typically consists of the predefined separating space that is designed with help of probable extreme (worse case of expansion and contraction that may occur) conditions cause in the entire life span.
Such space is either leave freely or filled with plastic and elastomers materials like Neoprene, bitumen pastes etc. popularly known as joint fillers.
These joints are required to be provided with joint edge protection which is generally done by an armoured angle section reinforced properly with concrete. The joint is completely throughout the slab or structure.
They are generally provided between two differently functioning structural units like deck and approach slab of bridge, Abutment and wing walls etc. Such expansion joint is necessarily needed to be implemented over the structure having a span greater than 45m.
These joints are generally used in bridges, railway tracks, and buildings having an extremely large area and span.
These joints are unstable in nature, i.e. there is horizontal movement due to continuous expansion or contraction or temperature change or horizontal and vertical movement due to inappropriate load transfer.
In rigid pavement, the longitudinal bays are connected with dowel bars and transverse adjacent bays with tie bars along with the joint space (separated by plywood) to allow movement along with load transfer while dynamic load (traffic load) is allowed over them.
Control joints
Control joints are joints provided in concrete to prevent unnecessary cracks in predefined paths or patterns so that structural efficiency doesn’t get affected.
These joints are also known as contraction joints as the joint helps to decrease the cracks due to contraction and shrinkage of concrete. These joints are generally formed after drying of concrete with the help of cutting tools or during a plastic state of concrete placement with the help of grooved tools with a period of 8-24 hrs of placement.
The joints are not much deeper but treated superficially with a depth of 25mm to 1/4th of slab depth. These joints are generally made in intermediate space of concretes flat surface and the section with sudden dimension changes.
These joints generally don’t require any type of joint filler; however, they can be filled with joint sealant for aesthetic purposes and dust control.
These joints are spaced at regular intervals not exceeding two to three times the thickness of the slab where spacing is considered in feet and thickness in inches.
For example, for the 5-inch thick slab, the control joint spacing should be limited up to 10ft or 15 ft. The joint is expected to be located in a high tensional zone that may cause shrinkage. Such joints are needed to be provided in drawing during construction.
There is often confusion over the difference between expansion joint and control joint for general people. However, they both are very different and have each specific characteristic and purpose. The major difference can be pointed out in the table below.
Control joint vs Expansion joint
| Parameters | Expansion joint | Control joint |
|---|---|---|
| Purpose | Provide to decrease effects in structural movement due to expansion and contraction by temperature or other cause Reduction in deformation and crack | Provided to decrease effects due to unexpected crack due to shrinkage by laying out in predefined path Only control over the excessive crack |
| Another name | Also called Movement joint as it allows free movement of structural units | Also called contraction joint as it decreases the cracks due to contraction and shrinkage |
| Spacing | Must be needed to be provided if the span of structure is greater than 45m. Provided between two separately functioning units of structure | Spacing shouldn’t exceed two to three times (in feet) of the thickness of the slab (in inches) |
| Depth of joint | Depth of joints is throughout the slab or structure | Depth is generally from 25mm to 1/4th of the thickness of the slab. |
| Location | General located at the end of the two separate structures | Located at the intermediate place of the slab or the structure |
| Edge protection | Edge of joints are generally protected with angle armored section | Join depth is small and usually in grooved form so no edge protection is provided |
| Filler material | Joint are needed to be sealed with the plastic and elastomers joint filler | Not necessarily to be filled with the joint sealant unless required for aesthetic beauty and dust control |
| Uses | Generally adopted for bridges, railway tracks, building with large spans, rigid pavement | Adopted for the surface of concrete and bitumen, Masonry construction |
| Degree of supervision | A high degree of supervision under an experienced Engineer is needed. The joint arrangements are done along with Autolevel or T.S. | The only general layout of the designed joint and manpower who can study it and maintain work are sufficient. |
I hope this article on “Control joint vs Expansion joint” remains helpful for you.
Happy Learning – Civil Concept
Contributed by,
Civil Engineer – Rajan Shrestha
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