Control joint vs Expansion joint | Why Joints are Provided in Construction?

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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

Control joint vs Expansion joint | Why Joints are Provided in Construction?
Fig:- Expansion joint arrangement and instalment process in bridge

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 joint vs Expansion joint | Why Joints are Provided in Construction?
Fig:-Expansion joint along with dowel bars

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.

Fig: Control joint formed by the method of grooving in a fresh state

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.

Control joint vs Expansion joint | Why Joints are Provided in Construction?
Fig: Control joint along with desired crack formation after hardening of concrete

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

ParametersExpansion jointControl joint
PurposeProvide to decrease effects in structural movement due to expansion and contraction by temperature or other cause Reduction in deformation and crackProvided to decrease effects due to unexpected crack due to shrinkage by laying out in predefined path   Only control over the excessive crack
Another nameAlso called Movement joint as it allows free movement of structural unitsAlso called contraction joint as it decreases the cracks due to contraction and shrinkage
SpacingMust be needed to be provided if the span of structure is greater than 45m. Provided between two separately functioning units of structureSpacing shouldn’t exceed two to three times (in feet) of the thickness of the slab (in inches)
Depth of jointDepth of joints is throughout the slab or structureDepth is generally from 25mm to 1/4th of the thickness of the slab.
LocationGeneral located at the end of the two separate structuresLocated at the intermediate place of the slab or the structure
Edge protectionEdge of joints are generally protected with angle armored sectionJoin depth is small and usually in grooved form so no edge protection is provided
Filler materialJoint are needed to be sealed with the plastic and elastomers joint fillerNot necessarily to be filled with the joint sealant unless required for aesthetic beauty and dust control
UsesGenerally adopted for bridges, railway tracks, building with large spans, rigid pavementAdopted for the surface of concrete and bitumen, Masonry construction
Degree of supervisionA 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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