Joints in construction are the separation or discontinuity created in concrete or sometimes steel structures in order to neglect the effect of contraction, expansion, movement, and settlement in structure with a change in the external environment.
These joints are left hollow or filled with resilient and flexible materials like wood, elastomers, fiber, rubber, plastic, bitumen asphalt mix, etc. Joint can be observed in between wall and column, slab and beam, long walls, concrete floor, and rigid pavement, Bridges, railway tracks.
But it must be noted that such joints are not made randomly, despite they are planned with great care under the supervision of the structural engineer as such joints are affected to shear, tension, and moment created in the structure.
In general, they are located to points of minimum shear and minimum moment as far as possible.
Joints in concrete are made in order to break the very wide span concrete structure so that each separated unit of the concrete structure acts solely and doesn’t affect the entire massive concrete structure.
Generally, such concrete joints always need to be formed in the vertical planes so that align perpendicular to the principal lines of tensile stress. In such vertical joints, only horizontal displacements are allowed to occur.
And some joints in concrete are rarely and specially designed in a horizontal plane in order to allow both vertical and horizontal movement.
Sometimes concrete joints are made in order to separate the two different functioning parts within the structure. This we can more distinctly be observed in bridges. Those joints are designed with the mathematical and analytical calculations of the stress, strain, and flexural properties.
Why construction joints are necessary?
The necessity of joints can be summarized in points as the different kinds of joints functions differently. The overall summary of the necessity of a joint is.
- Joints are very essential to prevent the cracks due to strain produced. The strain can be either positive, negative, thermal, structural, or any.
- Joints prevent unnecessary cracks. In fact, joints sometimes are pre-plan cracks with minimum damage to the structure.
- Joints ease in the massive construction work.
- Joints sometimes prevent the entire failure of the structure. For example, the joint retaining wall if unfortunately hit by the massive automobile then, only the section that is between the joint retaining wall section would fail and need maintenance.
- Joint helps to prevent unnecessary deformation as well.
- Joints also allow a free moment of two different structural elements functioning differently.
Types of joints in construction
There are basically four types of joints based on their function and construction features. They are listed as,
- Construction joint
- Expansion joint
- Contraction joint
- Isolation joint
1. Construction joint
These joints in the concrete structure are designed to separate the large concrete mass work into a divisional pattern.
These joints’ main purpose is to divide the large voluminous concrete work in a specialized way so that there would be no problem with in-work during construction as well as the structural strength of structure wouldn’t be affected. Construction joints are subjected to create intentionally, but during the actual work of construction different unfavorable conditions might occur.
At such time a construction joint also may induce. The unfavorable condition generally seizes the work, and thus the part of concrete in structure might get harden with time. Thus such old concrete (harden concrete) and new concrete to be placed during the resume of work are necessarily separated by unintentional instructions joints.
In such a situation such a construction joint is specially named as “Cold joints”. However such a joint must be consulted with the structural engineer, and cannot be randomly made. The engineer studies the stress conditions at that point and only decides whether to keep there joint and continue to work or not.
If joints on such points are analyzed to be not feasible then, the whole concrete work done previously might need to be demolished.
The construction joint is generally adopted in long construction works as the formation of very large slabs, in construction of long retaining walls along the road, irrigation channels, etc.
The construction joints should not be located near the centroid level of section as there transverse shear stress is very high. The construction joints are provided with “keys” at suitable spacing. The keys are interlocking structure made within the end of construction at the joint.
In resuming the work at joint (especially cold joints) the surface of the joint must be cleaned.
The construction joints are needed to be supplied with the load transfer device if the entire load is to be distributed overall structure. This generally happens in the rigid slab where there is a high amount of traffic movement. At such conditions, they are connected with dowel bars and tie bars.
The construction joints are further categorized into different types according to the method of the end of the joint face constructed. They are,
- Butt type
- Tong and groove type
- Butt type with dowels
- Butt type with tie bars
2. Expansion joint
Expansion joints are made in structure in order to reduce the effect of deformation due to the expansion of construction material due to changes in temperature and weather.
Such joints are generally formed in bridges, railway tracks, rigid pavements, etc. The joints are constructed to prevent unnecessary stress creation in structure due to their expansion. Moreover, this joint also helps to prevent the deformation of the structure due to thermal strain.
This deformation can create a disastrous effect on railway tracks. The expansion joint is mostly used in two separately functioning parts of a single unit like a slab of deck and approach slab, abutment and the return wall, and so on.
How to treat Expansion joint in the building?
According to the reference of IS 456:1978, if the structure length of any structure is greater than 45m then the expansion joint must necessarily be implemented.
The expansion joint should not contain any reinforcement within it. The sufficient space or gap should provide the considering expansion in the worst conditions.
Expansion joints are filled with elastomers like neoprene, bitumen filler, or any resilient material. Sometimes the end of the slabs or concrete at the gap of joints are provided with the armor angles section to prevent the crushing of the end.
For example, the expansion joint gap in the bridge has a single span deck slab is determined as below,
Expansion movement of superstructure at end
- due to total temperature rise/fall of 40°C = +-4.5 mm
- due to Residual shrinkage of concrete after 90 days = – 1.25 mm
Assuming expansion joints are installed after 90 days of casting of deck concrete, then
-total expansive movement at each end of span =4.5 – 1.25 =3.25 mm (say 4mm)
-total contraction at each end of span = – 4.5 – 1.25 = – 5.75mm(say – 6mm)
Hence expansion joint arrangements above abutment is,
If Initial gap between dirt wall and end of deck = 10mm
Due to expansion gap can reduce to (10-4) =6mm and can increase to (10+6)=16 mm
3. Contraction joint / Control Joint
Contraction joints are the joints provide to prevent the excessive cracks due to shrinkage and contraction of concrete material. These joints are also called control joints as it helps to control the excessive unnecessary cracks in concrete.
These joints are provided in the prediction of a crack in the concrete structure after proper analysis of the design.
These joints form the cracks in control planned way preventing it from propagating in the whole structure. Such cracks occur due to shrinkage and contraction caused by temperature fluctuations.
The joints are generally made in the groove over the concrete surfaces where the probable cracks are about to occur. The groove on concrete can be made on a fresh wet state as well as after setting by cutting it.
The joint depth is generally about ¼th of the slab thickness. And such joints are spaced at intervals not greater than 2 or 3 times the slab thickness. For example,
If the slab thickness is 8 inches, then 1/4th of the slab thickness would be 2inch. So the groves of depth 2inch are must necessarily be made in predicted are of crack. And the spacing of consecutive joint must be less than 2 or 3 times of slab thickness, i.e. 16inch or 24 inches.
4. Isolation joint
Basically, isolation joints are formed at an unsymmetrical intersection of the components of the structure to reduce the compressive strength.
These joints can allow both horizontal and vertical movement between the two components of the structure. Thus their main purpose is to prevent disturbance in two adjacent components if structure functioning differently.
Isolation joints are included in the foundation, ground-level floor slabs and respective columns and beams, the outer stairs up to the plinth level, etc.
The space for the isolation joints is often filled with the elastomers like neoprene, plastic, rubber, asphalt bitumen paste, which are very resilient and unaffected to friction.
Isolation joint in the column are arranged diagonally or circular form, in the slab near to the column. And isolation joint between the wall and footing of the foundation or beam is created by resting wall on the joint material filler.
According to the level of understanding, one can simply analogy the Bearing in the bridges are also kind of isolation joints. However, the bearing in bridges are the separate topic of study due to the involvement of different structural phenomena within it
Sometimes the massive building construction is divided into the components especially those which have complex structures as L- shaped, T- shaped building in order to decrease the vulnerability due to. Seismic activities.
At such conditions, the joint is form along with whole structure which later may be filled with elastomers like neoprene or the resilient substances giving single structure shape.
I hope this article remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha