Construction
joints
Introduction,
Types,
Purpose of Joints
–
and
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
necessary?
joints
are
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,
1.
2.
3.
4.
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
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
tracks. The expansion joint is
functioning parts of a single
approach slab, abutment and the
disastrous effect on railway
mostly used in two separately
unit like a slab of deck and
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)
And,
-total contraction at each end of span = – 4.5 –
5.75mm(say – 6mm)
1.25 = –
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
Contributed by,
Civil Engineer – Rajan Shrestha
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