EXPANSION JOINT IN CONCRETE STRUCTURES, AND ITS DETAIL ANALYSIS AND STEP BY STEP PROCEDURE AND TREATMENT
In this article the main point of focus is expansion joint or movement joints , how to treat them, what are the advantages and disadvantages of it and why we need to provide the expansion joint in the structure.
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What is
Expansion Joint ?
An expansion joint is a gap in the building structure provided by an architect or engineer to allow for the movement of the building due to temperature changes.
It is an assembly designed to safely absorb the heat-induced expansion and contraction of various construction materials.
Expansion joints are periodic breaks in the structure of the buildings or it can be as simple as a caulked separation between two sections of the same materials
Criteria for Expansion Joints
Concrete expands slightly when the temperature rises. Similarly, concrete shrinks upon drying and expands upon subsequent wetting. Provision must cater for the volume change by way of joint to relieve the stresses produced.
The movement of the building is caused most frequently by temperature changes, the amount of expansion and contraction of building depends upon the type of material it is constructed out of.
A steel
framed building will move by a different amount then a concrete framed one.
In case of a small building, the magnitude of expansion is less and therefore, no joint is required either in the floor or roof slab, But in case of the long building, the expansion is very large and may be as much as 25 mm.
Therefore, buildings longer than 30 m are generally provided with one or more expansion joints.
In case of a small building, the magnitude of expansion is less and therefore, no joint is required either in the floor or roof slab, But in case of the long building, the expansion is very large and may be as much as 25 mm.
Therefore, buildings longer than 30 m are generally provided with one or more expansion joints.
Problems Due to Expansion Joint
A. Problems due to Ingress of Water, Leakages in walls of expansion joint areas.
B. Problems due to Pest attack.
C. Problems due to poor workmanship.
D. Problems due to Ingress of Water, Leakages in walls of expansion joint areas-
1. In rainy
season water travels from the expansion joints and goes into the walls which
creates discomfort for people.
2. Also the
paints of the walls are affected by weather.
3. The
steel members get corroded and results in to risk of structural failure.
4. The
electric lines in expansion joints can be short circuited.
E. Leakages
due to Expansion Joint
F. Problems due to Pest attack
The pest
attack on the wooden pads or the Shalitex board of expansion joints and also
travels from electric pipes and spreads in the whole structure.
G. Problems due to poor workmanship
1. The
expansion joints provided only on the superstructure can cause failure of
foundation.
2. The
expansion joints not provided on the parapet walls can result into uneven
cracks on parapet walls.
Factors affecting Expansion Joints
The different materials in buildings expands and contracts according to their different co-efficient of expansion related to temperature change so stresses devolved in such conditional can be minimized by expansion Joints.
Wind
The stresses developed in storms and hurricanes can be minimize.
The stresses developed in storms and hurricanes can be minimize.
Loads
Bending moments due to load snow, rain, vibrations, can be decreased by expansion joints.
Bending moments due to load snow, rain, vibrations, can be decreased by expansion joints.
Earth
quake
The thrust on the building can be decreased by expansion joints .
The thrust on the building can be decreased by expansion joints .
During
earthquake.
Requirements
of Expansion joints
- The expansion joints in the floor should be straight and should align, without offset, with expansion joints in vertical planes such as double columns and walls.
- The expansion joint separation should not be used as a place for tolerance build-up from other construction activities.
- The expansion joint gap should have a consistent width throughout. If the gap is cast at a temperature other than the specified mean temperature, and/or post-tensioned concrete is used, the adjustment in the gap width may be needed to ensure that the specified joint has the specified movement capability.
- Forms should be strong with tight joints so as to allow concrete next to the forms to be thoroughly vibrated to ensure proper consolidation, to prevent seepage of concrete and irregularities in joint shape, and to avoid voids within concrete or on concrete surface.
- The forms should be removed promptly after initial curing of concrete to prevent them from being squeezed or becoming dislodged due to the joint movement.
- Once formed, the expansion joint gaps in the decks and floors need to be protected from damage by construction traffic throughout their length. At crossing points, joints should be protected with plates or ramps.
- Joints in the walls should be free of mortar protrusion, masonry ties, protruding shelf angles, and other obstructions that might hinder the movement or obstruct installation of the expansion joint system.
Materials
and Techniques for Expansion Joint water proofing
The one side of the expansion joint is first constructed to desired level, then the
Fiberboard is placed where Expansion joint is to be provided then the other
side is constructed. The fiberboard is sealed with sealing compounds.The gap of expansion joints is never left open. It is filled with a compressible material so as to make it water tight. The following materials are required to render the expansion joint watertight.
The
following materials are required to render the expansion joint watertight
- Sealant
- Joint filler
- Water bars
Treatment
of Expansion Joint –Water proofing ( Roof )
Case Study –
Due to Poor
workmanship and lack of technical knowledge The expansion joint on terrace was
neither treated and brick bat filling done over it which resulted in cracks
and heavy seepage of water during monsoon through it , which resulted in dampness
and pooling of water in below flats under expansion joint.
Following methodology was adopted in treatment of expansion Joint
Step 1
The terrace
has been waterproofed with china mosaic tiles.to begin with treatment for water
proofing over expansion joint chipping was done over the china mosaic and brick bat coba
Step 2
Surface preparation-Involves thorough cleaning and removal of laitance
Step 3
Cement
mortar was prepared with mix proportion 1:6 cement sand ratio with 500ml of
acrylic polymer so as to ensure bonding with old concrete.
The cement
mortar was plastered in such a way showing crown in the gap portion as shown in
fig.so as when expansion joint when comes in contact with water shall be
drained away from gap (Extra precautionary)
Curing shall
be done to the cement plaster for 2-3 days thrice in a day/
Step 4
The
expansion joint gap is filled with 40 mm thick thermocol. After filling the
joint with thermocol fully
the
cement water mixed with contentious polymer is duly poured by means of
beaker inside of thermocol so as to fill the minor gap too make it water
tight.
the
cement water mixed with contentious polymer is duly poured by means of
beaker inside of thermocol so as to fill the minor gap too make it water
tight. Step 5
The excess
thermocol is thereby removed from top. A slight portion of thermocol is removed
keeping it 1.5inch down from surface.
Step 6
After the
removal of thermacol as shown . The primer A and primer B solution is mixed 1:1
in proportion and stirred thoroughly for best results. The sealant is thereby
applied to the sides of joint and top portion as shown in figure to achieve
good bonding to breaker rod.
Step 7
The breaker
rod is inserted in the joint.A pair of broker rod stuck together with abro tape
is inserted so as to ensure thorough packing on account waterproofing. The
breaker rod is keep 0.75 inch down from the surface.
Step 8
after
the successful insertion of beaker rod, the silicon sealant is thereby
filled into gaps of beaker rod running through the length of joint.
after the completion of process allow the silicon to dry for an hour
Step 9
After that pour the pour- grade polysulphide sealant in to
the joint.
Thorough mixing of the additives shall be done of
polysulphide sealant by means of mechanical mixer. The sealant should be
allowed to dry for period of 4 hours under heat.
The vertical portion of parapet wall joint is thereby filled with gun-grade sealant and allowed to set for 4 hours
Step 10After final setting of polysulphide sealant. Aluminum Sheet of 6 inch wide is fixed over expansion. During the fixing of aluminum Patti. The silicon sealant is sued below the Patti on all the sides of edge to ensure through water tightness.
Step 11
Aluminum Patti is fixed with 1.5 inch screws. The fixing of
screws is done on one side.so as to have free movement due to expansion of
joint. The screws are fixed at interval of 1.5 foot apart.
Step 12
After the fixing of aluminum patti .use gun-grade
polysulphide sealant to seal to all four sides of plate
Step 13
Brown tape is applied on all corners edges of the plate
.(note –while applying tape silicon is adhered to tape so as to assure good and
life long adhesion.
Sand is evenly placed over expansion joint area .So as to
ensure de-bonding in said area to allow free movement after fixation of bitumen
sheet. ( Ensure the beside of plates to be thoroughly cleaned)
Step 14
bitumen sheet membrane of width 1.5 foot is spread-ed over
covering the width of joint in center of sheet and fixed with screws with
washer
Step 15
The Edges of Bitumen sheets are sealed with silicon
Ponding is done to ensure water tightness and then the same
is filled with brick batt and Ips and later followed with china mosaic
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