Joints and its types in Building Construction

 Joints

Usually buildings are large and need to be constructed part by part. The points of connection are termed as the joints in a building. Joints are unavoidable. It is because, the materials of constructions may not be available to the required dimension. The effect of weather and thermal phenomena has also serious consequence on the bulk volume of the work. All these facts call for the provision of joint in construction work.

The need of provision of joints:

• Between different materials in composite structure
• Between the same materials with different parts requiring different fabrication timing
• To allow space for shrinkage and expansion due to thermal and moisture movements 
• To control cracking due to the possible structural movements of the fabric.

Types of Joints

The various types of joints in a structure are;

1. Expansion Joint
2. Isolation Joint
3. Contraction Joint
4. Sliding Joint
5. Construction joint

1. Expansion Joint:

It is provided to accommodate the expansion of adjacent building parts and relieve compressive stresses that may otherwise develop mainly due to change in temperature. Expansion Joints are necessary when continuous length of a structure exceeds 45 M. The recommended maximum centre to centre spacing of expansion joints is 30 M. The gap varies from 10 mm to 40 mm. The gap is filled with compressible material for wind and water tightness as well as aesthetic.
The various materials available for treating the expansion joints are;
Joints Filler (bitumen, rubber, fibre etc.): Durable/ flexible material used to fill the gap if desired.
Sealing Compound  (bituminous sealant): These are applied in layers to seal the exposed slits against passage/ penetration of moisture, dust, etc.
Water Bars  (Rubber, P.V.C, Metallic): These are seal against passage of water.

2. Isolation Joint:

The details are similar to Expansion Joint, Where a part of a structure is higher than the other, it is desirable to separate the two structures. Isolation joints are provided to avoid damages to the structure due to differential settlement cause by varying soil pressure. It is sometimes provided in a regular structure, which has varying soil condition over its length. It is also provided to separate foundation of machines from rest of the structure.

 3. Contraction Joint (Shrinkage Joint):

The property of concrete to set and harden results in shrinkage in volume (contraction). It mainly depends on the amount of water in the mix. Contraction creates tensile stresses which may result in formation of random cracks.
Different types of Contraction Joints are
Complete contraction joint: Discontinuity of  both concrete and steel creates a complete joint.
Partial contraction Joint: Discontinuity of concrete only, creates a partial joint.
Dummy joint: These are provided in PCC/RCC pavements and flooring. The groove is of @ 3mm wide and 1/3rd to 1/5th of thickness (deep). It is normally filled with bitumen or rubber to provide a plane surface.     

4. Sliding Joint:

It is provided between part of a structure which has a sliding tendency over the other part due to variation in temperature or moisture. Example; RCC Slab and beam over masonry wall.

5. Construction Joint:

Generally casting of concrete is planned and preferred to be completed at one stretch within a day. Casting is seen to start early in the morning and are extended into late evenings, under flood lights. However, as the size of the building increases casting at a stretch becomes impossible and extends into two to three days. In such cases provisions are made for strategic location of construction joints in the cast to provide a water tight joint.

Factors to be taken into consideration during preparation of construction joints;

• Joints should be located near plane of minimum shear.
• Joints should be at right angle to the axis of the member for horizontal, vertical or inclined members.
• Prior to laying new concrete the stop-board is removed and the face is cleaned with wire brush and  water and treated with a coat of cement mortar.

Location of Construction Joints

In case of slabs:

The joint should be located at points of minimum shear, which is at mid way or with the center between support with the vertical plane at right angles to the direction of the main reinforcement. It also can be located directly over the center of the beams making a vertical joint at right to the span. The layer of concrete at joint should be finished against a properly fixed stop-board to ensure a vertical clean surface.

In case of Beams:

The joint should be located at the point of minimum shear, which is at mid way or within the center between the support, with vertical plane at right angle to the direction of main reinforcement. It can also be formed at the center of column along the vertical plane at right angles to the length of beam, allowing one half of the beam to become the bearing surface of the future adjoining beams. In no case should be concreting be terminated in beam or slab where shearing action will be greatest i.e. near the ends or directly under the concentrated loads. The layer of concrete at joint should be finished against a properly fixed stop-board to ensure a vertical clean surface.

In case of column:

These joints can be provided at a level surface 10 to 15 cm below the junction with the beams. This is particularly important in flat slab construction. Normally, two hours elapse between pouring of concrete in beams/ slabs to allow for settlement of shrinkage of the column concrete.

In case of walls:

The horizontal construction joint in walls should be located at the top of plinth, top or bottom of window opening or at any other convenient height

Methods of joining new concrete to old one:

1. If the stoppage in concreting work is for a short duration, then the joining is done by applying mortar grout of similar composition to that contained in the new concrete.
2. In case of floor and roof slabs, if the slabs are cast in section then to prevent water seepage.
3. If the surface is partially hardened, the joining surface should scrubbed with wire or hard bristle brush, removing dirt and other undesirable materials. The joining surface is then moistened with water and cement slurry (1:1) , applied with a brush followed by 150mm thick layer of cement mortar (1:2).
4. If the surface is partially hardened, the joining surface should scrubbed with wire or hard bristle brush, removing dirt and other undesirable materials. The joining surface is then moistened with water and cement slurry (1:1) , applied with a brush followed by 150mm thick layer of cement mortar (1:2).