JOINTS in building construction occur where two successive placements of concrete meet. They are designed to permit movement. These are physical break or gap between concrete members or buildings.
Joints are a common source of weakness and, therefore it is desirable to avoid them. If this is not possible their number shall be minimized.
Concreting shall be carried out continuously up to construction joints, the positions shall be indicated by the structural designer as per inputs of site engineer.
Need of Construction & Expansion Joints
- Member or structure cannot be constructed as monolithic unit in single placement of concrete
- To relieve the stresses and increase the serviceability of building
- Detailing of joint is an important aspect of building design, well designed and constructed joint will enhance the building’s appearance
Construction Joints in Buildings
Construction joint provides continuity to structure between the members meeting at joint. In big projects there is a starting and stopping point, the entire concrete work may not be done at once, hence concrete pouring need to be stopped causing a joint in element known as construction joint.
They are typically placed at the end of days work or when concrete placement is stopped for longer time than the initial setting time of concrete. These joints are formed between successive building element parts during construction work, in which one part is allowed to harden before the next is placed.
Construction joints are placed in a concrete beam or column to define the extent of the individual placements, generally in conformity with a predetermined joint layout. Construction joints shall be placed at accessible locations to permit cleaning out of laitance, cement slurry, and unsound concrete, in order to create rough surface.
For construction joint, fresh concrete shall be thoroughly vibrated near joint so that mortar from the new concrete flows between large aggregates and develops a proper bond with old concrete.
If high shear resistance is required at the construction joint , shear key may be provided.
Construction Joints are made before and after interruptions in the placement of concrete or through the positioning of precast units. Locations are usually predetermined so as to limit the work that can be done at one time to a convenient size, with least impairment of the finished structure, though they may also be necessitated by unforeseen interruptions in concreting operations.
Construction joints may run horizontally or vertically depending on the placing sequence prescribed by the design of the structure.
Expansion Joints in Buildings
In building construction, an expansion joint is a mid structure separation. Expansion joints must be designed in order to allow displacements between both sides of the slab but, at the same time, they have to relieve stress on building materials produced by building movement . The building longer than 45m are generally provided with one or more expansion joint.
These joints are provided to ensure separation between the adjacent measures to allow one member to move independently to the other. These joint bisects the building element and are designed to safely absorb thermal expansion and contraction and to relieve stresses due to wind or seismic events.
Since the joint bisects the structure into two, it creates a gap and this must be filled to restore the functions of buildings such as sound and water proofing. The structure adjacent to the joint should preferably be supported on separate columns or walls but not necessarily on separate foundation. Expansion joint should be completely clear and reinforcement shall not extend across expansion joint.
Expansion Joints are designed to prevent the crushing and distortion of the abutting concrete structural units that might otherwise occur due to the transmission of compressive forces that may be developed by expansion, applied loads, or differential movements arising from the configuration of the structure or its settlement.
Expansion joint movement may be high (up to 30 % of joint width). The term movement joint has been widely adopted over expansion joint as building movement takes place in both expansion and contraction.
Materials Used in Expansion Joint
Asphalt being durable and flexible, is mixed with vegetable fiber and minerals and used as expansion joint material. Asphalt creates a permanent waterproof sealing and thus results in a good expansion joint material.
Fiber is a resilient, versatile, non extruding and a flexible material, when compressed to half of its original thickness, it can recover to a minimum of its 70% of its original thickness. High range flexibility of fiber makes it a great expansion joint material.
Ceramar is a light weight, highly flexible, and resilient material composed of isomeric polymers.
4. Sponge Rubber
It is easily compressible and has a recovery of 95% of original thickness which makes its use as a expansion joint material.
Cork has great expansion properties, it can expand upto 140% of its original thickness and thus can compensate for concrete shrinkage.