Expansion and Contraction Joints in Concrete Construction
Reinforced concrete is an important construction material, it has strength, durability and can be formed into a variety of shapes. Careful design is essential to ensure correct performance of the structure. Reinforced concrete must be designed and constructed to accommodate the inevitable movements which will take place:
• Shrinkage - Initially rapid, less severe as the concrete ages.
• Creep - A phenomenon which causes a redistribution of stress away from points of intensity.
• Thermal movement - Expansion and contraction of the structure with temperature changes.
Structures are designed with expansion and contraction joints at appropriate places to allow this movement to take place. The design of the joint is important for the overall design to function correctly.
Conventional Dowelled Joints
Dowels are used to transfer shear across construction and movement joints in concrete. They are usually either cast or drilled into the concrete. A single row of short thick dowels provides reasonable shear transfer but suffer from deformation. This can lead to stress concentrations, resulting in subsequent spalling of the concrete.
Where dowels are used across expansion and contraction joints, half the length of the bar is debonded to allow movement to take place.
Difficulties with Conventional Joints
Dowelled joints either require formwork to be drilled for the dowels to pass through, or concrete to be drilled for dowels to be resin fixed in one side.
Where dowels are used at movement joints the dowels will need to be accurately aligned in both directions to ensure movement can actually take place, otherwise cracking is likely to occur.
Single dowels are not very effective when used across joints wider than 10mm.
Keyed joints require complicated formwork to create the tongue and groove. If the joint is not formed correctly, differential movement can take place. Load is transferred through the locally reduced section of the joint which can at times result in cracking.
The DSD Solution to Joints
In most cases conventional dowelled or keyed joints can be replaced by joints incorporating DSD shear connectors. These are more effective at transferring load and allowing movement to take place, easier to fix on site and a far more cost effective solution. They can be used for movement joints in floor slabs, suspended slabs, and for replacing double columns and beams at structural movement joints. Applications in civil engineering include joints in bridge parapets, bridge abutments and diaphragm wall construction.
Traditional Construction Joints Ancon Engineered Solutions
Structural Movement Joint
Floor to Wall connection
Designed to minimise thermal bridging in masonry cavity walls, the patented Ancon Thermal Windpost (TWP2) from Leviat offers up to an impressive 70% reduction in thermal transmission through the span of the windpost.
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