Introduction of Movement Joint In Concrete
All buildings move. If there is no restraint to movement, then it can occur freely without the development of internal stresses that could lead to damage.
In practice, some restraint will always be present, even in the smallest building element.
The concrete designer is concerned to identify sources of movements, assess their magnitude, and then to consider whether the structure will be damaged if and when movements occur.
If so, joints offer one solution for avoiding or controlling such damage. (Other solutions include the use of prestressing to prevent or limit tensile stresses, and the use of additional reinforcement to control cracking.)
Failure to provide movement joints where needed may lead to the structure making its own – by cracking.
Why Required Movement Joint In Concrete
Sources of movement can be external or internal.
External sources include:
• Temperature variation
• Loading (Static and Dynamic, Including Gravity, Wind, and Earthquakes)
• Atmospheric humidity changes
Sources of movement also arise from within the concrete itself.
These are principal:
• Irreversible drying shrinkage
• Creep under stress.
Types of Movement Joints / Joints in Concrete
• Isolation Joints / Free Joints
• Free Contraction Joints
• Partial Contraction Joints
• Tied Partial Contraction Joints
• Debonded Partial Contraction Joints
• Expansion Joints
• Hinged Joints
• Sliding Joints and Bearings Joints In Concrete
• Seismic Joints
Isolation Joints / Free Joints In Concrete
As its name implies, this joint allows free translation and rotation in all directions. See the above figure.
It is most commonly used in the following circumstances:
• In structures above ground level (particularly in roof slabs and exposed soffit slabs) to accommodate temperature variation movements.
Temperature variations are both of smaller magnitude and slower to occur below ground level, due to the ‘heat sink’ effect of the subsoil and reduced exposure to climatic extremes.
• In structures generally, to accommodate differential ground movements, especially when adjacent elements or blocks exert different levels of bearing pressure and/or have foundations with different settlement characteristics, such free movement joints are also known as settlement or isolation joints.
Free Contraction Joints In Concrete
Free Contraction Joints
This type of joint is formed and has no initial gap. It is for use where the movement will lead to opening of the joint only (see above figure ).
It is most commonly used in water-containing structures to deal with early-age thermal movements and irreversible drying shrinkage, where no load transfer or equalizing of deflection in the plane of the joint is required.
Such a joint may be applicable in circumstances where some expansion may occur, for example, due to temperature rise – but only when this follows and is of lesser magnitude than initial contraction.
Partial Contraction Joints In Concrete
In this type of contraction joint, reinforcement continues across the butt joint in the concrete, although it is either reduced in section or debonded so that the joint will indeed be able to serve as a plane of weakness at which contraction can occur.
Steel is provided to ensure that shear loads can be transferred across the joint and/or when equalizing of deflection in the plane of the joint is required.
Such joints are provided to deal with early-age thermal movement and irreversible drying shrinkage in water-containing structures, retaining walls, and large ground-bearing slabs.
There are two basic variants – the tied and the debonded joint. They may be formed by placing concrete either side of the joint in two pours, or alternatively, the concrete is placed in a single pour.
Tied Partial Contraction Joints In Concrete
Tied Partial Contraction Joints
The tied contraction joint has a reduced area of reinforcement across the joint.
This assists shear transfer and prevents overall free opening of the joint while permitting relief of early-age thermal movements and shrinkage on the surface.
This reduces the possibility of unwelcome surface cracking from such causes occurring elsewhere.
This type of joint is much used between adjacent pours in water-containing structures and large ground-bearing slabs.
Debonded Partial Contraction Joints In Concrete
Debonded Partial Contraction Joints
In this variant, some reinforcement is provided across the joint as for the tied contraction joint.
It is debonded on one side of the joint so that unrestrained contraction can occur across the full thickness of the section to allow early-age thermal movements and shrinkage to occur.
This type of joint is used less often than tied contraction joints.
It is most commonly used in large ground-bearing floor slabs, roads, and hard-standing.
The reinforcement is usually provided in the form of dowel bars.
Expansion Joints In Concrete
This term is commonly, but loosely, used to describe any formed gapped joint, whatever its structural role and irrespective of whether opening or closing movements are expected.
Free Expansion Joints
Such a joint is intended to allow expansive movement to occur freely. It may be free (See above figure free expansion joints) – in which case it is essentially a free movement or isolation joint – or have reinforcement (See below figure reinforced Expansion joints) to transfer shear and equalize deflections.
Reinforced Expansion Joints
The reinforcement, if present, must be debonded to allow free axial movement.
This type of joint is used for elements exposed to significant temperature variation (notably solar gain) such as roof slabs, footbridges, and ground-bearing slabs outdoors.
Hinged Joints In Concrete
The hinged joint (See above figure) is more common in bridges, particularly arches, than in building structures.
It has a narrow concrete throat with concentrated reinforcement to allow rotational freedom (minimizing moment transfer) while providing shear and axial load transfer and equalizing deflections across the joint.
It has been used in a few structures to provide effectively ‘pinned’ concrete connections, for example, to minimize potentially damaging horizontal shear forces in the lowest of column carrying floors down to a progressively – post-tensioned long-span transfer (See below figure)
Sliding Joints and Bearings Joints In Concrete
Sliding / Bearing Joints
Sliding joints and bearings are generally used in precast concrete structures and in larger civil engineering structures such as bridges, although sliding joints are also found at the wall-floor and wall-roof junctions in water-retaining structures.
Such joints are similar to free movement joints except that loads are carried from one element to the other by bearing.
This may be achieved either directly across smooth concrete surfaces or (preferably, to minimize friction) by purpose-made bearings or membranes designed to allow the required freedom of movement while still transmitting loads.
Such bearings, suitably designed, can also provide acoustic and vibration isolation (e.g., for a hospital built next to a railway line) and base isolation to protect structures in seismic zones.
A common example of a sliding joint is the slip membrane provided under a ground-bearing slab to allow shrinkage contraction. This is discussed in Deacon”.
Figure sliding and bearing joints show a typical sliding joint at a floor-wall junction in a water-retaining structure.
Earthquakes can cause large movements to occur in buildings and other structures.
This calls for particular attention where open movement joints have been provided; these joints will have to be of sufficient width to accommodate the cumulative anticipated movement across the joints.
This may be 100mm or more, which is well beyond the capability of any orthodox joint sealant or gasket.
Choosing a suitable bridging material for such joints may be complicated by the need to accommodate pedestrian or vehicular traffic.
Construction Joint Vs Expansion Joint
|Sr.No.||Construction Joint||Expansion Joint|
|1||A construction joint occurs when there are multiple concrete placements.||An expansion joint is used in concrete and steel.|
|2||It can occur between different days of concrete placements.||An expansion joint allows the concrete or steel to expand or contract with daily temperature variations.|
|3||If you don’t allow this, you may get crake to develop in concrete||If you don’t allow this, you may get buckling, or spalling, or total failures.|
|4||Type of Construction Joint|
1. Free Contraction Joints
2. Partial Contraction Joints
2.a. Tied Partial Contraction Joints
2.b. Debonded Partial Contraction Joints
|Type of Expansion Joint|
1. Free Expansion Joints
2. Reinforced Expansion Joints