Movement Joints in Concrete: Types and Necessity

Introduction of Movement Joint In Concrete 

All buildings move, especially when considering factors like expansion joint in concrete. 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:

1. Temperature variation
2. Loading (Static and Dynamic, Including Gravity, Wind, and Earthquakes)
3. Atmospheric humidity changes
4. Ground movements (Settlement, Consolidation, Shrinkage, Heave, etc.).

Sources of movement also arise from within the concrete itself.

These are principal:

1. Early-age thermal movement from the rise in concrete temperature during cement hydration and, more significantly, the subsequent drop back toward ambient temperature
2. Irreversible drying shrinkage
3. reep under stress.

Types of Movement Joints / Joints in Concrete

1. Isolation Joints / Free Joints.
2. Free Contraction Joints.
3. Partial Contraction Joints.
   1. Tied Partial Contraction Joints.
   2. Debonded Partial Contraction Joints.
4. Expansion Joints.
5. Hinged Joints.
6.  Sliding Joints and  Bearings Joints In Concrete.
7. Seismic Joints.

#1. Isolation Joints / Free Joints In Concrete:

Isolation Joints

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:

1. In structures above ground level (particularly in roof slabs and exposed soffit slabs) to accommodate temperature variation movements.
2. 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.
3. 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.

#2. 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.

#3. 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.

In the latter case, separation at the joint is achieved by the use of a crack-inducing strip and/or sawing a groove in the concrete surface.

#3.1. Tied Partial Contraction Joints In Concrete:

Tied Partial Contraction Joints

1. The tied contraction joint has a reduced area of reinforcement across the joint.
2. 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.
3. This reduces the possibility of unwelcome surface cracking from such causes occurring elsewhere.
4. This type of joint is much used between adjacent pours in water-containing structures and large ground-bearing slabs.

#3.2. Debonded Partial Contraction Joints In Concrete:

Debonded Partial Contraction Joints

1. In this variant, some reinforcement is provided across the joint as for the tied contraction joint.
2. 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.
3. This type of joint is used less often than tied contraction joints.
4. It is most commonly used in large ground-bearing floor slabs, roads, and hard-standing.
5. The reinforcement is usually provided in the form of dowel bars.

#4. Expansion Joints In Concrete:

This term, often referred to as expansion joint in concrete, 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. 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.

#5. Hinged Joints In Concrete:

Hinged Joint

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)

Hinged Joints

#6. 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.

#7. Seismic Joints:

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

FAQs on Movement Joints in Concrete

What are movement joints in concrete?

Movement joints in concrete are designed gaps between concrete sections that allow for free movement due to external or internal forces without causing damage to the structure.

Why are movement joints required in concrete structures?

Movement joints are required to accommodate various sources of movement such as temperature variations, loading, atmospheric humidity changes, and ground movements. They help prevent the development of internal stresses that could lead to cracking or other damage.

What are the main types of movement joints in concrete?

The main types of movement joints in concrete include:

1. Isolation Joints / Free Joints
2. Free Contraction Joints
3. Partial Contraction Joints (Tied and Debonded)
4. Expansion Joints
5. Hinged Joints
6. Sliding Joints and Bearings
7. Seismic Joints

What is an isolation joint?

An isolation joint, also known as a free joint, allows free translation and rotation in all directions. It is commonly used to accommodate temperature variations and differential ground movements in structures.

How do free contraction joints function?

Free contraction joints are formed without an initial gap and allow for opening of the joint due to movements such as early-age thermal movements and irreversible drying shrinkage. They are often used in water-containing structures.

What is the difference between tied and debonded partial contraction joints?

Tied partial contraction joints have reduced reinforcement across the joint to assist in shear transfer and prevent free opening. Debonded partial contraction joints have reinforcement that is debonded on one side, allowing unrestrained contraction across the joint.

When are expansion joints used in concrete?

Expansion joints are used to allow expansive movement to occur freely, accommodating significant temperature variations and other expansive forces. They can be either free or reinforced to transfer shear and equalize deflections.

What are hinged joints and where are they commonly used?

Hinged joints have a narrow concrete throat with concentrated reinforcement to allow rotational freedom while transferring shear and axial loads. They are commonly used in bridges, particularly arches, and occasionally in building structures to minimize damaging shear forces.

What is the role of sliding joints and bearings in concrete structures?

Sliding joints and bearings are used in precast concrete structures and large civil engineering structures such as bridges. They allow for freedom of movement while transmitting loads and can also provide acoustic and vibration isolation.

How do seismic joints accommodate earthquake-induced movements?

Seismic joints are designed to accommodate large movements caused by earthquakes. They must be sufficiently wide to handle the cumulative anticipated movement, which can exceed the capabilities of standard joint sealants or gaskets.

What is the difference between a construction joint and an expansion joint?

1. Construction Joint: Occurs when there are multiple concrete placements on different days. It prevents cracking by allowing controlled movement between pours.
2. Expansion Joint: Used to accommodate daily temperature variations in concrete and steel, preventing buckling, spalling, or total failure.

Why is it important to properly design and place movement joints in concrete structures?

Proper design and placement of movement joints are crucial to prevent uncontrolled cracking, ensure the structural integrity, and extend the lifespan of concrete structures by accommodating various movements and stresses effectively.