Concrete expansion joints explained: When and why are they needed?
Concrete is strong and durable, but it is not completely static. Changes in temperature, drying shrinkage and movement within a structure can all create stresses in a concrete slab.
Joints are used to accommodate or manage this movement. However, terms such as expansion joint, control joint and contraction joint are sometimes used interchangeably even though they do not necessarily perform the same function.
Understanding the difference can help when planning concrete driveways, patios, floors and other slabs. In this guide, we explain what concrete expansion joints are, why joints are used and why their position and type should be considered before concrete is poured.
Key takeaways
- Expansion joints are designed to allow movement between sections of concrete or between concrete and adjoining structures.
- Control or contraction joints are used to encourage drying shrinkage cracking to occur at planned locations.
- The type, position and spacing of joints depend on the slab and project design, so there is no single joint layout suitable for every concrete project.
What are concrete expansion joints?
Concrete expansion joints are intentional separations that allow sections of concrete to move without pressing directly against each other or an adjoining structure.
Concrete can expand and contract as temperatures change. Movement can also occur between a slab and other parts of a building or paved area.
An expansion joint provides space for this movement and normally contains a compressible joint material rather than simply leaving an open gap.
They may be required where a concrete slab meets another structure or where movement needs to be accommodated within a larger concrete installation.
The precise joint arrangement should be determined as part of the design and installation of the slab.
Why does concrete need joints?
Concrete changes after it has been placed.
As concrete hardens and loses moisture, drying shrinkage can occur. Temperature changes can also cause hardened concrete to expand and contract.
Because concrete has relatively low tensile capacity compared with its compressive strength, restraint of these movements can contribute to cracking.
Joints allow movement to be accommodated or cracking to be managed at predetermined locations.
The important point is that not every joint serves the same purpose. The joint required for thermal or structural movement may be different from one intended to manage drying shrinkage.
Expansion joints vs control joints
Expansion joints and control joints perform different functions.
An expansion joint creates a separation between sections of concrete or between the concrete and another structure so movement can be accommodated.
A control joint, often called a contraction joint, creates a planned weakened line in a slab. As the concrete shrinks, this encourages a crack to form beneath the joint rather than randomly across the visible surface.
This does not mean control joints can guarantee that no other cracking will occur. Concrete performance is influenced by the mix, slab design, reinforcement, ground conditions, curing, weather and workmanship.
Correct joint design is one part of good concrete construction.
What are concrete contraction joints?
Contraction joints are specifically intended to manage shrinkage movement.
Concrete can reduce slightly in volume as it dries. If this movement is restrained, tensile stresses can develop within the slab and cracking may occur.
A contraction joint introduces a planned line of weakness. If shrinkage cracking develops, the intention is for it to occur at or beneath this joint where it is less disruptive to the appearance and performance of the slab.
Contraction joints may be formed during placement or cut into the slab after finishing at an appropriate stage.
The correct timing, depth and spacing depend on the slab design and should be determined by the contractor or project specification.
Are expansion joints and contraction joints the same?
No.
Although both deal with movement, they address different types of movement and should not automatically be treated as interchangeable.
Expansion or isolation joints provide separation so adjoining concrete sections or structures can move independently.
Contraction joints are primarily intended to control where cracking caused by shrinkage occurs.
A concrete installation can therefore contain more than one type of joint depending on its size, layout and relationship with surrounding structures.
Understanding which type is required is important before work begins.
Where are concrete expansion joints used?
Expansion or isolation joints may be required where movement needs to be accommodated between concrete and another part of a structure.
Examples can include locations where a slab meets:
- Existing buildings
- Walls
- Columns
- Kerbs or other fixed elements
- Separate concrete sections
Whether a joint is required in a particular location depends on the design.
For structural or larger commercial projects, joint requirements should follow the drawings and specification prepared for the work.
For domestic concrete, the installer should still plan the joint arrangement before placement rather than deciding after cracking has occurred.
Do concrete driveways need expansion joints?
Concrete driveways need an appropriate joint layout, but that does not mean every joint in a driveway will necessarily be an expansion joint.
Contraction joints are commonly used to manage shrinkage cracking across concrete slabs. Isolation or expansion joints may also be appropriate where the driveway meets a building, another fixed structure or an independently moving section.
The exact requirements depend on the driveway dimensions, layout, thickness, surrounding structures and design.
AMIX supplies concrete for driveways, patios and walkways for domestic and commercial projects. Correct ground preparation, concrete specification, placement, finishing, jointing and curing all contribute to the performance of the finished surface.
What about expansion joints in patios and paths?
Patios and paths are also exposed to changes in temperature and moisture, so movement and shrinkage need to be considered.
Joint placement is particularly relevant where a concrete area meets a house, wall or another fixed element.
For larger areas, planned contraction joints can help manage shrinkage cracking and divide the surface into more manageable sections.
The layout should also consider the finished appearance. Poorly positioned joints can be visually disruptive even when they perform correctly.
Planning the joint arrangement alongside the shape and dimensions of the area helps achieve a more practical finished result.
Do concrete floors and slabs need joints?
Many concrete floors and slabs require some form of jointing strategy.
Large horizontal areas can be particularly susceptible to shrinkage stresses because there is a greater area of concrete undergoing dimensional change.
The appropriate approach depends on factors such as:
- Slab dimensions
- Thickness
- Reinforcement
- Sub-base
- Expected loads
- Connections to surrounding structures
- Intended use
- Environmental conditions
AMIX supplies concrete for slabs and floors for domestic and commercial applications including garage floors, base slabs, internal floors, workshops and outbuildings.
The required joint arrangement should be considered as part of the slab design rather than determined solely by the concrete mix.
How far apart should concrete expansion joints be?
There is no universal spacing that should be applied to every concrete slab.
The correct spacing depends on what type of joint is being discussed and the design of the concrete.
Expansion joints, isolation joints and contraction joints perform different functions, so applying one standard spacing to all of them can be misleading.
Slab dimensions, thickness, reinforcement, geometry, restraint and intended use can all affect joint design.
Where joint positions and spacing are specified on project drawings, those requirements should be followed. For other projects, the installer or suitably qualified professional should determine an appropriate joint layout.
When should concrete joints be planned?
Joint requirements should be considered before the concrete arrives.
This allows the installer to account for joint locations when preparing formwork, reinforcement and the pour sequence.
Some joints are formed as the concrete is placed. Others, such as saw-cut contraction joints, may be created after finishing at the appropriate stage.
Timing matters. If contraction joints are cut too late, uncontrolled shrinkage cracks may already have developed.
The contractor responsible for placement should therefore have a clear plan for forming or cutting the required joints before the pour begins.
Can expansion joints prevent concrete from cracking?
Expansion and other concrete joints can help manage movement and reduce the risk of uncontrolled cracking, but they cannot guarantee that concrete will never crack.
Cracking can be influenced by numerous factors, including:
- Drying shrinkage
- Thermal movement
- Ground movement
- Inadequate preparation
- Poor curing
- Excess water
- Loading
- Slab design
- Reinforcement
- Workmanship
A good joint layout is therefore one part of the overall construction process.
Correct preparation, an appropriate concrete specification, effective placement and compaction, suitable finishing and proper curing are also important.
Does reinforcement stop the need for concrete joints?
Not necessarily.
Reinforcement can help control cracking and provide structural capacity, but it does not prevent concrete from shrinking or responding to temperature changes.
The need for joints should therefore be considered separately from the decision to reinforce a slab.
AMIX supplies reinforced concrete mixes for projects where structural reinforcement is specified. The steel bars or mesh used as structural reinforcement are specified and installed separately by the contractor or engineer.
Jointing and reinforcement should both follow the project design where structural requirements apply.
Does the concrete mix affect cracking?
The concrete mix is one factor in the performance of a finished slab, but it is not the only factor.
AMIX supplies paving concrete for external surfaces such as driveways, patios and paths. The company also supplies standard, fibre reinforced, reinforced and high early strength concrete for different applications.
Selecting the appropriate concrete provides the starting point for the project.
The concrete then needs to be placed, compacted, finished and cured correctly. Ground preparation, slab design, reinforcement and jointing can all influence how the finished concrete performs.
This is why concrete specification and site workmanship should be considered together.
Why curing still matters
Joints do not replace proper curing.
Concrete needs appropriate curing after placement to support strength development and durability. If a fresh slab loses moisture too quickly, the risk of shrinkage and surface problems can increase.
External concrete can be particularly exposed to changing weather conditions, including sun, wind and temperature variations.
AMIX advises that correct installation and curing are important for achieving reliable long-term performance from paving concrete.
The contractor should therefore plan both jointing and curing before the pour takes place.
Planning joints before concrete delivery
Site preparation should be completed before concrete arrives.
Formwork should be secure, the sub-base should be properly prepared and any reinforcement or membranes required by the design should already be in place.
The joint layout should also have been considered.
AMIX delivers its concrete batched to order and ready for placement. Labour should be available so the concrete can be placed and finished promptly after arrival.
Planning these details in advance helps avoid delays during the pour and gives the contractor the best opportunity to complete jointing and finishing at the appropriate time.
Choosing concrete for external slabs
For a driveway, patio or path, the concrete needs to suit the loads, exposure and intended use of the finished area.
AMIX’s paving concrete is designed for external applications including driveways, patios, paths and paving bases. It is supplied batched to order and delivered ready to use.
Other projects may require standard, fibre reinforced or reinforced concrete depending on their design.
The correct concrete should therefore be selected according to the application and specification rather than using the same mix for every slab.
Joint design remains a separate consideration and should be planned alongside the concrete specification.
Speak to the AMIX team
AMIX has supplied ready mix concrete across the North West since 1993, supporting domestic and commercial construction projects.
The team can help customers identify an appropriate concrete mix based on the application, expected use, quantity and delivery requirements.
Where a project has specific structural, reinforcement or jointing requirements, these should be determined by the relevant contractor, designer or engineer and incorporated into the project specification before concrete is ordered.
Planning the mix, site access, placement and finishing requirements in advance can help the pour run more smoothly.
Frequently asked questions
What is an expansion joint in concrete?
A concrete expansion joint is a deliberate separation that allows adjoining sections of concrete or a concrete slab and another structure to move without pressing directly against each other.
What is the difference between an expansion joint and a control joint?
An expansion joint accommodates movement between sections or structures. A control or contraction joint creates a planned weakened line that encourages shrinkage cracking to occur at a controlled location.
Do concrete slabs need expansion joints?
Concrete slabs often require a planned jointing strategy, but the type of joints required depends on the slab design. Not every joint is an expansion joint. Contraction and isolation joints may also be required.
How far apart should concrete joints be?
There is no single spacing suitable for every project. Joint spacing depends on the type of joint, slab dimensions, thickness, reinforcement, layout and other design factors. Project specifications should be followed where provided.
Do expansion joints stop concrete cracking?
Joints can help accommodate movement and manage where cracking occurs, but they cannot guarantee a crack-free slab. Concrete specification, preparation, reinforcement, placement, curing, ground conditions and loading also affect cracking.
Does AMIX install concrete expansion joints?
AMIX supplies ready mix concrete for domestic and commercial projects. Joint design, formation and installation form part of the concrete construction work and should be handled by the contractor or other suitably qualified professional responsible for the project.