Concrete is strong in compression, but it still shrinks, expands, contracts and moves. Joints are deliberately included so that movement and cracking can occur in planned locations instead of appearing unpredictably across the slab.
Expansion joints, control joints and construction joints are not interchangeable. Each serves a different purpose, and some joint systems also provide load transfer between adjoining slab sections.
Important: Joint type, location, spacing, depth, dowel size, load-transfer system and sealant details must follow the engineer’s design, approved plans and project specification. This article is a general guide only.
Quick comparison
| Joint type | Main purpose | How it works |
|---|---|---|
| Expansion or isolation joint | Allows adjoining concrete sections or structures to move independently | Creates a full-depth separation, normally incorporating a compressible filler or purpose-designed joint system |
| Control or contraction joint | Encourages drying-shrinkage cracking to occur along a planned line | Creates an intentional weakened plane using a formed joint, proprietary system or saw cut |
| Construction joint | Provides a planned stopping and restarting point between concrete pours | May include a keyed profile, dowels, sleeves or another designed load-transfer detail |
What is an expansion joint?
An expansion joint—often also called an isolation joint—separates concrete from another slab section, wall, column or fixed structure. Its purpose is to provide space for movement and reduce the transfer of unwanted pressure between adjoining elements.
A true expansion joint generally extends through the full slab depth and includes a compressible material or a purpose-designed system. Our adhesive expansion-joint foam is a closed-cell cross-linked polyethylene foam available in 75, 100, 125, 150 and 200 mm widths, supplied in 25 m rolls.
For continuous slab-on-ground pours, the Connolly Expansion Joint provides a straight metal joint profile designed to work with dowels and sleeves. It is available in 100, 125, 150 and 200 mm depths in 3 m lengths. A Connolly Expansion Joint Complete System is also available.
For detailed product installation information, see our existing guide: How to Install the Connolly EXJ Expansion Joint System.
What is a control or contraction joint?
“Control joint” and “contraction joint” are commonly used to describe the same basic function: creating a planned weakened line so shrinkage cracking is more likely to occur neatly at that location.
A control joint does not guarantee that a slab will never crack elsewhere. Its performance still depends on the concrete mix, slab design, reinforcement, sub-base, curing, weather, timing and joint layout.
Control joints may be formed during the pour, created using a proprietary system or saw-cut after finishing. Saw cutting that occurs too late can allow random cracking to begin first, while incorrect joint depth or location may reduce effectiveness. Follow the project requirements rather than relying on a single rule for every slab.
The Connolly Straight Up Contraction Joint is a continuous-pour slab-on-ground system available in 125, 150 and 200 mm depths in 3 m lengths. Complete-system options are also available through the Straight Up Contraction Joint Complete System.
What is a construction joint?
A construction joint is the planned interface between separate concrete placements—for example, where one pour ends and the next begins. It needs to be positioned and detailed so the two slab sections behave as intended.
Depending on the design, a construction joint may use a keyed profile, dowels, sleeves or plate dowels to transfer vertical loads while allowing the required movement. A construction joint is not automatically an expansion joint; it only serves that function when it has been specifically designed and detailed to do so.
The Connolly Key Joint is a continuous-pour solution for contraction and planned construction-joint applications in slabs on ground. Store options include 100, 150 and 200 mm profiles in selected 3 m and 6 m lengths, supplied with matching pegs and wedges.
Why dowels and sleeves matter
A joint creates a break between concrete sections, but adjoining slab edges may still need to share loads. Correctly specified dowels can transfer load across the joint and help limit vertical displacement while sleeves allow the movement intended by the design.
Gippsland Concreting Supplies stocks:
- Hot-dip galvanised round dowel bars
- Connolly universal sleeves for round dowels
- Connolly universal sleeves for square dowels
- Connolly biscuit plate dowels and sleeves
Do not guess the dowel shape, diameter, length, spacing, alignment or corrosion protection. These details must match the plans and the selected joint system.
Cappings, rebate moulds, pegs and wedges
A complete joint installation may require more than the main metal profile. Connolly Expansion Joint cappings and rebate moulds help form and protect the top of the joint or create a rebate for the specified sealant system.
Connolly pegs and wedges hold expansion-joint and Key Joint profiles at the required height and alignment during the pour. Correct setup is important because a joint that moves, twists or sits at the wrong height may not provide the intended finished result.
Common joint mistakes
- Using “expansion joint” as a general name for every line in a slab.
- Changing the joint layout without approval from the engineer or designer.
- Placing joints after random cracking has already started.
- Misaligning dowels or using the wrong sleeves.
- Allowing a joint profile to move during concrete placement.
- Bridging a movement joint with reinforcement when the design requires separation.
- Using rigid filler where a compressible material or flexible sealant is specified.
- Ignoring joints around columns, walls, penetrations and changes in slab geometry.
Which joint system do you need?
- Need full separation and room for movement? The plans may call for an expansion or isolation joint, expansion foam or a purpose-designed expansion-joint system.
- Need to control where shrinkage cracking occurs? Look for the specified contraction-joint system, formed joint or saw-cut detail.
- Stopping and restarting a pour? Follow the construction-joint detail, including any Key Joint, dowels, sleeves or plate-dowel system.
Check the latest drawings before ordering. Gippsland Concreting Supplies stocks high-quality Connolly joint systems, expansion foam, cappings, pegs, wedges, dowels and sleeves to help you complete the specified joint detail properly.
