Most Commonly Used Joints in Concrete
In this post, we will cover the different types of construction joints used in concrete. By the end, you will understand why we need construction joints, the different types of joints you will have to use and the details of the joints. As a bonus, we will look at industry recommended guidelines.
Why Do We Need Joints in Concrete?
Both heat and load can cause displacements in the concrete. We need to prevent any major displacements in the concrete or else cracking would occur, detracting from the usability and the aesthetics. Unfortunately, we cannot stop all the concrete movement or displacement. However, joints placed at strategic locations in the concrete will allow for movement and therefore reducing the risk of cracking.
Joints also stop force from transferring between one part to the other. These are known as isolation joints.
During concrete pours, concrete undergoes chemical reactions as the concrete sets. This causes thermal expansion and therefore displacement/movement in the concrete. Another cause of thermal expansion is due to changes in temperature. To limit thermal expansion, we have a few options available:
- Design the concrete mix using admixtures.
- Pour smaller quantities of concrete.
- Use reinforcement bars.
If you haven’t checked out the post about concrete design mixes, you can do so here. Using admixtures can stop/reduce thermal expansion in the concrete, but this is unfortunately not economical. On some occasions, client briefs and specifications may state that no joints are permitted.
Therefore, the primary method is to pour smaller quantities of concrete. Construction joints allow for multiple pours. If required, we can also dowel the concrete slabs together to provide restraint and sliding resistance.
Where Do We Find Joints in Concrete?
Concrete joints can be found most commonly in slabs, but they also occur in other locations. The list below shows where you may encounter them:
- Concrete Slabs.
- Retaining Walls.
- Garage driveways (Concrete Surfacing).
The next time you are in a building with a concrete slab, look out for the joints. In the recommendations below you will see where they should be expected.
Types of Materials Used in Concrete Joints
Different types of materials can be used in concrete joints. It is important to use elastic materials for isolation joints to allow movement.
For construction joints we can use:
- Precast Concrete
For isolation joints we can use:
- Asphalt-impregnated fiberboard
- Closed cell neoprene
- C2 products
If a good quality of finish is required for the isolation joint, elastomeric sealers can be used to replace a thin layer of the joint.
Construction, Expansion and Contraction Joints
Concrete joints can be separated into three main categories, or three types; construction joints, expansion joints and contraction joints.
Construction joints as described above, allow for the concrete member to be poured in multiple phases.
Concrete expansion joints separate the concrete into two parts, which stops cracking in the concrete by allowing movement. Movement in concrete typically occurs due to loads and thermal expansion. Generally speaking, expansion joints are not required for internal slabs as the temperatures are more controlled. The amount of concrete poured in slabs generally does not induce a significant amount of chemical reaction either.
Contraction joints are different to expansion joints since they allow the concrete to crack by inducing cracking in a structural weakness. Controlling the cracks in this way causes them to generally occur in straight lines, which will cause minimal disruptions to the use (serviceability) of the concrete.
Details and Types of Concrete Joints
The first two details below show two types of construction joints. They both allow for multiple pours to make up the complete concrete element. A butt-type configuration (shown below) is the most common. If load transfer is required between the two slabs, then a dowel can be used. Reinforcement steel is stopped before the joints on either side.
Construction Joint with a Dowel
Saw Cut Joints
The saw cut joint creates a weakened area in the concrete, which in turn invokes cracks at locations that are deemed most satisfactory. The area is weaker due to the lower area, reducing the amount of permissible stress.
Saw Cut Joint
Isolation joints are located around columns (steel, timber etc.), drain pipes and in any location where it is not desired for the force to enter the concrete member. Normally the force would be transferred directly into the substructure such as the foundations.
Recommendations For Best Practises
Here is a checklist that we should follow to ensure we use the correct types of joints in concrete to prevent/control cracking:
- The engineer should mark on the concrete general arrangement the location and type of all concrete joints, including the details.
- The areas of the concrete should be kept as close to square as possible. Typically we follow the rule that the joints should be at 6m (approx. 20 ft) intervals maximum. Others have suggested it to be between 24 and 36 times the depth of the concrete.
- Saw cut joints should be between ¼ (one quarter) to ⅓ (one third) of the depth of the concrete.
- Isolation joints should be used if no force is to transfer between the separate concrete elements, such as around columns.
- Reinforcement should be stopped before the joint and continued after.
- If load is required to be transferred through the slabs, a mechanism should be provided, such as a dowelled connection.