What is shrinkage?
As fresh concrete hardens it loses part of its water and gets smaller in volume. This is called shrinkage. A ground slab rubs against the ground beneath it, which restrains the shrinkage and creates tensile stress inside the slab.
If the stress exceeds the tensile strength of the concrete, a crack forms. Joint cutting is the way to choose in advance where that crack will form.
The main types of joint
Three joint types are common in ground slabs:
- Control (saw-cut) joint: cut into the slab surface to guide the crack
- Isolation (expansion) joint: around columns and walls, letting the slab move independently of the structure
- Construction joint: formed where the pour stops
Why does cutting time matter?
If cut too early, the joint edges can ravel. If left too late, the crack can form by itself before the joint. The right window varies with the concrete mix, air temperature and wind, so the decision is made on site by looking at the state of the concrete.
Joint spacing also depends on slab thickness, the reinforcement system (steel fibre or mesh) and the subgrade. That is why we do not quote one spacing; the joint plan is prepared for the project.
Joint filling and curing
On floors with forklift traffic, joint edges can wear over time. Joint filler protects those edges. Curing helps reduce the rate of shrinkage and the risk of cracking. For our service details see the ground slab page.
