Restraint

Summary

Restraint - Restraint inhibits the movement of the slab, leading to an build up of stresses within the concrete.

Technical Information

Excessive restraint is potentially damaging for floor slabs and can lead to extensive cracking.

A slab will contract during and after hydration; therefore attachment at its edges will induce tensile stresses. Because newly placed concrete is very weak in tension, this can easily cause mid-panel cracks to form. The same can be seen where the slab abuts a fixed element and cannot move during drying shrinkage.

The following measures can be utilised to reduce restraint and help control associated issues, such as random, uncontrolled cracking:

Casting the slab over a sub-base with a controlled surface tolerance, in accordance with the recommendations in TR34.
Laying the slab over a slip membrane to further reduce friction.
Installing an isolation joint around any fixed elements that interact with the slab, such as columns and walls.
Installing sawn restrained-movement joints to incrementally relieve the build up stresses.
Careful consideration must be given to the joint layout as a whole.