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
- 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.