During curing, water and air move towards the
top of the slab - due to their low relative densities. If a slab is finished
too soon it closes the slab surface preventing bleed water and/or air from
escaping, which results in the accumulation of water and/or air voids just
below the top surface, leading to the development of a weak plane. The concrete
above the weak plane can then break off very easily during the use of the
concrete slab, causing the surface to become rough and unusable.
The factors affecting delamination occurring
are primarily based on the exit of the water and/or air. Some of these factors
- Differential setting throughout the slab - Accelerated drying of the slab surface due to cross winds can
significantly increase the chances of delamination if the surface is
- Differential temperature throughout the slab – The internal temperature of the slab must be considered before
trowelling. The lower the internal temperature of the slab, the slower it will
cure and the longer it will bleed. This must be considered before trowelling.
- Air content – Air entrained from admixtures should be minimised by careful
selection of the admixture, before power trowelling. Entrapped air from the
concrete mixing or agitation must be minimised by efficient compaction and
consolidation. Compacted concrete will generally retain <1% of entrapped air
but some admixtures can entrain additional unwanted air.
- Bleed characteristics of the concrete - Bleed is very important in
relation to the escape of the excess air. Adjustments to the fine aggregate grading will permit the air to escape early before the
surface has any chance of sealing. An early bleed is required especially when dry-shake topping is used.
- Application of Dry-Shake toppings - The risk of delamination
is increased when using a dry-shake topping. Sufficient bleed water is required
to thoroughly wet the dry-shake. This indicates that the water is present to
hydrate the cement and that bleed channels exist through the topping to allow
air to escape.
Delamination can be
repaired by removing the entire surface in the affected area, preparing and
cleaning the newly exposed surface (by scabbling or similar) then filling with
a cementitious or resin-based mortar system.