Concrete Delamination

Summary

Concrete Delamination - is the debonding of a thin layer of concrete from the surface of a concrete slab.

Technical Information

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 include:

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 prematurely sealed.
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.