Applied exterior finishes

External finishes to concrete structural elements are many and varied, offering different performance characteristics, appearances and costs. During the post-war period, several products were popular:

• tesserae;
• ceramic tiles; and
• render, in particular Mineralite render.

All 3 finishes share a common thread in that they rely on adhesion firstly to a suitable substrate (usually render) and secondly good adhesion between the substrate and the background (often concrete). Typically, points of failure are:

• tile adhesive adhesion failure;
• background adhesion failure;
• cracking;
• background failure;
• water penetration and associated frost or salt damage;
• poorly positioned movement control joints or lack of continuity of joints; and
• unsuitable movement joint materials.

Failures in the adhesion of the background render can give rise to serious health and safety risks.

To overcome the difficulties of achieving a plain, regular background, it was often necessary to introduce an intermediate layer between the background material and the finish. Usually this was of cement render, perhaps 10-15mm in thickness and preferably no more than 20mm. Normal rendering techniques and design rules applied in limiting shrinkage and suction. However, it is the interface between the rendering and the background material that is the critical part of the whole finish and when serious failure of surface finishes occurs it is more often the failure of the adhesion of the render rather than the tile adhesive.

From the late 1950s various experiments were tried with bonding agents - PVA initially, and later acrylic resins and SBRs. Both PVA and acrylics were water soluble and so vulnerable to water ingress, some of which was bound to occur. Weakening of the bond between the render and the background could create plates of delaminated materials that would suffer from frost action and fail suddenly.

Other failures could result from traces of mould oil left on the surface of the concrete background, or alternatively failure to remove laitance or, if a bonding agent was not used, a failure to provide adequate mechanical key or suction.

The combined weight of finish and rendering is significant (probably around 50kgm-2), so the consequences of failure are much higher than with localised failures of individual tiles.