Masonry facades

The alternative to support by brick nibs or support angles is to continue the floor beam through to the face of the brickwork or to stop it short of the face by 50mm or so and to tile it with slips of cut bricks. Such construction was popular in the 1970s.

Failures in the adhesion of brick slips had led to caution over the use of this material. For several years, brick slips were included in lists of deleterious and hazardous materials simply for the fact that slips could become detached from a building and cause a safety hazard.

The use of 20mm brick slips became common in the 1970s as a means of disguising the edge of a floor slab or downstand beam in situations where an expressed concrete frame was considered to be undesirable. The slips were often bedded on polymer modified mortar and attached as tiles or in some cases had additional mechanical fixings (although to begin with this was rare).

A failure to accommodate movement in the frame and brick cladding can lead to serious failures. The mechanism of failure mainly involves shrinkage of the concrete frame and expansion of the clay brickwork. A failure to provide a soft joint between the normal courses of brick and the brick slips could lead to the transfer of forces to the thin slips which in turn are then forced off the building. Even the provision of soft joints could be problematic if they are locally of insufficient thickness or have become bridged by mortar.

The bonding of slips to the concrete frame could be problematic if there was insufficient key, or if the face of the concrete was contaminated with mould oil or other contaminants. Similarly, if the slips were cut on site, it was vital to remove brick dust from the bonded face to ensure that a suitable bond was achieved. The use of PVA building adhesives was unsatisfactory since PVA is water soluble; SBR adhesives may have given a better performance. Epoxy based mortars were expensive and so during the 1970s were unpopular. Furthermore, variability of the line of the concrete frame could result in the backing adhesive being of varying thickness, meaning that some slips would need to be supported temporarily while the adhesive set. More importantly, drying shrinkage resulting from varying thickness could create planes of weakness.

Illustration of potential cracking due to shelf angles being too short

Brick slip failure due to contraction of frame and expansion of brickwork

Brick slips spalling off; note outward bowing of remaining slips to the left of the image

If the expected life of the external fabric could reasonably have been predicted at 60 years, it is doubtful whether any adhesive-based systems could have given satisfactory performance for that period of time. Indeed, one current manufacturer of polymer modified adhesives predicts a life of around 20 years.

Given uncertainty over the life expectancy or effectiveness of adhesive-based systems, some form of mechanical fastener could have been expected to give better performance. One type of fastener comprised a stainless steel clip that fitted into a slot in the top edge of a floor slab and then hooked into a small slot in the bottom edge of a slip. Another alternative was to provide a flat metal plate beneath the slips to deal with the effects of gravity and to use a mortar or epoxy modified mortar although this is subject to the limitations expressed above.