Repairs to reinforced autoclaved aerated concrete

Reinforced autoclaved aerated concrete (RAAC) was a popular choice as structural material in educational, commercial and industrial buildings between 1950 and 1980. It was primarily used for pre-cast wall panels and flat roof planks in factory and warehouse blocks. 'Siporex', for example, was a common proprietary brand of RAAC wall panel.

However, the term autoclaved aerated concrete (AAC) is a slight misnomer in that it is not a true form of concrete. AAC is not concrete in its constituent materials or in its physical properties (Noy and Douglas, 2005).

AAC was also used for blocks in blockwork walling as well as for pre-cast wall and roof panels in low-rise residential properties. It is made under high-pressure steam-curing conditions by introducing bubbles of gas into a cement or lime mix. The finished product is a uniform cellular material that could be classed as a 'foamed mortar' - although it is sometimes described erroneously as 'foamed concrete' (Noy and Douglas, 2005). In one sense it is analogous to a 'no-coarses' concrete, in contrast to 'no-fines' concrete. As a result RAAC is relatively lightweight and has good thermal insulating properties.

RAAC, though, like ordinary Portland cement (OPC) is susceptible to water-induced degradation. Interstitial condensation and rainwater penetration are its main moisture-related deterioration mechanisms (Noy and Douglas, 2005). These can lead to corrosion of the reinforcement. Along with creep this can cause elements such as roof planks to sag by over 50 mm - depending on the span. Structurally AAC blocks and planks are susceptible to following main problems:

• Cavity walls containing AAC blocks may have insufficient flexural strength to transfer wind loads or be poor at resisting impact loads, all of which would be exacerbated by poor condition of the masonry or the lack of ties between the leaves or the inadequacy of restraint fixings.

• As their modulus of elasticity is low AAC planks are not as strong as reinforced concrete slabs and are therefore more prone to sagging. When they are used as the structural deck on flat roofs this results in ponding.

• 'Siporex' flat roof planks may have a lower factor of safety against uplift than required by the current British Standard owing to inadequate holding down straps.

• There is a risk of shear failure occurring at the bearing of roof planks on the wallhead.

The extent of distortion in RAAC panels found during the initial survey of a building will determine the required response. Generally, the following actions would apply in the circumstances outlined:

• Deflections causing significant ponding, replace the roof.

• Deflections greater than 1 in 100, replace the roof.

• Deflections greater than 1 in 150, monitor annually.

• Deflections greater than 1 in 200, monitor every 5 years.

Traditionally, the repair method would involve replacing the defective deck. This is of course an expensive, time-consuming and disruptive option.

However, 'Metsec Building Products', have devised a suitable repair method that obviates the need for re-roofing these decks. It involves installing under the soffit of the RAAC planks a lightweight arrangement of steel castellated and lattice beams. The castellated beams are about 175 mm deep (depending of course on the span) and positioned at 2.4 m centres. A sub-grid of lattice beams 100 mm deep is positioned at

800 mm centres between the castellated beams. Special tubing is installed between the 50 mm nominal gap between the top of the beams and the soffit of the planks. It is then inflated to jack up the defected planks using a patented process called 'precise air lifting'. Non-shrink grout is then forced into the gap between the top of the beams and soffit to keep the planks in position once they have been lifted.

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