Upper floor and roof decks

In February 1999 the Standing Committee on Structural Safety advised owners of both school and non-school buildings that have pre-1980 RAAC plank roofs to arrange for these roofs to be inspected if it had not been done since 1994, although generally the deterioration of RAAC planks may not jeopardise structural safety (SCOSS Bulletin 3, February 1999). Tests by BRE have supported this view, but firm recommendations have been given for the regular inspection of roof planks of this type - particularly those erected prior to 1980.

The stability of more modern imported panels has been called into question and because of this RAAC planks are not considered to be durable materials. References to them in BS 8110 and prEN 12602 have been dropped for fear of lending 'respectability' to what is viewed to be a relatively short-life material.

Possible defects to look out for include:

• slippage of the reinforcement under load (because the material is soft and easily crushed, it does not secure the reinforcement as solidly as with conventional concrete), slippage could lead to deflection;
• crushing of bearing ends;
• corrosion of light steel reinforcement;
• transverse cracking on the soffit.

Tests undertaken by Matsushita et al. have shown that the microstructure of AAC changes during carbonation, resulting in degradation such as cracking, with the degree of carbonation increasing over time. (Matsushita, F., Aonon, Y. and Shibata, S., Microstructure Changes of Autoclaved Aerated Concrete during Carbonation under Working and Accelerated Conditions, Journal of Advanced Concrete Technology, Vol. 2, No. 1, Japan, Concrete Institute, 2004, pp. 121–129)

Following the identification of RAAC planks, the following initial checks should be undertaken:

• Check for evidence of cracking on the soffit.
• Check for evidence of deflection in the planks (it may be necessary to measure this accurately as part of a more detailed investigation).
• Determine the age of the structure - planks used before 1980 are more susceptible to deflection and cracking.
• Check the condition of the roof coverings - prolonged leakage could encourage corrosion of the steel reinforcement.
• Check the bearing widths. If less than 40mm seek further advice.

Should the above suggest that a more detailed investigation be in order, the procedures in BRE Information Paper IP10/96 can be followed.

Possible recommendations may include:

• Inspect every 5 years, or if located in a harsh environment check each year.
• Reduce dead loads by removing stone chippings, felt over-layers, plant, etc.
• Check for transverse cracking and if appropriate measure deflection of the slabs.