RAAC shear: the question is at what load

Shear is a force, not a label

Shear failure is not a property a material has or does not have. It is what happens when the shear force on a section exceeds what that section can carry. Every material has that limit. The engineering question for a RAAC roof is the same as for any other structure: what load does it carry in service, and how does that compare with the load it can take?

That is why a statement that RAAC "fails in shear" is not useful on its own. Without the force involved, and without comparing it with the working load on the roof, it cannot tell an estates manager whether a plank is safe or what it needs.

What our testing shows

We have carried out our own shear tests on RAAC. Under normal working loads we have never observed RAAC failing in shear. We are very aware of the flexural issues RAAC can have, and that is where our assessments and our designs concentrate.

Every roof is unique

As with any engineering question, the answer depends on the roof. We are not saying that no RAAC plank can ever fail in shear, and we are not saying that every plank does. Treating every RAAC plank as a shear failure waiting to happen condemns an entire group of materials by default, and that is not engineering. Each roof, and each plank type within it, is assessed on its own evidence and designed for on its own merits.

The real risks in a RAAC roof

• Flexure and deflection across the span.
• Water, which weakens the aerated concrete, adds weight and corrodes the reinforcement.
• Condition: cracking, spalling and corrosion.
• Bearings that are short or damaged.
• Planks that have been cut for roof lights and services, or loaded with plant they were never designed to carry.

We survey and measure all of these, plank by plank, and assess the findings against the current IStructE RAAC guidance. Every assessment is signed by a Chartered Engineer and Fellow of the Institution of Structural Engineers.

Why more strength is not the answer

It is tempting to treat RAAC as a material that simply needs more strength added. That is the wrong approach, and it can be a dangerous one. Aerated concrete has a low compressive strength. Keep adding strength to the tension face of a plank and the weak point moves into compression. A plank that has been over-strengthened is not a safer plank.

The right amount of strengthening is the amount that particular plank needs. That is why we tailor make every CFRP solution for each roof and each plank type, designed by John Staves and manufactured in the UK to that design. Ours is the UK's first CFRP system designed specifically for RAAC, insurer approved and extensively tested.