Concrete crisis blighting NHS hospitals echoes 1970s HAC debacle

Now history is repeating itself, according to safety body CROSS-UK. Building owners across Britain are being advised to determine whether their premises have precast elements of RAAC (no easy task) and if they do, to consider what temporary then permanent measures must be taken to restore safety.

Already, schools and hospital with RAAC are having to be propped following a number of high profile structural failures. A major weakness is becoming apparent in what was originally seen (as was HAC) as an innovative and profitable building development.

In common with HAC, RAAC ‘borrowed’ technology from other industrial processes in a move to speed the manufacturing turn around of precast concrete units which over time appears to have backfired. This almost exactly mirrors the case of precast elements made with high alumina cement.

RAAC roof collapse in Essex school in 2018

Engineers now appreciate there is a weakness common to both forms of concrete: that of loss of strength while performing in situ.

Just as HAC could ‘convert’ over time into a less stable condition, meaning the concrete elements cast with it could no longer sustain their design loadings, so super heat cured, aerated RAAC’s vulnerability to moisture ingress means any reinforcement lacking sufficient protection becomes ultra liable to corrosion.

This being so, loss of tensile strength appears to be major factor in the failure of RAAC floor and roof panels. There could also be other factors at play - see below.

“The stories of precast concrete units made with HAC and those of RAAC do seem remarkably the same in some areas,” said Sydney Lenssen, founder editor of NCE which reported HAC events at the time.

“Nobody to my knowledge was killed in HAC collapses and I understand that, in the recent school roof collapse of RAAC ceiling planks, nobody was hurt there either. Which is the good news.”

The bad news began in 1973 when the assembly hall roof of the Camden School for Girls collapsed, fortunately in the early morning while the hall was empty. Desks and chairs had been laid out for girls scheduled to sit exams that day.

Very substantial long span traverse precast prestressed beams made with HAC crashed down from their supports. A few months later, a similar collapse occurred, of the HAC roof above the swimming pool at Sir John Cass school in Stepney. Again the beams came down in the early morning, with nobody in the pool at the time.

The Building Research Establishment (now BRE) investigated the chemical conversion process deemed to bring about strength loss and confirmed that changes to HAC’s mineralogical composition were the cause; and that the presence of water (with pooling not unknown on flat roofs, and condensation common above swimming pools) was a contributory factor.

Also, in the case of the school in Camden, the supporting nibs were far too small: the roof beams only had to bow a little before they lost support. In addition, lateral ties between the roof structure and the assembly hall’s walls were missing or otherwise inadequate.

High alumina cement was initially produced and marketed by Lafarge for the heat resistant linings of industrial chimneys. It was noted, however, that HAC concrete cured and hardened more rapidly than ordinary Portland cement (OPC).

Precasters used the material for beam and column products destined for the private as well as public sectors; aiming to increase turnaround and thereby profits by more quickly striking and re-using concrete forms.

Development of RAAC followed a not dissimilar path (although the material was initially designated AAC, autoclaved aerated concrete, without reinforcement. The decision to add re-bar came later).

The intention was to create a lightweight concrete, with OPC and aggregate mixed with aluminium as an expansion agent. The aluminium creates air bubbles thus generating a low density material. Again the intention was to speed full curing as much as possible.

The Swedish developers turned to autoclaves, which are heated containers traditionally used for chemical reactions and other processes. They employ very high temperatures and pressures and are probably best known as sterilisers of medical implements.

Curing aerated concrete under autoclave heat and pressure takes as little as eight to 12 hours. Whether the combination of aluminium aeration and autoclave techniques contributes in some way to long term strength loss and failure of RAAC is not yet known.

Research into the cause of elements made by the material collapsing is on-going

“HAC conversion took some time to establish,” Lenssen said. “Who knows: perhaps a similar phenomenon will emerge as the cause of RAAC becoming weak.”

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