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Foamed concrete explosion - HSE interim position 03 December 2009

This document has been produced to provide information on an explosion
involving foamed concrete and provide interim advice whilst HSE continues its
investigation.

There was an explosion which injured two people in August 2009. A contractor
had filled a pit with about 6m depth of foamed concrete. Whilst the concrete was
setting, workers started removing steelwork using angle grinders. There was an
explosion underneath the steel walkway on which two contractors were standing
and which blew the steel plates and the workers up into the roof.

HSE is investigating. The foaming agent in this concrete mix was air produced
by the addition of surfactants and agitation. It appears that while the concrete
was setting, the flammable gas hydrogen was produced. The particular location of
the pour within a relatively confined area beneath a walkway appears to have
allowed a flammable/explosive mixture to have developed.

HSE has been investigation the means by which hydrogen was generated.
Aluminium is known to react with cement/concrete mixtures to form hydrogen. The
particular concrete mix included incinerator bottom ash (IBA) which is suspected
of being the source of aluminium. Tests on raw materials and the mixed concrete
revealed the presence of aluminium. Tests to confirm the suspected mechanism
have not yet been completed.

In the meantime the supplier of the IBA has been asked to advise customers of
the following:

foamed concrete mixes containing IBA or other recycled materials should be
poured in the open air;

if such mixes are to be poured within a building or confined area,
adequate ventilation must be provided and the entire surface of the poured
concrete should remain visible until it has set;

sources of ignition such as naked flames or spark-generating tools (e.g.
disc cutters, angle grinders) should be kept away from the concreted area
during the pouring and setting process.

HSE has not taken any action requiring the removal of IBA from concrete
products.

HSE is not investigating any other similar explosions.

HSE estimates that the scientific tests will be completed by the end of 2009
at which time we will consider what further action needs to be taken and will
update this information note.

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Updated 03.12.09

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