Incident Report

Subject:                     Foamed concrete explosion - HSE investigation update

Date of Email report:   Fri 30/07/201

Report Detail:

Updated Information 07 July 2010

This text provides an update to information published by HSE on 3 December 2009 following an explosion in August 2009 involving foamed concrete in which two persons were injured. A contractor had filled a pit to a depth of some 6m with foamed concrete. Whilst this was setting, workers started to remove steelwork using angle grinders. There was then an explosion beneath a steel walkway on which two contractors were standing, and this blew the steel plates and the workers up into the roof of the building. The cause of the explosion was ignition of the flammable gas hydrogen evolved from foamed concrete. The liquid concrete had been aerated, or foamed, by mixing air into it by agitation in the presence of a foaming agent (surfactant). There is evidence that hydrogen was produced from the concrete as it was setting. The particular location of the pour appears to have allowed a flammable/explosive atmosphere to accumulate within a relatively confined space beneath the walkway. HSE has investigated the mechanism by which hydrogen was generated. The particular concrete mix included incinerator bottom ash aggregate (IBAA), which has been shown to contain a significant proportion of aluminium. (Tests on other raw materials involved, e.g. crushed concrete and glass frit, showed insignificant presence of aluminium). Aluminium is known to react with cement/concrete mixtures to form hydrogen gas.
Where use of IBAA is being considered it is advisable to adopt the following precautions:

• Test the IBAA for the production of hydrogen by adding a sample to a solution of sodium hydroxide. If no bubbling is observed after several hours, the aggregate can be used in foamed concrete mixtures;
• If aggregates which produce bubbling when added to sodium hydroxide solution are to be used in foamed concrete, further checks should be made for hydrogen evolution when the aggregate is included in a test sample of the intended concrete mixture. If no bubbling is observed after several hours on laboratory-scale, the IBAA can be used in applications of foamed concrete;
• If concrete mixtures which produce hydrogen are to be used in civil engineering projects, the risk of fire and explosion must be assessed. Adequate natural or forced ventilation should be provided to keep the concentration of hydrogen in air well below the lower explosive limit. The ventilation requirements can be established using maximum rates of hydrogen production per unit mass measured in laboratory-scale experiments. In addition, sources of ignition should be avoided in the working area.

HSE confirms that it has taken no action requiring the omission of IBAA from foamed or general concrete mixtures for use in civil engineering works, and has thus not 'banned' IBAA from being used.

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.