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    CONCRETE EXPLOSION CAUSE AND PRECAUTIONS CONFIRMED

    Flammable gas generated by cement reaction with incinerator bottom ash

    In August 2009 an explosion occurred on a construction project in Hertfordshire during the use of foamed concrete. Two workmen were injured in the blast.

    Foamed concrete had been poured into a pit to a depth of some 6m. During the setting process workers started to remove steelwork using angle grinders.

    An explosion occurred beneath a steel walkway on which two contractors were standing thereby forcing the walkway and the workers into the roof of the building.

    Flammable nitrogen gas ignited

    The cause of the explosion has now been confirmed as the “ignition of the flammable gas hydrogen evolved from foamed concrete”.

    The liquid concrete had been aerated (foamed) by mixing air with the foam by agitation in the presence of a foaming agent (surfactant). Evidence is that hydrogen was produced from the concrete as it was setting.

    The particular location of the pour allowed a flammable/explosive atmosphere to accumulate within a relatively confined space beneath the walkway.

    Bottom ash aggregate escapes ban 

    HSE has investigated the mechanism by which hydrogen was generated.

    The 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.

    The HSE report states that the investigation is concluded and no action has been taken requiring the omission of IBAA from foamed or general concrete mixtures for use in civil engineering works.

    It is now confirmed that HSE has “not ‘banned’ IBAA from being used”.

    Precautions recommended

    Where use of IBAA is being considered HSE advise the following precautions: 

    1. “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;
    2. 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;
    3. 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.”

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