CONCRETE FOUNDATIONS on brownfield sites worldwide could be under threat from 'concrete-eating bugs' which leave affected structures vulnerable to accelerated sulphate attack, an American forensic civil engineer has warned.
A 15 year old six storey office block on the US East Coast has been demolished because its foundations were eaten away, Dr Naysan Emami of Exponent Failure Analysis Associates told delegates at a London forensic engineering conference last month. 'There's no reason why the same problem shouldn't be found anywhere in the world - and sulphate resisting cement is no answer,' she warned.
Following demolition of the building - its location was not revealed for legal reasons - EFAA was called in. The building had been condemned after one corner was found to be sagging more than 200mm and many of the precast concrete piles had virtually disintegrated. Sulphate attack was blamed, but investigations showed a more complicated picture.
'Sulphate levels were only moderate and only some of the piles were affected,' Emami said. 'It soon became evident that the sulphate attack was confined to areas where the piles passed through anoxic silts with very high organic contents.'
Further investigations identified high levels of sulphate-reducing bacteria. Crucially, however, it was found that free iron levels were very low. Emami explained that in the absence of oxygen, bacteria were more likely to use iron than sulphate to complete the oxidisation of organic matter.
With low iron levels in the silts, the bacteria used the iron in the concrete pile matrix. But research has proved that the way to increase sulphate resistance in cement paste is to increase the ratio between the iron and alumina content. 'Conversely, if the iron content is reduced significantly, the rate of sulphate attack increases dramatically,' Emami said.
Although normal Portland cement was used in this case, she added, the steam-cured piles had a low water/cement ratio compliant with US and British codes for sulphate resistance in moderate exposure conditions.
EFAA detected a distinctive 'iron front', a very thin iron-rich layer between corroded and sound concrete, in all the piles which passed through the silt, but not in those that were founded in uncontaminated soils. It also found that the silt concerned was contaminated by sewage and by diesel fuel spillage, but any organic material could produce the same effect.
Emami said checks for thaumasite formation in samples from the piles were negative.