CONCRETE-EATING BUGS which attack foundations on brownfield sites are threatening to leave structures worldwide vulnerable to accelerated sulphate attack, an American-based forensic civil engineer has warned.
A 15 year old six storey office block on the east coast of the US has already been demolished after its foundations were eaten away, Dr Naysan Emami of Exponent Failure Analysis Associates told delegates to a London forensic engineering conference this week hosted by ICE.
'We could be sitting on a ticking bomb,' she said. 'There's no reason why the same problem shouldn't be found anywhere in the world - and sulphate resisting cement is no answer.'
For legal reasons the location of the building attacked could not be released, and Emami said EFAA was only called in to investigate after the building was demolished on the orders of the municipality.
The building was condemned after one corner was found to be sagging more than 200mm. Many of the precast concrete piles turned out to have virtually disintegrated. Sulphate attack was blamed, but when EFAA began its investigations a more complicated picture emerged.
'Sulphate levels were only moderate and only some of the piles were affected,' Emami explained. '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 site investigations identified high levels of sulphate-reducing bacteria. Crucially, however, it was found that free iron levels were very low.
'In the absence of oxygen, bacteria need another element to complete the oxidisation of organic matter that produces their energy,' Emami pointed out. 'And given a choice between iron and sulphur, they will use iron every time.'
With low iron levels in the silts, the bacteria would use the iron available in the concrete pile matrix, said Emami. But research has proved that the way to increase sulphate resistance in cement paste is to increase the ratio between the iron and alumina contents of the cement.
'This is what happens in the manufacture of sulphate-resisting cement,' she said. 'Conversely, if the iron content is reduced significantly, the rate of sulphate attack increases dramatically.'
Although a normal Portland cement was used in this case, she added, the steam-cured piles had a low water/cement ratio and complied with all US and British codes for sulphate resistance in moderate exposure conditions. 'And there was no history of any problems with piles of this type anywhere else in the area,' Emami assured.'
EFAA, she explained, was able to detect 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 later that after reading reports of the discovery of the thaumasite form of sulphate attack on bridges in the UK, she had asked for a check for thaumasite formation in the samples from the piles. 'But it was all ettringite,' she reported.
'This doesn't mean that microbial activity couldn't be a factor in the reported UK cases,' said Emami. 'Adding tests for bacteria and iron contents to normal site investigations would cost relatively little and provide valuable reassurance.'
Delegates from Ireland are expected to rush to a London conference on the accelerated low water corrosion of sheet steel piling after the reported discovery of the mysterious phenomenon in the Republic's main fishing port of Killybeg. Experience of 'steel-eating bugs' from as far away as Japan will be discussed at the conference, to be held at Great George Street on 12 October.