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CROSS Safety Report

Chemical attack on structural enclosures

Report ID: 912 Published: 20 May 2024 Region: CROSS-UK


Overview

A reporter has witnessed instances where the corrosive atmosphere created by waste water treatment and refuse recycling plant being enclosed has caused the structure to become initially unserviceable and, in due course, hazardous.

Key Learning Outcomes

For asset owners/managers, and civil and structural design engineers:

  • If a chemical attack on a structure is possible, specialist literature or expertise may need to be consulted
  • As highlighted in this report, the best solution may be to manage the environment in a way that reduces the demand on the structure
  • Part of durability is building in the capacity to inspect structures to ensure they can be regularly inspected during the lifetime of the structure

Full Report

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The reporter believes the problem of airborne chlorine attack is well known for the structures enclosing indoor swimming pools. However, they have witnessed instances of a similar phenomenon with enclosed plant for waste water treatment and for refuse recycling. In both cases, what are typically ‘outdoor’ process plants have been enclosed and the effect of the corrosive atmosphere has caused the reinforced concrete structures to become initially unserviceable and, in due course, hazardous (see Figure 1). The reporter states that mechanical and electrical equipment also suffered accelerated degradation.

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Figure 1 - badly deteriorated concrete enclosure to utility pipe

In the waste recycling example, the reporter shares that a number of significant structures had to be replaced after just two years; galvanising was found to have been no defence to the corrosive environment. The cost incurred was not just for the structural, mechanical and electrical works but also for maintaining the recycling operation in temporary facilities.

In the wastewater treatment example, hydrogen sulfide (H2S) in the atmosphere was responsible for considerable degradation of reinforced concrete roof structures over a period of around 30 years. The levels of H2S were subsequently reduced by implementing a more stringent regime of daily plant operational management. The reporter suggests this exemplifies why a structural engineer should think carefully about what atmosphere might occur, not just what atmosphere it is intended will occur.

The reporter’s challenge to the industry is to better recognise when process plant enclosure will lead to accelerated, and potentially hazardous, degradation.

Expert Panel Comments

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An Expert Panel comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-US Expert Panel page.

Concrete design standards highlight the need to consider chemical attack arising from the use of the building or structure. However, standardised methods for resisting chemical attack of concrete are mainly concerned with attack from soils and ground water. This is because, while uncommon, the range of potential chemical actions is large. Therefore, where chemical attack is possible it is likely that specialist literature or expertise will need to be consulted.

There have been a number of technical reports over the years to suggest that durability of structures is really not properly considered. All 'design weight' is applied to numbers, whereas design life and structural capacity are profoundly affected if the structure lacks durability. A net zero agenda involving reuse would be undermined if, rather than its life being prolonged, a structure fails before its notional design life is up. 

Part of 'durability' is building in the capacity to inspect structures, and there have been several CROSS Safety Reports over the years describing where inspectability was lacking, resulting in extremely bad deterioration or failure.  Anything by the sea is at also at risk (saline environment). Galvanising is no panacea, as noted in this report, and we know the historical problems of cavity walls which incorporated galvanised wall ties.

The Panel agrees that the design community needs more guidance on the issue. As highlighted in this report, the best solution may be to manage the environment in a way that reduces the demand on the structure.

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