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CROSS Safety Report

Further report on freezing of water in hollow sections

Report ID: 579 Published: 1 January 2017 Region: CROSS-UK

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Overview

A reporter has a further warning on the issue of freeze/thaw cracking following report 434 after their firm encountered a similar problem on a UK project and investigated the matter.

Key Learning Outcomes

For asset owners and managers:

  • Regular inspections and maintenance can help keep a structure and its elements safe and detect any obvious safety issues

  • Consider including a risk assessment for internal corrosion in the inspection and maintenance regimes for external hollow section members

  • A check for internal corrosion should be carried out by a suitably qualified person where internal corrosion has been assessed as a significant risk

For civil and structural design engineers:

  • Where there is a potential risk of moisture build-up in external hollow section members, consider using a different section type that may be more appropriate for the given environment

  • When carrying out structural inspections, be aware that water build-up in external hollow sections is a possibility and, where appropriate, consider specific investigation

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A reporter has a further warning on the issue of freeze/thaw cracking following report 434. Their firm encountered a similar problem on a UK project and investigated the matter. The units had originally been plated but liquid metal assisted cracking (LMAC) was ruled out as the crack faces had no zinc on them. This was evident due to the corrosion which would have been prevented by the zinc.

The metallurgical investigation provided no clear evidence of the failure mechanism. There had been however a very cold winter that was also wet. The drain holes were originally only intended to drain excess zinc and not for water drainage. The hollow sections had large amounts of water in them which had not escaped. Freezing an undamaged unit was tried but a similar failure could not be reproduced.

While the investigation concluded that LMAC was not the source of the problem it could not be confirmed that failure was due to freezing of water. All units had been exposed in the same way but not all cracked, so it might be a small difference in the unit that was tested that avoided the phenomenon. The hollow section was cold formed not hot formed so there would have been strain ageing on the corners. This is known to reduce toughness and, coupled with the pressure of the frozen water may have been enough to cause cracking.

Expert Panel Comments

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Something of a forensic mystery but there have been previous reports of freeze/thaw cracking on hollow sections. You can search for safety information on hollow sections on the CROSS website.

Here the conventional wisdom is that hollow structural members should be effectively capped/sealed to prevent water ingress in the first place but problems with imperfect sealing have been known. Alternatively, there could be drain holes so that any water ingress does not build up to the degree that freezing might cause damage.

Feedback 1 February 2017

A correspondent has queried the term ‘plating’ used in report 579 in issue 45 of the CROSS Newsletter.

The reporter replied:

‘I can confirm that the section had been hot dipped galvanised (HDG). The use of the word ‘plating’ is not actually incorrect as HDG is plating. Further reading of the report states that drain holes had only been intended for zinc, which makes it obvious that it would be HDG and electroplating. The report is about the issues in regard to hollow sections filling with water, freezing, then failing. The corrosion protection only helped to eliminate one possible failure mechanism and is therefore not really important.’

The correspondent responded with:

‘Plating, as in electro-plating, is not the same as hot dipped galvanizing.  In the former the zinc is applied by passing a current through a plating solution, in the latter then part is dipped in molten zinc with no current passing.  There are significant are metallurgical differences between the two coatings. The mentioned LMAC mechanism occurs with hot dipped galvanizing because the zinc is liquid rather than in plating where it is not.’

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