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926 Emergency motorway lane closure during concrete repairs

Report ID: 926

Published: Newsletter 59 - July 2020

Report Overview

Confusion during concrete repair works led to an unplanned emergency lane closure of a bridge carrying a motorway.

Report Content

The incident involved an unplanned emergency lane closure of a bridge carrying a motorway. This was required to mitigate the risk of overload of the bridge after excessive concrete removal to the soffit during repair works. The bridge consisted of 3 spans with half joints for a central suspended span. Leaking road surface water through the half joint and longitudinal central reserve joint had contributed to extensive concrete repairs being required to mitigate the risk from falling spalled concrete and further reducing structural capacity (Figures 1 and 2).

A structural assessment was carried out prior to the concrete repairs which put detailed constraints on the amount of concrete to be removed from the soffit. These constraints included limits on the extent of repairs over-night, which were highlighted on the drawings and as the first key risk on the Pre-Construction Information passed to the Contractor and subsequently the Sub-Contractor.

A sequence of misunderstandings led to a larger area of removal, which exceeded the specified constraints. The extensive areas of spalling on the central span near the half joint led the site team to believe, incorrectly, that the constraints were only applicable to this area, not the side spans. The hydro-demolition team cut out a much larger area of concrete than had been planned (Figure 3).

An assessment was then carried out by the designers which found that the structure did not now have enough capacity for the required live loading. Poor deck reinforcement detailing, combined with the extensive concrete removal, meant that permanent, significant deformation and damage could have occurred. This led to an instruction to immediately close the lane above the repair area and to repair the soffit the following nights (Figure 4).

The incident highlighted specific lessons learnt for the scheme, but also wider lessons learnt for concrete removal schemes:

  • The start of shift briefings to the hydro-demolition sub-contractor were not highlighting key constraints.
  • A clearer visual method of highlighting the constraints on the drawings may improve clarity on site to cover, where operatives may not be reading the full details of the agreed procedures.
  • Hydro-demolition teams should always be made aware of constraints and the maximum areas they can remove. The areas should be approved and accepted by a competent supervisor from the designer’s team prior to its removal, especially when working in a protected area enclosed by non-transparent sheeting.
  • An Emergency Procedure Document needs to be prepared before the start of works and briefed to the site supervision team to avoid leaving a site team uninformed of possible risks and/or placing them in a situation where they are forced to make an un-informed decision under intense pressure.
  • The sample hammer survey used to form the original scope of works indicated repair areas significantly smaller than the final repair areas found on site with the full, detailed hammer survey. This led to constant programme and cost pressure on teams and less use of the previous repair extent drawing which highlighted the constraints. Full hammer surveys of the whole structure immediately prior to scheme procurement would allow proactive planning of works instead of reactive planning.
  • Concrete repairs recommended in several previous Principal Inspections and Management Strategies were not given funding until extensive deterioration that risked affecting bridge capacity had occurred. This possibly highlights limited funding to manage a large stock of deteriorating structures, forcing reactive instead of cost-effective proactive maintenance.

Comments

Apart from the technical issues in this report, recurring themes can be identified:

  • Communication of design intent which is vital but not always easily achieved;
  • As in the first report of this Newsletter (Report 894), designer absence from site is detrimental, particularly on a project like this where there must be uncertainty;
  • An objective when planning must be to detect error before it progresses too far.

More widely, this report shows yet again that the concept of structural safety must include recognition that structures deteriorate. Some structural forms are more prone to this than others and bridge deck half joints are one such arrangement. As well as reducing the load-carrying capacity, the exposure and debonding of a significant amount of reinforcement may well change the failure mechanism of that span to one where there is less warning of failure.

It is of interest to note that concrete repairs previously recommended had not been carried out. It is probable that if the repair had been done earlier, then the extent would have been less. Whilst financial models often advocate delaying expenditure, this may not be the most economical solution if the subsequent repair is more extensive/complex.

All asset owners aspire to maintain their asset to maximise the life, however available budgets do not always permit this. Therefore, it is important to determine the condition of the asset and then identify what work can be done to address any issues within the budget. Large asset owners, responsible for hundreds of structures, use management processes to prioritise work. This relies on regular inspections and interventions must always take place before any structure deteriorates beyond repair.

The execution of the works such as those described need to be supervised by a competent person, who is able to advise on the changing extent of the repairs, and how these would affect structural safety. Often, more substantial demolition and repairs are required than were originally planned. The reporter is right about the necessity for a readily available Emergency Procedure Document (contingency plan) – and everyone being aware of its contents.

 

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Images

Figure 1: Initial damage


Figure 2: Loose concrete removed


Figure 3: Hydro-demolition


Figure 4: Repaired section



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