CROSS Safety Report
Inspection and maintenance of Super-T bridge girders
This report is over 2 years old
Doubts exist about the durability and maintenance of bridge Super-T girders as a result of lack of access for inspection of sealed internal cavities.
Unseen and potentially dangerous corrosion of steel reinforcement and deterioration of concrete may be occurring in some safety critical elements of Super T-girders.
Key Learning Outcomes
For structural and civil engineers:
Ensure that safe access is provided to all safety critical elements of a bridge (or any other structure) to allow close inspection
Advise client and/or bridge owners of the need for detailed close inspection of critical structural elements both outside and inside at regular timely intervals, especially in sections where corrosion and material deterioration may have occurred
For bridge authorities:
Categorise risks of bridges built using Super-Ts depending on type of construction and location in relation to marine environments
Plan and execute an inspection plan of the cavities of any bridges in the higher risk category more than 20 years old
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Precast Super-T girders have been used in bridge construction in Australia for around 20 years and the reporter has some concern about their long-term durability and maintenance due to lack of access to inspect the sealed internal cavities. This type of girder incorporates large internal cavities, typically 5m long x about 800mm wide and up to 1.4m deep. The external webs of the girder are 100mm to 120mm thick reinforced with one central layer of reinforcement.
With the open flange Super-T that is most commonly used, sacrificial formwork is used to bridge the open box section and about 200mm reinforced concrete is placed over the girders to form the deck slab. On completion there is no access available to the internal cavities to inspect the internal surfaces of the girders.
No inspection since construction
The reporter's concern is that to their knowledge, none of these girders has been inspected internally since their introduction some 20 years ago to check for any possible spalling, cracks, or other defects on the internal surfaces of the girders due to the effect of fatigue, freeze-thaw, or other factors.
Trapped moisture may freeze
It appears to be accepted that moisture can be trapped within the cavities as drainage holes are required at the bottom of each void in the girders to discharge the water. Thus, the possibility of the creation of ice on the internal surfaces of the girders is a real possibility in subzero temperature conditions.
Poor maintenance can lead to bridge failure
There have been several bridge failures reported around the world, some with deadly consequences, with poor maintenance often being cited as the reason. Accordingly, the reporter is proposing that at least one of these bridges using Super-T girders, 20 years or older, should be inspected to see the condition of the internal cavities and to check whether the anticipated 100 years life duration can be relied upon.
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The manual 'Hidden defects in bridges - guidance for detection and management' published by CIRIA addresses voids in concrete structures amongst many other issues. It is a valuable resource; https://www.ciria.org/ItemDetail?iProductCode=C764&Category=BOOK.
- Quality on the inside face can be checked before the slabs are installed. Hence quality is not the concern. Exposure may be more severe than design but unlike outside surface this may not be evident as no visual inspection undertaken. Main issue would be contaminated leaking water.
- Core a 25mm a hole (save bits for tests) and inspect with a borescope to see if there is any any evident deterioration. This can be undertaken on many beams and would be far more informative than taking apart one beam. Consider making it a requirement that 5 such inspection holes are used per bridge with 'Tee Roffs' at 20 year inspection. I will consider specifying such a hole at construction in future bridges.
- We need to embrace reliability based durability design in Australia. A risk assessment would provide the reliability required for internal surfaces with no inspection possible. That reliability could be used to determine the cover increase necessary if no inspection holes for example.
Expert Panel Comments
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There have been many failures of bridges and other structures because of challenges associated with inspecting and problems with access to hidden regions. CROSS has recommended for years that all safety critical structures must be capable of being examined both during construction and during their operational life. Designers should take this into account whatever type of structure is under consideration and make appropriate provision for access to hidden voids.
Potential problems cannot be identified
As bridges are generally designed for a nominal design life of 100 years, durability is a primary concern and regular inspections are carried out to check their condition and to identify any maintenance requirements. However, the reporter has raised concerns about Super-T bridge girders that have large internal cavities with no access and whether these cavities should be inspected.
Poor control of dimensions in earlier construction of some Super T bridge girders
When Super-T beams were introduced around 20 years ago, there were two types, closed-flange and open-flange, both used in conjunction with an in-situ concrete deck slab - for typical details refer to National Precast Concrete Association Australia (NPCCA). To form the void in the closed-flange type, sacrificial formwork (typically polystyrene) was used that was difficult to hold in place during pre-casting, resulting in poor control over member dimensions and cover to reinforcement; and this type is no longer allowed by bridge authorities.
The open-flange type can be cast with reusable rigid (usually steel) internal forms fixed securely in place and there is much better control over member dimensions and cover to reinforcement. The NSW Department of Transport standard drawings for Super-Ts specifies web dimensions, reinforcement arrangement, concrete grade, cover, and the provision of drainage holes. The QLD Department of Transport Drafting and Design Presentation Standards Manual (DDPSM) Chapter 14 has similar requirements. The current version of AS5100 Bridge Design also give guidance including recommendations for different climatic zones.
Expected 100-year design life may not be reliable
The question posed by the reporter is whether the anticipated 100-year design life for these Super-T girders can be relied upon without inspection of the internal cavities and an assessment of their condition?
Greater risk in/near marine environment
For bridges in a non-coastal environment (exposure class B1), that have used the open-flange type of Super-T, cast in rigid steel moulds, the risk would appear to be very small. Where bridges are in a coastal environment (exposure class B2), the risk is likely to be greater. Also, if there are bridges in use that have used the earlier closed-flange type of Super-T these are potentially a greater risk due to the casting problems identified earlier.
Assessment of risk and subsequent inspection
Therefore, if not already being done by the relevant bridge authorities, the following would appear to be reasonable precautionary steps:
Categorise the risk of bridges using Super-Ts; e.g. those that have used closed-flange type; bridges in a coastal environment; bridges subject to freeze-thaw environment especially where de-icing salts are used;
Instigate an inspection plan of the cavities of any in the higher risk category that are 20 years old or more. This could be carried out using a cavity inspection camera or similar probe although if the voids are filled with foam this could be difficult.