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AUS-21 Inspection and maintenance of Super-T bridge girders

Report ID: 956

Published: CROSS-AUS Newsletter 4 - August 2020

Report Overview

Durability and maintenance of bridge Super-T girders with particular reference to the lack of access to inspect the sealed internal cavities.

Report Content

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.

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. 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.  

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.

Comments

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.

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.

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.

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?

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.

Therefore, if not already being done by the relevant bridge authorities, the following would appear to be reasonable precautionary steps:

  • Categorize 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.

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