Use of weathering steel in construction of bridges in suitable environments could slash maintenance costs and eliminate the need for bridges to be repainted, an online forum has heard. (image: 100 m weathered steel bridge on Pestells Lane over Princes Highway, south of Kiama, NSW East Coast. source: Seymore Whyte)
During an online discussion hosted by Engineers Australia last week in partnership with BlueScope, panelists highlighted the use of weathering steel on the design and construction of two bridges as part of a $450 million upgrade of the Princes Highway between Berry and Bomaderry on the east coast of New South Wales (south of Sydney).
The discussion was moderated by Georgina Baxter, Associate Bridge Engineer at Aurecon.
Panelists included David Talbot, Principal Engineer, Civil Structures at Arcadis Australia Pacific; Andrew Sams, Quality Manager, CIVMEC; and Rob Danis, National Engineering Manager – Infrastructure at BlueScope.
According to Talbot, weathering steel is much like any other form of structural steel with a few differences.
The product is a steel alloy which contains some additional trace metals that help to form a strong rust coating (called a patina) on the surface.
Whilst any form of steel rusts when exposed to moisture and oxygen, the rust coating on normal carbon manganese steels is relatively weak, brittle and porous. Over time this detaches from the steel – leaving fresh steel exposed and the cycle of rust to recommence.
By contrast, the alloying metals in weathering steel form a stable protective coating. This coating – which adheres to the base metal – is much stronger, more durable and much less porous. As a result, the material effectively forms its own paint coating and does not need to be painted.
The patina takes about eight years to form. Once it has formed, the steel will be subject to very little corrosion damage over the product’s approximately 100-year life span.
Weathering steel is new to Australia and its use on the Princes Highway project was the first not only for Transport for NSW but also in Australia. Since then, Transport NSW has used the material on James Ruse Drive bridge as part of the Parramatta Light Rail Project. Another weathering steel bridge is being constructed on the Sydney Gateway project. Several more are in the Transport NSW pipeline.
On the Princes Highway project, the product was used on two of the project’s twelve bridges. These included:
- A 55m single span bridge over the highway on Strongs Road at 21.8 kilometres south of Kiana (bridge on a 20m skew); and
- A 100m bridge (three spans) over the highway on Pestells Lane 28 kilometres south of Kiama (top image).
Arcadis was the lead designer for the project. Fabrication and welding for six girders (three on each bridge) was completed by CIVMEC, BlueScope supplied the weathering steel material. Construction was undertaken by the Downer Seymour Whyte Joint Venture.
When used in a suitable environment, Talbot says weathering steel offers several advantages.
As the material forms its own protective coating, it does not need to be repainted. Indeed, the only maintenance which is needed is periodic inspection and cleaning to ensure that the patina is forming correctly and that there are no other issues or concerns.
This saves time and money as it removes the need for repainting (2-3 times) over the bridge’s life and thus avoids the need for temporary scaffolding, road lane closures or stripping back of existing paintwork.
This is particularly advantageous for sites where future repainting may be difficult or dangerous or where traffic disruption needs to be minimised (such as over a freeway).
In addition, the material develops an attractive appearance as the patina ages and darkens over time and tends to blend in with the surrounding environment.
Third, as with any other steel bridge (as opposed to in-situ concrete), there are savings in terms of construction time as steel girders can be fabricated offsite before being transported to site and assembled.
On the two bridges referred to above, Talbot said a key driver for use of steel (in this case, weathering steel) revolved around the requirement to have a clear span over the highway.
For these longer spans, standard or precast concrete girders would have necessitated designs which were bespoke and inefficient.
Meanwhile, the ability to fabricate the girders offsite not only saved on construction time but enabled the fabrication to cater for the 20-degree skew on the Strongs Road bridge.
From a detailing perspective, Talbot says weathering steel has a similar performance profile compared with regular steel when it comes to strength, serviceability and fatigue. Accordingly, working through the Australian design standard is similar to any steel.
Nevertheless, there are some things which need to be considered when working with weathering steel as the patina the patina needs cyclical wet and dry to help it form evenly. If exposed to fully saturated environments, the patina and will not form properly.
As a result, it is important to avoid any dripping or ponding of water on any of the detailing elements.
On the bridges referred to above, three strategies were used.
These included (refer diagram below):
- Provision of a drip inducer on the soffit of the concrete deck panels to intercept any water which may be on the underside of the deck and allow this to drip off before reaching the steel box girders.
- To avoid the potential for trapped areas where water may be caught on the welds, these were ground down to produce a smooth flush weld.
- To avoid potential for dirty or stained water coming off the weathering steel bridges from spilling over and staining the concrete on the two piers on the Pestals Lane bridge, a fall was provided along with a downpipe from the top of the pier.
During fabrication, Sams says one challenge was the procurement of weathering steel bolts. These were not manufactured locally and needed to be source from the US. This presented challenges in terms of long lead times, minimum order quantities and availability in imperial rather than metric measurements. On this last point client permission was needed to amend the specifications to cater for bolts in imperial measurement and for increase in the diameter of the bolt hole to suit the larger imperial equivalent measurement.
According to Talbot, Danis and Sams, there are several broader considerations when dealing with weathering steel.
These are that:
- Not all locations are suitable for weathering steel use. Weathering steel requires cyclic wet and drying in order to form properly. The Patina also performs poorly in high chloride or other highly aggressive environments. It is therefore generally not recommended for marine environments. In fact, Danis says the material is recommended for use only in in a corrosivity environment of C1 to C3 according to the corrosivity zones as specified in Australian Standard AS4312.
- During construction, care should be taken to look after the coating when girders are being transported, stored and installed. Any localised damage could result in one section colouring differently and at a different rate compared with the remainder of the bridge.
- As mentioned above, certain aspects of detailing are important when dealing with weathering steel. This includes grinding back the welds along with drip pans, drip plates to avoid staining at abutments (steel corrosion often occurs at the joins where moisture and dirt can accumulate. Should any build-up of moisture or dirt be uncovered during inspections, this can either be washed away or blown away with a compressed air jet.
- Pre-treatment is recommended once the product has been fabricated and given an abrasive blast, Danis says. This involves a minimum three cycle process of wetting and drying the weathering steel structures. This can be as easy as hosing it down and letting it dry three times before sending to site.
- As weathering steel is a new product, there is currently little Australian guidance on its detailing by way of specific standards for weathering steel (several standards deal with steel more generally). However, BlueScope in 2017 published a useful HERA design guide to provide detailing advice for weathering steel in Australia.
Danis says the case for greater use of weathering steel is strong.
He says uptake will grow with more awareness and education sessions, such as last week’s session hosted by Engineers Australia.
“I think that’s what make weathering steel a much more viable offering for those infrastructure projects moving forward,” Danis said, referring to the aforementioned advantages associated with the material’s use.
“With painted steel bridges, you need to recoat two or three times over the life span of the bridge. Weathering steel removes the need for that. Fron a maintenance and cost perspective, it should work out a lot cheaper …
“I think just having more of these sessions will give people awareness and familiarity with it moving forward.”
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