After a complex, multi-stage, 18-month process, Duratec recently completed remediation works to Rawdon Island Bridge near Port Macquarie in New South Wales, in turn, restoring safe road access to the local community.

Bridges – a community essential

From city centres to remote territories, roads, rail and other transport infrastructure are essential to the social and economic wellbeing of communities around Australia. Bridges form an important part of this infrastructure and Duratec is at the forefront of bridge remediation technology.

While bridges are theoretically designed to last in excess of 50 years, those built prior to the 1990s typically underwent minimal durability planning at construction. A prime example is the 60-year-old
Rawdon Island Bridge, upon which Duratec recently completed successful remediation works.

Rawdon Island Bridge project

Twenty kilometres inland from Port Macquarie, along the Hastings River, is a picture-perfect pocket of New
South Wales encompassing Rawdon Island and Little Rawdon Island. Connecting the islands – and their
150 inhabitants – to the mainland is the 164-metre-long, reinforced-concrete Rawdon Island Bridge. Following prolonged flooding to the area in March 2021, Port Macquarie Hastings Council (PMHC) undertook inspections of all its bridges. At Rawdon Island, engineers discovered major structural issues
with the pylons that support the bridge, as well as significant concrete delamination, mainly to the blade
walls above water level.

At the time of these inspections, Duratec was working on a different, albeit much smaller, bridge maintenance project for PMHC and was able to provide the client with options regarding the remediation of Rawdon Island Bridge. Duratec had also previously delivered a similar project for the Council – the remediation of Dunbogan Bridge. In the end, Duratec’s services were engaged by the Council via an early contractor involvement (ECI) contract.

Level 3 investigation

What started as an ECI contract progressed into a level 3 investigation of the bridge, consisting of both  underwater and above-water inspections. Duratec’s technical investigation not only assessed the condition of the bridge but also enabled a greater understanding of any active or potential concrete deterioration mechanisms.

Below-water inspection findings

  • Significant voiding to piles with exposed reinforcement
  • Up to 90 per cent loss of pile section and significant loss of exposed steel
  • White residue seen throughout pile surfaces (petrographic analysis and XRD at deteriorated test locations indicated white residue likely formed due to excessive leaching of portlandite)
  • Significant chloride content in tidal zone
  • Sulphate content determined to be low risk

Above-water inspection findings

  • Blade wall above tidal area at highest risk of corrosion
  • Up to 35mm carbonation depth recorded for bladewalls/head stocks, up to 25mm depth recorded for
  • Evidence of poor compaction from original construction
  • Evidence of previous patch repairs to cover areas of honey combing, typically at construction joint locations
  • Delamination and spalling in low-cover areas

Scope of works

  • Engagement of structural engineer to devise new pile-strengthening system
  • Installation of suspended scaffolding on bridge piers
  • Construction of informal temporary boat ramp
  • Strengthening of piles through ‘jacketing’ – installation of steel formwork system consisting of micro-piles, steel reinforcing cage and underwater grout infill
  • Breaking out of delaminated and spalled concrete on face of blade walls
  • Application of blade-wall thickening
  • Installation of ICCP (impressed current cathodic protection) system
  • Concrete repair to and protective coating of headstocks and blade walls
  • Rehabilitation of bridge joints
  • Concrete repair to bridge deck soffit
  • Rock scour replacement

Initial critical repairs

Once the extent of the structural issues had been identified, the bridge and waterway underneath were closed to traffic both for safety reasons and to prevent further damage. This occurred on 5 July 2021 and, of course, impacted access for residents and their on-island businesses.

With the local community front of mind, Duratec expedited investigations, design and procurement to allow repairs to be completed. In the meantime, PMHC staff worked around the clock to provide residents with supplies, waste collection and assistance with transport on and off the island, which involved the procurement of a barge for access via the river.

On 30 August 2021, prior to repairs being undertaken, PMHC reopened the bridge to light vehicles only under strict conditions. Bridge sensors and live monitoring were installed to allow immediate closure in the case of unexpected movement or flooding.

Underwater works

Underwater works began in November 2021 and involved strengthening the existing pile foundations through ‘jacketing’, a process which involves the installation of a temporary steel formwork system consisting of a micro-pile, steel reinforcing cage and underwater grout infill.

Jackets were installed to both piles at each of the seven piers, the most deteriorated undergoing works first. Upon completion of these works in February 2022, the Council was able to reopen the bridge to full traffic reinstating the bridge load limit of 42.5-tonne gross vehicle mass (GVM), while Duratec continued with above-water repairs.

Works begin above water

Above-water works involved breaking out the deteriorated concrete on the face of the bridge’s blade walls and applying blade-wall thickening, particularly at pile connection points. Concrete repair and protective coating were also undertaken to headstocks and blade walls.

To prevent further corrosion to the structural steel elements of the bridge, Duratec designed and installed a custom impressed current cathodic protection (ICCP) system.

2022 Eastern Australia floods

In February, March and July 2022, the Rawdon Island Bridge remediation project was delayed by significant flooding. During these events, the water level would rise above the suspended scaffolding system – the main access point to the bridge piers. As a result, debris would cause major damage to the scaffolding, putting the system under immense pressure.

The team would then have to clear the debris and have the scaffolding contractor come to site to repair the system. Assessments were required to confirm safe access prior to works re-commencing after a flooding event. In March and April, works were paused, causing a six-to-eight-week delay.

In an effort to reduce the impact of further delays from flooding, Duratec implemented an under-bridge inspection unit (UBIU) at the start of June. The machine was tracked from the compound to bridge deck daily and folded out underneath the bridge to create an adjustable working platform.

While the July flood still caused delays as a result of its impact on the scaffolding, the crew was able to use the UBIU to access the bridge.

On the home stretch

In September 2022, a seven-month effort to strengthen the bridge piers above the waterline drew to a close. This involved the removal of damaged concrete, the installation of steel reinforcement, repairs to the
remaining concrete and the installation of the ICCP. The final stage of the rehabilitation process comprised
improvements to the bridge deck.

Mission accomplished

Duratec successfully delivered the Rawdon Island Bridge remediation project to the PMHC in February  2023. Through a collaborative delivery model, the project team was able to maximise the project outcomes, while minimising the impact to stakeholders, especially the local community.

The remediation of the bridge has extended its life for another 25 years. Upon completion of the project,
Duratec extended its gratitude to the Rawdon Island community for their patience and encouraged them to “enjoy the safer and smoother ride that the upgraded bridge provided”.

“A terrific success”

Port Macquarie Hastings Council group manager community infrastructure planning and design Blayne
West said the Rawdon Island Bridge rehabilitation was a terrific success under very trying circumstances.
“Projects like this usually take up to two years of planning before boots are on the ground. However, given the critical nature of these repairs, we were able to fast-track the design and construction stage, including acquiring all the necessary permits within five months of the initial discovery.

“We dedicated resources quickly and engaged highly skilled, meticulous contractors. We met the requirements of this urgent rehabilitation while supporting the residents through a difficult time.

“To achieve a complete project of this calibre in less than two years – the timeline that typically would be allocated to planning alone – is a defining result for our organisation.”