With geography as steep and rugged as New Zealand’s, bridges have played a significant role in our country’s development. Across urban and rural settings, or rail and road applications, we boast some truly magnificent bridges.

Grafton Bridge in central Auckland, the Fairfield Bridge in Hamilton, the Opawa Bridge in Blenheim and the Kopuawhara Viaduct are classic examples of bridges from the early 20th century. The north and south Rangitikei Viaducts, the new Hapuawhenua Viaduct, and the Otira Viaduct are notable contemporary structures from the late 20th century.

Concrete is the common construction material across all these bridges – a trend maintained with the assistance of Firth Industries in two recent projects, both of which posed many logistical challenges.


Restricted access and harsh working conditions were only some of the issues faced by the teams at Firth and MacMillan Drilling, the subcontractor appointed by Downer, for repairs to Bridge 27 on the Midland Line of the TransAlpine Railway between Springfield and Craigeburn.


The bridge was originally constructed at the beginning of the 20th century, and formed the last link in the railway between the east and west coasts of New Zealand.

Six of the original mass piers were constructed in concrete, with the remaining two piers and abutments in timber. The replacement of the timber elements with new concrete piles and pile caps was non-negotiable in terms of ensuring the long-term stability of Bridge 27.


“The conditions were pretty tough as we were subjected to heavy frosts and snow during the six months of repair work,” says Jaron McMillan, Company Director of McMillan Drilling. “We were required to permanently case piles (4 x 1200mm in diameter and 4 x 1050mm in diameter) with casing depths to 14m. This also involved using temporary driven UC piles with steel truss supports, to hold the viaduct in position while the permanent concrete pile caps and bridge piers were poured.”

Adding to the project’s difficulties was the remote location of the bridge. “The site is extremely isolated, about a two hour drive from our Darfield plant in Christchurch,” says Dominic Sutton, Chief Operating Officer - Southern for Firth. “We were also required to pump the concrete over 100m across a live rail line as there was no vehicle access to the south side of the bridge.”

In order to meet the clients pour rate requirements Firth also brought concrete from its Greymouth plant. This was the first time the company had supplied a single project from plants located on either side of a mountain range.


“A key characteristic required of the concrete on this job was that it had to have early high strength,” explains David Jackaway, Contract Manager for Downer. “The site required a concrete strength of 40MPa and the concrete had to achieve a minimum strength of 30MPa before departure.”

Independent concrete testing carried out by Telarc/IANZ accredited Downer Laboratory in Christchurch, showed the concrete achieved the required strength. The tests also demonstrated that the strength was developed very quickly, with a minimum seven day result of 41MPa and a maximum 28 day strength result of 60.5MPa.

“Firth did a good job as the site provided many challenges for concrete work, including the cart distance from the Darfield plant, difficult access conditions, pumping over 100m across the bridge and the cold weather,” adds David. “The concrete was generally of a high quality and together we achieved an excellent end product.”


In mid-2009 Firth Industries were awarded the concrete supply for the new Kopu Bridge near Thames. Involvement with this estimated $47 million dollar project required the upgrade of a small rural plant to ensure the production of consistently high quality concrete.


The project is a new 580m long two lane bridge over the Thames estuary, adjacent to the old single lane bridge. The bridge itself consists of piles up to 50m deep, which support 15 pile caps and piers. A steel beam arrangement has been adopted, with a 200mm thick 12.8m wide concrete deck slab. The span has two lanes for vehicles and a separated pedestrian and cycle way. The main single span of the bridge is 42.8m long, and sits approximately 6.5m above mean sea level to allow for the passage of boats.


The Firth Industries plant at near-by Thames is over 25 years old, and was designed to cope with the demands required of a small rural plant. This new project however, required larger volumes of high specification concrete that the existing plant could not supply without significant modification.

As such, alterations were made to the plant, including electrical upgrades and a larger capacity cement weigh hopper. A water weigh hopper replaced the old flow meter, and a new electronic batch card system was introduced to ensure accurate moisture control. The ability to comfortably service up to seven trucks from the wider Firth fleet called for further plant alterations.


Project specification required an onsite testing regime. This meant that during the initial months of supply, Firth had to test each truck load of concrete both at the yard and onsite. Once client confidence in comparable results had been established the regime reverted to yard testing.  However, one test per 50m3 still had to be conducted onsite.  This meant that testing staff had to be drawn from other Firth plants, while training was also provided to HEB staff.


As many pours were out over the estuary on the temporary staging (and in the dark on occasions), all drivers had to undergo an extensive site safety induction to address the significant hazards present..

The large pours were all around the 130m3, taking about 5 hours to complete. Typically, the first concrete was on site ready for pumping at 5:00 am.

Although there was always a truck waiting for the previous truck to discharge, pour times on the deck slabs were extended due to the long wait as each truck reversed slowly out along the staging.

The barriers on the east side of the bridge were cast in-situ. These have Wai etching and Waka designs cast into them. Firth worked closely with HEB to achieve the required finish and early age strength so that moulds could be used every day.

Concrete was sent to site at 50mm slump, and patterns added to achieve the desired workability. The intricate patens required a highly workable consistency, whilst maintaining strength to allow quick turnaround of forms.


For the duration of this project the Firth Thames staff had a very close working relationship with HEB Structures, ensuring the concrete was on time and to specification, with all the required additional testing completed to the client’s satisfaction. These efforts have helped deliver Kopu Bridge six months ahead of schedule and $6 million dollars under budget.

Taken from the April 2013 NZRMCA Newsletter
Images - Firth Industries