In early 2013 Firth Industries were awarded the concrete supply (to main contractor McConnell Dowell) for the Waitaki Bridge Replacement Project near Kurow. The project was to replace the 132 year old twin bridges crossing the Waitaki River.

Demanding technical requirements combined with an isolated location in an often extreme environment meant the Firth team had to go the extra mile for an extended period.

The supply of foundation, pile, pier head, abutment and deck concrete in significant quantities was from Firth’s small rural concrete plants, Hilderthorpe and Oamaru.


Firth’s existing plant at Oamaru is over thirty years old, although upgrades over the years have meant production of high spec concrete at a moderate output was possible.

The Hilderthorpe plant (15 minutes north on the main road to Timaru), is a modern and compact Gough single weigh hopper plant with a satellite grading to Oamaru.

Both plants required additional admixture storage and dispensing upgrades to cater for project demands. The majority of concrete supplied to the project was from Hilderthorpe, with Oamaru helping out on large pours as required.

The incumbent trucking fleet of North Otago catered for typical pours to the bridge project, however large deck and pile pours required trucking assistance from Timaru, Cromwell and Dunedin. Travel to site is a near continuous climb for 57km and 1 hour, large pours required 6-8 trucks per day.


Compression and slump tests were taken in-yard and at site at a rate of 1 set per 50m3 of concrete supplied. Air/yield and material testing were as per NZS 3104 Specification for Concrete Production.

Bridge abutments were over 2m thick in places therefore heat of hydration was a concern for the project engineers URS. Data loggers (with up to 4 probes) were cast in for each of the 6 pours to monitor core and cover temperatures for 7 days.

The in-house designed data loggers transmitted multiple probe data every 30 minutes, resulting in heat profile curves being continuously available to client and engineer.


Bridge support piles 1.8m diameter used a 40MPa specifically designed tremie mix. Up to 6 hours hydration control was required for the pours of up to 55m3 per pile. BASF Delvo hydration control admixture was used on each pour, resulting in ideal slump life.

Tremie pouring 20+ m deep piles in the middle of the often raging Waitaki River presented plenty of challenges for Firth staff and main contractors McConnell Dowel. Mix design was crucial to limit concrete mixing with displaced water down steel casings. The top water was pumped into 1000 litre totes and transported to land for safe disposal.

Deck slab pours were up to 93m3 , and used a hydration controlled 40MPa pump mix. A total of 6 pours over the 2 bridges were completed.

Due to extremely low winter temperatures, 40MPa bridge deck concrete required 5% air content to resist internal forces from  freeze/ thaw cycles.

The challenge of maintaining high air content in set retarded and plasticized 40MPa concrete proved a challenge the Firth operations and technical team were up to.


At the project’s completion in June 2014, Firth Hilderthorpe, a small satellite graded plant to NZRMCA guidelines successfully completed significant volumes of highly specified and controlled concrete within the requirements of NZS3104, McConnell Dowell and URS requirements.

Over the 1000+ m3  supplied, zero loads were rejected at site, and all compression results were above design f’c, and within CoV parameters. The project’s successful completion is a testament to the fact that technical demand or location is no barrier for projects requiring certified concrete to NZRMCA and NZS3104 requirements.

Taken from the December 2014 NZRMCA Newsletter
Images - Firth Industries