Firth Hastings produce the goods for Kererū Gorge culvert project
Cyclone Gabrielle devastated many parts of the country in February 2023 with the Hawke’s Bay taking the brunt of this major weather event. Nineteen bridges and culverts in the region were damaged, including the Kererū Gorge culvert, which was washed out, completely wiping out the existing 1.5m culvert and making the road impassable.
“We actually put out a request to the various batch plant concrete providers within Hawke’s Bay and then we spoke with each of them putting forward a series of requirements,” says Michael Stiebel, Project Manager Civil (Structures) Fulton Hogan Napier. “The Firth Hastings team were chosen, and we are glad they were.”
Firth’s technical expertise shines
“Once we were chosen to be the provider for the Kererū gorge project we started designing the mixes that would be used for the various stages of the culvert project,” explains Tim Bulmer, Regional Materials Manager (Bay of Plenty) for Firth. “Initially, the plan was to use a 30MPa concrete with a fly ash addition for the main structure of the culvert to manage the huge heat normally generated on large pours.”
Various trials were undertaken at the Firth Mt Maunganui laboratory and the Hastings plant to achieve both the low density and low heat characteristics, Firth proposed using GGBS (slag).
“The laboratory trials with a 60% slag replacement yielded promising results, leading to another plant trial,” says Tim. “This time the temperature probes showed a much lower temperature and met the other performance criteria. We had come up with a design which was then signed off by Stantec.”
After achieving the desired mix design the work began for Firth Hastings to get the GGBS from Tauranga to Hastings, a six-hour journey, and then to allow for the 90-minute trip for deliveries of concrete to the project site.
“Despite all of these complexities we successfully supplied large quantities of concrete, including pours of approximately 280m³ the week before Christmas and then another 250m³ on 4 Jan this year,” says Lester Wagner, Area Operations / Sales Manager (Hawke's Bay) for Firth.
“Especially in the early stages we were presenting the concrete’s stats to Stantec to show them that the targets were not just being met, but bettered,” adds Michael. “The initial requirement was not to exceed the 65 degree mark and to achieve 3MPa in the first 48 hours. From the early stages the concrete was hitting 10MPa. This gave everyone confidence that everything was going well.”
“Layering either side of the installation evenly is essential and of course we couldn’t work on the next layer until the previous layer had cooled enough. Firth and the concrete performed very well.”