NZ’s largest single-project floor slab

As Contech approaches their three millionth square meter of post-tensioned floor slabs, Mike Lawson, Manager – Technical, Quality and Health & Safety, shares details of a record-breaking floor slab recently completed by the team and some further background to the story – which is all about food!

The Contech team has just completed the largest floor slab ever produced in New Zealand in a single project. In terms of the scale of this amazing post-tensioning project, the adjacent statistics say it all.

The floor was installed as the foundation of a brand-new distribution centre for Foodstuffs – a 100% New Zealand-owned company established in 1922 that’s now the country’s leading grocery distributor and one of its largest organizations.

State–of-the-art distribution

Foodstuffs operates through regional co-operatives that together run more than 700 owner-operated businesses. These include full-service supermarkets, retail food warehouses, neighbourhood grocery stores, small convenience stores, large- and small-format liquor stores, fuel sites and specialist supermarkets that source goods from local growers and farmers.

Each regional cooperative supports and services its local stores through integrated warehouse and transport centres. These centres use a state-of-the-art ‘voice picking’ warehouse management system, which has enabled them to increase production, reduce repairs and maintenance and improve their health and safety records.

In February 2018, the company announced its intention to consolidate three of its North Island distribution centres into a single, purpose-built, temperature-controlled distribution centre. Its chosen location was Auckland Airport’s ‘The Landing’ – a world-class business park that comprises more than 100 hectares of development land catering for the logistics, technology and light industrial sectors.

Early stage involvement

Contech became involved with the Foodstuffs project in the early stages of its development, mainly in relation to the suitability and layout of the floor slab. However, the warehouse’s design and layout later changed, making the original joint layout less than ideal.

Working with Conslab – the industrial concrete flooring contractor and an industry partner – Contech redesigned the slab to minimize the number of armoured floor movement joints and maximize the size of the floor pours. The originally specified floor joints were replaced with a patented joint system developed by Conslab in New Zealand that offers superior performance and durability. For Foodstuffs, this meant a robust and highly usable floor, low whole-of-life maintenance costs and enhanced operational efficiency.

The 74,000m2 warehouse is approximately 360m by 230m at its most extreme dimensions – and overall the space equates to around the size of seven rugby football pitches. It uses two slab designs, as one area has a taller racking system than elsewhere, and the floor is required to handle the heavier loads.

Floor slab specifics

The floor comprises nine separate post-tensioned slabs of 6,000m2 to 10,000m2 in area, with heavy-duty movement joints around the perimeter. Each of the nine slabs comprises three or four smaller, continuous slabs with coupled tendons or stop-end joints.

Each slab was poured over two consecutive days. In all there were 31 pours ranging from 300m3 to 530m3, with most slabs being 180mm thick. Given the size of the job and working closely with Auckland Airport and main contractor Macrennie Commercial Construction, the Contech team leapt into action to source materials and manpower. The latter was a particular challenge, as the project required a full-time crew of four to six members with the skills and experience to deliver a perfect result.

The whole thing went like clockwork. Our team was delighted to be working on this massive project, and we were very proud to emerge with an immaculate safety and quality record.

Latest BBR post-tensioning

The project signalled a new ‘first’ for Contech with the use of the BBR VT CONA CMF S2 flat post-tensioning system. The system leads the market in offering all the benefits of a wide anchorage and coupler size range, with very thin slab depths and low reinforcement requirements, plus a very small minimum centre spacing and slab thickness at low concrete strength.

It all adds up to smaller, lighter and stronger tendons for post-tensioning flat slabs – and at lower cost than previously. We had the confidence of knowing it had been rigorously tested – and were really pleased with the result.

Given the scale of the Foodstuffs post-tensioned floor, not to mention its place as one of the largest in the world and the requirement for it to withstand extremely heavy loads, the new CONA CMF S2 system is a welcome addition to the BBR range. The team is looking forward to using it for other projects – and perhaps adding to the tally of post-tensioned flooring undertaken for Foodstuffs, which now stands at a whopping 190,000m2.

Project Summary:

Owner: Auckland International Airport

Architect: Eclipse Architecture

Structural Designer: Day Consultants

Main Contractor: Macrennie Commercial Construction

Technology: BBR VT CONA CMF flat