The relocation of the local waste and recycling centre to make way for the £357 million Emirates stadium at Ashburton Grove posed technical challenges. Opened in 2004, the three-storey £60m facility provided a brand new, multipurpose waste transfer and recycling hub for the London borough of Islington and the North London Waste Authority (NWLA).
Run by the NWLA’s waste contractor, London Waste Ltd, the centre in Hornsey Street has a 401,500 tonnes capacity and is licensed to process 275 tonnes of waste per hour. It was built to handle all household waste from Islington as well as a proportion from neighbouring boroughs, Camden and Haringey, and has scope to process 50,000 tonnes a year of recyclable waste.
With a large proportion of waste handling located in a tipping hall on the first floor, the concrete slab faced tough performance demands of exceptional characteristics. The floor needed to withstand substantial wear and tear from heavy vehicles and plant as they tip and shovel waste, and off-load and shunt laden skips and containers. The project team also required good resistance not just to water but also to the chemical residues – mostly acidic – leached by the waste.
The tipping hall presented three particular areas of concern, as Ian Flewitt, a partner at the consulting engineers, Price & Myers, explained, “While no waste is loaded or stored directly on the floor of the tipping apron, we had to protect against spills which potentially might have chemical content from, for example, car batteries. In the tipping area for glass, cardboard and other recyclable waste, the floor is subject to heavy loads, abrasion and impact from plant scooping and moving material between pens. The third area, the domestic waste transfer centre, includes a deposit for batteries, so again, there was the possibility of the floor being exposed to acid residues. There is also a physical element with skips being loaded on and off lorries, and scraping the floor as they are pushed into place.”
As well as being engineered for use by all types of waste vehicle, including bulk articulated vehicles, floor performance criteria also needed to account for future flexibility. The facility is designed to accommodate different recyclable fractions and recycling bays are moveable to adapt to future collection systems, incurring additional wear and tear. Sound waterproofing of the first floor slab was also essential to protect the workshops, plant room and parking areas below from wash-down water used to clean the first floor slab.
Ian Flewitt said: “PUDLO were the only people who came back with ideas for a solution and who were keen to spend time working on a mix that would provide all the performance characteristics we required.” He added: “They provided us with very good upfront technical service in developing and refining a design for the mix. This was followed by further service support inspecting the mix, the batching and attending site to oversee that it was placed correctly.”
The final mix included fine steel fibres to improve abrasion resistance, polypropylene reinforced fibres for impact resistance, and the addition of a pozzolanic compound for acid resistance. These were carefully formulated and tested to ensure the resulting gains added to the water-proofing performance of the PUDLO additive. Since the application was unique and charted new territory in terms of concrete additive formulation, there were no set criteria for performance. The project team needed to see as much gain as possible in the characteristics required so that they could confidently validate the engineering and adopt the innovative mix.
PUDLO carried out painstaking formulation and testing of the mix design at microstructure level, looking at the potential problems of acid leachate penetration and arriving at a pozzolan content that would provide effective defence. The sample demonstrated high resilience to chemical attack, proven by ensuring the free lime (calcium hydroxide) was physically bound and converted to calcium silicate. The use of fibres to create impact resistance is well-known, but enhancing concrete to counter abrasion is far less common. A sample of the special formula was cast and evaluated for abrasion by assessing the rate of loss of thickness subject to a laboratory scrape test (using Taber analysis). Testing showed a three-fold increase in abrasion resistance over ordinary C40 concrete. Alternatives, such as a traditional waterproof mix with an applied powder on top as used for warehouse floors, would not have addressed all of the performance issues as neatly as the PUDLO solution.