Withstanding a Harsh Chemical Environment
PUDLO concrete has been used in the construction of mechanical biological reactors at Southwark’s Integrated Waste Management Facility (IWMF) which have been designed to withstand chemical leachate and high process temperatures. Veolia Environmental Services was awarded the 25-year £60M PFI contract to deliver and operate the groundbreaking facility for London Borough of Southwark. Main contractor, VolkerFitzpatrick, undertook the project as a design and build working closely with consulting engineers, Royal Haskoning.
Located on a former gasworks on the Old Kent Road, the site also includes a materials recovery facility, household waste, reuse and recycling centre, and waste transfer station. It handles up to 200,000 tonnes of material per year, processing all of Southwark’s household waste as close to source as possible and reducing the amount sent to landfill.
The mechanical biological treatment (MBT) plant, with 87,500 tonnes per annum capacity, is Veolia’s first in the UK. Composting takes place in two sets of six reinforced concrete tunnels, each structure measuring 36m by 6m by 6m high. One set bio-dries the waste to reduce moisture content; the other carries out bio-stabilisation to produce a final residue suitable for fuel recovery.
Process vapour is extracted via four concrete bio-filter chambers, cast insitu in strategic locations on the tunnel roofs. Chemical leachate is conveyed to a substantial concrete underground tank, measuring approximately 10m by 20m by 5m deep. The concrete for the tunnels, biofilters and storage tank needed special formulation to cope with the effects of chemical leachate and condensate aggravated by temperatures potentially as high as 85°C.
The presence of slot drains in the spigot floor slab of the composting tunnels ruled out the use of a coating to achieve chemical resistance, so this needed to be built into the concrete. Although the addition of microsilica to the concrete would have provided some of the required chemical resistance, it would not have waterproofed the structures. Highly resistant to the corrosive effects of chemicals, PUDLO concrete waterproofing system was specified for concrete in all of the critical areas of chemical exposure within the MBT plant. BBA approved, it promotes the integral development of important performance characteristics in concrete, including water- and chemical-resistance, without the need for supplementary membranes or coatings.
The bottom slab and kicker of the process tunnels were constructed in normal concrete. PUDLO was included in the C40 concrete used above for the movement joint and spigot floor, as well as for the walls and roof. Royal Haskoning used a finite element software package to model the structure in three dimensions with the temperature gradients applied. This allowed them to calculate stresses due to thermal effects, and to predict the expansion and contraction of the concrete to help design an effective solution.
Benefits of the Mix
The altered microstructure of PUDLO concrete, with its greatly reduced capillary network, offered benefits here, too. By minimising the ingress of air/fluid, it is dimensionally very stable and tolerant of heat extremes, helping to inhibit stress cracking. Naturally increased strength gains provided when adding PUDLO to the concrete without the need for additional cement were another advantage providing up to 30% additional compressive strength gain. The type of mix used (DC-4m) had a high cement replacement content, around 66-80% ggbs. This slows down the rate of strength gain especially in the early age of the concrete. PUDLO test results at 7 days were well above 30 MPa where figures of around 15 to 20 MPa would have been expected using the DC-4m mix with no admixture. At 28 days, tests were up to 50-60 MPa for C40 concrete. With replacement content as high as 75%, the hydration process in concrete will be slower, making early strike of falsework and formwork potentially problematic.