Concrete Additives Add Strength and Reduce Carbon Footprint

(Journal of Commerce 5-9-2011) The National Research Council of Canada - Institute for Research in Construction (NRC-IRC) has developed a new project aimed at reducing the environmental impact of concrete by replacing part of the cement with supplementary cementing materials (SCM).
It is generally agreed that the production of one tonne of cement emits one tonne of CO2. Materials like fly ash and slag from steel production are already used to replace a percentage of the Portland cement, resulting in high-performance concrete (HPC) or ultra high-performance concrete (UHPC), depending on the ratio.
The NRC-IRC study introduces different fine powders into the mix, rather than coarse aggregates. “The bigger particle is sand,” said Pierre-Claver Nkinamubanzi, research officer with NRC-IRC. “Here we try different types of byproducts in powder form, like metal insulator ash, slag from non-ferrous steel production and metal kaolin.” So far, the result is a higher-strength product with a longer set-up time because the supplementary materials are less reactive than cement. “It reacts slowly for the first 21 days, but later has much better performance (than traditional concrete) because the metrics are compact and less permeable to aggressive agents and components like rebar that can cause corrosion,” said Nkinamubanzi.
Chemical add mixtures off-set longer set-up time, enhance workability and resist freezing and cracking, but the wrong combination can retard or accelerate the hydration process. “We need to control this and understand the mechanics of what is happening,” he said. “This is the basic element of our research here.”
Conventional concrete measures around 35 Mega Pascals (MPa), high-performance concrete measures up to 100 MPa and ultra high-performance concrete can reach between 100 and 200 MPa. NRC-IRC has produced concrete up to 800 MPa in a lab setting. The study is aimed primarily at developing pre-cast elements for use in bridges.
The first structure to use UHPC was the Sherbrook Pedestrian Bridge in Sherbrook, Quebec. The structure’s deck, supplemented by stiffening ribs, is only three centimetres thick and spans 60 metres. While the long-term performance of SCM concrete is dramatically improved, the longer setting time may affect construction scheduling.
Carolyn Campbell, executive director of the BC Ready-Mixed Concrete Association said it is generally accepted that in western Canada, about 20 per cent of the cement in concrete is routinely replaced with supplementary materials, typically fly ash. “You can go as high as 50 per cent,” she said. “In B.C. we are stripping the forms quickly. High fly ash concrete is going to gain strength over a longer period of time and this would affect the ability to strip the forms quickly.”
Another issue is the need for greater attention to the curing process. “Because it is a slow strength gain, in cold weather, you would want to heat and cure it properly,” said Campbell, adding that this may off-set gains made in reducing a project’s carbon footprint. “What you don’t want to do is have an architect or an engineer design a structure to be built in summer using high-fly ash concrete, and then see the project get bumped into the colder weather,” she said. “Then, you have to spend the energy to heat and horde and have you really displaced any energy?”
B.C.’s Little Mountain Reservoir in Vancouver’s Elizabeth Park was constructed with high fly ash concrete to meet significant seismic and structural deficiencies. The six-sided concrete box, with no expansion joints, is B.C.’s largest joint-free structure. A specific temperature tolerance during construction and an insulated roof prevented built-in thermal strains. A one-meter by 400-meter perimeter gap was in-filled to tie the roof to the walls and the temperature of the concrete was controlled using cooled water and 20,000 square metres of hording.
The use of slag is more common in central Canada, where more building owners are asking for it in projects with green mandates, said John Hull, president of the Ready-Mixed Concrete Association of Ontario. The percentage of slag used to replace the cement is weather dependent, with summer projects using up to 50 to 60 per cent and winter projects dropping down to 20 per cent. Adding SCMs is almost like adding ball bearings to the mix, said Hull. The size of a cement particle is about the same as a slag particle. “You can reduce the water content because while a cement particle is angular a slag particle is rounded and a fly ash particle is even more so – it is almost egg-like,” he said. “When you reduce the water content, you reduce shrinkage and increase strength and durability.”
The benefits of using concrete are measurable, he added, citing an NRC fuel study conducted with Ontario’s Ministry of Transportation, which tested fuel consumption on a 183 km stretch of concrete highway between London and Toronto. Trucks saved 70 million litres of fuel and produced 196,000 fewer metric tonnes of CO2 in a year. 
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