Recommendation: Permit the use of recycled or secondary aggregates but do not over-specify.
When specifying recycled and secondary aggregates, the factors to balance are resource depletion, transportation CO2 impacts and implications on mix design. These are all affected by availability, and concrete producers are well placed to ensure the most sustainable aggregates for each project are used. Depending on the application and the type of concrete there is often an opportunity to incorporate recycled aggregates previously used in other projects and secondary aggregates that may be by-products from other processes e.g. 'stent' from china clay production and granulated slag from steel making. Environmentally it is preferable to use these materials as close as possible to their origin, although their embodied CO2 value can exceed virgin materials. This decision should be made on a project basis to ascertain benefit. For this reason the concrete strategy does not include a numerical target for the proportion of recycled/secondary aggregates of the total aggregates used but reports on the trend. This has been consistent at around 5% by mass in concrete production, although some products can have significantly higher levels.
Recycled concrete can be used as aggregate in new concrete, particularly the coarse portion. When using the recycled concrete as aggregate, the following should be taken into consideration:
• Recycled concrete as aggregate will typically have higher absorption and lower specific gravity than natural aggregate and will produce concrete with slightly higher drying shrinkage and creep. These differences become greater with increasing amounts of recycled fine aggregates.
• The chloride content of recycled aggregates is of concern if the material will be used in reinforced concrete. The alkali content and type of aggregate in the system is probably unknown, and therefore if mixed with unsuitable materials, a risk of alkali-silica reaction is possible.
Recycled steel reinforcement
Steel reinforcement manufacturing BAR members used around 94% of recycled ferrous metal waste as a proportion of the raw materials consumed in their electric arc furnaces (EAF). While reinforcement fabricators used 100% EAF material in producing and supplying rebar for use in concrete.
Concrete manufacture uses by-products from other industries, such as fly ash from power stations and ground granulated blastfurnace slag (GGBS) from the steel industry. These materials have a lower embodied CO2 than the cement they replace and can have a positive effect on the appearance and performance of concrete.
To support designers in achieving the benefits of additional cementitious materials guidance is available in Specifying Sustainable Concrete published by The Concrete Centre.