Recycled aggregate in fresh concrete mixes
By Tom Offerman
In most European countries it is nowadays virtually impossible to construct a building without bringing sustainability into the picture. Except for the urge of the public, this is a consequence of the latest European Union laws and regulations. EU regulations strive to prepare about 70 per cent of all construction and demolition waste by 2020 for re-use; recycling or undergoing any other material recovery, compared to the year 1990. This is one of the most crucial rules stated in the Waste Framework Directive (2008/98/EC), made by the European Union.
Recycling of construction materials is an excellent way to reduce the impact on the environment. The consequence of recycling is that the amount of raw materials used and therefore the amount of demolition waste will be reduced. Within the building sector, the concrete industry can contribute. All over Europe there are a lot of abandoned office buildings which are being renovated into residential buildings, or demolished to make space for new building projects. The waste material, the recycled aggregates, recovered during this demolition process can be used in new concrete structures.
Even though the subject of reusing recycled aggregates isn’t new, it is still not widely used in practice. Many studies have shown that the material made with recycled aggregates have mechanical properties quite similar to those made with ‘regular’ aggregates. Nonetheless the materialistic properties of the aggregates do differ. The recycled aggregates are characterized by a high water absorption capacity which can be linked to the presence of the remained unreacted cement attached to the surface of the gravel. (see picture 1). The difference in water absorption capacity has an effect on both the fresh and the hardened state of the concrete mixture.
Within the fresh state, the higher amount of water absorption results in a reduction of workability, as part of the water is instantously absorbed by the attached cement on the aggregates. This means that water can’t react with the newly added cement and thus in consequence reduces the water-cement-factor (w/c). As a result the concrete in the hardened state, will reach a higher compressive strength. While a having higher compressive strength is not a bad thing, the concrete will likely fail on the workability demands during compacting.
This means that there has to be a way to take the amount of attached cement into consideration while preparing the concrete mix. One way to take this variable into the mix is, prior to use, saturate the aggregate for a period of time. This eliminates the effect of the unreacted cement using water intended for the new mix.
This would mean that even with an unknown amount of variable unreacted cement contents on the aggregates, this recycled material can be used for structural concrete. This will provide a helping hand towards the goal of a more sustainable construction industry.