27/04/2010

recycling concrete

WHAT?

The recovery of concrete falls between standard definitions of reuse and recycling: concrete is broken down into aggregates (granular material), generally to be used in road works, but also as aggregates in new concrete. [1]


WHY?

It is estimated that roughly 25 billion tonnes of concrete are manufactured globally each year, over 3.8 tonnes per person in the world each year. 

Twice as much concrete is used in construction around the world than the total of all other building materials, including wood, steel, plastic and aluminum.

About 1,300 million tonnes of waste are generated in Europe each year, of which about 40%, or 510 million tonnes, is construction and demolition waste (C&DW). 

Returned concrete (fresh, wet concrete that is returned to the ready mix plant as surplus) can also be successfully recycled. Recovery facilities to reuse the materials exist on many production sites in the developed world. Over 125 million tonnes are generated each year. 

Recycling concrete reduces natural resource exploitation and associated transportation costs, and reduces waste landfill. However, it has little impact on reducing greenhouse gas emissions as most emissions occur when cement is made, and cement alone cannot be recycled.[2]

Concrete was once routinely trucked to landfills for disposal, but recycling has a number of benefits that have made it a more attractive option in this age of greater environmental awareness, and the desire to keep construction costs down.

Recovering concrete has two main advantages: it reduces the use of new virgin aggregate and the associated environmental costs of exploitation and transportation, and it reduces landfill of valuable materials.


HOW?

When structures made of concrete are demolished or renovated, concrete recycling is an increasingly common method of utilizing the rubble. 

Concrete aggregate collected from demolition sites is put through a crushing machine. Crushing facilities accept only uncontaminated concrete, which must be free of trash, wood, paper and other such materials. Metals such as reinforcement bars are removed with magnets, and melted down for recycling elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. After crushing has taken place, other particulates are filtered out through a variety of methods including hand-picking and water flotation.

Crushing at the actual construction site using portable crushers reduces construction costs and the pollution generated when compared with transporting material to and from a quarry. Large road-portable plants can crush reinforced concrete rubble at up to 600 tons per hour, compact mini-crushers can handle up to 150 tons per hour, and crusher attachments connected to construction equipment, such as excavators,  can process up to 100 tons/hour. 





In our project for the new railway station in 1999 we persuaded the Contractor to reuse the reinforced concrete from the redundant retaining walls as the aggregate for the car park adjacent to the station. The required road portable plant was brought from Valencia, the concrete crushed, and we obtained 60% of the aggregate necessary for the base of the car park. The Client and the Contractor contacted the local press, and the local council joined in the festivities: we successfully avoided landfill, transport costs, aggregate extraction, and the subsequent publicity gave "Sustainable" credentials to the Contractor, Client and Local Authority.

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