Build 133 — December 2012/January 2013 — 75

Concrete contributes 7% of all construction waste.

Departments/Sustainability

CONCRETE IS THE SECOND most consumed material after water, with an estimated 25 billion tonnes of concrete manufactured globally each year. Twice as much concrete is used in construction against all other building materials combined.

In New Zealand, construction and demolition waste is over a quarter of total waste generated, with concrete 7% of that total.

Recycling of concrete encouraged

The $10/tonne landfill levy acts as an incentive in New Zealand to find other uses for waste concrete. Green Star New Zealand provides credits for using recycled aggregate to reduce the resource depletion associated with manufacturing concrete.

The Cement and Concrete Association, supported by BRANZ, recently published the Best practice guide on the use of recycled materials in concrete to raise awareness of concrete recycling. It contains technical guidelines on recycled aggregate use in concrete and looks at the use of crushed concrete – returned leftover concrete or demolition concrete – and waste materials from other industries, such as glass, as aggregate in new concrete.

Several options for excess concrete

It is estimated that, in Auckland alone, around 30,000 m3 of fresh concrete, mostly over-ordered surplus, has to be disposed of yearly. Ready-mix trucks can no longer wash out at the construction site – controlled disposal is required.

Whether it is surplus material or demolition waste, the reuse of concrete is actively encouraged. A best practice guide outlines how to do this.

By Derek Chisholm, Solid Concrete Solutions, Otaki

New life for concrete

If flushed out of the returned concrete, the recovered aggregate can be recycled while the alkaline slurry can be reused as part of the batch water in new concrete.

However, the concentration of slurry in the mix water needs to be restricted, as setting times in the new concrete can be adversely affected. Alternatively, surplus concrete can be left to harden and subsequently crushed as leftover concrete aggregate.

Normally, the compressive strength of concrete is controlled by the cementitious phase. However, recycled concrete aggregate typically has mortar adhered to the aggregate from the original concrete (termed parent concrete), which can have a limiting effect on compressive strength depending on the coarse aggregate replacement rate and the strength of the parent concrete. This adds to uncertain-ties, particularly for demolition aggregate where the properties of the parent concrete are not typically known. Also, testing for contaminants such as gypsum or asbestos may be required.

Guide includes replacement rates

Tables in the best practice guide specify maximum recycled coarse aggregate replacement rates from 100% for 17.5 MPa concrete to 30% recycled demolition concrete aggregate for 30 MPa concrete.

The recommended replacement rates for demolition aggregate are lower than for leftover concrete aggregate due to greater uncertainty about aggregate properties.

Issues with natural aggregate supplies

Natural concrete aggregate supplies are increasingly marginal in New Zealand, and haulage is an issue. In Auckland, quarried aggregate is transported over 100 km to construction projects.

The use of concrete aggregates and waste aggregates is likely to increase in ready-mix concrete. The best practice guide is a resource for concrete specifications, and New Zealand standards for concrete aggregate supply (NZS 3121) and concrete supply (NZS 3104) are being amended to facilitate the use of recycled aggregates. For more The best practice guide can be downloaded at www.ccanz.org.nz

under publications.