Chipping away concrete’s carbon footprints

Beef and dairy farmers are popular targets in the debate about how best to reduce New Zealand’s greenhouse gas (GHG) emissions. However, there are plenty of other opportunities to reduce New Zealand’s carbon footprint that will give pause to environmentalists fixated on cows.

Take for example the current residential construction boom. This, coupled with commercial and large-scale public infrastructure build projects, will lead to further increases in construction and demolition waste.

Concrete receives special attention thanks to its core ingredient, cement, which by industry estimates produces 900kg of CO2 for every tonne manufactured. New Zealand produces approximately 4.1 million cubic metres of ready-mixed concrete each year, representing millions of tonnes of cement and a staggering amount of CO2, without even considering the high fuel and energy consumption required to mine sand and virgin coarse aggregate for concrete manufacture.

Reprocessing concrete products as many times as possible to maximise their lifetime in built environments can play a critical role in reducing CO2 emissions. While a handful of local demolition companies chip away at the edges of concrete recycling, most demolished concrete ends up in the ground, in clean or managed landfill.

However, the situation will soon change, with EnviroWaste set to unveil an industrial scale concrete recovery and processing facility at its Bombay Resource Recovery facility in South Auckland. The development is part of the company’s broader strategy to expand the range of materials it can reprocess.

“Our customers are looking for more sustainable solutions,” says James Rutter, EnviroWaste’s General Manager of Infrastructure. “We’ve led the way in re-purposing green waste, residential kerbside food waste, and inedible food from commercial customers, and we expect to make similar inroads into concrete waste.”

To jump start its ambitions, EnviroWaste approached industry experts Simon Lee and David Kenny to steer the company’s newly formed concrete recovery operation.  Ten-plus years working in construction, most recently as owners of Simken Limited, the pair has played an instrumental role in refining the development of concrete crushing, screening, and cleaning processes now operating at EnviroWaste’s recovery facility.

The Bombay site includes concrete processing equipment and a tipping facility for demolished, waste, and leftover concrete, which is pre-processed, crushed, and screened into various aggregate and drainage grades known as Recycled Crushed Aggregate (RCA).

Lee’s initial interest in concrete recycling was piqued during a development project involving the excavation of a 140-metre-long concrete driveway, which it turned out had been built over two additional driveways lost to the ravages of time and traffic.

Facing the prospect of transporting and disposing of many tonnes of broken concrete, and then having to import a similar tonnage of aggregate to replenish the void, Lee and his business partner used a small mobile crusher on site to re-purpose the concrete into aggregate – a move that all but eliminated the cost of concrete removal, trucking, dumping, and replacement aggregate.

The experience led the duo to establish a permanent site for concrete tipping and crushing, setting in motion further development and investment in plant to process demolition concrete into various grades of aggregate used as a direct substitute for virgin quarry sourced aggregates. Applications include sub-base, fill, slab preparation for new homes, drainage, and road building.

Local contractors and carriers bought their recycled aggregate in such volumes that Lee was unable to source enough usable waste concrete to keep up with customer demand. The situation led him to EnviroWaste, where he was able to provide the know-how and blue-chip customers to quickly scale up operations.   

He says concrete recycling produces various grades of aggregate at prices comparable to the virgin product – but without the significant carbon footprint. “There are few if any discernible differences between virgin aggregates and our recycled product,” he says, crediting processes to remove contaminants, such as plastic and wood.

Lee expects demand for recycled aggregate to escalate as customers weigh up the environmental impact of virgin aggregate and factor in the haulage costs of quarried aggregate.

Government’s proposal to "encourage more reuse and recycling" by progressively increasing the levy rate for landfills will further shift economics in recycling’s favour. From July 2023 a $10 per tonne levy will apply to waste concrete tipped at managed fill sites.

With the wheels now turning to produce commercial volumes of RCA, EnviroWaste is set to further refine processing with a view to develop RCAs for concrete manufacture. However, processing is not without challenges. RCAs typically have cement paste adhered to the aggregate from the original concrete (or parent concrete), which can impact compressive strength, depending on the coarse aggregate replacement rate and the strength of the parent concrete. A critical and extremely complex step in the process is to determine the properties of the parent concrete.

In the meantime, growing capacity to keep up with demand for RCAs is the principal focus. “Customer feedback has been extremely positive,” says Lee. “Everyone wishes they’d discovered recycled aggregate earlier. They love the idea of diverting concrete from landfill and ensuring aggregate does another lap of the construction lifecycle.”

Below: Stockpile of Recycled Crushed Aggregate (GAP 7)

Concrete Recycling