Supergrip aggregate

Driving over certain areas of Auckland’s motorway, your tyres will be gripping the latest in skid resistant asphalt containing a special aggregate from Winstones Aggregates branded ‘Gripchip’.

Produced for specialist surfacing applications and conforming to Transit TNZ M6 specification, Gripchip is not crushed from a rock quarry but produced from ‘electric arc furnace’ (EAF) slag from Winstone’s sister company, Pacific Steel, in Auckland.

In a great example of company collaboration and sustainability, Winstone Aggregates and Pacific Steel have been working together since the end of last year to turn the steel work’s manufacturing waste into a asphalt/sealing chip product with a very high resistance to skidding and with the ability to remain intact under high stress conditions. 

The joint waste recovery and aggregate processing (WRAP) plant has been in operation at the back of Pacific Steel’s Otahuhu factory for a year now, producing a range of sealing chip and PAP product, all tested and conforming to TNZ M6 specification.

Using steel slag for chip is not new and blast furnance slag, a related product, is produced overseas. However, EAF steel slag is considered to be the preferred material when skid resistance is the objective.

Winstone says its Gripchip has been lab tested to determine a polished stone value (PSV) of plus 65, a level with limited resource options in this country, so is ideal for skid resistance applications such as highway off-ramps and roading areas leading up to pedestrian crossings.

The slag comes out of the steel plant at 1500 degrees and is cooled to about 400 degrees before loaded on a truck and taken around the back of the mill to the quarrying plant. The slag feedstock is aged for six months before processing, which conforms to accepted world’s best practice and safeguards against possible expansion issues. Winstone says it backs this ageing process up through testing by experienced slag consultants in the UK.

The operation can handle 25,000 tonnes of material a year, although this is obviously a high value, low volume product in a limited market. A bespoke plant was designed and installed by Rocktec with the aim of producing a finished product at a throughput of 40 tonnes per hour. The plant has to date actually averaged 50 tonne an hour.

The matured slag is loaded into a hopper with bars spaced 150mm apart and is carried by a conveyor under four magnets that pull out any steel. It is essential that the finished product is 100 percent free of steel. The recovered steel is sent back to the mill for reprocessing by Pacific Steel. The aggregate then passes over a scalping screen, through a 40mm by 10mm granulator then goes under a second magnet, before final processing through a 840 Barmac crusher.

The first screen splits off the fine aggregate product (PAP 5) and any oversize is re-circulated back into the Barmac. Each of four grades of chip product goes under a final magnet before it gets to the wash screen. The plant was specifically designed to provide flexibility in product sizing, and currently the plant is producing grade 3, 4, 5 and 6. Winstone says the demand has been for the finer chips.