Skid Resistant Roading Material: looking for the God particle

AQA technical advisor BILL BOURKE updates the continuing search for the ideal skid resistant roading material.

Bill Bourke
Bill Bourke

The AQA’s technical group has been consulting with the Transport Agency in regards to a review of M6 and other skid resistant roading material specifications and standards for nearly two years.

The initial aim was to have the process wrapped up by July, but AQA technical advisor Bill Bourke says it could take longer.

“We still need to test a number of New Zealand aggregates. Historically we have tested our aggregates with methods from the UK, but our aggregates are generally younger than most other markets and it is important that we choose the best materials for our road surfaces.”

Much effort has already gone into the process he adds. Bill is a long-time champion of the Micro-Deval skid resistant test and, with many other experts, is keen to see the testing method progressed here.

“As an international test to determine the durability of roading aggregate, it has been adopted in Europe and is used increasingly in North America,” he says.

The difference between this test and the more common ‘Los Angeles Abrasion’ test is that the material is tested wet. The aggregate is soaked before being rotated in a drum with steel balls. The fines that drop off are then measured as a percentage and the lower the number the better.

A company familiar with Micro-Deval testing is Opus, says Bill, and it has been particularly interested in the problem of flushing – when the bitumen comes through the chip to form a smooth patch on the road surface.

“Their theory is that aggregate breakdown is contributing to the problem of flushing, but they also admit they have more work to do to prove this.”

Opus has one of the two sets of Micro-Deval testing equipment in this country and it has slightly modified the drum with a larger ball in it to simulate the attrition of an aggregate on a road.

Bill is convinced that through testing methods we will find better aggregates that will improve both the safety and maintenance of our roads.

“We could be looking for the ‘God particle’ that distinguishes an aggregate for making safe, hard wearing roads, but I believe the way to that discovery is through Micro-Deval and studying the properties of our unique aggregates in more detail than we have done before.

A passion for melter slag

Bill’s own experience with Micro-Deval testing melter slag was at the Glenbrook mill some 12 years ago, while working for NZ Steel.

Melter slag (see footnote) is a by-product from the iron-making process at NZ Steel.

“We don’t use a blast furnace in New Zealand so the iron-making process and the ironsand raw material means the slag at Glenbrook is unique and has many of the characteristics of ‘steel making’ slags used overseas in asphalt. And it has properties that make it an excellent material for roading.”

Bill says NZ Steel was very thorough with its melter slag tests that went beyond the standard ‘polished stone value’ requirements of the day.

“And I knew through detailed testing that melter slag was a good aggregate.”

Jokingly comparing his efforts to convince the Transport Agency at the time of this value to “Mao’s long-march”, Bill says it took nine years of lobbying before the Kiwi slag was deemed a suitable roading material.

“Now it is one of the NZTA’s top aggregates for this end use and it’s only a matter of the ability of NZ Steel to keep up the supply.”

Melter slag has been used extensively by the Auckland Motorway Alliance and Fulton Hogan has been working with melter slag as a roading material since 2004.

Another characteristic of Glenbrook’s melter slag is its ability to remove heavy metals from degraded water – including zinc, copper and lead.

This property was discovered when melter slag was first used as a filter bed for the Waiuku treatment plant raceway and it was noticed that the water coming out of the raceway had no phosphorus in it. While this filtration ability does have a tendency to drop off, it still produces a very good, clean outfall says Bill.

“I did a lot of work on this aspect of our melter slag and made at least three international presentations on the subject.”

Meanwhile, the search for the ‘God particle’ continues.

“At the risk of sounding poacher turned gamekeeper, I personally believe the best result for skid resistance will be found with a number of natural aggregates that have yet to be properly identified.

“The AQA is working with NZTA and the industry to better identify the source of aggregates that are specified for this end use, which can then be entered into the RAMM database to be reviewed by the Agency and other stakeholders to pinpoint those aggregates that are performing well.”

Melter slag is a major by-product of the steel manufacturing process, and its potential for use as an aggregate in open-graded emulsion mixes (OGEMs), is the topic of an Opus report that is available from Crushed melter slag was first tested in 1991 for use in constructing access roads within the Glenbrook steel mill complex. Its performance was later trialled on a state highway intersection for about four years (1994-1997). The report details the results of the monitoring and evaluates its use as aggregate.

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