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Introduction

Melter chip is a premium synthetic chip produced by New Zealand Steel for surfacing high demand skid environments. Melter chip seals constructed at high demand and accident black spot sites have shown a reduction in wet road crashes. However, there are some material properties of melter chip that impact on treatment selection, chip seal design and chip seal construction that needs to be considered for the successful completion of melter chip seals.

Description

Melter chip is crushed from the non-metal material that comes from the iron refining process. Traditionally this product has been referred to as slag.

Image 1 - Melter chip in stockpile

Properties

Melter chip has the following properties:

• It is generally hard and tough
• Has a vesicular 'honeycomb' structure
• Can have very flat smooth faces
• Is quite dense so is heavier than the same volume of natural aggregate.

Hardness / Toughness

Melter chip is generally hard and tough although it does not always meet the NZ Transport Agency M6 crushing resistance standard test. Additional fines can be produced during the crushing test as fine edges are broken from the vesicles in the chip. Also fines produced during the crushing process can be retained within the vesicles and shaken out during the crushing test.

The Agency's M6A, Amendment to specification for sealing chip for use with melter slag from New Zealand Steel has been developed to allow for this anomaly. Although, there is often a higher percentage of fines produced in the crushing test, melter chip does not break down like some other softer high polished stone value (PSV) natural aggregates. The initial skid coefficient for melter chip is high so while it does polish at a similar rate to some natural aggregates it generally retains a skid coefficient above other natural aggregates under the same traffic stresses.

Vesicular (honeycomb) structure

Gas vesicles are formed in the slag during the smelting and cooling phases of the iron production. These vesicles give the chip a honeycomb look similar to scoria. While the edges of the vesicles add to the frictional properties of the rock the vesicles themselves make the rock more porous and can contribute to potential stripping in melter chip seals. This is possibly due to water retention in the vesicles and binder absorption.

Flat / smooth surfaces

Melter chip can have some very smooth, glassy, flat faces that make up 5 to 10% of any sample. If the majority of these faces are used in the PSV test the result will be potentially much lower than expected. Care needs to be taken to ensure that the PSV specimen is made up of a representative sample of the melter chip with broken faces exposed to the test wheel. On road performance is the best measure of the skid performance of melter chip.

Cleanness

While melter chip is generally free from organic dust it can have a coating of fine slag dust probably generated during the crushing process. This can be seen on your skin if you hold a handful of wet melter chip. Additional dust may be caused during long cart distances. This may require rewashing or pre-coating of the melter chip at the final destination prior to use to ensure good adhesion of the chip.

Density

Melter chip is denser than natural aggregates. Care needs to be taken in ordering and transporting melter chip to ensure sufficient chip is delivered to site to complete the required area of chip seals.

Treatment selection

Melter chip seals should be reserved for high demand skid sites. Chip retention and exposure of uncontaminated chip to vehicle tyres is most important. First coat seals, holding seals and sandwich seals would therefore not normally be completed using melter chip unless there is a significant risk of wet crashes occurring at the site.

Perceived efficiencies by mixing melter chip with natural chip need to be pursued with caution. If skid resistance is important at the site, melter chip should be used in both layers of multi-layer chip seals.

Small (2 to 5mm) melter chip 'friction' seals have been trialled to improve skid resistance particularly on asphalt surfaces where the aggregate has polished. These 'friction' seals can be very successful if attention is paid to the binder selection, binder quantity and the timing of construction.

Chip seal design

Absorption testing has shown that melter chip absorbs approximately 40% more bitumen than natural aggregates. This is due to the vesicular (honeycomb) structure of the melter chip. For this reason consideration should be given to increasing binder application rates for melter chip seals. Binder application rates (excluding texture allowances) for two coat seals should be increased by 10 to 15% and 15 to 20% for racked-in and single coat seals where there is no free binder on the surface to be sealed. These rate increases may be reduced where there is free binder on the surface to be sealed that will provide an additional bond to the melter chip before the onset of colder night temperatures.

Image 2 - Racked-in 3/5 melter chip seal

Tyre forces on melter chip will be higher than for most natural chip seals due to the increased grip that melter chip provides. Polymer modification to toughen the binder should be considered.

NZ Steel produce a limited range of chip sizes (generally grades 3, 4 and 5). Chip seals need to be designed around the chip sizes available at the time. Smaller chip sizes can be achieved by further processing of PAP material.

Chip seal construction

Melter chip seals appear more prone to chip loss (stripping) than natural aggregate chip seals if there is any rain during or within hours following the construction of the seal. This may be due to water retained within the vesicles of the chip excluding binder absorption into the vesicles promoting water induced stripping. Avoid sealing with melter chip if there is any chance of rain during or within hours of the completion of the seal.

Experience has shown that when using emulsion binders. cohesion can take longer to happen when using melter chip than natural aggregates. This can be a problem with chip pick up particularly when polymer modified emulsions are being used. Use of water to reduce adhesion of the chip to vehicle tyres must be treated with caution as it can increase the time for the emulsion to cure.

Chip pickup appears to be a greater problem with melter chip than with natural aggregates particularly when using polymer binders. On Two Coat seals the higher binder application rate should be placed first to ensure a better bond of the large chip to the substrate to reduce the risk of the large chip being plucked from the seal. There may also be a need for additional sweeping resource to sweep back chip that has been tracked off each end of the sealing site.

Although melter chip is generally hard and tough, avoid over chipping particularly of the second chip in multi-layer seals. It appears that the smaller chip sizes may be variable in hardness and possible crushing of the small chip may occur from excessive over chipping.

The first application of road marking paint can appear quite faded or dirty in colour with obvious rust spots. Allowance should be made for a second application of paint within three to four months of the initial application of paint.

Image 3 – First paint application (06/03/2015 LHS) and second paint application (06/05/2015 RHS) on a melter chip seal

Note:

• Density of melter chip is approximately 1.54 tonne per cubic metre
• Density of natural chip is approximately 1.47 tonne per cubic metre.

Further information

For further information, contact the National Pavements team at pavements@nzta.govt.nz.

Neil Cree