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#19-08 Release of the new M6 specification for sealing chip

Published: | Category: Technical advice note | Audiences: Road traffic engineers & consultants, Roading contractors

This notice advises that the new sealing chip specification is available to be used for all future chipsealing works.  The new specification will come into effect immediately and should be applied to all upcoming chipsealing works in the State Highway Programme.

Publication details

  • Author:
  • Published: 24 June 2019
  • Reference: 19-08
  • Version: 1
  • Contact:


This notice advises that the new sealing chip specification is available to be used for all future chipsealing works.  The new specification will come into effect immediately and should be applied to all upcoming chipsealing works in the state highway programme.

This document sets out best practice for the production of aggregate including minimum frequency of sampling and testing.  The new specification controls the quality of the rock feedstock used for the manufacture of sealing chip and sets limits around criteria for five grades of chip.

This notice replaces Technical Advice Note #17-08.

Over the past several years alternative materials have been used to manufacture sealing chip to create polishing-resistant seals that retain their skid resistance over the design life of the seal.   This update to the M6 specification also regularises the use of such materials.

There are other changes to M6 specification covering the control of the source rock quality used for manufacturing sealing chip.  These changes have become more prescriptive, and include:

  • An increase in the frequency of testing based on volumes of sealing chip produced
  • A composite bulk sample consisting of a blend of grades 2 to 6 chip is used for source rock property testing
  • Source rock tests are Crushing Resistance, Weathering Resistance and now include regular Polished Stone Value testing. 

The specification is now available online at

General information

The drive to provide skid resistant pavement surfacings that retain their skid resistance over their lives has created a demand for chip aggregates that do not polish.  While there are some natural rock aggregates that perform well in high stress situations where aggregate polishing can occur, synthetic materials such as Glenbrook Melter Aggregate (GMA), have been found to perform very well in comparison.  The changes to M6 allow the use of GMA chip aggregates, to reflect current chip sealing practice.  The changes include:

  • Specific reference to Glenbrook Melter Aggregate in the Source Properties Section
  • Adjustment of the Crushing Resistance requirement for GMA allowing up to 13% fines to be produced under a standard load.

The Polished Stone Value test, BS EN 1097-8 specifies a UK-sourced control stone, used as a reference aggregate in the test to standardise the test results.  This control stone is difficult to obtain, so a New Zealand sourced control stone has been identified and M6 specifies amendments to the Polished Stone Value test to allow the use of this stone.

Some changes have been made to the specified properties of the sealing chip.  These include:

  • A new requirement to measure and report the Cleanness Value for Grades 5 and 6 chip.  At this stage no specification limits have been determined for these Cleanness Values
  • Grade 5 chip has been added to the Size and Shape testing requirements with the results as report only at this stage.

Changes to the notes

The notes to M6 have been extensively rewritten.  Changes include:

  • A rewrite of the Skid Resistance section to align it with T10 and to improve clarity.
  • A new section discussing the Aggregate Performance method for selecting a surfacing aggregate.
  • A rewrite of the Polished Stone Value section to reflect current knowledge.
  • Photographs of correctly prepared PSV test specimens to assist testing agencies with the preparation of representative test specimens.
  • New sections on Production Properties.

User benefits

The value to producers, chip sealing practitioners and asset owners include:

  • Greater clarity on requirements for source rock pre-qualification and testing frequencies going forward
  • The explicit inclusion of high skid resistant premium synthetic aggregates in the specification with some specific requirements for the GMA chip which regularises its continuing use
  • The specific allowance of a New Zealand-sourced “control stone” for the Polished Stone Value test, and guidance for using this material.  This will allow lowered testing costs and more efficient use of resources when conducting the Polished Stone Value test
  • M6 is now aligned with T10, particularly in respect of providing guidance on selecting an appropriate aggregate for the maintenance of skid resistance over the life of a chip seal
  • The collection of more test data for Grade 5 chip to improve the design of chip seals that use Grade 5 sealing chip
  • The notes have been extensively rewritten to improve the guidance and advice given to engineers, testing laboratories, practitioners and the aggregate industry.

Publication details

Author: New Zealand Transport Agency
Published: June 2019
Version: 1
Found at:

Further information

To submit documents or if you have any questions or feedback, please contact the NZTA Principal Surfacings Engineer, Grant Bosma at


David Darwin                 Vanessa Browne