Skip to content

Access keys for nzta.govt.nz

  • h Home
  • m Menu
  • 0 Show list of access keys
  • 2 Skip to content
  • 3 Skip to top

Resources

Back to Resources

#20-16 New specifications for dense graded asphalt concrete and stone mastic asphalt

Published: | Category: Technical advice note | Audience: Road traffic engineers & consultants

This note advises that Waka Kotahi NZ Transport Agency specification M10: 2020, Specification for dense grade asphaltic concrete (AC) and specification M27: 2020 Specification for stone mastic asphalt (SMA) have been released for general use and are available for download.

Publication details

  • Author:
  • Published: 10 November 2020
  • Reference: 20-16
  • Contact: pavements@nzta.govt.nz

Purpose

This note advises that Waka Kotahi NZ Transport Agency specification M10: 2020, Specification for dense grade asphaltic concrete (AC) and specification M27: 2020 Specification for stone mastic asphalt (SMA) have been released for general use and are available for download.

The key changes are:

  • Changes to the mix design process
  • Strengthening of requirements for quality control testing
  • The separation of SMA into its own specific specification
  • Inclusion of an option to use the Superpave™ method of asphalt mix design.

General information

M10 specification was last reviewed in 2014.  At that time, it underwent substantial change, permitting the use of the Austroads method of asphalt mix design, the addition of coarser-graded AC designated asphalt mixes and including, for the first time, specified criteria for SMA mixes.  Over the past six years a number of issues have been identified that required a review of the specification.

The intent of the review and specification update was to improve the quality of asphalt mixes, reducing risk and cost for client and supplier alike.

Changes to the Specifications

M10 and M27 now allow the use of the Superpave™ laboratory specimen compaction apparatus and conditions for the preparation of test specimens during the mix design process with the agreement of the Engineer.

A requirement to conduct regular volumetric testing mixes at production has been introduced into both specifications and acceptance limits have been set around the air voids content of compacted mix specimens.  While this new requirement will increase the cost of asphalt testing it is expected that consequent improvements in quality and reduced remedial costs will outweigh the testing cost.

A statistical process is used to determine density and air voids compliance of mix placed in the field.  The statistical process has been adjusted to a proportion defective risk of 10% (as previously) and a probability of acceptance of 90%. 

M10 and M27 now explicitly require the use of binders compliant with M01-A specification.

Minimum asphalt layer thicknesses have been adjusted upwards for dense asphalt mixes in M10 specification.  The intention is to specify a minimum thickness, based on research, such that the aggregate structure is properly formed in the asphalt mix layer to achieve good compaction, density and air voids, and hence durable mixes. 

The “Very Heavy” design target of 5% air voids has been amended in M10 to a design air voids content of 4%. 

A more detailed procedure to optimise the aggregate structure for SMA mixes has been included in M27. 

Other incremental improvements include:

  • Updated and clarified rules over asphalt mix design revalidation
  • A production trial is mandated for all new mix designs
  • The assessment of layer thickness from core specimens is assessed for compliance using a statistical approach
  • More information is required to be provided prior to acceptance of an asphalt mix design.

Benefits

It is expected that, over time, the Superpave™ method will become the norm in both New Zealand and Australia.  This will align our practices with international norms which are based on extensive research and field validations.

Adding a requirement to do volumetric testing of asphalt mixes at production will lift the quality of asphalt pavements.  It has long been considered best practice but its absence from specifications until now has increased the risk of non-compliances with mix placed in the field.

The statistical process to determine compliance of asphalt placed in the field has been adjusted to reduce the risk of non-compliant mix being accepted by Waka Kotahi.  This approach is consistent with the approach of other road agencies and specifications.

A requirement to use asphalt binders as specified by NZTA M01-A specification has been added.  This has the double advantage of linking binder choice to site climate and traffic loading and provides a rational basis for the use of polymer modified binders in asphalt mixes where they are necessary.

The design target of 5% voids has been removed from the specifications.  This amendment was made to ensure that mixes had enough binder for good fatigue behaviour, with any risk of increased susceptibility for deformation managed by the mandated Wheel Tracking testing.

A detailed process for designing SMA mixes, based on AASHTO R 46 has been included in the new M27 specification.  This process ensures that a “stone-on-stone” coarse aggregate skeleton is formed in the SMA mix.  Formation of such a skeleton is necessary to prevent densification, flushing and deformation of SMA mixes in service.  It is believed that using R 46 will improve the quality and durability of SMA mixes in New Zealand.

Implementation

The review of M10 and the development of M27 were completed by a working group, with representatives from client, consultant and contractor organisations, working under the aegis if the National Surfacings Technical Group.  As such there has been wide and extensive consultation with the new specifications developed by consensus.  It is hence expected that uptake of these new specifications will be smooth, resulting in better quality asphalt mixes and better outcomes for client and supplier alike.

The new specifications are now available for use.  To aid the implementation support and advice is available from Waka Kotahi Principal Surfacings Engineer at pavements@nzta.govt.nz

Effectiveness review

To ensure that these specifications have been implemented effectively, it is intended to undertake a review in twelve months’ time to find out how aware practitioners are of the specifications and how well they have been used.

Publication details

Author: Waka Kotahi NZ Transport Agency
Published: October 2020
Version: M10: version 6 and M27: version 1
Found at:
M10 Specification for dense grade asphaltic concrete (AC)
M27 Specification for stone mastic asphalt (SMA)

Further information

To submit documents or if you have any questions or feedback, please contact the NZTA Principal Surfacings Engineer, Grant Bosma at pavements@nzta.govt.nz

 

[signed]

David Darwin - Chair, Ratification Group

Vanessa Browne - Delegated Approver

Top