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Introduction

The Austroads Guide to pavement technology part 2: pavement structural design(external link) and the associated New Zealand supplement (Transit 2007) are used to design pavements in New Zealand. These documents consider the effect of vertical stresses at the pavement surface, and the impact on the tensile strains at the bottom bound layers (asphalt or cemented materials) and compressive strains at the subgrade level (figure 1). 

Figure 1: Pavement model for mechanistic design procedure (Austroads 2012)

Recent pavement failures, where designers have not followed the advice contained in these guidelines in relation to high stress areas have prompted this technical advice note.

Mechanistic modelling indicates that both a thin (< 50 mm) and thick asphalt surfaced pavement can be adopted (Austroads 2012). However, consideration needs to be given to the shear stress the pavement will be under.

Where thin asphalt is being considered in areas of high shear stress, the Austroads pavement design guide provides the following advice:

• During certain manoeuvres, such as braking, turning and travelling uphill, heavy vehicles apply horizontal loads to the pavement, which are currently not taken into account in the design model. For thin surfaced granular pavements, the stresses generated by these loads are concentrated in the upper pavement layers and can have a significant detrimental impact on the performance of the surfacing.
• Caution is advised in adopting the thin asphalt surfaced pavement option because the dominant damage types are not necessarily those addressed by the design model.

In thick asphalt pavements, the interface bonding and the strength/modulus of the layers allow horizontal shear stresses to be dissipated throughout the pavement, therefore have relatively minimal effect on performance (Austroads 2012).

Asphalt thickness

For those state highway sites where an asphalt surface is considered the most appropriate treatment at locations that are subject to high shear stress, the NZ Transport Agency has provided guidance on the desired minimum asphalt depth (table 1).

a) on any leg where there is a potential for traffic to brake or change direction
b) laid in at least two layers, including a dense graded or SMA surfacing

Table 1: Minimum asphalt depth at intersections and roundabouts

The minimum asphalt depth in table 1 should not be seen as a default value. The designer, in meeting the requirements of this technical advice note, will still be required to ensure that the thickness and configuration of the pavement layers are sufficient to achieve the critical strain criteria required by the Austroads Guide to pavement technology part 2: pavement structural design.

Design considerations

The designer should ensure that the asphalt mixes in each layer are selected to achieve the required construction and serviceability requirements. Guidance on the minimum asphalt layer thickness for different mix sizes is provided in the Transport Agency’s M10 specification: Dense graded and stone mastic asphalts. 

Surface texture and skid resistance requirements must also be considered. Designers should refer to the Transport Agency’s T10 specification: Specification for state highway skid resistance management for guidance. Particular attention should be paid to selection of the aggregate source for the surfacing mixes. It should be noted that the aggregate performance method is the preferred means of selecting aggregate in surfacing applications.

The length treated should include the approaches to the intersection or roundabout that are subject to significant horizontal stresses. At a minimum, the skid assessment length for Category 1 sites defined in the Agency’s T10 Specification should be used.

Reference should be made to the Transport Agency’s M1-A specification: Performance specification for asphalt binders for the selection of an appropriate binder. It may be necessary to improve the shear resistance of the surfacing mix by using a polymer modified binder to resist shear forces in areas subject to high temperatures and large numbers of heavy vehicles.

These requirements may not be applicable at non-signalised intersections where the majority of traffic will not be required to brake or change direction.

Implementation

It is intended that the technical note will take immediate effect. For projects that have already been awarded at the time of publication of this document, Project Managers will need to assess the risk of not adopting these recommendations and the potential for premature failure against a variation for any additional costs. For future works it is expected that these requirements will be adhered to and built into the cost of the project.

The requirements in this technical advice note will be included in the next revision of the New Zealand supplement to the Austroads Guide to pavement technology part 2: pavement structural design.

References

• Austroads (2012) Guide to pavement technology part 2: pavement structural design.

• Transit NZ (2007) New Zealand supplement to the document, pavement design – A guide to the structural design of road pavements (Austroads 2004).

Further information

Contact Principal Surfacings Engineer Robert Busuttil from the Highways and Network Operations’ Pavement team. 

Kevin Reid