Previous projects

Projects have been undertaken at CAPTIF for a variety of clients. These clients have included the National Roads Board, BP Oil International, Transit New Zealand, OECD Road Transport Research programme, Transfund New Zealand, Opus International Consultants, the Ministry of Transport, the Cement and Concrete Association of New Zealand and ARRB TR Ltd on behalf of Austroads. A quick summary of CAPTIF data driven projects is provided below.

The relationship between vehicle axle loadings and pavement wear on local roads.

In New Zealand heavy vehicles are charged for using the road based on the damage caused passing over the road. The current approach to charging has its origins in American research that found doubling an axle load increased the damage as a power function with an exponent of 4, known as the Fourth Power Law. This was developed with limited pavement and vehicle load types not representative of most of the roads in New Zealand. This research provided reliable evidence on the wear characteristics of New Zealand local road pavements from accelerated pavement loading studies at the Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF). The aim was to determine the relative damage on different pavement types/strengths. The data was extended with rut depth modelling with repeated load triaxial data and validated with field data from the nationwide long-term pavement performance sites. A relationship was found between pavement life tested at CAPTIF plus the rut depth modelling and the damage law exponent for the 4 and 6 tonne equivalent axle loads. For short-life pavements the damage law exponent increased.

Effects of water on chipseal and basecourse on high-volume roads

The objectives of this study were to investigate the relationship between permeability of chipseals, waterfilm thickness, basecourse moisture sensitivity, heavy traffic volumes, and premature pavement failure following construction through the use of accelerated pavement testing at CAPTIF. The research has produced some surprising results in that the traditional M/4 basecourse was the worst performer in all cases. However, it must be borne in mind that this research can only be considered applicable to first coat seals, with high water film thicknesses at very high traffic volume. The recommendations resulting from the research are to:

• prime all new pavements before first-coat sealing to reduce the risk of early failure
• condition new seals before they are loaded in wet conditions, ie avoid the practice of sealing just before it rains as this is likely to increase the probability of failure
• avoid geometric designs that generate large water film thicknesses
• not delay in placing second-coat seals on high-volume roads
• use unsaturated hydraulic models for modelling moisture movement in pavements
• review first-coat seal failures for the factors observed in this report
• undertake field trials of lower permeability M/4 alternatives.

Epoxy-modified porous asphalt

Undertaken as part of a larger collaborative research programme conducted under the auspices of the OECD/ ECMT (European Conference of Ministers of Transport) Joint Transport Research Centre, this research focused on the economic evaluation of long-life pavements. The aim of the research was to investigate the potential of epoxy-modified asphalt as a low-maintenance long-life (>30 years) surfacing material. The NZTA’s contribution to the research focused on the potential benefits of epoxy-modified open-graded porous asphalt (EMOGPA).

Pavement thickness design charts derived from a rut depth finite element model

The two finite element models, validated with data from CAPTIF, were used to calculate rutting for a range of pavement depths on a number of subgrade soils. This information was used to develop pavement thickness design charts from the rut depth predictions and these were compared to the chart for granular pavements in the Austroads design guide.

The design of stabilised pavements in New Zealand

This research showed that modifying the tested aggregates with 1% cement could reduce rutting and improve the rutting life of the pavement by 200–300% compared to the traditional unbound pavement. Foamed bitumen and cement reduced rutting and created a 500% improvement in life compared to the unbound pavement without any loss of stiffness (fatigue) during the project.

The benefits of modified aggregates can be included in the current Austroads design framework with the empirical procedures developed in the report. The procedures can be used to estimate design life from laboratory mix design data on any proposed material.

Development of tensile fatigue criteria for bound materials

The purpose of this research was to develop a methodology from laboratory beam fatigue tests to obtain the tensile fatigue design criteria of aggregates bound by stabilising agents for use in pavement design to guard against cracking and/or a return to an unbound condition within the design life.

The testing found that stabilised aggregate beams can be successfully compacted in a the laboratory resulting in similar strengths (maximum tensile stress) to saw cut beams at CAPTIF and Australia’s accelerated loading facility (ALF).

The analysis method developed calculated conservative fatigue lives for the CAPTIF pavements. The surface of the CAPTIF tests did not crack, but the strain and deflection measurements increased to a typical level for a fully unbound pavement.

Fatigue design criteria for low noise surfaces on New Zealand roads

This research showed that, if pavements are constructed well, then applying low noise surfaces immediately after construction is possible. From analysis of the first test falling weight deflectometer readings, a conservative approach would consider all deflections having curvatures over 0.25mm to be unacceptable and that such results would require additional analysis. Where the criteria was not met – a short period of trafficking of the underlying pavement would generally lead to acceptable surface life.

Effect on pavement wear of an increase in mass limits (stages 1–4)

Research into potential axle mass limit changes showed that for state highway pavements – the increase to a 8.8 tonne single axle dual tyre for high productivity motor vehicles (HPMV) is unlikely to be significantly damaging to pavement structures or chipseal surfacings. The research also showed that the 4th power law for road user charges is conservative for state highways. Conversely, damage law exponents for low-strength low-volume roads could increase. The most appropriate damage law exponent for use in road user charges should be determined as a function of the pavements strength.

Benchmarking pavement performance between Transit’s LTPP and CAPTIF programmes

This programme includes analysis of the NZ long-term pavement performance (LTPP) programme and the CAPTIF accelerated pavement testing programme to develop a rutting model for New Zealand conditions. Previous work highlighted some data limitations in the LTTP programme.

Fundamental behaviour of unbound granular pavements under various loading conditions

The purpose of this project was to examine specific fundamental loading parameters (load magnitude and number of repetitions, tyre inflation pressure and basic tyre type) that influence the behaviour of thinsurfaced granular pavements. The pavement response and performance measurements included continuous surface deflection basins, longitudinal and transverse profiles, and vertical strains in the granular layers and subgrade.

Effect of binder modification on asphalt pavement performance

The trial involved constructing six test sections of various asphaltic concrete mixes over 200mm of unbound granular basecourse and a silty clay subgrade. The design life of all test sections was 1.0 x 10⁶ ESA, so the depth of the asphaltic concrete varied from 80mm to 125mm, depending on the characteristics of the different mixes.

Dynamic wheel forces and pavement wear

The objective of this research programme (three pavements) was to compare the pavement deterioration caused by dynamic loads generated under different types of suspensions: steel parabolic leaf spring and shock absorber, multi-leaf steel suspension, and air bag suspension with shock absorber.

The dynamic interaction of vehicle and infrastructure experiment (DIVINE)

The OECD Scientific Expert Group IR/6 has completed a coordinated international research programme investigating the effect of vehicle dynamic loading on pavement deterioration. The experiment consisted of six interlinked research elements undertaken by researchers from more than 10 countries. Element 1 of the DIVINE programme was an accelerated pavement testing project undertaken at CAPTIF to investigate the relationship between the dynamic loading produced by different suspensions and the resultant pavement performance.

A selection of projects and papers using CAPTIF data