Dr Greg Arnold – Pavement Specialist

Basecourse Aggregates for High Trafficked State Highways

Pavespec Ltd is working with Transit New Zealand to revise their TNZ M4 specification for basecourse aggregate that incorporates the Repeated Load Triaxial Test to determine whether or not the aggregate will be suitable for high trafficked roads (>10 Million ESAs over 25 years).

Recent research has shown that even if your aggregate complies with M4 that they do not perform equally.  The aggregate may fail by shear (see Figure below) within 6 months after the pavement has been constructed especially if it gets wet.



Figure - Shear failure within the aggregate layer

Small quantities of cement, lime or other additive are classified as a modified aggregate in the Transit New Zealand Supplement to the Austroads Pavement Design Guide.  Modified aggregates are recommended in the NZ Supplement for use for high trafficked state highways as an economical alternative to structural asphalt.  Using the RLTT will identify the most appropriate amount of additive and using rut depth modelling can predict the amount of extra life that can be obtained through modification compared with the source aggregate