Dr Greg Arnold – Pavement Specialist

Basecourse Aggregates and Suitable Alternatives

Pavespec Ltd is working with Transit New Zealand to revise their TNZ M4 specification for basecourse aggregate that would allow alternatives to TNZ M4 provided the alternative material passed the Repeated Load Triaxial Test.

Types of materials that now can be considered as alternatives to TNZ M4 for basecourse through RLTT include:

GAP 40 – These are the as received materials from the quarry before additional processing to meet the recipe requirements in TNZ M4 – they may have the necessary rut resistance for a basecourse without the need for further processing and thus saving the quarry costs;

Modified Aggregates – Cement, Lime or similar additive may be added to lower quality/previously discarded aggregates to improve their resistance to rutting and allow their use as alternatives to TNZ M4 – this allows quarries to consider using overburden aggregates previously discarded as road basecourse aggergates. 

Hydrated Cement Treated Crush Rock (HCTCR) – HCTCR is a modified aggregate with a unique process of adding cement or similar additive as commonly used by Western Main Roads in Perth. Cement and water is added at the quarry and the material is left in the stockpile for 7 days, while in the first 24 hours the stockpile is moved to break the cementitious bonds.  During the 7 days water is sprayed on the stockpile to keep moist.

The aim of the Perth process, named Hydrated Cement Treated Crushed Rock (HCTCR) or 7day hydration is to ensure the final cement modified aggregate still remains flexible and not too “stiff” to prevent cracking of the pavement.  For more information please click here to download the report.