Durability and Volume Stability

The U.K. Transport Research Laboratory frost heave test indicates that CBM show negligible heave when their compressive strength exceeds 2N/mm². In other words the specimens still maintain their integrity at this strength and can be considered as bound materials and thus superior to unbound materials.

With other HBM designed to achieve a lower strength i.e. CBR values in the range of 15 – 50% past practice has submitted the mixture to frost heave testing and apply the maximum heave criteria – about 12mm that is applicable to unbound materials. However, with such a degree of heave the integrity of the HBM is actually destroyed during the test and it is impossible to know whether the HBM can still be considered "bound" and thus perform as "bound" in the pavement.

Recently using data accumulated during the drafting of the European Standards for HBM, it has been possible to formulate more relevant durability recommendations for these lower strength HBM also including a limit for indirect tensile strength based on French and Belgian experience. The recommendations shown in the table, it should be noted, apply to all HBM irrespective of the frost susceptibility of the aggregate or the soil in the unbound condition.