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Bitumen curing in asphalt bases

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Bitumen curing in asphalt bases

Contrary to the effects of bitumen hardening in the surface course, a gradual hardening of the main structural layers of the pavement appears to be beneficial, and is described as ‘curing’. As with all hot mixed asphalts, the penetration of the bitumen in a base will harden by approximately 30% during the mixing and laying process, and, despite asphalt bases being locked inside the pavement construction and shielded from exposure to the environment, the penetration of the bitumen will continue to exhibit varying rates of hardening. The Transport Research Laboratory (TRL) carried out a wide ranging investigation into the behaviour of asphalt pavements in service (Nunn et al., 1997). This very important study noted in section 2.1 that One of the findings of this research was that changes occurring in asphalt over the life of the road are crucial in understanding its behaviour. These changes, which are referred to as curing, can help to explain why conventional mechanisms of deterioration do not occur and why, provided the road is constructed above a minimum threshold strength, it should have a very long, but indeterminate, structural life. The increase in the stiffness of the asphalt base causes the traffic-induced strains in the pavement structure, which control fatigue and structural deformation, to decrease with time. Therefore, a road will be more vulnerable to structural damage in its early life, before curing has increased the structural strength of the material. If the road is designed and constructed with sufficient strength to prevent structural damage in its early life, it has been found that curing doubles the stiffness of DBM roadbase [now base, of course] in the first few years in service and this will substantially improve the overall resistance of the pavement to fatigue and structural deformation. The improvement in the bearing capacity of the road, asdetermined by deflection measurements, provides confirmation of this improvement.. As can be seen by examining the figure, the stiffness of an asphalt concrete base (the most common type of base in UK roads), which is initially 2 GPa at the time of laying, increases to around 4 GPa 4 years after laying. So, the stiffness has doubled in a period of 4 years. Clearly, the increase in the stiffness of asphalt bases over time will improve the structural competence of a road pavement, and has implications for the structural design of flexible pavements (see section 13.3).