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Hardening of bitumen during mixing with aggregate

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Hardening of bitumen during mixing with aggregate

This is described as ‘short term ageing’, a term that is also applied to hardening that occurs during laying. During the mixing process, it is estimated that all the aggregate and filler is coated with a thin film of bitumen, usually between 5 and 15 mm thick. If the bitumen from 1 tonne of a dense asphalt concrete was spread at 10 mm thick, it would occupy an area of around 10 000 m2, the equivalent of over one and a half average size football pitches. Thus, when bitumen is mixed with hot aggregate and spread into thin films in a paddle mixer, conditions are ideal for the occurrence of oxidation and the loss of volatile fractions within the bitumen. Hardening of bitumen during this process is well known, and is taken into account when selecting the grade of bitumen to be used. As a very rough approximation, during mixing with hot aggregate in a paddle mixer, the penetration of a paving grade bitumen falls by about 30%. However, the amount of hardening depends on a number of factors, such as temperature, duration of mixing and bitumen film thickness. Minimising the hardening during mixing depends on careful control of all these factors. Control of the temperature and the bitumen content are particularly critical. clearly shows increasing bitumen hardening, measured by higher values of softening point, as mixing temperatures are raised. Similarly, shows that reducing the thickness of the bitumen film significantly increases the viscosity of the bitumen. The latter is measured by the ageing index, which is defined as the ratio of the viscosity of the aged bitumen, ha, to the viscosity of the virgin bitumen, ho. Note that the ageing index is not a fundamentally defined parameter – it is usually a ratio of two values (e.g. viscosity, stiffness or penetration) measured at different times. The type of mixer used also affects the amount of hardening during mixing. It has been recognised for some time that the amount of hardening in a drum mixer is often less than that which occurs in a conventional batch mixer (Haas, 1974). This is due to the presence of steam in the drum, which limits the availability of oxygen. However, the multiplicity of different designs of drum mixers means that variation in the amount of hardening between different designs of plant is almost inevitable. Modern drum mix plants, notably those with counter flow drum mix and double drum configurations with very high mixing efficiencies, result in minimal hardening of bitumen compared with older drum mix plants. Notwithstanding, a study carried out by Shell Bitumen (1973) on two different drum mixers showed that, for equivalent mixing temperatures, the overall reduction in penetration and increase in softening point can be less than half of that which occurs in a conventional batch mixer.