Drum mixers, which produce
large volume of asphalts, require silo storage for the mixed material to
account for peaks and troughs in demand. In such circumstances, mixtures are
stored in hot silo as well as in the delivery vehicle during transportation.
Some hardening of the bitumen
will take place during hot storage whether it is a silo or in a truck. It was
stated above that the amount of hardening will depend principally upon the
duration of exposure to oxygen, the thickness of the bitumen film and the
temperature of the mixture. When a mixture is discharged into a storage silo,
air enters with the mixture and some is trapped in the voids of the material. During
the storage period, some of the oxygen in this entrained air will react with
the bitumen. If no additional air enters the silo, oxidation of the bitumen
will cease.
It is important that the
entry and discharge gates are airtight and that there are no other openings
where air can enter the silo. If the discharge gate is not airtight the silo
may behave like a chimney drawing air in at the discharge gate (which exits the
loading gate) resulting in oxidation and cooling of the stored material. In addition,
the silo should be as full as practicable in order to minimize the amount of
free air at the top of the silo. Air remaining at the top of the silo will
react with the top surface of the material. This reaction forms carbon dioxide
that, because it is heavier than air, tends to blanket the surface of the
mixture protecting it from further oxidation. In the USA, some silos have the
facility to be pressurized with exhaust gases, containing no oxygen, from a
burner. These exhaust gases purge the silo of entrained air and provide a
slight positive pressure preventing more air entering the silo.
Studies carried out in the USA
suggest that if oxidation in the silo is limited to that induced by entrained
air then little or no additional oxidation will occur during transportation and
laying. It is hypothesized that this is because no significant quantity of
fresh air is entrained in the mixture during discharge into the truck. Thus, little
or no additional air is available for oxidation. In fact, it was observed that
if the mixture was discharged directly from the pugmill into the delivery
vehicle, the amount of hardening during transportation was very similar to that
which occurs during silo storage.
If materials are being laid
at low ambient temperatures or if the mixture has to be retained for a period
in hot storage, there is a temptation to increase mixing temperatures to offset
these two factors. However, increasing the mixing temperature will be lost because
of additional oxidation of the bitumen which may adversely affect the long-term
performance of the material.
