Bitumen, like any organic material, is affected by factors such as the presence of oxygen, ultraviolet rays, and changes in temperature. These factors are responsible for the hardening of bitumen. dire consequences
- reduction in entry price
- softening point temperature increase and
- Increase in penetration index (PI).
To increase the life of bituminous pavement it is necessary that there is no excessive hardness.
Factors affecting the aging of bitumen
The hardening of bitumen under the influence of external factors occurs in the following ways:
1. Oxidative hardening,
When bitumen is exposed to the atmosphere for a long time, oxygen begins to react with the bitumen components and higher molecular weight molecules are formed. Larger molecules result in less ductility and therefore increased stiffness. The degree of hardness is dependent on factors such as ambient temperature, exposure time and the thickness of the bitumen film. It is observed that 10. For0C in temperature 100 . rise above0c, the rate of oxidation doubles.
2. Hardening Due to Loss of Volatile:
Over time the volatile components in bitumen evaporate. The rate of evaporation depends only on the temperature. Bitumen has relatively few volatiles and therefore the hardening due to the loss of volatiles is relatively small.
3. Body Hardening:
At ambient temperature the bitumen molecules slowly reorient themselves. This gives physical strength. This process is an extremely slow process and hence the actual hardening due to the above factor is very low.
4. Exudative Hardening:
Exudative hardening occurs as the oily components precipitate out of the bitumen over time. The rate of hardening due to this process also depends on the type of bitumen and the porosity of the aggregate.
5. Hardening of Bitumen During Storage,
Bitumen is stored in above-ground tanks at high temperatures and the presence of oxygen are the two primary factors responsible for the hardening of bitumen. It is therefore very important that bitumen is handled at the lowest possible temperature consistent with efficient use. Also storage tanks should have a low surface-to-volume ratio to reduce the exposed surface area. Less exposed surface area would mean a lower oxidation rate.
When designing tanks it should be ensured that recirculation pipelines always enter the tank below the bitumen surface. This will reduce splatter during recycling. When the recirculation line enters the tank above the surface of the product, all three factors that promote oxidation. Higher temperatures, access to oxygen, and higher exposed surface-to-volume ratios are present. Therefore the quality of bitumen deteriorates very rapidly.
If handled properly, hardening in tanks can be negligible as the product is stored for short periods. If the bitumen is to be stored for a long time (4 to 5 days), the temperature should be set at 20 °C. should be reduced to0C to 250C above the softening point.
In the case where the bitumen is to be reheated to raise the temperature, adequate precautions should be taken. Bitumen should not be heated continuously in the beginning. Continuous heating can result in very high localized temperatures in the area near the heat source.
6. Hardening of Bitumen on the Road:
There may also be some hardening of the bitumen on the road due to oxidation. The level of oxidation purely depends on the accessibility of oxygen. If the pavement is well graded and well compacted then the hardening is nominal as the void content will be less.
HP Bitumen Handbook