Aging of Bitumen Binders
Bitumen, like any organic matter, is affected by factors like presence of oxygen, ultraviolet rays and changes in temperature. These factors are responsible for hardening of bitumen. Hardening results in
- Decrease in penetration value
- Increase in softening point temperature and
- Increase in penetration index (PI).
For increased life of bituminous pavement it is essential that excessive hardness does not take place.
Factors Influencing Aging of Bitumen
Hardening of bitumen takes under the influence of external factors in the following ways:
1. Oxidative hardening:
When bitumen is exposed to atmosphere for a prolonged period the oxygen starts reacting with the bitumen constituents and higher molecular weight molecules are formed. Larger molecules results in lesser flexibility and hence increased hardness. The degree of hardness is dependent on factors like ambient temperature, exposure time & thickness of bitumen film. It is observed that for 100C increase in temperature above 1000C, the rate of oxidation doubles.
2. Hardening due to loss of volatiles:
Over a period of time the volatile components in bitumen evaporate. The rate of evaporation is dependent on temperature only. The volatiles in bitumen are relatively very low and hence hardening due to loss of volatiles is relatively small.
3. Physical hardening:
At ambient temperatures bitumen molecules slowly reorient themselves. This results in physical hardening. This process is an extremely slow process and hence actual hardening due to the above factor is very low.
4. Exudative hardening:
Exudative hardening takes place due to the movement of oily components out of bitumen over a period of time. The rate of hardening due to this process is dependent on the type of bitumen and also on the porosity of the aggregate.
5. Hardening of bitumen during storage:
Bitumen is stored in above ground tanks at high temperatures and presence of oxygen are the two primary factors responsible for hardening of bitumen. Hence it is very important that bitumen be handled at the lowest possible temperature, consistent with efficient use. Also the storage tanks should have low surface to volume ratio so as to minimize the exposed surface area.. Lower exposed surface area would mean lower oxidation rate.
While designing the tanks it should be ensured that the recirculation pipelines always enter the tank below the bitumen surface. This will reduce splashing during recirculation. When the recirculation line enters the tank above the product surface all the three factors which promote oxidation viz. high temperature, access to oxygen and high exposed surface to volume ratio, are present. Therefore bitumen quality deteriorates very fast.
If handled properly the hardening in tanks can be insignificant as the product is stored for shorter durations. If bitumen is to be stored for long durations (4 to 5 days) then the temperature should be reduced to 200C to 250C above softening point.
In case where bitumen is to be reheated to increase the temperature adequate precautions have to be exercised. Bitumen should not be heated continuously in the beginning. Continuous heating can result in very high localized temperatures in area close to the heating source.
6. Hardening of bitumen on road:
Some hardening of bitumen can also take place on the road due to oxidation. The level of oxidation is purely dependent on the access to oxygen. If the pavement is well graded and well compacted the hardening is nominal as the void content will be low.
HP Bitumen Handbook