Asphalt

Question 1

Why does the industry use performance-based specifications for asphalt cement and asphalt concrete rather than prescription-based specifications?

Answer 1

1. Asphalt is a very variable material
2. Instead of assigning a specific ratio of components which is hard to regulate
3. Assign a performance criteria like strength required which can be easier to regulate
4. Adjust performance by controlling aggregate gradation, production method

Question 2

Why should aggregates be dry when being bound to asphalt
cement?

Answer 2

1. Asphalt molecules are polar which makes them attracted to aggregates surface
2. Problem is water is even more polar: the attraction of water to aggregates is stronger than the attraction of asphalt to aggregates
3. Water will bind with aggregate first leaving the asphalt unbonded

Question 3

What is an open-graded and dense-graded asphalt concrete?

Answer 3

1. Open-graded: Too much asphalt cement and not enough aggregates
2. Dense-graded: Too many aggregates and not enough asphalt cement

Question 4

What is the mechanism of aggregate bonding in asphalt?

Answer 4

Adsorption: polarity of asphalt cement crystals will cause asphalt cement to be attracted to aggregates

Question 5

How does rutting occur in asphalt and why is it dangerous?

Answer 5

1. Rutting: large channels formed due to the constant load of vehicule tires applied on the same surface
2. Rutting is dangerous when it is raining heavily because the tracks become filled with water
3. Tires are no longer in contact with the asphalt, car hydroplanes which causes sudden loss of control of the car

Question 6

How are potholes formed?

Answer 6

1. Water enters underneath asphalt into sub-base which is mostly loose aggregates
2. Water pockets are formed in sub-base from erosion of aggregates due to the water
3. When temperature drop, asphalt becomes brittle
4. Water freezes and expands causing cracking in asphalt
5. Due to vehicle driving over cracks and pockets, asphalt fails and creates a hole