Which one of the following statements is correct regarding Marshall test ?
- AWith increase in bitumen content, flow value increases, air voids decrease and voids filled with bitumen increase.Correct
- BWith increase in bitumen content, flow value increases, air voids decrease and voids filled with bitumen decrease.
- CWith increase in bitumen content, flow value decreases, air voids decrease and voids filled with bitumen decrease.
- DWith increase in bitumen content, flow value decreases, air voids decrease and voids filled with bitumen decrease.
Solution
Concept
Trends of Marshall properties with bitumen content
Step-by-step solution
1. More bitumen lubricates the mix: flow value increases and stability first rises then falls. 2. More bitumen fills the voids: air voids (VIM) decrease. 3. The voids filled with bitumen (VFB) increase.
Explanation
As the bitumen content rises the binder fills more of the void space, so VFB rises and the air voids fall. The mix becomes more plastic, so the flow value increases.
Why the other options are wrong
Options (b), (c) and (d) show VFB decreasing or flow decreasing, which is the opposite of the actual trend.
Shortcut
More bitumen: flow ↑, VFB ↑, VIM ↓; stability ↑ then ↓.
Common mistake
Thinking VFB decreases as air voids decrease.
Correct answer: A) With increase in bitumen content, flow value increases, air voids decrease and voids filled with bitumen increase.