GATE 2023 CE (CE2) – Question 51
Consider the Marshall method of mix design for bituminous mix. With the increase in bitumen content, which of the following statements is/are TRUE?
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Correct answer: (B) the Flow increases monotonically; (D) the voids filled with bitumen (VFB) increases monotonically
Explanation
In the Marshall method, the properties are plotted against bitumen content, and the typical trends are as follows.
- **A. Stability decreases initially and then increases.** False. Stability **increases to a peak** at some bitumen content and then decreases, because excess binder lubricates the aggregate.
- **B. Flow increases monotonically.** True. A richer mix is more plastic, so it deforms more under the load. ✓
- **C. Air voids ($V_a$) increase initially and then decrease.** False. More bitumen fills the voids, so air voids **decrease** continuously.
- **D. Voids filled with bitumen (VFB) increase monotonically.** True. More binder fills a larger share of the voids in the mineral aggregate. ✓
Correct: **B and D**.