The Marshall method of mix design for bituminous mix involves determining the optimal bitumen content that offers a balance between stability, flow, air voids, and voids filled with bitumen. Interpreting the given options with respect to the effect of increasing bitumen content, we can analyze each statement as follows:

Option A: the Stability decreases initially and then increases

This statement is inaccurate. Typically, with the increase in bitumen content, the stability of the mix initially increases up to an optimum point and then starts to decrease as the additional bitumen acts as a lubricant and makes the mix more pliable.

GATE CE 2023 Set 2 Transportation Engineering - Highway Materials Question 2 English Explanation 1

Option B: the Flow increases monotonically

This statement is correct. The flow value, which indicates the deformation undergone by the specimen before failure, generally increases continuously with the increase in bitumen content, making the mix more flexible and deformable.

GATE CE 2023 Set 2 Transportation Engineering - Highway Materials Question 2 English Explanation 2

Option C: the air voids (VA) increases initially and then decreases

This statement is incorrect. Air voids (VA) in the mix tend to decrease as the bitumen content increases because the bitumen fills the spaces between the aggregate particles, reducing the amount of air entrapment.

GATE CE 2023 Set 2 Transportation Engineering - Highway Materials Question 2 English Explanation 3

Option D: the voids filled with bitumen (VFB) increases monotonically

This statement is correct. With increasing bitumen content, the voids filled with bitumen (VFB), which represents the percentage of the void spaces in the compacted aggregate mass that are filled with bitumen, increases steadily because more voids get filled with the additional bitumen.

GATE CE 2023 Set 2 Transportation Engineering - Highway Materials Question 2 English Explanation 4

Therefore, the correct statements with the increase in bitumen content are:

Option B: the Flow increases monotonically

Option D: the voids filled with bitumen (VFB) increases monotonically