In highway engineering, flexible pavements achieve their structural stability primarily through which mechanism? Choose the most complete combination that reflects how granular layers and bituminous binders work together.

Difficulty: Easy

Correct Answer: Aggregate interlock, particle-to-particle friction, and a measure of cohesion within the bituminous matrix

Explanation:


Introduction / Context:
Flexible pavements distribute wheel loads through a layered granular system topped by a bituminous surfacing. Unlike rigid pavements that rely on slab action, flexible systems mobilize aggregate interlock, friction, and binder-related cohesion to resist shear and rutting. This question checks your understanding of the fundamental stability mechanism in asphalt/bituminous pavements used in civil engineering practice.


Given Data / Assumptions:

  • Typical flexible pavement structure: surface (bituminous mix), base and sub-base (granular or treated), subgrade.
  • Load transfer occurs by grain-to-grain contact and matrix action, not by bending as a beam.
  • Bitumen provides adhesion and some cohesion but does not replace aggregate skeleton stability.


Concept / Approach:
Stability in flexible pavements arises mainly from the aggregate skeleton. Angular particles lock together (aggregate interlock) while normal forces generate frictional resistance; the bituminous binder coats and glues particles, adding cohesion and durability. The layered system gradually reduces stresses with depth by spreading the load over a larger area, preserving subgrade performance.


Step-by-Step Solution:

Identify the load path: wheel load → surface mix → base → sub-base → subgrade.Recognize that the surface and base layers rely on aggregate interlock and friction for shear resistance.Acknowledge that bitumen contributes cohesion and waterproofing, enhancing the aggregate skeleton's stability.Conclude that the complete mechanism combines interlock + friction + cohesion, not any single factor alone.


Verification / Alternative check:

Marshall and gyratory mix designs target stability and flow by optimizing aggregate gradation (skeleton) and binder content (cohesion, durability), confirming the combined mechanism.


Why Other Options Are Wrong:

Cohesion alone ignores the dominant role of aggregate skeleton.Binder's adhesive power alone is insufficient without proper gradation and interlock.Flexural strength of surface course suggests rigid-slab behavior, not applicable to flexible pavements.


Common Pitfalls:

Over-asphalting mixes, which reduces interlock; or using rounded aggregates that impair friction and stability.


Final Answer:

Aggregate interlock, particle-to-particle friction, and a measure of cohesion within the bituminous matrix

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