Flexible pavements — probable cause of rutting Which of the following is a common and probable cause of rutting (permanent longitudinal depressions) in flexible pavements?

Introduction / Context:
Rutting is a structural/permanent deformation in wheel paths of flexible pavements caused by densification and shear flow under repeated traffic loads. It compromises safety and ride quality and accelerates moisture damage by trapping water in the depressions.

Given Data / Assumptions:

• Asphalt layers and granular bases are susceptible to post-construction densification if not compacted to target density.
• Heavy axle loads and high pavement temperatures exacerbate shear deformation.
• Aggregate gradation and binder properties influence rutting resistance.

Concept / Approach:
Achieving target air-voids by adequate compaction at construction is the first defence against rutting. Under-compacted layers densify rapidly under traffic, causing depressions. Shear-susceptible mixes (poor VMA, high binder at high temperature, rounded/weak aggregates) and inadequate confinement in base/subgrade further contribute, but compaction shortfall is the most frequently cited root cause in practice.

Step-by-Step Solution:
Relate rut depth growth to plastic strains in HMA and underlying layers.Identify compaction as controlling initial air-voids and aggregate interlock.Conclude inadequate compaction produces early densification and wheel-path grooves.

Verification / Alternative check:
Performance tests (Hamburg wheel-track, flow number, dynamic modulus) correlate rutting to mix design and density; forensic studies routinely implicate insufficient in-place density.

Why Other Options Are Wrong:
Stripping (a) can cause ravelling and potholes; it contributes but is not the most typical direct cause of rutting.
Flaky aggregates (b) degrade stability but compaction deficiency is more fundamental.
High wind (d) is irrelevant to rut formation.
Excess binder (e) can worsen rutting; it does not eliminate risk.

Common Pitfalls:

• Confusing rutting (permanent deformation) with shoving or bleeding, which are different distress modes.
• Ignoring subgrade rutting; weak subgrade under wheel loads can mirror as surface ruts.

Final Answer:
Inadequate compaction of pavement layers during construction