Pavement Distress Identification – Where is alligator (map) cracking most commonly observed?

Introduction / Context:
Alligator (or map) cracking is a characteristic fatigue distress pattern that resembles the scales of an alligator. Recognizing its typical occurrence helps engineers diagnose causes and select appropriate maintenance strategies for pavements.

Given Data / Assumptions:

• Flexible pavement layers with bituminous surfacing under repeated wheel loads.
• Subgrade and base properties influence tensile strain at the bottom of bituminous layers.
• Distress identification based on standard pavement condition surveys.

Concept / Approach:

In flexible pavements, tensile strains at the bottom of the bituminous layer under repeated traffic lead to initiation of micro-cracks. With continued loading, these cracks propagate and interconnect to form a characteristic polygonal pattern—termed alligator or map cracking. This is fundamentally a fatigue phenomenon of the asphaltic layer, exacerbated by inadequate thickness, poor drainage, aging and oxidation, or overloaded traffic. While rigid (concrete) pavements exhibit cracking, their patterns and mechanisms differ (e.g., transverse, longitudinal, corner breaks) and are not called alligator cracking in standard terminology.

Step-by-Step Solution:

Verification / Alternative check:

Field manuals (e.g., HDM, PCI for roads) illustrate alligator cracking exclusively for flexible pavements; corresponding concrete distress categories are different (faulting, pumping, slab cracking).

Why Other Options Are Wrong:

Concrete pavements (a, e) do not use “alligator cracking” as a standard distress term; gravel roads (c) show raveling/washboarding rather than fatigue polygons; WBM (d) may ravel or rut but not exhibit classical asphalt fatigue maps.

Common Pitfalls:

Confusing block cracking (due to aging and thermal cycles across larger blocks) with fatigue alligator cracking (fine polygons in wheel paths).

Final Answer:

Bituminous surfacing