In a bituminous pavement, alligator cracking is a mainly due to

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Option B: Fatigue arising from repeated stress applications

Explanation & Concepts:
Alligator cracking, also known as fatigue cracking, is a distress pattern in bituminous pavements resembling the skin of an alligator. It is primarily caused by the repeated application of traffic loads (fatigue) that exceed the load-carrying capacity of the pavement structure, leading to structural failure.

Working Principle: As a wheel load moves over a bituminous surface, it creates tensile stress at the bottom of the asphalt layer. If the pavement is repeatedly subjected to loads that induce tensile strains exceeding the fatigue life of the mix, micro-cracks initiate at the bottom and propagate upward to the surface. Eventually, these cracks interconnect to form the characteristic polygons associated with alligator cracking.

Key Formulas: $N_f = k \cdot (\frac{1}{\epsilon_t})^n$ — The basic fatigue equation where $N_f$ is number of cycles to failure, $\epsilon_t$ is tensile strain, and $k, n$ are material constants.

Key Points: It is a structural distress indicating that the pavement is no longer capable of supporting traffic loads.; Usually starts as longitudinal cracks in the wheel paths and progresses to alligator patterns.; Commonly associated with weak subgrade or inadequate structural thickness.; Water infiltration through these cracks accelerates base and subgrade deterioration.

Applications: Pavement condition index (PCI) evaluation; Pavement maintenance and rehabilitation planning

Additional Info: Option A is incorrect because inadequate wearing course mainly leads to raveling or surface friction issues.. Option C is a contributing factor to the lack of structural support, but the primary cause categorized as 'alligator' is specifically the repeated fatigue stress.. Option D typically leads to bleeding or flushing of the bitumen, not alligator cracking.

Answer: B is correct — Alligator cracking is fundamentally a structural failure resulting from the cumulative fatigue caused by repeated traffic load applications.

Exam Tip: Remember: Alligator cracking is a 'Structural' failure, whereas 'Raveling' or 'Bleeding' are typically classified as 'Functional' failures.

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Option A: inadequate wearing coarse

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Option C: Inadequate thickness of sub-base course

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Option D: use of excessive bituminous material

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