According to Indian practice (ISI/IS specifications for paving bitumen), into how many standard grades is bitumen classified for roadwork selection and quality control? (Focus on the conventional grade count as per IS-based classification.)

Introduction / Context:
Road specifications rely on standard grading systems for bitumen so that engineers can choose binders appropriate to climate and traffic. The question checks recall of the conventional number of ISI grades historically referenced in Indian road engineering literature and exams.

Given Data / Assumptions:

• Grading systems (penetration or viscosity) categorize binders by measurable properties.
• Indian Standards have specified a discrete set of grades for procurement and QC.
• The question targets the standard grade count associated with IS-based practice, not the specific grade names.

Concept / Approach:
Standardization simplifies design charts, binder selection, and interchange across suppliers. The ISI classification enumerates fixed grades so that climatic suitability and mix design parameters can be matched consistently on projects across regions.

Step-by-Step Solution:

Verification / Alternative check:
Indian exam-prep and construction-handbook tables align with a six-grade classification for paving bitumens under IS practice in legacy references, even though modern revisions may use viscosity grades by code number.

Why Other Options Are Wrong:

• 2 or 4 grades: too few and do not reflect the broader climatic coverage intended.
• 8 or 10 grades: exceed the historical ISI enumeration relevant to this question.

Common Pitfalls:
Mixing up modern viscosity-grade labels with older penetration-grade lists; the question asks specifically for the number of grades in ISI classification for paving bitumen as traditionally taught.

Final Answer:
6 grades