IRC Code: Guidelines for the Design of Flexible Pavements for Low Volume Rural Roads (IRC:SP-72-2015)

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India’s vast rural road network is the backbone of economic development and social integration. The Indian Roads Congress (IRC) has formulated guidelines for designing flexible pavements for low-volume rural roads under IRC:SP-72-2015. These guidelines provide essential recommendations on traffic evaluation, material selection, subgrade preparation, and pavement thickness to ensure durable and cost-effective rural road infrastructure.

This blog delves into the key provisions of IRC:SP-72-2015 and how they impact the design and maintenance of flexible pavements for rural roads.

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Why Flexible Pavements for Rural Roads?

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Flexible pavements are widely used for rural roads because they are:✅ Cost-effective compared to rigid pavements.✅ Easier to construct with locally available materials.✅ Capable of withstanding varying loads and environmental conditions.✅ Easier to maintain and repair.

The IRC:SP-72-2015 guidelines ensure that these pavements are designed for sustainability and durability while optimizing costs.

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Key Design Considerations as per IRC:SP-72-2015

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1. Traffic Classification & ESAL Applications

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The design of flexible pavements is primarily based on Equivalent Standard Axle Load (ESAL) applications, which estimate the cumulative vehicle load over a 10-year design life. The guidelines categorize traffic into nine levels (T1 to T9) based on the total ESAL applications:

• T1: 10,000 – 30,000 ESALs
• T5: 200,000 – 300,000 ESALs
• T9: >1,500,000 ESALs

For roads expecting traffic above 1 MSA (Million Standard Axles), detailed traffic surveys are recommended.

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2. Subgrade Strength Evaluation

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The subgrade forms the foundation of any pavement. Its strength is measured using the California Bearing Ratio (CBR) test, which helps determine the required pavement thickness. IRC:SP-72-2015 classifies subgrade strength into five categories:

• Very Poor (S1): CBR = 2%
• Poor (S2): CBR = 3-4%
• Fair (S3): CBR = 5-6%
• Good (S4): CBR = 7-9%
• Very Good (S5): CBR = 10-15%

🔹 If CBR < 5%, the subgrade should be stabilized using lime or cement treatment to improve its bearing capacity.

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3. Pavement Composition & Materials

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The pavement structure consists of multiple layers, each serving a distinct purpose:

🔸 Sub-base Course: Composed of natural sand, moorum, gravel, or crushed stone. Lime-treated or cement-treated sub-bases are recommended for weak subgrades.

🔸 Base Course: Typically consists of Water Bound Macadam (WBM), Wet Mix Macadam (WMM), or crusher-run macadam.

🔸 Surfacing:

• Bituminous Surface Treatment (for traffic > 100,000 ESALs)
• Gravel Surfacing (for lower ESAL applications)

Proper selection and compaction of materials ensure a strong, long-lasting pavement.

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4. Recommended Pavement Thickness

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The thickness of pavement layers is determined based on traffic category and subgrade CBR. Key recommendations:✅ Minimum 150mm base thickness for roads with ESAL > 100,000.✅ Incorporation of stabilization techniques for weaker subgrades.✅ Proper drainage measures to prevent waterlogging and subgrade failure.

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5. Drainage & Maintenance Strategies

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Effective drainage plays a critical role in pavement durability. The IRC code recommends:✔ Cross-drainage structures to prevent water accumulation.✔ Well-compacted subgrades to minimize moisture retention.✔ Regular condition surveys to assess and repair damages in time.

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Conclusion

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The IRC:SP-72-2015 guidelines are essential for constructing and maintaining durable, cost-effective, and efficient rural roads. By adhering to these recommendations, engineers and policymakers can ensure improved rural connectivity, reduced maintenance costs, and enhanced road performance.,For engineers and infrastructure planners, implementing these guidelines is key to ensuring sustainable rural development.

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