Materials Used in WMM: Specifications and Selection Criteria as per IRC Code 109

Introduction
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Wet Mix Macadam (WMM) has emerged as a preferred method for constructing strong, stable road base and sub-base layers. Its advantages over traditional Water Bound Macadam (WBM) — such as faster construction, higher strength, and better performance under traffic loads — have led to its widespread adoption.

To achieve the desired quality in WMM, material selection plays a critical role. The Indian Roads Congress (IRC), through IRC Code 109-2015, specifies clear standards and selection criteria for materials used in WMM.

This blog explores these specifications in detail, helping engineers make informed decisions during road project execution.

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Why Material Selection Matters in WMM
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• Ensures strength and durability of the pavement.
• Affects compaction and load distribution.
• Prevents segregation and premature failure.
• Maintains surface uniformity.
• Ensures compliance with national standards (IRC Code 109-2015).

Key Materials in WMM Construction (as per IRC Code 109-2015)
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1. Coarse Aggregates

Physical Requirements

• Source: Crushed stone, crushed gravel, or shingle.
• At least 90% retained on 4.75 mm sieve must have two fractured faces.
• Aggregates must meet the following physical criteria:

TestRequirementTest MethodLos Angeles Abrasion Value≤ 40%IS:2386 (Part IV)Aggregate Impact Value≤ 30%IS:2386 (Part IV) or IS:5640Combined Flakiness & Elongation Index≤ 35%IS:2386 (Part I)Soundness (if water absorption > 2%)≤ 12% (with Sodium Sulphate)IS:2386 (Part V)

Why It Matters

• Provides mechanical stability.
• Resists wear and deformation under traffic.
• Prevents surface cracking.
• Promotes dense compaction.

2. Grading Requirements

IRC Code 109-2015 provides precise grading limits (Table 2) to ensure proper particle distribution.

IS Sieve Designation% Passing53 mm10045 mm95–10022.4 mm60–8011.2 mm40–604.75 mm25–402.36 mm15–30600 micron6–1875 micron4–8

Additional Requirements:

• 75 micron fraction must be ≤ 2/3 of 600 micron fraction.
• Plasticity Index (PI) for material finer than 425 micron must not exceed 6.

Why It Matters

• Ensures dense, impermeable mix.
• Controls void ratio for better compaction.
• Minimizes settlement and rutting.

3. Fine Aggregates (if required)

• If crushing does not generate enough fines, non-plastic angular sand can be added.
• Must be free of deleterious materials.

Why It Matters

• Fills voids between coarse aggregates.
• Improves compaction and surface finish.
• Enhances load-bearing capacity.

4. Water

• Must be clean and potable (no harmful salts or impurities).
• Used to bring mix to Optimum Moisture Content (OMC) during mixing.
• Variation in moisture during compaction must not exceed ± 2% from OMC.

Why It Matters

• Ensures proper lubrication during compaction.
• Helps achieve maximum dry density.
• Prevents segregation and layer instability.

Selection Criteria Summary (as per IRC Code 109-2015)
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Material Key Selection Criteria Coarse Aggregates Grading, strength, durability, flakiness, elongation, soundness Fine Aggregates Angular, non-plastic, proper grading Water Clean, free from harmful substances Overall Mix Meets grading envelope, proper fines content, low PI

Tips for Engineers

• Source aggregates from approved quarries with a consistent supply.
• Conduct preliminary tests on all materials to ensure compliance.
• Perform trial mixes to verify grading and compaction behavior.
• Maintain strict quality control at the crushing plant and WMM plant.
• Regularly monitor moisture content during transport and laying.

Conclusion

Selecting the right materials — and ensuring they meet the stringent requirements of IRC Code 109-2015 — is essential for constructing high-quality Wet Mix Macadam layers.

By adhering to the specified grading limits, physical standards, and moisture control guidelines, engineers can deliver durable, stable, and high-performance road bases — critical for long-lasting pavements.

For project success, material selection should never be compromised. Following IRC 109-2015 is the key to ensuring best-in-class WMM construction.

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