IRC Code: Tentative Specifications for Stone Matrix Asphalt (IRC:SP:79-2008)

‍

Stone Matrix Asphalt (SMA) is a highly durable, rut-resistant bituminous mix used for heavy traffic roads. It was first developed in Germany in the 1960s and later adopted worldwide, including India, for pavement surfacing and intermediate courses. The Indian Roads Congress (IRC) introduced IRC:SP:79-2008, which provides tentative specifications for SMA suited to Indian conditions.

In this blog, we will explore the materials, design process, construction techniques, and quality control aspects of IRC:SP:79-2008 and how it ensures the longevity and performance of bituminous roads.

‍

‍

What is IRC:SP:79-2008?

‍

The IRC:SP:79-2008 manual sets specifications for Stone Matrix Asphalt (SMA) as a high-performance gap-graded bituminous mix designed to resist rutting, cracking, and water damage. Unlike conventional bituminous mixes, SMA relies on a stone-on-stone aggregate skeleton that enhances its stability and load-bearing capacity.

‍

Key Features of Stone Matrix Asphalt (SMA)

‍

✅ Superior Rut Resistance: Designed for high-traffic roads
✅ Increased Durability: Prevents fatigue cracking and moisture damage
✅ High Skid Resistance: Improves road safety in wet conditions
✅ Longer Service Life: Reduces maintenance costs
✅ High Binder Content: Improves flexibility and resistance to aging

‍

Scope of IRC:SP:79-2008

‍

According to the IRC guidelines, SMA is recommended for:
✔ Heavy traffic roads as a wearing course or binder course
✔ Highways, expressways, and urban arterial roads
✔ Areas with extreme weather conditions (hot summers and heavy rainfall)
✔ Airport runways and taxiways requiring high durability

✔ Two Types of SMA Mixes:

• 13mm SMA: Used as a wearing course, thickness 40-50mm
• 19mm SMA: Used as a binder course, thickness 45-75mm

‍

Materials Used in SMA (As per IRC:SP:79-2008)

‍

1. Bituminous Binder

The bitumen used in SMA should conform to IS:73 or IRC:SP:53 standards. The viscosity-graded (VG) paving bitumen or Polymer Modified Bitumen (PMB) is recommended based on climatic conditions.

✔ Bitumen Type:

• VG-30 Bitumen (for standard conditions)
• PMB-40 (for higher durability and stress resistance)

2. Coarse Aggregates

✔ Crushed stone retained on a 2.36mm sieve
✔ Cubical, strong, and durable particles
✔ Aggregate Impact Value: ≤ 18%
✔ Los Angeles Abrasion Value: ≤ 25%
✔ Water Absorption: ≤ 2%

3. Fine Aggregates

✔ 100% crushed sand (manufactured sand)
✔ Non-plastic with Sand Equivalent Test value ≥ 50

4. Mineral Filler

✔ Stone dust and/or hydrated lime
✔ Plasticity Index ≤ 4
✔ Fly ash is not permitted in SMA

5. Stabilizing Additive (Fibers)

✔ Pelletized cellulose fibers are added to prevent binder draindown
✔ Minimum dosage: 0.3% of total mix weight

‍

SMA Mix Design Process (As per IRC:SP:79-2008)

‍

The SMA mix design follows the AASHTO MP8 & PP41 standards, ensuring an optimal balance of stability, voids, and durability.

✔ Air Voids: 4.0%
✔ Minimum Binder Content: 5.8%
✔ Voids in Mineral Aggregate (VMA): ≥ 17%
✔ Voids in Coarse Aggregate (VCA): Ensures stone-on-stone contact

Bitumen Content Selection

• Initial bitumen content: Minimum 6.0%
• Adjusted based on mix volumetrics
• Final bitumen content ensures 4.0% air voids

Draindown Test (ASTM D6390)

• Ensures bitumen does not drain from the mix during transport and placement
• Maximum draindown allowed: 0.3%

Tensile Strength Ratio (TSR)

• Evaluates moisture resistance of SMA
• Minimum TSR required: 85%

‍

Construction of Stone Matrix Asphalt (SMA)

‍

1. Weather Limitations

🚫 SMA should not be laid when:

• The temperature is below 10°C (for VG bitumen)
• The temperature is below 15°C (for PMB bitumen)
• Rain, standing water, or dust storms are present

2. Surface Preparation

✔ The existing bituminous surface must be clean and dry
✔ Potholes and cracks must be repaired
✔ Tack Coat Application:

• Bitumen Emulsion (IS:8887) or VG-10 bitumen
• Application rate: 0.20-0.40 kg/m²

3. Mixing and Laying

✔ SMA mix is prepared in a hot mix plant
✔ Bitumen temperature: 150-165°C (VG-30), 160-180°C (PMB-40)
✔ Mix is transported in insulated trucks to prevent cooling

4. Compaction Process

✔ Rolling must begin immediately after laying
✔ Compaction must be completed before temperature drops below 100°C
✔ Rollers Used:

• Breakdown Rolling: 8-10 tonne steel wheel roller
• Intermediate Rolling: 8-10 tonne steel wheel or vibratory roller
• Final Rolling: 6-8 tonne smooth wheel roller

✔ Minimum compacted density: 94% of theoretical max specific gravity

‍

Quality Control & Acceptance Criteria

‍

To ensure durability and long-term performance, strict quality control measures are followed:

✔ Surface Smoothness: Variation ≤ 6mm under a 3m straight edge
✔ Air Voids in Mix: Must be 4.0% ± 0.5%
✔ Binder Content: 5.8% minimum
✔ Moisture Susceptibility (TSR): ≥ 85%
✔ Density of Compacted Layer: ≥ 94% of max theoretical density

‍

Advantages of Using Stone Matrix Asphalt (SMA)

‍

✅ Superior Strength: Ideal for high-traffic roads
✅ Longer Life Span: Reduces maintenance costs
✅ Better Skid Resistance: Enhances road safety
✅ Resistant to Rutting & Fatigue Cracking
✅ Effective for Harsh Weather Conditions

‍

Conclusion

‍

The IRC:SP:79-2008 code provides clear guidelines for designing, constructing, and maintaining Stone Matrix Asphalt (SMA) roads in India. By following proper mix design, material selection, and quality control measures, SMA offers superior durability, reduced maintenance costs, and enhanced road performance. For high-traffic highways and expressways, Stone Matrix Asphalt (SMA) is an ideal long-term solution to withstand heavy loads, extreme weather, and continuous wear and tear.

‍

RoadVision AI is revolutionizing road infrastructure development and maintenance with its innovative solutions powered by computer vision AI. By leveraging advanced technologies, the platform conducts comprehensive road condition monitoring and traffic surveys, enabling early detection of surface issues like potholes and cracks for timely repairs and enhanced roads. Through traffic congestion analysis, RoadVision AI provides data-driven insights to address traffic congestion challenges and optimize road usage. With a focus on building smarter and more efficient road infrastructure, RoadVision AI ensures full compliance with IRC Codes, helping engineers and stakeholders reduce costs, minimize risks, and improve road safety and transportation efficiency.