Tentative Specifications for Stone Matrix Asphalt
2008 Edition

IRC SP 79 specifies the following material and quality requirements for Stone Matrix Asphalt (SMA):

1. Bitumen: Use VG-30 grade per IS:73 or Polymer Modified Bitumen (PMB) Grade 40 as per IRC:SP 53.
2. Coarse Aggregates must meet cleanliness (<2% passing 0.075 mm), flakiness and elongation (<30%), Los Angeles abrasion (<25%), impact value (<18%), polished stone value (>55% except 19 mm SMA), soundness (Sodium sulphate <12%, Magnesium sulphate <18%), and water absorption (<2%) standards.
3. Mineral Filler should comprise finely divided material like stone dust or hydrated lime; fly ash is prohibited. Grading must conform to 100% passing 0.6 mm sieve, 95-100% passing 0.3 mm, and 85-100% passing 0.075 mm.
4. SMA Mix Requirements include air void content of 4.0%, minimum bitumen content of 5.8%, cellulose fibre content at least 0.3% by total mix weight, VMA minimum 17%, and VCA mix less than VCA dry rodded.

These specifications ensure the SMA pavement's durability, resistance to moisture damage, and mechanical stability.

The SMA mix design per IRC SP 79 involves:

• Determining Voids in Coarse Aggregate (VCA) via dry rodded unit weight measurements as per ASTM C29, calculated by:

  VCA_DRC = ((G_ea × γ_w - γ) / (G_ca × γ_w)) × 100

  where G_ca is bulk specific gravity of coarse aggregate, γ_w is unit weight of water (998 kg/m³), and γ is dry rodded aggregate unit weight.

• Adhering to specified aggregate gradation limits for 13 mm or 19 mm SMA.

• Ensuring the VCA of the mix is less than the VCA of the dry rodded aggregate to maintain aggregate skeleton stability.

• Maintaining minimum voids in mineral aggregate (VMA) of 17%, air voids at 4.0%, bitumen content at or above 5.8%, and cellulose fibres at least 0.3% by weight.

This design approach provides a stable aggregate framework with adequate binder and fibres to prevent binder draindown and ensure long-lasting pavement performance.

According to IRC SP 79, SMA placement and compaction procedures include:

• Handling mineral filler dry with accurate proportioning.
• Adding cellulose fibres (minimum 0.3%) either dry into the weigh hopper or via a dedicated feeding system to ensure uniform distribution.
• Using an asphalt paver for consistent spreading of the SMA mix.
• Checking the rate of spread after every fifth truckload.
• Compacting longitudinally with rollers close behind the paver, overlapping subsequent passes by at least one-third of the rear wheel width.
• Maintaining roller speed at or below 5 km/h and avoiding stationary rolling on uncompacted material.
• Keeping roller wheels moist through water or spray to prevent mix adhesion, but avoiding excessive water pooling.
• Conducting density tests at intervals of one test per 250 m².
• Carrying out trial sections of at least 150 meters to verify mix and compaction quality before full-scale operations.

These steps ensure proper compaction, uniformity, and durability of the SMA pavement.

IRC SP 79 specifies that:

• SMA with conventional bitumen should be laid only when the surface temperature is at least 10°C, and with polymer-modified bitumen at least 15°C.
• SMA placement must be avoided if there is standing water, imminent rain, fog, or dust storms.
• The base or binder course must be dry before laying SMA.
• Laying should not occur if wind speed exceeds 40 km/h measured at 2 meters height.
• Traffic is prohibited on the SMA surface until it cools to the ambient temperature throughout its depth.
• Transport vehicles must be clean, insulated, and covered with asphalt release agents to prevent mix contamination or cooling.

These provisions help ensure optimal bonding, compaction, and long-term pavement performance under suitable environmental conditions.

IRC SP 79 mandates comprehensive quality control tests including:

• Fibre quality verification with manufacturer’s test data and protection from moisture.
• Preparation of Marshall specimens with target air voids of 7.0 ± 0.5% for moisture susceptibility testing via indirect tensile strength and calculation of Tensile Strength Ratio (TSR).
• Draindown testing using ASTM D6390 or the Schellenberg test at production temperatures, with a maximum allowable draindown of 0.30%; fibre content is adjusted if necessary.
• Aggregate and binder tests per Table 5, including impact value, Los Angeles abrasion, flakiness index, soundness, water absorption, sand equivalent, plasticity index, polished stone value, and fractured faces.
• Mix grading, air voids, VMA, binder content, temperature monitoring, and density of compacted layers at specified frequencies.
• Conducting trial paving sections of at least 150 meters to validate mix design and compaction quality before main works.

This rigorous testing regime ensures the SMA meets durability, stability, and performance criteria.