IRC SP 79:2008 is the Indian Standard (IRC) for tentative specifications for stone matrix asphalt. SMA is a premium wearing course mix — gap-graded with stone-on-stone contact for rutting resistance and high binder content for durability. Used on expressways and heavy traffic NH as the top wearing course. Costs 30-40% more than BC but lasts 2-3× longer.
Specifications for Stone Matrix Asphalt (SMA) — a premium bituminous wearing course for heavy traffic highways and expressways.
Key properties for aggregates, binder, mix design criteria (volumetrics, Marshall), and construction temperature/compaction controls for Stone Matrix Asphalt.
| Reference | Value | Clause |
|---|---|---|
| LA Abrasion Value (Aggregates) | ≤ 30% | Cl. 4.2.1 (Table 2) |
| Combined Flakiness & Elongation Index | ≤ 30% | Cl. 4.2.1 (Table 2) |
| Water Absorption (Aggregates) | ≤ 2.0% | Cl. 4.2.1 (Table 2) |
| Polished Stone Value— For wearing course applications. | ≥ 55 | Cl. 4.2.1 (Table 2) |
| Gradation: Passing 4.75 mm Sieve— For 13 mm Nominal Maximum Aggregate Size (NMAS). | 20 - 28% | Cl. 5.1 (Table 3) |
| Gradation: Passing 2.36 mm Sieve— For 13 mm NMAS. | 16 - 24% | Cl. 5.1 (Table 3) |
| Gradation: Passing 0.075 mm Sieve (Filler)— For 13 mm NMAS. | 8 - 12% | Cl. 5.1 (Table 3) |
| Minimum Binder Content— By weight of total mix. | ≥ 5.8% | Cl. 5.2 |
| Recommended Binder Grade | VG-30, VG-40, or Modified Binder | Cl. 4.5 |
| Stabilizing Additive (Cellulose Fibre)— By weight of total mix. | ≥ 0.3% | Cl. 4.6.1 |
| Filler / Binder Ratio— By mass. | 1.2 - 1.6 | Cl. 5.2 |
| Air Voids (Vv) in Mix | 3.0 - 4.0% | Cl. 5.3 (Table 4) |
| Voids in Mineral Aggregate (VMA) | ≥ 17.0% | Cl. 5.3 (Table 4) |
| Voids in Coarse Aggregate (VCA_mix)— VCA in mix must be less than VCA of dry rodded coarse aggregate. | < VCA_DRC | Cl. 5.3 |
| Marshall Stability (75 blows) | ≥ 9.0 kN | Cl. 5.3 (Table 4) |
| Marshall Flow | 2.0 - 4.0 mm | Cl. 5.3 (Table 4) |
| Binder Draindown Limit— Tested at anticipated production temperature. | ≤ 0.3% | Cl. 5.4 |
| Mix Laying Temperature (Min)— Varies with binder type and haul distance. | 145 °C | Cl. 6.3 |
| Compaction Requirement— Of the average laboratory Marshall density. | ≥ 96% | Cl. 6.4 |
| Layer Thickness (13 mm NMAS) | 40 - 50 mm | Cl. 6.3 |
| Permissible Variation in Binder Content | ± 0.3% | Cl. 7.1 (Table 6) |
IRC SP 79 (2008) provides Tentative Specifications for Stone Matrix Asphalt (SMA) — a premium gap-graded wearing course originally developed in Germany for heavy-traffic highways and now standard on Indian NH/expressway projects. SMA combines a coarse-aggregate stone skeleton with high binder content + stabiliser fibres to deliver rut resistance, fatigue resistance, and long service life.
Use IRC SP 79 when you are: - Specifying premium wearing course on heavy-traffic NH / expressway (AADT > 5,000 commercial vehicles/day) - Doing rehabilitation overlay on rutted pavement - Specifying noise-reducing pavement in urban arterial near hospitals / residential - Considering SMA vs CRMB (IRC:SP-107:2015) vs dense-graded BC (IRC:107:2013) - Doing NHAI / state-NH 4/6-lane wearing-course design
SMA characteristics: - Gap-graded aggregate (heavy coarse, gap in middle sizes 2.36-4.75 mm) - High binder content (6.0-7.5 %) enabled by stone-on-stone contact - Polymer-modified or fibre-stabilised binder (typical SBS-PMB or CRMB) - Stabiliser: cellulose / mineral fibre (0.3-0.5 % by mix mass) to prevent binder drain-down - Marshall stability > 7.5 kN; air voids 3-5 % - Premium cost: 30-50 % more than dense-graded BC - Design life: 8-12 years; > 12 years with PMB
Why SMA works: - Stone-on-stone contact carries the traffic load through the coarse aggregate skeleton; binder isn't critical to load-bearing - High binder content (6.0-7.5 %) fills voids + provides flexibility + durability - Stabiliser fibres prevent binder drain-down during transport + placement - Better rutting + fatigue resistance than dense-graded BC - Surface texture is coarser → better skid resistance in wet conditions - Noise reduction ~3-4 dB compared to dense-graded
Aggregate gradation (SMA, 13.2 mm max — typical): - 19 mm sieve: 100 % - 13.2 mm: 90-100 % - 9.5 mm: 50-75 % ← significant - 4.75 mm: 20-30 % ← deep gap - 2.36 mm: 16-26 % - 1.18 mm: 14-22 % - 0.6 mm: 12-18 % - 0.3 mm: 10-14 % - 0.15 mm: 8-13 % - 0.075 mm: 7-10 %
The deep gap (sharp drop between 9.5 mm and 4.75 mm) is the defining characteristic.
Binder: - Plain bitumen VG-40 + 1-2 % stabiliser fibre (acceptable but minimum) - Polymer-modified bitumen (PMB-120 or PMB-70) — preferred for SMA - Crumb Rubber Modified Bitumen (CRMB-55 / CRMB-60) — alternative - Modified bitumen provides better elastic recovery + softening point
Stabiliser: - Cellulose fibre: softwood pulp fibre, 0.3-0.5 % by mix mass - Mineral fibre: glass / rock wool, 0.3-0.4 % - Function: holds bitumen + prevents drain-down; binder content stays in mix during transport + placement
Marshall mix design: - Optimum binder: 6.0-7.5 % - Air voids (Va): 3-5 % - VMA: ≥ 17 % (much higher than dense-graded) - VFB: 70-80 % - Stability: ≥ 7.5 kN - Flow: 2-4 mm - TSR: ≥ 0.80 - Drain-down (Schellenberg test): ≤ 0.3 %
Construction temperatures: - Mixing: 170-185 °C - Transport: minimum 165 °C - Laying: ≥ 160 °C at paver - Compaction completion: complete before mix < 130 °C
Layer thickness: - Single SMA layer: 30-50 mm typical wearing course - Aggregate-to-thickness ratio: 1/3 to 1/2 - For 13.2 mm SMA: layer ≥ 40 mm
Tack coat: 0.2-0.3 kg/m² (residue basis) between underlying layers + SMA
Compaction: - Tandem vibratory roller: 4-6 passes - Static pass at start + finish - Avoid pneumatic roller — rubber tyres pick up + smear mix
Density: - Field density: ≥ 96 % of Marshall density (preferred); ≥ 95 % minimum - Density measurement: nuclear gauge + cores
Surface texture: - Mean Profile Depth (MPD): 0.8-1.2 mm (coarser than BC) - Skid resistance (BPN): ≥ 55 BPN at 28 days
Acceptance criteria: - Layer thickness: ± 5 mm of design - Surface evenness: 3 mm max under 3-m straight-edge - Density: per above - Texture + skid: per above - TSR + drain-down: per design
1. No stabiliser fibre. Binder drains during transport; mix arrives binder-poor; performance degraded. Mandatory cellulose / mineral fibre 0.3-0.5 % by mass. 2. Drain-down not tested. Schellenberg test not done; mix loses binder during transport; final mix below design. Mandatory Schellenberg drain-down test. 3. Pneumatic roller used. Tyres pick up bitumen; surface texture damaged. Tandem vibratory only. 4. Aggregate gradation outside envelope. Coarse-aggregate skeleton incomplete; load-bearing impaired. Strict gradation control. 5. Mix laid at low temperature. Below 160 °C; compaction inadequate; density poor. Reject cold mix. 6. Insufficient binder. Below 6 % despite spec; mix dry; ravelling. Verify binder content per batch. 7. No TSR test. Moisture sensitivity; first monsoon stripping. Mandatory TSR ≥ 0.80. 8. Layer thickness inadequate. Below 40 mm for 13.2 mm SMA; aggregate broken during rolling. Strict thickness compliance. 9. Old / wrong modified bitumen. PMB stored too long; degraded; mix performance reduced. Fresh bitumen + storage protocols. 10. Cost not justified for traffic. SMA on low-traffic road; cost not recovered. SMA for traffic > 5,000 commercial veh/day. 11. Marking compatibility. Standard paint chips off SMA texture; markings fail. Specify compatible marking material. 12. Maintenance schedule. SMA + PMB has long life but still requires periodic mid-life dressing. Schedule micro-surfacing at year 6-8.
SMA project — IRC SP 79 touchpoints:
1. DPR / pavement design: SMA specified on heavy-traffic NH / expressway wearing course; cost-benefit vs dense-graded BC.
2. Mix design (laboratory): - Aggregate gradation within SMA envelope (13.2 mm typical) - Modified bitumen (PMB-70/120 or CRMB) per IRC:SP-53:2010 - Cellulose/mineral fibre at 0.3-0.5 % - Marshall test at trial binder contents (5-8 %) - Volumetric verification (Va = 3-5 %, VMA ≥ 17 %) - Stability ≥ 7.5 kN; Drain-down ≤ 0.3 %; TSR ≥ 0.80 - Final mix proportion
3. Plant setup: - Batch mixer capable of 170-185 °C operation - Cement-bound silos for aggregates - Modified bitumen storage tank with agitation + temperature control - Fibre dosing system - Calibrate before contract + monthly
4. Trial section: - 100-200 m on representative subgrade + DBM - Validate plant + paver + roller pattern - Adjust for site conditions - Cores for density + mix-design conformance
5. Mass production: - Plant mix at 170-185 °C - Transport in covered insulated dumpers within 1.5-2 hrs - Tack coat 0.2-0.3 kg/m² ahead of paver - Paver-laid at 160-175 °C - Rolling: static + vibratory; complete before 130 °C
6. Quality control + acceptance: - Plant: aggregate + binder + fibre + temperature per batch - Site: laying temperature + roller pass count + density (nuclear + cores) - Cores: 1 per 1000-2000 m² - Surface: texture + evenness + skid resistance
7. Operations + maintenance: - First-year: light surface settling expected - 5-year: detailed inspection - Mid-life: micro-surfacing at year 6-8 to refresh texture - 10-12 year design life with PMB
IRC SP 79 is the premium wearing-course standard for Indian heavy-traffic highways — replacing dense-graded BC on NH 4/6-lane projects + expressways. The 2008 specification remains current + widely-applied.