Stone Matrix Asphalt (SMA) is a gap-graded bituminous mixture designed to provide exceptional rutting resistance, durability, and long-term pavement performance under heavy traffic loading. It consists of a stone-on-stone aggregate skeleton, rich bituminous mortar, mineral filler, and stabilizing fibres that work together to create a highly stable and durable pavement surface.
Originally developed in Germany, SMA has become one of the most preferred surfacing materials for national highways, expressways, urban arterial roads, intersections, climbing lanes, and other heavily trafficked pavements. In India, the requirements for SMA are specified under MoRTH Clause 515, which covers materials, mix design, production, laying, compaction, and quality control requirements.
This comprehensive guide explains the composition, design philosophy, material requirements, construction methodology, special performance tests, and quality control procedures for Stone Matrix Asphalt in accordance with IRC and MoRTH specifications.
This clause covers the construction of fibre-stabilized Stone Matrix Asphalt (SMA) in single or multiple layers over a previously prepared bituminous bound surface. SMA may be used as either a wearing course or a binder/intermediate course depending upon the aggregate gradation and layer thickness requirements.
The SMA mixture consists of a gap-graded aggregate skeleton, rich bituminous mortar, mineral filler, and stabilizing fibres. The fibres help prevent binder drain down and ensure uniform distribution of the higher bitumen content characteristic of SMA mixes.
| SMA Type | Maximum Aggregate Size | Recommended Use | Nominal Layer Thickness |
|---|---|---|---|
| 13 mm SMA | 13.2 mm | Wearing Course | 40–50 mm |
| 19 mm SMA | 19 mm | Binder / Intermediate Course | 45–75 mm |
Before laying SMA, the underlying bituminous layer should be structurally sound, properly compacted, clean, and provided with an approved tack coat to ensure adequate bond between layers.
The performance of Stone Matrix Asphalt (SMA) depends significantly on the quality of constituent materials. MoRTH Clause 515.2 specifies stringent requirements for bitumen, aggregates, mineral filler, and stabilizing additives to ensure the development of a durable stone-on-stone aggregate skeleton capable of resisting rutting and moisture damage under heavy traffic loading.
The binder used in fibre-stabilized SMA shall be viscosity grade paving bitumen conforming to IS:73 or Modified Bitumen conforming to IS:15462 and IRC:SP:53. The selected binder grade shall be capable of satisfying all mix design requirements and shall conform to the requirements specified in MoRTH Table 500-2.
Coarse aggregates shall consist of crushed rock retained on the 2.36 mm sieve. Aggregates shall be clean, hard, durable, cubical in shape, and free from dust, clay, organic matter, and other deleterious substances.
| Property | Test Method | Requirement |
|---|---|---|
| Cleanliness | IS:2386 Part 1 | < 2% passing 0.075 mm sieve |
| Combined Flakiness & Elongation Index | IS:2386 Part 1 | < 30% |
| Los Angeles Abrasion Value | IS:2386 Part 4 | < 25% |
| Aggregate Impact Value | IS:2386 Part 4 | < 18% |
| Polished Stone Value* | IS:2386 Part 114 | > 55% |
| Soundness (Na₂SO₄) | IS:2386 Part 5 | < 12% |
| Water Absorption | IS:2386 Part 3 | < 2% |
*Polished Stone Value requirement is not applicable for SMA used as binder/intermediate course.
Fine aggregate passing the 2.36 mm sieve and retained on the 0.075 mm sieve shall consist entirely of crushed manufactured sand obtained from approved crushing operations.
Mineral filler shall consist of finely divided mineral matter such as stone dust or hydrated lime. Use of fly ash as filler is not permitted.
| IS Sieve | Cumulative Passing (%) |
|---|---|
| 0.600 mm | 100 |
| 0.300 mm | 95–100 |
| 0.075 mm | 85–100 |
The Plasticity Index of filler shall not exceed 4. This requirement does not apply when hydrated lime is used.
Only pelletized cellulose fibres shall be used as stabilizing additives in SMA. The fibres prevent binder drain down and ensure uniform distribution of the rich bituminous mortar.
0.3% by weight of total mix (on loose fibre basis)
The fibre dosage shall be sufficient to ensure that binder drain down does not exceed 0.3% when tested in accordance with ASTM D6390.
| Property | Requirement |
|---|---|
| Maximum Fibre Length | 8 mm |
| Ash Content | Maximum 20% |
| Oil Absorption | More than 4 times fibre weight |
| Moisture Content | Less than 5% |
After selection of suitable materials and aggregate gradation, the Stone Matrix Asphalt (SMA) mixture shall be designed in accordance with AASHTO MP-8 and AASHTO PP-41. Marshall specimens shall be prepared using 50 blows on each face. The final mix shall satisfy the volumetric and performance requirements specified below.
| Mix Design Parameter | Requirement |
|---|---|
| Air Voids (Va) | 4.0% |
| Bitumen Content | Minimum 5.8% |
| Cellulose Fibre Content | Minimum 0.3% by weight of total mix |
| Voids in Mineral Aggregate (VMA) | Minimum 17% |
| Voids in Coarse Aggregate (VCA Mix) | Less than VCA (Dry Rodded) |
| Asphalt Drain Down (ASTM D6390) | Maximum 0.3% |
| Tensile Strength Ratio (TSR) – AASHTO T283 | Minimum 85% |
Air voids represent the percentage of air spaces within the compacted asphalt mixture. Maintaining approximately 4% air voids ensures adequate durability while preventing bleeding and excessive permeability.
SMA contains a significantly higher binder content than conventional Bituminous Concrete. The rich bituminous mortar improves durability, flexibility, and resistance to cracking.
Cellulose fibres act as stabilizing additives and prevent binder drain down during production, transportation, and laying operations.
VMA represents the intergranular void space between aggregate particles. SMA requires high VMA to accommodate the rich mortar consisting of bitumen, filler, and fibres.
This is one of the most important requirements in SMA design. It confirms that stone-on-stone aggregate contact exists within the compacted mixture.
When this criterion is satisfied, traffic loads are carried primarily by the aggregate skeleton rather than the asphalt mortar.
Drain down measures the tendency of binder to separate from the aggregate skeleton during storage and transportation.
TSR evaluates moisture susceptibility by comparing tensile strength before and after moisture conditioning.
Unlike conventional Bituminous Concrete, SMA relies on a stone-on-stone aggregate skeleton combined with a rich bituminous mortar. Therefore, compliance with Air Voids, VMA, VCA, Drain Down, and TSR requirements is essential to achieve:
Stone Matrix Asphalt (SMA) shall be produced in a hot mix plant having adequate capacity and capable of manufacturing a uniform, homogeneous mix with complete coating of aggregates. Due to the gap-graded nature and higher binder content of SMA, strict control of mixing operations is essential to achieve the desired stone-on-stone aggregate structure and prevent segregation or binder drain down.
When conventional viscosity grade (VG) bitumen is used, the mixing temperature of the SMA shall generally be maintained between 150°C and 165°C. This temperature range ensures proper coating of aggregates while preventing excessive aging of the binder.
In the case of modified bitumen, the mixing and compaction temperatures are generally higher than those used for VG bitumen. The exact temperature shall be established based on the type of modifier, dosage, and recommendations provided by the binder manufacturer.
To ensure consistent quality and compliance with the approved Job Mix Formula (JMF), the hot mix plant shall be calibrated periodically. Calibration shall verify the accuracy of aggregate feeders, bitumen pumps, filler feeding systems, and fibre dosing equipment.
Proper calibration helps maintain:
Mineral filler plays a critical role in Stone Matrix Asphalt (SMA) by forming part of the rich asphalt mortar that fills the voids between coarse aggregate particles. Since SMA mixtures require significantly higher filler content than conventional dense graded bituminous mixes, proper storage, handling, and proportioning of filler are essential for achieving the desired mix properties.
Suitable arrangements shall be provided to accurately meter and uniformly introduce the required quantity of mineral filler into the SMA mixture. Variations in filler content may adversely affect mix stability, workability, durability, and moisture resistance.
Cellulose fibres are incorporated into SMA mixtures as stabilizing additives to prevent asphalt drain down caused by the relatively high binder content of the mix. Proper fibre addition and uniform distribution are critical for achieving the desired performance characteristics of SMA.
In batch-type hot mix plants, pelletized cellulose fibres shall be added directly into the weigh hopper located above the pugmill. Adequate dry mixing is essential to ensure uniform dispersion of fibres throughout the heated aggregate mass.
The increased mixing duration ensures complete fibre distribution and prevents the formation of fibre clumps within the mixture.
For drum mix plants, a separate fibre feeding system shall be provided. The system shall be capable of accurately and uniformly introducing fibres into the drum without restricting normal production rates.
The fibre feed system shall be calibrated regularly to maintain the dosage specified in the approved Job Mix Formula (JMF).
Proper surface preparation, tack coat application, transportation, paving, and compaction are essential for achieving the desired performance of Stone Matrix Asphalt (SMA). Due to its gap-graded structure and high binder content, SMA requires stricter construction controls than conventional bituminous mixes.
Before laying SMA, the underlying bituminous surface shall be thoroughly cleaned and prepared to ensure proper bonding between layers. Any contaminants or surface defects may adversely affect pavement performance and lead to premature failures.
Prior to SMA placement, all pavement defects shall be rectified.
A tack coat shall be applied on the prepared bituminous surface in accordance with the requirements of MoRTH Clause 503. The tack coat promotes adequate bond between the existing surface and the new SMA layer.
Transportation of SMA mixture shall comply with the requirements of MoRTH Clause 501.4. Special care shall be taken to prevent segregation, contamination, excessive cooling, and loss of material during transit.
The temperature of SMA shall be maintained within the limits specified in the approved Job Mix Formula. Excessive cooling may result in poor workability and inadequate compaction.
| Problem | Cause | Effect |
|---|---|---|
| Segregation | Improper loading or unloading | Non-uniform surface texture |
| Temperature Loss | Long haul distance | Poor compaction |
| Contamination | Dirty truck body | Reduced pavement quality |
| Binder Loss | Improper handling | Reduced durability |
Stone Matrix Asphalt shall be laid only under suitable weather conditions to ensure proper placement, compaction, and long-term performance. Paving operations shall comply with the requirements of MoRTH Clause 501.5.1.
Spreading of SMA shall be carried out using a self-propelled sensor-controlled paver capable of laying the material uniformly to the specified line, grade, crossfall, and thickness. The requirements of MoRTH Clause 501.5.3 shall apply.
Compaction of SMA shall be carried out in accordance with MoRTH Clause 501.6. Rolling operations shall begin immediately after paving while the mix remains within the specified compaction temperature range.
Compaction develops aggregate interlock, reduces air voids, improves durability, and enhances rutting resistance. Inadequate compaction can significantly reduce the service life of SMA pavements.
The density of the finished SMA layer shall be determined using 150 mm diameter core samples.
The compacted density shall not be less than:
94% of the Average Theoretical Maximum Specific Gravity (Gmm)
Theoretical Maximum Specific Gravity (Gmm) shall be determined in accordance with ASTM D2041 using loose mix samples obtained on the same day.
The maximum allowable air void content in the compacted SMA layer shall be:
Maximum 6% Air Voids
Longitudinal and transverse joints in SMA shall be constructed in accordance with MoRTH Clause 501.7. Special attention shall be given to achieving proper alignment, bonding, and compaction at joints.
| Defect | Cause | Impact |
|---|---|---|
| Open Joint | Insufficient overlap | Water ingress |
| Low Density Joint | Poor rolling | Premature failure |
| Segregated Joint | Improper paving practice | Ravelling |
| Uneven Joint | Poor screed control | Poor riding quality |
Quality control of Stone Matrix Asphalt (SMA) is essential to ensure compliance with the approved Job Mix Formula (JMF), material specifications, and construction requirements. Regular testing of materials, plant-produced mix, and finished pavement layers helps maintain uniformity and long-term pavement performance.
| Sl. No. | Test / Inspection | Frequency |
|---|---|---|
| 1 | Quality of Bitumen | Number of samples and tests as per IS:73, IRC:SP:53, IS:15462 |
| 2 | Aggregate Impact Value / Los Angeles Abrasion Value | One test per 100 m³ of aggregate |
| 3 | Flakiness & Elongation Index | One test per 100 m³ of aggregate |
| 4 | Soundness Test (Sodium / Magnesium Sulphate) | One test for each source and whenever aggregate quality changes |
| 5 | Water Absorption of Aggregate | One test for each source and whenever aggregate quality changes |
| 6 | Sand Equivalent Test | One test for each source |
| 7 | Plasticity Index | One test for each source |
| 8 | Polished Stone Value | One test for each source |
| 9 | Percent Fractured Faces | One test per 50 m³ of aggregate when crushed gravel is used |
| 10 | Mix Gradation | One set for individual constituents and mixed aggregate from dryer for every 400 tonnes of mix, subject to minimum two tests per day per plant |
| 11 | Air Voids, VMA & Theoretical Maximum Specific Gravity (Gmm) | Three tests per day |
| 12 | Moisture Susceptibility (TSR) – AASHTO T 283 | One test for each mix type whenever aggregate source or quality changes |
| 13 | Temperature of Binder, Aggregate, Mix at Laying & Compaction | At regular intervals |
| 14 | Binder Content | One test for every 400 tonnes of mix, subject to minimum two tests per day per plant |
| 15 | Rate of Spread of Mix | After every 5th truck load |
| 16 | Density of Compacted Layer | One test per 250 m² area |
Since SMA relies on stone-on-stone aggregate contact, aggregate strength, shape, durability, and polishing resistance directly influence pavement performance.
Maintaining the approved SMA gradation is critical for achieving the desired aggregate skeleton and stone-on-stone contact.
Any variation in gradation may result in:
Routine monitoring of Air Voids, VMA, and Gmm ensures compliance with the approved mix design.
| Parameter | Requirement |
|---|---|
| Air Voids | 4% |
| VMA | Minimum 17% |
| Density | Minimum 94% of Gmm |
The Tensile Strength Ratio (TSR) test evaluates resistance to moisture-induced damage and stripping.
TSR ≥ 85%
If the mix fails the TSR requirement, hydrated lime may be incorporated as an anti-stripping additive.
Temperature monitoring shall be carried out throughout production, transportation, laying, and compaction operations.
Stone Matrix Asphalt (SMA) is a gap-graded bituminous mixture characterized by a coarse aggregate skeleton with stone-on-stone contact, a rich mortar of bitumen and filler, and stabilizing additives such as cellulose fibers. While conventional Marshall parameters verify volumetric properties, they do not fully assess field performance against rutting, moisture damage, and binder drain down.
Consequently, performance-oriented tests such as Binder Drain Down, TSR, Wheel Tracking, Hamburg Wheel Tracking, VCA, Dynamic Creep, IDEAL Rutting, and IDEAL Cracking have gained importance in modern SMA mix evaluation.
Marshall Stability and Flow primarily measure:
However, they do not directly evaluate:
Therefore, additional performance tests are required to verify actual field behavior.
SMA contains significantly higher binder content than conventional Bituminous Concrete. The stabilizing effect of fibers must prevent binder migration during storage in silos, transportation, and placement operations. Excessive drain down destroys the uniformity of the SMA matrix.
ASTM D6390
Drain Down ≤ 0.30%
The primary load-carrying mechanism in SMA is the coarse aggregate skeleton. The aggregate framework should remain in direct contact after compaction.
VCA Mix < VCA Dry Rodded
If VCA Mix exceeds VCA Dry, aggregate particles float in mortar, rutting susceptibility increases, and SMA behaves similarly to conventional Bituminous Concrete.
Moisture damage occurs when water displaces bitumen from aggregate surfaces. TSR quantifies retained tensile strength after moisture conditioning.
AASHTO T283
TSR ≥ 85%
Permanent deformation develops under repeated wheel loads at elevated temperatures. Wheel Tracking directly measures rutting susceptibility.
Lower rut depth indicates superior rutting resistance.
Wheel Tracking is considered one of the most representative laboratory simulations of field rutting.
Unlike conventional Wheel Tracking, Hamburg testing combines water conditioning and repeated wheel loading to simultaneously evaluate rutting, moisture susceptibility, and stripping resistance.
Provides realistic simulation of pavement behavior under wet traffic conditions.
Repeated compressive loading is applied to determine the accumulation of permanent strain.
A lower creep slope indicates higher resistance to permanent deformation.
Evaluation of rutting susceptibility under repetitive loading.
Developed as part of Balanced Mix Design (BMD).
Assessment of cracking resistance.
A mix should simultaneously resist rutting and cracking. Balanced Mix Design aims to achieve both objectives.
| Test | Property Evaluated |
|---|---|
| Drain Down | Binder Stability |
| VCA | Aggregate Skeleton |
| TSR | Moisture Resistance |
| Wheel Tracking | Rutting Resistance |
| Hamburg Wheel Tracking | Rutting + Moisture Damage |
| Dynamic Creep | Permanent Deformation |
| IDEAL-RT | Rutting Performance |
| IDEAL-CT | Cracking Resistance |
Performance testing complements conventional Marshall design by evaluating actual pavement behavior under traffic and environmental conditions.
Among all special tests, Drain Down, VCA, TSR, and Wheel Tracking are the most critical for ensuring durable SMA pavements capable of resisting rutting, moisture damage, and premature distress.
As NHAI and MoRTH increasingly adopt performance-based specifications, these tests are expected to play a larger role in the approval and quality control of SMA mixtures on expressways and high-traffic National Highway projects.
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