Stone Matrix Asphalt has huge potential in constructing sustainable roads in India.

Vikas Sharma

- Director, Genesis Infra Project Consultants

Genesis Infra Project Consultants are the pioneers of implementing SMA in India as implementing and supervisory consultants. “We plan to take stone matrix asphalt technology to the most interior corners of India,” says Vikas Sharma, Director, Genesis Infra Project Consultants. Excerpts from the interview…

Could you brief us on Stone Matrix Asphalt technology (SMA)?

Stone-Matrix Asphalt Technology (SMA) was developed in Germany in the 1960s with the first SMA pavements placed in 1968.

There are three major types of asphalt surfacing, characterized by a mixture of bitumen and stone aggregate. These are Dense Graded asphalt (DGA), Stone Matrix Asphalt (SMA) and Open Graded Asphalt (OGA). Asphalt surfacing differ by the proportion of different size aggregate, the amount of bitumen added and the presence of other additives and material. Stone Matrix asphalt (SMA) is a stone-on-stone like skeletal structure of gap graded aggregate, bonded together by mastic, which actually is of higher binder content, filler and fibre to reduce the binder drain. This structure improves the strength and the performance of SMA even higher than the dense graded and open graded asphalt mixtures. High percentage of binder content and bitumen coated cellulose fibres are important to ensure the durability and laying characteristics of SMA.

How sustainable and eco-friendly is this technology and what is the scope and potential of the same in the construction sustainable pavements in India?

Stone matrix asphalt technology has high stability to deformation and high wear resistance. Thereby, SMA has longevity and durability to premature cracking and ravelling because of high content of bitumen and void less mastic mortar, which fills the voids of stone skeleton to bind it. High quantity and quality of binder are found to be useful for long service life.

SMA technology is the ideal technology for India as the life cycle cost compared to conventional bituminous concrete works out to be economical because of rut resistance characteristics which helps it to become sustainable and have a longer life.

SMA technology is environmentally better than the conventionally used concrete roads (rigid pavements). Also, SMA takes less construction time than rigid pavements. Hence, we can conclude it to be ideal for Indian conditions.

What is the bearing of SMA technology on the mix design, and construction process like compaction and paving etc? 

The SMA mix design is very close to the conventional BC design that we all are aware of. Few macro level differences that are evident - especially when the SMA design, mix and paving is quality conscious and requires a few careful methodologies.

Materials

Bitumen: The bitumen for SMA shall be viscosity grade 30 VG - 30 to complying with Indian Standard Specification for paving bitumen complying with Indian Roads Congress Specification IRC: SP: 53.

Coarse Aggregate: Aggregates used in stone matrix asphalt are coarse and consist of crushed rock retained on 2.36 mm sieve. It shall be clean, hard, durable of cubicle shape and free from dust and soft organic and other deleterious substances (Ref to table 1 of page no.2 of IRC SP 79- 2008) attached herewith.

Fine Aggregates: Fine aggregate (passing 2.36 mm sieve and retained on 0.075 mm sieve) shall consist of 100% crushed, manufactured sand resulting from crushing operations. The fine aggregate shall be clean, hard, durable, of fairly cubical shape and free from soft pieces, organic or other deleterious substances.

Mineral Filler: Mineral filler shall consist of finely divided mineral matter such as hydrated lime.

Stabilizer Additive: Only pelletized cellulose fibres shall be utilized. The dosage rate for cellulose fibres is 0.3 % minimum by weight of the total mix.

Mixing: The stone matrix asphalt mix shall be prepared in a hot mix plant of adequate capacity and capable of yielding a mix of proper and uniform quality with thoroughly coated aggregate.

SMA Placement and Compaction: The existing bituminous surface shall be cleaned of all loose extraneous matter by means of mechanical broom and high-pressure air jet from compressor or any other approved equipment/method. Any potholes and/or cracks shall be repaired and sealed.

Tack Coat: A bitumen emulsion complying with IS: 8887 of a type and grade or viscosity grade bitumen VG-10 shall be applied as a tack coat on the existing bituminous layer.

Transportation: The stone matrix asphalt shall be transported in clean, insulated covered vehicles.

Compaction shall commence immediately after laying and shall be completed before the temperature falls below 100 degree Celsius. Rolling of the longitudinal joints shall be done immediately behind the paving operation. After this, the rolling shall commence at the edge and progress towards the center longitudinally except at sections with unidirectional camber, where it shall progress from lower edge to upper edge parallel to centreline of the pavement. 

Compared to the conventional dense-graded mix what are the superior features of SMA and what are the techno-commercial benefits?

The primary advantage of SMA is the extended life with improved pavement performance compared to conventional dense-graded hot-mix asphalt (HMA). The other advantages are noise reduction, improved frictional resistance, and improved visibility.

SMA surface has high frictional resistance, which provides improved safety to motoring public when traveling on wet pavements.

A life-cycle cost analysis was conducted which compared SMA with dense-graded HMA as an overlay on an existing PCC pavement. The analysis used rehabilitation intervals of 7.5 and 10 years for dense-graded HMA and SMA, respectively. This comparison corresponds to the 30-40 percent expected increase in SMA pavement life as generally experienced in Europe.

How do you rate the success of this technology in the national / state highways so far? 

The SMA technology has been used in India since as early as 2009 and has been adopted in Draft Specification (IRC SP 79-2008) and MoRTH Specification 515 (2013). Since the first use of stone matrix asphalt on national highway, state highway and rail over bridge and underpasses. It can be termed as a success as till date, SMA projects, works, pavements have been performing as per expectations and thereby, in the India context the potential of SMA is high.

What are the hurdles in faster adoption of SMA in highway projects? 

Hurdles mainly emanate from resistance of complacent agencies incorporating new technologies, and contractors looking to find ways by compromising on quality of work.

Do you expect any upgrades in codes and specs that may help faster adoption of this technology?

The IRC SP 79-2008 was adopted at IRC 2018 fourth revision and is recommended as wearing course in place of conventional BC. If adopted in principal and practically, the SMA has huge scope and potential in India.

What are the steps initiated to spread awareness on this technologies from your side?

Genesis Infra Project Consultants are the pioneers of implementing SMA in India as implementing and supervisory consultants. We work with agencies like NHAI, NHIDICL, state PWDs and many municipal corporations to educate, implement and lay SMA technology. We are working with IIT’s and polytechnic colleges to educate future engineers and planners.

What is the core strength and competencies of Genesis Infra? Please also brief us on your road map for next couple of years?

We have a well conversed team with on hand practical experience of marketing, selling and technically implementing SMA in India. We guarantee our projects as well as are committed to hand hold contractors, agency engineers and all stake holders of road construction.

We plan to take stone matrix asphalt technology to the most interior corners of India and prove the world that sustainable roads are the roots of infrastructure development of India. If we have good roads, we will have a great connectivity and a highly enhanced economy in India.

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