Sant Kumar Sehgal, Technical Consultant For Stone Matrix Asphalt Roads, J. Rettenmaier & Sohne, Germany
IntroductionOver the years India has seen a tremendous rise not only in the traffic volume, but heavy loading as well. Vehicles carrying heavy loads, have special designed tyres, which are inflated to high pressure, causing high stress on pavement. Under these conditions, conventional dense grades mixes are not rut resistant. STONE MATRIX ASPHALT – SMA, a gap graded mix, is highly rut resistant, tough, stable, skid resistant, with high quantity of coarse aggregates, relies on stone on stone contact, to provide strength & rich in binder, to provide durability. SMA was developed in Germany in the late 1960's & since 1970's has been used extensively in Europe & since 1990's in USA. SMA is used for wearing courses and binder courses (DBM).
SMA - ConceptThe SMA-CONCEPT can be briefly summarised as:
- High stability to permanent deformation & high wear resistance by an excellent particle interlock and a high content of crushed premium aggregates.
- Longevity & durability to premature cracking and ravelling by a very high content of bitumen and a void less mastic mortar fills the voids of the stone skeleton and binds it together: high quantity & quality of binder are prerequisite for a long useful service life.
- Stabilizing additives [cellulose fibres] addition assures homogeneity [no binder drainage], during the manufacturing, transportation, laying and compaction.
- High quality & high quantity of bitumen (thick binder films) & coarse aggregates are the essentials for a long service life of SMA.
Dense Graded Asphalt Mix (BC) vs Gap Graded Asphalt Mix (SMA)BC which is a dense graded asphalt mix, the coarse aggregates do have no contact with each other & are separated by fine aggregates [sand] particles which have to carry the load. SMA is a gap graded asphalt mix, which lacks certain sizes of aggregates, it has abundance of coarse aggregate particles [material retained on 4.75 mm sieve]. SMA has high filler content, [passing on 0.075 mm sieve], usually between 8-12%, compared to BC which has filler content less than 5%. SMA has a higher bitumen content > 6%, hence require the use of cellulose fibre pellets, to inhibit high amount of bitumen from draining out.
Pic 1 - Structure of a typical dense graded asphalt mix (BC)
Pic 2 - Stone on stone skeleton structure of coarse aggregate in gap graded asphalt mix (SMA)
SMA MaterialsAll materials used must be of high quality & confirming to their respective codes. Bitumen used is either VG30 or PmB40 confirming to IS:73:2013/IRC:SP:53. Coarse aggregates should be crushed rock confirming to IS:2386 in all parameters of, cleanliness, combined flakiness & elongation index, Los Angeles abrasion value, aggregate impact value, polished stone vale, soundness & water absorption. Aggregates should not show any stripping. Fine aggregates should confirm to IS:2720. Mineral filler should be either stone dust or a combination of stone dust and maximum 2.5% hydrated lime. Stabilizing additive, cellulose fibres, are dosed in pellets form like VIATOP fibre pellets, the minimum dosage is 0.3% on the mix.
SMA - Mix Design
- All material should be selected after thorough laboratory checks & meeting the desired parameters.
- Aggregate gradation should be within the specified band.
- Establish optimum binder content, by checking the air void level at 4% & voids in mineral aggregates (VMA) higher than 17%.
- Evaluate binder drain down, should be less than 0.3% (Schellenberg Drain Down Test)
- Evaluate moisture susceptibility should be >85.
- Perform test for voids in coarse aggregates under dry rodded condition, to ascertain stone skeleton.
Pic 3 - SMA gradation as per IRC:SP:79
|Composition of Stone Matrix Asphalt|
|Course||Wearing course||Binder course|
|Sieve mm||Cumulative weight passing||Cumulative weight passing|
SMA - Production, Laying & CompactionSMA mix is best produced in batch plant, in the available Indian drum plants, it is not recommended to produce SMA, since it does not have place to introduce VIATOP cellulose fibre pellets properly. However in India, Ammann Apollo are working on, a drum plant where dosing of VIATOP cellulose fibre pellets will be possible.
In batch plant, there are two methods of dosing of VIATOP cellulose fibre pellets [i] manually [ii] automatic. The heated aggregates, when come in the pug mill, at that time filler is dosed & at this stage VIATOP cellulose fibre pellets are also dosed (a dry mixing time is not required for the VIATOP fibres) & get mixed with the aggregates & then bitumen is dosed & wet mixing time of 25-30 seconds is given. In case of manual dosing of VIATOP cellulose fibre pellets, most of the batch plants have an opening over the pug mill, where from a pre weighed bag of VIATOP cellulose fibre pellets can be dosed easily, see Pic 4.
Pic 4 - Attachment made in Apollo plant for manual dosing of VIATOP fibre pellets
Manual dosing is an effective method of dosing, since the bag containing fibre pellets, goes directly in the middle of the pug mill & mixing of the fibres is easy, the critical point of manual dosing is, what if the worker forgets to add the fibre bag, in any of the batches, 1-2 workers are required to be attentively present for dosing fibre pellets, it is work oriented process.
The other dosing is automatic, where alongside silo for mineral filler, another hopper can be erected, where from the fibre pellets are pumped to silo, which has a weight cell & connected to the control computer. From silo fibre pellets are pumped into the pipe bringing in the mineral filler, so that both the materials get dosed simultaneously, see Pic 5.
Pic 5 - Automatic dosing of cellulose fibre pellets (Source: Rettenmaier & Söhne, India)
SMA is typically produced in a batch plant at temperature of 165°C when using VG30 & at 175°C when using PmB40. Mineral filler & fibre should be dosed in the pug mill, as soon as the aggregates reach & fibre get homogenously mixed with the aggregates, filler and the bitumen. A normal 25-30 seconds of wet mixing time should be given, before downloading the material on the truck. Truck should be covered before it leaves the plant to avoid premature oxidation. The mix temperature should be checked before laying, it should be 145-150°C. A standard paver with sensor, can be used, but should have heating system in its screeds. 8-10 MT steel wheel tandem rollers are to be used no rubber tyre rollers for compaction. Minimum 3 rollers to be put into service, first roller to do the break down rolling, should be just behind the paver & should compact on slow vibration or static. The other two rollers should do the rolling & a maximum of 6 passes should be given, the 6th pass should be the final pass. More passes can not be given as they may break the aggregates. Before final pass, the surface should be checked for density, with the help of a nuclear gauge & remaining passes can be adjusted as per that. Must ensure that compaction is completed before the mix temperature comes down to approx. 100°C.
Indian ExperiencesOver the last seven years, we have been associated with laying of SMA on national highways, state highways, as well as city roads. Some of the sections done are now, over 5 years & have not shown any distress, although these sections have very high axle loads.
A major challenge that has been encountered is of usage of rollers, the roller operators are typically used to rolling BC surfaces, no matter what, they are not able to understand the meaning of first pass as vibratory, a strict check needs to be done by the contractor. Because of high amount of bitumen, no rubber tyre rollers were used for compaction of SMA.
Tandem rollers, as needed for compaction (Source: Rettenmaier & Söhne, India)
A strict check also has to be kept on the operator, adding VIATOP cellulose fibre pellets, that these are added in every batch & at the time of aggregates are downloaded in the pug mill.
Find below some photographs of SMA section, immediately on compaction, after gap of 12 months, 30 months & 60 months.
A freshly compacted SMA surface-NH14, Palanpur-Deesa section laying done April 2009
SMA surface after 12 months of laying-NH13 Bijapur-Hungund section laying done June 2011
SMA surface after 30 months of laying –NH8, Kishangarh-Ajmer, laying done June 2012
SMA surface after 60 months of laying – NH14, Palanpur-Deesa, laying done April 2009
Binder drainage test (according to Schellenberg Drain Down Test) must be done 2-3 times every day, to see the effectiveness of fibres.
Normally during compaction, before ending the compaction, density of the wearing course should be checked with the help of nuclear gauge, to see if any more passes are required or further any number of pass can be reduced. Actually, we did not find any nuclear gauge with most of the contractors, where than we have to rely on the extraction of cores, next day & checking them for density, to see if desired air voids are obtained & accordingly adjust the number of passes, the next day. The withdrawn cores show a very close stone skeleton.
Water logging in the underpass
Distressed rigid pavement
SMA surfaces are highly impervious to ingress of water due to thick binder films around the aggregates. We had the opportunity to help Ahmedabad Municipal Corporation, in laying SMA, over a rigid pavement, in an underpass, called Parimal Underpass, in the Ahmedabad city. The underpass, remains water logged, with heavy rain & may take some time before water is completely drained out & the underpass becomes motorable again, but immediately after the water is removed, two wheelers driving in the underpass, meet with the accident, as the two wheelers skid, because of accumulation of algea. We helped in designing a 50 mm thick SMA wearing course, laying to be done over rigid pavement, keeping in mind, about water logging, the air voids were kept between 2 & 3%. The laying was done in May 2014. More than 2 years & more than 20 water loggings, not even a single aggregate has moved out of place, neither does it show any distress. No two wheeler accidents have occurred since then, due to skidding of their vehicles.
SMA is a very robust, rut resistant pavement, with low life cycle costs and highly quality conscious, can not tolerate any deviation from its mix design & check should be done, that no traffic is allowed on the surface at least for 24hrs & that in case if the earlier surface has been milled & fresh SMA laying is being done, profile correction of the surface must be done. We witnessed this problem, on one of the surfaces, where there was no control on the traffic & all heavy loaded trucks were coming on the surface, even when the compaction was going on, resulting in slippage of the material. On the same highway, no profile correction was done, before SMA laying, resulting in settlement of the mix in the canals created by milling. Later this was corrected, by first laying 30 mm thick surface & next day laying another 30 mm, on the first surface. At this stage no settlement is observed.
SummaryThe concept of SMA was introduced successfully to India since a few years. Conventional dense graded asphalt pavements (BC) will not be able to withstand the increasing heavy loads on the Indian road network in future. SMA, gap graded mixes, based on present Indian specifications (e.g. MORTH) show a much longer service life without any maintenance compared to Indian BC mixes. Therefor gap-graded mixes are more economical. STONE MATRIX ASPHALT – SMA, a gap graded mix, is highly rut resistant, tough, stable, SMA covers all needs of the Indian roads.
These are author' personal experiences & observations.