Stone mastic asphalt for heavy-traffic highways

There are three major types of asphalt surfacing, characterized by a mixture of bitumen and stone aggregate. These are Dense Graded asphalt, Stone Mastic Asphalt and Open Graded Asphalt. Stone mastic asphalt (SMA) is a stone-on-stone like skeletal structure of gap graded aggregate, bonded together by mastic, which actually is 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 is important to ensure the durability and laying characteristics of SMA. Researchers BE Gite and Momin Soyal Abjal explain.

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SMA is best explained as two-component hot mix asphalt HMA which comprises a coarse aggregate skeleton derived from a gap-graded gradation and a high bitumen content mortar. Since 1960s, Stone Mastic Asphalt (SMA) pavement surfaces have been used successfully in Germany on heavily trafficked roads. In recognition of its excellent performance a national standard was set in Germany in 1984. Since then, because of its excellent performance characteristics, the use of SMA increased in popularity amongst the road authorities and asphalt industry.

These two components combined address the concerns of both mixture stability and durability. SMA can provide an extremely high rut resistant and durable HMA mixture as compared with dense-graded asphalt mixture. This improvement is realized through the formation of a stone-to-stone aggregate skeleton in SMA. Based on many research reports and engineering case studies has on road surfaces can achieve better rut-resistance and durability. The SMA mixtures are designed to have high aggregate content, high asphalt content typically 5.5? 7% and high filler content. For ordinary SMA, the use of unmodified bitumen together with fibrous material as a drainage inhibitor is sufficient. Under high temperatures and heavy loading, a harder bitumen grade will also suffice.

1.2 Difference Between SMA & Conventional Mixes

SMA is successfully used by many countries in the world as highly rut resistant bituminous course, both for binder (intermediate) and wearing course. The major difference between conventional mixes and SMA is in its structural skeleton .The SMA has high percent about 70-80 percent of coarse aggregate in the mix .This increases the interlocking of the aggregates and provides better stone to stone contact which serves as load carrying mechanism in SMA and hence provides better rut resistance and durability. On the other hand, conventional mixes contain about 40-60 percent coarse aggregate. They does have stone to stone contact, but it often means the larger grains essentially float in a matrix composed of smaller particles, filler and asphalt content .The stability of the mix is primarily controlled by the cohesion and internal friction of the matrix which supports the coarse aggregates .It can be followed from diagram of the grain size distribution of the mixes given below.

The second difference lies in the binder content which lies between 5-6 percent for conventional mixes. Below this the mix becomes highly unstable. Above this percent will lead to abrupt drop of stability because the binder fills all the available voids and the extra binder makes the aggregates to float in binder matrix. The SMA uses very high percent of binder > 6.5 percent which is attributed to filling of more amount of voids present in it, due to high coarse aggregate skeleton. The high bitumen content contributes to the longevity of the pavements.

The third difference is the use of stabilizing additives in SMA which is attributed to the filling up of large no of voids in SMA so as to reduce the drain down due to presence of high bitumen content. On the contrary, there is no stabilizing agent in conventional mixes since the bitumen content is moderate, which only serves the purpose of filling the moderate amount of voids and binding the aggregates

2. METHODOLOGY

2.1Composition
1. Asphalt(Binder)
2. Aggregate
3. fibres
4. Mineral filler

2.1.1. Asphalt (Binder)

SMA contains very high content of bitumen as compared to conventional mixes i.e.> 6.5 %. It is used to bind the aggregates, fillers and stabilizing additives. Different studies on SMA have been conducted by using different bitumen grades namely bitumen of grade 60/70.

Heavy duty performance is usually enhanced with polymers and fibres, this helps to provide a thick aggregate coating to the aggregate, and the prevention of drain down during the transportation and placement.

The type of PMB most commonly used with SMA is styrene, butadiene styrene (SBS) which is an elastomeric polymer type. Advantages of polymer in SMA:

1 .To increase the resistance to permanent deformation,
2. To increase the life span of the pavement,
3. To reduce the application and damage risk in thin layers,
4. To reduce the need for drainage inhibitor.

2.1.2. Aggregate

The strength and rut resistance of SMA is mostly depends upon on the aggregate mix is 100% crushed and with good shape (cubical) and strength limits for abrasion resistance and crushing strength. The sand used must be crushed sand as the internal friction contributes into the overall strength.

Design Aggregate Structure -

The selection of a design aggregate structure is one of the major features of the SMA mixture design method. The procedures for the blended aggregates outlined in the methodology provided a direct ability to proportion aggregates by volume to achieve varying degrees of aggregate packing. In this study, five different design unit weights of the coarse aggregate selected to evaluate aggregate packing characteristics.RUW ranging from 80% to 90%.

2.1.3. Fibres

Fibres are being used in SMA as a stabilizing agent and to reduce the drain down significantly. Various fibres have been used in various studies showing very good results. Here we have tried to use bagasse fibre as a stabiliser in the mix. The fibres were collected locally from sugar mills. The fibres were having a thickness of approximately 1mm to 3mm which is determined from a rough estimation. 0.3% of fibre content by mass of total mix.

The inclusion of cellulose or mineral fibres during the mixing process as a stabilizing agent has several advantages including:
a) Increased binder content.
b) Increased film thickness on the aggregate by 30-40%.
c) Increased mix stability.
d) Interlocking between the fibres and the aggregates which improves Strength.
e) Reduction in the possibility of drain down during transport and paving.

Commonly used fillers:
1. Hydrated lime,
2. Fly-ash,
3. Reactivity of fine material.

2.2. Design

2.2.1 Asphalt mixture testing and analysis
With the selection and testing of the individual dry coarse and fine aggregate gradations completed, the aggregates were combined in precise percentages to produce SMA asphalt mixtures that should exhibit control levels of coarse aggregate stone to stone contact. Mix designs with respect to various combinations of coarse and fine aggregate gradations and asphalt bi