SMA

Stone Matrix Asphalt (SMA) was initially invented in Germany in the 1960s.  The first technical guideline was issued out in 1984.  Its usage spread significantly in Europe and the US in 1990 and in Russia in 1994.  Currently, SMA is one of the most popular types of hot mix asphalt prescribed for a broad range of technical and climatic conditions.  With a mostly coarse aggregate structure, SMA contains aggregates, bituminous binder, filler and fiber.  As a modern asphalt material, SMA carries more coarse aggregate and binder and less voids when compared to traditional asphalt mixes.  Such a special asphalt mix, implemented for 50 years in Europe, holds several tailored characteristics which brand this material for its specific advantages.

Structural Characteristics of SMA

• Stone-on-stone contact and significant aggregate interlock gifting the material its structural integrity

• Further use of coarse and high quality aggregate providing higher loading capacity

• Further use of binder leading in exceptional durability

• Less voids and more filler, thus higher compactness and better aggregate-binder interface, resulting in pavement longevity

• Presence of fibers as material reinforcement preventing binder drainage and bleeding as well as decreasing voids and increasing material integrity

• Great macro (overall pavement texture) and micro (chipping texture) causing substantial skid resistance

Performance of SMA is mainly based on its specific gap-graded aggregate skeleton and its stone-on-stone load transferability.  The mastic, filling in SMA aggregate voids and reducing layer voids, is made of binder, filler and fibers.  Due to the high binder content and coarse aggregate structure of SMA, binder can easily run off the surface of aggregate and change SMA mix properties; therefore, a type of stabilizer is needed to avoid binder draindown.  Fibers have proved the ability to provide stability, reinforcement and homogeneity as well as preventing binder drainage.

Fibers, technically called cellulose fibers, are directly added into the mixer.  Once they are in, the thin cover around the fibers is melted and the fibers are spread around inside the mixing drum.  Shear forces exerted by the aggregates cause the fibers to settle around and between aggregates and binder in a complex manner.  This will prevent the binder to drain off freely.  As another advantage, fibers thicken the binder film covering the aggregates and by longer preserving bitumen on the aggregate particles hinder asphalt aging and fatigue.

SMA Advantages

• Excellent durability and high strength against permanent deformations

• Longer service life

• Resistance against rutting, bitumen bleeding and fatigue

• Significant noise reduction

• Longer maintenance-free performance periods, thus, lower maintenance costs

• Substantial inherent skid resistance

• Reduced runoff splash, hence, extended road visibility for the drivers

• No aquaplaning and less surface water accumulation leading in safer roads and less skidding

• Longevity of pavement marking

• lesser light reflection disturbing particularly during night time

SMA Specifications:

The aggregate gradation of SMA is of vital importance due to the coarse nature of SMA and its stone-on-stone load transfer characteristic.  Lack of medium size aggregate means SMA aggregate gradation is classified as gap-graded where the opportunity of stone-on-stone contact is provided.

Use of high quality aggregates in SMA to provide material strength and stability is very crucial.  Aggregate shape, angularity, crushed surface percentage, resistance to crushing and fragmentation (LA coefficient), resistance to polishing (PSV value), resistance to surface abrasion (AAV value) and resistance to wear (Micro-Deval coefficient) affect SMA quality directly.  SMA contains more coarse aggregate, binder, filler and fibers when compared to the normal classic asphalt mixtures.

Cellulose Fibers

In addition to preventing binder draindown, fibers play role in material stabilization, reinforcement and integrity.  Granulated cellulose fibers commercially entitled as TOPCEL® are produced by CFF GmbH & Co in Germany.  These fibers having least moisture susceptibility and very long durability are coated with wax within a special industrial process.

Role of Fibers in SMA

Cellulose fibers absorb significant amount of the free bitumen in asphalt concrete mixture and in conjunction with filler develop mastic which then further holds the aggregates together. Fibers are very light materials although are of considerably large volume.  Such properties in addition to their interconnecting network help reduce the voids in SMA as well as enhancing aggregate-mastic adhesion.

Main Duties of Fibers in SMA

Fibers are expected to cover three mechanically important tasks when incorporated in the SMA mix:

1. Prevent binder draindown during transport and construction stages of paving the road with SMA and also avert binder bleeding after construction

2. Increasing the thickness of binder film on and around the aggregates

3. Filling the mixture voids

The Advantages

• Preventing binder drain down

• Mixture reinforcement and increasing mixture tensile strength

• Great influence in mitigating segregation

• Helping material uniformity and homogeneity, hence preventing development of fat spots

• Prolonging bitumen cover around the aggregate particles and hindering asphalt concrete fatigue and aging

• Non-adhesive fibers

• Inexpensive fibers, hence no huge financial burden

• Reducing binder oxidization to maintain binder functionality

• Easy application with no sensitivity to mix temperature

• Absolutely recyclable

• Easy storage, environmentally friendly and no health and safety issues

Applications

Due to the nature of SMA where coarse aggregates of low special surface dominate and also larger amount of binder present in the SMA mixture, there is a necessity for a binder holder to prevent binder runoff.  This function is fulfilled by the cellulose fibers without leaving any negative effects on the performance of SMA.  In addition to preventing binder draindown (which otherwise will change mixture design, causing significant problems to the pavement performance), use of fibers will assist in mixture uniformity and homogeneity as well as ease of compaction.  Presence of cellulose fibers in the mixture will eventually enhance asphalt ductility, elasticity, plasticity limits and tensile strength.  Research has shown that  use of TOPCEL® products in SMA mixture creates a uniform, stable and strong material so much so that its usage can be technically extended to thin surface courses and manual asphalt laying for local purposes.

Mixing

CFF fibers can be manually added directly into the mixing drum or can be fed in automatically along with the filler.  Therefore, no change to the asphalt plant will be required to use CFF fibers.  We recommend use of automatic batching which is of higher quality and accuracy necessary for large scale projects.

Fiber performance mechanism and influence

The wax covering the surface of the fibers assists in making the pellets.  This will allow keeping the fibers apart during the process of mixing the ingredients.  In addition, the wax will perform as a separator ensuring the fibers will be distributed in the mix evenly and uniformly within the stages of asphalt production process.  Once the pellets are added, the wax melts and the fibers develop a complex network confining the bitumen to remain within the mixture.  In the final product, presence of fiber network causes a thicker bitumen film coverage around the aggregates which will ultimately hinder asphalt concrete aging and fatigue.

Dosage

To create a uniform and homogeneous mixture with no bitumen drain down, research and experience are unisonous to recommend the popular dose of 0.3% fiber by the weight of asphalt mixture; i.e. tipping 3 kg of TOPCEL to one ton of SMA mixture in the mixing drum.  Nevertheless, dosage can be justified depending on the plant and/or asphalt production conditions based on accurate and reliable laboratorial testing.

Conclusion

SMA may initially deem more expensive than the traditional asphalt mixes due to its higher binder requirement and material quality.  However, this is completely compensated by its longer lifespan, its better safety and structural serviceability, less road closures and costs due to less maintenance requirements, and also easier laying and compaction. Previous performances of this material have proved that SMA is an economical material for the road authorities, contractors and road users, an evidence of which is the growing trend of the use of SMA in urban and rural roads worldwide.

More importantly, no change in the process of asphalt concrete production, transportation and laying is necessary to replace traditional asphalt concretes with SMA.  However, it is noticed that laying and compaction of SMA requires high accuracy and precision.  Therefore, it is strongly advised to provide appropriate training for production and laying processes to all the operational staff.  The necessity of using fibers to reinforce and stabilize SMA cannot be emphasized anymore; however, use of a suitable and fit-for-purpose type of fiber in order to prevent bitumen drainage as well as access to reliable technical support are other essential points for a successful SMA project.