Stone mastic asphalt
Encyclopedia
Stone mastic asphalt was developed in Germany in the 1960s. It provides a deformation resistant, durable surfacing material, suitable for heavily trafficked roads. SMA has found use in Europe, Australia, the United States, and Canada as a durable asphalt surfacing option for residential streets and highways. SMA has a high coarse aggregate
content that interlocks to form a stone skeleton that resists permanent deformation. The stone skeleton is filled with a mastic of bitumen and filler
to which fibres are added to provide adequate stability of bitumen and to prevent drainage of binder during transport and placement. Typical SMA composition consists of 70−80% coarse aggregate, 8−12% filler, 6.0−7.0% binder, and 0.3 per cent fibre.
The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional dense graded asphalt (DGA) mixes (see above picture). Improved binder durability is a result of higher bitumen content, a thicker bitumen film, and lower air voids content. This high bitumen content also improves flexibility. Addition of a small quantity of cellulose
or mineral fibre prevents drainage of bitumen during transport and placement. There are no precise design guidelines for SMA mixes. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of filler
and binder.
using individually wrapped press packs or bulk dispensing equipment. Mixing times may be extended ensure that fibre is homogeneous
ly distributed throughout the mix and temperatures controlled in order to avoid overheating or damage to the fibre. In drum plants, particular care must be taken to ensure that both the additional filler content and fibre additive are incorporated into the mixture without excessive losses through the dust extraction system. Filler systems that add filler directly into the drum rather than aggregate feed are preferred. Pelletised fibres may be added through systems designed for addition of recycled materials, but a more effective means is addition through a special delivery line that is combined with the bitumen delivery, so that the fibre is captured by bitumen at the point of addition to the mixture.
should be kept to a minimum to avoid fracture of coarse aggregate particles, or drawing of binder to the surface of the mix. The use of polymer modified binder may decreases mix workability and necessitate increased compactive effort to achieve high standards of compacted density. Achieving high standards of compacted density and low field air voids has been identified as an important factor in the performance of all SMA work. SMA is normally placed with a minimum layer thickness of 2.5 to 3 times the nominal maximum aggregate particle size. Greater layer thicknesses assist in achieving appropriate standards of compacted density.
Binders used in SMA include:
Cellulose fibre is most commonly used in SMA work in Australia. Other fibre types, including glass fibre, rockwool, polyester
, and even natural wool
, have all been found to be suitable but cellulose fibre is generally the most cost-effective. Fibre content is generally 0.3% (by mass) of the total mix.
to the flexible mastic.
Construction Aggregate
Construction aggregate, or simply "aggregate", is a broad category of coarse particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates. Aggregates are the most mined material in the world...
content that interlocks to form a stone skeleton that resists permanent deformation. The stone skeleton is filled with a mastic of bitumen and filler
Filler (materials)
Fillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material...
to which fibres are added to provide adequate stability of bitumen and to prevent drainage of binder during transport and placement. Typical SMA composition consists of 70−80% coarse aggregate, 8−12% filler, 6.0−7.0% binder, and 0.3 per cent fibre.
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
or mineral fibre prevents drainage of bitumen during transport and placement. There are no precise design guidelines for SMA mixes. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of filler
Filler (materials)
Fillers are particles added to material to lower the consumption of more expensive binder material or to better some properties of the mixtured material...
and binder.
Manufacture
SMA is mixed and placed in the same plant as that used with conventional hot mix. In batch plants, the fibre additive is added direct to the pugmillPugmill
A pugmill or pug mill is a machine in which materials are simultaneously ground and mixed with a liquid. Industrial applications are found in pottery, bricks, cement and some parts of the concrete and asphalt mixing processes. A pugmill is a fast continuous mixer. A continuous pugmill can achieve a...
using individually wrapped press packs or bulk dispensing equipment. Mixing times may be extended ensure that fibre is homogeneous
Homogenization (chemistry)
Homogenization or homogenisation is any of several processes used to make a chemical mixture the same throughout.-Definition:Homogenization is intensive blending of mutually related substances or groups of mutually related substances to form a constant of different insoluble phases to obtain a...
ly distributed throughout the mix and temperatures controlled in order to avoid overheating or damage to the fibre. In drum plants, particular care must be taken to ensure that both the additional filler content and fibre additive are incorporated into the mixture without excessive losses through the dust extraction system. Filler systems that add filler directly into the drum rather than aggregate feed are preferred. Pelletised fibres may be added through systems designed for addition of recycled materials, but a more effective means is addition through a special delivery line that is combined with the bitumen delivery, so that the fibre is captured by bitumen at the point of addition to the mixture.
Placement
The primary difference in placing SMA, compared to DGA is in compaction procedures. Multi-tyred rollers are not used due to the possible working of binder-rich material to the surface of the asphalt and consequent flushing and pick-up. Trafficking of the newly placed asphalt while still warm may have the same effect and it is generally preferable for surfaces to cool below about 40°C before opening to traffic. The preferred method of compaction is to use heavy, non-vibrating, steel-wheeled rollers. If these are not available, vibrating rollers may be used but vibrationshould be kept to a minimum to avoid fracture of coarse aggregate particles, or drawing of binder to the surface of the mix. The use of polymer modified binder may decreases mix workability and necessitate increased compactive effort to achieve high standards of compacted density. Achieving high standards of compacted density and low field air voids has been identified as an important factor in the performance of all SMA work. SMA is normally placed with a minimum layer thickness of 2.5 to 3 times the nominal maximum aggregate particle size. Greater layer thicknesses assist in achieving appropriate standards of compacted density.
Materials
Aggregates used in SMA must be of high quality – well shaped, resistant to crushing and of suitable polish resistance.Binders used in SMA include:
- Class 320 bitumen - used in many general applications.
- Multigrade binder - used to provide enhanced performance at higher traffic levels.
- Polymer modified binder - increasingly used in heavy traffic conditions to provide additional resistance to flushing and rutting.
Cellulose fibre is most commonly used in SMA work in Australia. Other fibre types, including glass fibre, rockwool, polyester
Polyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
, and even natural wool
Wool
Wool is the textile fiber obtained from sheep and certain other animals, including cashmere from goats, mohair from goats, qiviut from muskoxen, vicuña, alpaca, camel from animals in the camel family, and angora from rabbits....
, have all been found to be suitable but cellulose fibre is generally the most cost-effective. Fibre content is generally 0.3% (by mass) of the total mix.
Advantages
- SMA provides a textured, durable, and rut resistant wearing course.
- The surface texture characteristics of SMA are similar to Open graded asphalt (OGA) so that the noise generated by traffic is lower than that on DGA but equal to or slightly higher than OGA.
- SMA can be produced and compacted with the same plant and equipment available for normal hot mix, using the above procedure modifications.
- SMA may be used at intersections and other high traffic stress situations where OGA is unsuitable.
- SMA surfacings may provide reduced reflection cracking from underlying cracked pavements due
to the flexible mastic.
- The durability of SMA should be equal, or greater than, DGA and significantly greater than OGA.
Disadvantages
- Increased material cost associated with higher binderBinderThe reaper-binder, or binder, was a farm implement that improved upon the reaper. The binder was invented in 1872 by Charles Withington. In addition to cutting the small-grain crop, it would also tie the stems into small bundles, or sheaves...
and filler contents, and fibre additive. - Increased mixing time and time taken to add extra filler, may result in reduced productivity.
- Possible delays in opening to traffic as the SMA mix should be cooled to 40°C to prevent flushing of the binder to the surface (bleeding).
- Initial skid resistance (lack of FrictionFrictionFriction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
) may be low until the thick binder film is worn off the top of the surface by traffic. In critical situations, a small, clean grit, may need to be applied before opening to traffic.