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The secret of the stone skeleton: How did stone dust asphalt become a durable choice for highways?
Stone-milled asphalt (SMA) was first developed in Germany in the 1960s, and the first SMA pavement was laid near Kiel in 1968. The material's resistance to deformation and durability have quickly made it an ideal surface material for roads with heavy traffic and it is now widely used on residential streets and highways in Europe, Australia, the United States and Canada.
The high coarse aggregate content of stone powder asphalt enables it to form a stable stone skeleton and effectively resist permanent deformation. The asphalt texture formed by the combination of stone powder and filler within this skeleton can provide the necessary stability and prevent the loss of asphalt during transportation and paving.
The typical composition of stone powder asphalt includes 70-80% coarse aggregate, 8-12% filler, 6.0-7.0% binder and 0.3% fiber.
Stone dust asphalt's resistance to deformation comes from its higher aggregate contact ratio, providing improved performance compared to conventional dense grade asphalt (DGA) mixes. In addition, a high asphalt content can improve aging resistance and flexibility, and prevent asphalt loss during transportation and paving through a small amount of fiber.
Manufacturing process
Stone dust asphalt is mixed and laid in the same plant as conventional hot mix. In batch production, the fiber additives are added directly to the mixing box and a prolonged mixing time is used to ensure that they are evenly distributed.
In drum plants, special care must be taken with the incorporation of additional fillers and fiber additives to avoid excessive losses in the dust collection system. Some fill systems are designed to add fill directly into the drum rather than into the aggregate supply.
Laying method
The main difference in laying SMA compared to DGA is the compaction procedure. Multi-tire rollers should not be used because of the possibility that bitumen-rich material in the mix may float to the surface. The newly paved road surface should be cooled to below 40°C before it can be opened to traffic.
The most suitable compaction method is to use a heavy duty non-vibration steel wheel roller. Generally speaking, vibration should be kept to a minimum to avoid breaking up the coarse aggregate.
Material Selection
Aggregates used for SMA must be of high quality, requiring good shape, compression resistance and moderate resistance to calendering. Binders commonly used in SMA include No. 320 asphalt, modified asphalt, etc. to improve performance under high traffic volume.
The addition of fibre is also crucial and Australia typically uses cellulose fibre, which is chosen based on cost effectiveness and performance. Other fibers such as fiberglass, rock wool, etc. have also proven to be suitable.
Advantages and disadvantages of stone powder asphalt
SMA not only provides an ideal surface texture, but also has excellent durability and resistance to deformation. In areas with high traffic volume, SMA can effectively reduce the occurrence of reflective cracks, thereby extending the service life of the pavement.
The main disadvantages of SMA include relatively high material costs and increased mixing and laying time, which may cause delays in opening to traffic.
In addition, since the initial friction of the newly laid SMA surface may be low, it may be necessary to carry out certain treatments before opening to traffic to improve sliding safety. Stone dust asphalt may also require defect removal during subsequent renovations.
With the increasing demand for highway durability, the market prospect of stone powder asphalt will continue to expand in the future. What new technological developments will occur in the future that will allow it to be applied in more fields?
