Use of SMA in Hong Kong
SMA was developed in Germany in mid-1960s and it has spread throughout Europe and across the world in 1980s and 1990s. The excellent performances include resistance to mechanical and temperature deformation (cracking and rutting), resistance to weathering actions such as aging and low-temperature cracking. Durability is excellent even under slow moving heavy traffic. The textured surface increases skid-resistance and provides environmental and driving comfort by reduced noise level, and improved visibility in rainy days.
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In Hong Kong, trials of SMA began in mid-1990s. Subsequently, Highways Department adopted it as a standard mix. SMA has been widely used in recent years as a wearing course material, replacing the standard dense graded asphaltic concrete, in some of the most demanding local roads, for improved performance and durability when laid on top of a sound underlying pavement.
SMA, incorporating high performance PMB (polymer-modified binder), was laid on top of mastic asphalt on the two steel cable-stayed bridges of the Hong Kong-Shenzhen Western Corridor, giving good performance since the pavement was opened in 2007. PMB improves resistance of the mix to deformation so as to suit the environment of increased movement on steel deck. The SMA wearing course serves as a sacrificial course to facilitate future maintenance, and contributes to the cost-effectiveness of both the capital and maintenance costs. This SMA-mastic asphalt surfacing system has since been incorporated on Stonecutters Bridge. [HKC-2008-08]
Performance and development of robust SMA in Hong Kong
Despite wide application of SMA in its homeland since invention in Germany in 1960s and wide acceptance by many other countries in 1980s and 1990s, performance of SMA may not be so consistent in some parts of the world. Hong Kong has had different experiences.
SMA is renowned for its structure of “stone-on-stone contact” and thick binder film, which provides superior performances including mechanical and temperature deformation resistance, cracking resistance, rutting resistance, weathering resistance (such as aging and low-temperature cracking), and excellent durability, skid-resistance, noise reduction and reduced hydroplaning. Use of cellulose fibre is an outstanding feature that allows SMA to retain thick binder film which helps its durability.
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SMA was first trialed in Hong Kong in 1997. Upon initial trials, gradings and binder were adapted to conventional local aggregates and 60/70 pen. bitumen. The general performance was satisfactory and 20 mm SMA showed better performance than the 10 mm counterpart. Extensive use of SMA in Hong Kong began almost ten years after its initial trials, with the aim of replacing those dense asphaltic concrete wearing courses on demanding urban roads but severe deformation of SMA was then a frequent occurrence. The causes of failure may be complicated. They may include unsound base, excessive binder, inadequate binder properties, deficient compliance criteria, and deficient mix design criteria.
The presence of unsound base may be due to stripping and fatigue because the local environment is wet and traffic volume is heavy.
Unlike dense-graded mixes in which the mass is supported by continuously-graded aggregates; stability of SMA relies on the “stone-on-stone” skeleton. Under normal circumstances, the stone skeleton of SMA provides sound support to traffic loadings which often outweighs the support provided by the continuous gradings of dense graded mixes, and hence becomes more durable than dense-graded mixes. On the other hand, the stone skeleton may become unstable or collapse when stability of the base is deficient or in the presence of excessive shearing stress. SMA then becomes very unstable and may deform drastically and quickly within a few weeks after laying.
The binder-filler mortar can be critical in securing stability of the stone skeleton, in particular when the stone skeleton is experiencing severe stress. This requires that the binder provides adequate stiffness at high temperatures. Much failure has been found with SMA incorporated with 60/70 pen. bitumen, which is often graded to PG64-22, whilst use of PG76 binder has gained some improvement. This is understandable as pavement temperature reaching 65+°C is not unusual in the summer of Hong Kong under plenty of sunshine. SMA, incorporated with a grade of binder close to or exceeding PG82, that has been laid on the long-span steel bridges in Hong Kong (HK-Shenzhen Western Corridor in 2006 and Stonecutters Bridge in 2009) reveals no premature deformation so far.
Discrepancy between added and measured binder contents is an unresolved problem, which has led to addition of extra binder and, which eventually causes accelerated deformation and bleeding. The measured value is often lowered by around 0.2% but discrepancies at levels of 0.3—0.5% or even higher are not uncommon. The issue has been studied separately and jointly by various parties for years but the cause has yet to be determined. It is only reasonable to believe that loss of binder on handling the binder-rich samples is a major cause of the error.
VCA (voids in coarse aggregates) has been introduced to safeguard the quality of the stone skeleton. This is also helpful in securing surface texture. Yet maintaining the VCA alone without upgrading the binder property has been proved to be ineffective in preventing premature deformation.
The number of Marshall Blows normally adopted by other countries for SMA is 50 per side. Although compaction at such level may prevent excessive compaction leading to breakage of stones, it risks introducing excessive binder through inadequate compactive effort. Now new SMA mix design have been reverted to 75 blows per side, to reflect the environment of road traffic here.
After many years of development, SMA is still a treasure for exploration for enhancing pavement durability, driving comfort and safety, and cost-effectiveness. Noise reducing SMA has now been developed in Germany. The maximum nominal aggregate size of SMA is reducing which may deliver a thinner, durable, and more noise-reducing material. This calls for better shaped aggregates and new size fractions from quarries but in the long run it could be proved to produce cost effective material for the public, and may suit the dense environment of Hong Kong. [HKC-2011-08]