In Hong Kong, SMA trials began in the mid-1990s, and the Highways Department subsequently adopted it as a standard mix. In recent years, SMA has been widely used as a wearing course, replacing dense-graded asphaltic concrete on demanding roads to enhance the overall pavement performance and durability of local roads. New developments in Europe in the early 2000s introduced small aggregate SMA (4.75–6.3 mm), which demonstrated superior noise reduction of 3–5 dB(A). This led to the adoption of 6 mm SMA as a standard low-noise surfacing material for local roads in Hong Kong since the early 2020s.

SMA incorporating high-performance PMB (polymer-modified binder) was laid over mastic asphalt on the two steel cable-stayed bridges of the Hong Kong–Shenzhen Western Corridor. Since opening in 2007, the pavement has performed well. PMB enhances deformation resistance, making it suitable for the dynamic environment of steel decks. The SMA wearing course also serves as a sacrificial layer to facilitate future maintenance, contributing to cost-effectiveness. This SMA–mastic asphalt system has since been adopted on Stonecutters Bridge and Cross Bay Link. [HKC-2008-08, updated 2025]

SMA was first trialed in Hong Kong in 1997. Initial trials adapted gradings and binder to local aggregates and 60/70 pen. bitumen. Performance was generally satisfactory, with 20 mm SMA outperforming the 10 mm version. Extensive use began nearly a decade later to replace dense asphaltic concrete on demanding urban roads. However, severe deformation became frequent. Causes were complex, including unsound base, excessive binder, inadequate binder properties, and deficient design or compliance criteria.

Unsound base may result from stripping and fatigue, especially in Hong Kong’s wet climate and heavy traffic. Unlike dense-graded mixes supported by continuous gradings, SMA relies on a stone skeleton. While this structure offers superior load support and durability, it can collapse under excessive shearing stress or poor base conditions, leading to rapid deformation.

The binder-filler mortar can be critical in securing the stability of the stone skeleton, particularly when the skeleton is subjected to severe stress. This requires the binder to provide adequate stiffness at high temperatures. Much failure has been observed with SMA incorporating 60/70 pen. bitumen, typically graded as PG64-22, whereas the use of PG76 binder has shown some improvement. This is understandable, as pavement temperatures exceeding 65 °C are not uncommon during Hong Kong’s summer under strong sunshine. SMA incorporating a binder grade close to or exceeding PG82—such as those laid on the long-span steel bridges of the HK–Shenzhen Western Corridor (2006) and Stonecutters Bridge (2009)—has so far shown no signs of premature deformation.

Discrepancy between added and measured binder contents remains an unresolved issue, often leading to extra binder addition and resulting in accelerated deformation and bleeding. Measured values are typically lowered by around 0.2%, with discrepancies of 0.3–0.5% or more not uncommon. The issue has been studied by various parties, but the cause is still undetermined. It is reasonable to believe that binder loss during handling of binder-rich samples is a major contributor. The consequence has been alleviated by shifting the target measured binder content 0.2% below the designed value.

VCA (voids in coarse aggregates) has been introduced to safeguard skeleton quality and surface texture. However, maintaining VCA alone without improving binder properties has not prevented premature deformation.

The standard number of Marshall Blows for SMA is 50 per side. While this avoids stone breakage, it may cause excessive binder due to low compactive effort. Hong Kong has reverted to 75 blows per side to better reflect local traffic conditions.

After years of development, SMA remains a valuable material for enhancing pavement durability, comfort, safety, and cost-effectiveness. Noise-reducing SMA developed in Germany uses smaller aggregate sizes, offering thinner, durable, and quieter surfaces. This requires better-shaped aggregates and new quarry fractions, but may prove cost-effective and well-suited to Hong Kong’s dense urban environment. [HKC-2011-08, updated 2025]

Anderson Asphalt Limited (AAL) estimates Hong Kong reuses only ~24% of RAP by 2025. Annual RAP availability is ~240,000 to 290,000 tonnes, yet only ~70,000 tonnes are reused by batching plants. The remainder is either disposed of or underutilized, adding to landfill pressure and missing opportunities for carbon reduction.

This gap highlights the need to revise specifications and initiate demonstration projects to validate 50% RAP mixes under local conditions. Such efforts would support ESG goals, promote circular construction practices, and align Hong Kong with global sustainability trends.