Hong Kong’s skyline is globally recognised for its density and dynamism, yet beneath these iconic views lies a growing concern: the structural deterioration of ageing residential buildings. As many of the city’s older developments approach or exceed 50 years of age, issues such as concrete spalling and the corrosion of steel reinforcement have become increasingly common. These problems not only undermine structural safety but also affect residents’ quality of life, with districts such as Yau Tsim Mong—where ageing housing stock is concentrated—experiencing the issue more acutely.

Against this backdrop, innovation in materials engineering has become essential. In recent years, scientists and engineers have intensified efforts to develop more effective methods for restoring deteriorated building components, extending the lifespan of older structures, and improving living conditions. These advancements are particularly critical for vulnerable groups residing in subdivided units, ageing tenement buildings, or poorly maintained apartments.

One such breakthrough is the Repair Mortar Material developed by the Nano and Advanced Materials Institute (NAMI) prior to its merger with the Hong Kong Applied Science and Technology Research Institute (ASTRI). Enhanced with specially formulated mineral fibres, this advanced mortar is engineered to tackle two of the most critical issues in ageing buildings: steel corrosion and concrete spalling. The material has already been applied and has demonstrated promising results in district-based improvement projects, including the Yau Tsim Mong Scheme of Remedying Exposure of Reinforcement through Technology, where it was used to repair severely deteriorated ceilings and slabs.

Yet the significance of this initiative extends beyond technical innovation. It underscores the role of applied R&D in creating tangible and meaningful benefits for the community. Over the past year, the programme has supported ten families in resolving urgent structural hazards in their homes—many of whom had long struggled to find assistance.

The state of Hong Kong’s ageing buildings

Hong Kong’s rapid post-war development spurred an intense construction boom from the 1960s to the 1980s. Today, many of these buildings have reached an age at which structural deterioration has become increasingly evident. According to the Buildings Department, more than 9,600 private buildings across the city are over 50 years old—a number that continues to rise.

Recurring issues face by older residential structures

Common problems in ageing buildings

Older residential structures face a range of recurring issues, the most prevalent of which include:

• Concrete spalling

Moisture intrusion and carbonation gradually weaken the concrete, causing the protective layer that covers the steel reinforcement to crack and fall away.

• Exposed or corroded Reinforcement Bars

Once steel reinforcement bars are exposed, corrosion accelerates and quickly reduces their load-bearing capacity, posing risks to overall structural safety.

• Water leakage and moisture ingress

Persistent dampness, especially in ceilings and beams, further accelerates material deterioration and compromises structural elements.

• Lack of timely maintenance

For low-income households or those living in subdivided flats, essential repairs are often delayed due to financial constraints, insufficient knowledge, or limited access to qualified contractors.

These conditions pose serious safety risks, particularly for elderly residents, low-income households, new immigrants, and occupants of subdivided units, where falling concrete and ceiling failures are not uncommon.

The technological breakthrough: ASTRI’s Mineral Fibre-Enhanced Repair Mortar

To address these challenges, a Class 25 repair mortar specifically engineered for the rehabilitation of spalled concrete and corroded steel reinforcement bars was developed. The material is the result of extensive research in materials science, fibre reinforcement, and durability engineering, offering a high-performance solution tailored to the realities of ageing buildings.

High-performance industrial mixers for superior mixing, granulating, and processing

Key features of the Mortar Material

To address these challenges, a Class 25 repair mortar specifically engineered for the rehabilitation of spalled concrete and corroded steel reinforcement bars was developed. The material is the result of extensive research in materials science, fibre reinforcement, and durability engineering, offering a high-performance solution tailored to the realities of ageing buildings.

1. Mineral fibre reinforcementThe addition of specially designed mineral fibers significantly enhances the mortar’s tensile strength and resistance to cracking. Conventional mortar is prone to shrinkage cracks—particularly in overhead applications—the fibre reinforcement distributes internal stresses effectively.
2. Enhanced bonding strengthThe mortar forms a stronger bond with both concrete substrates and steel reinforcement, improving durability and reducing the risk of delamination and premature failure.
3. Restored structural performanceBy strengthening the interface between concrete and steel and providing tensile reinforcement, the mortar helps recover part of the load-bearing capacity lost due to reinforcement corrosion.
4. Low permeability and durabilityOptimised particle grading and dense microstructure reduce moisture ingress, slowing future corrosion and extending service life.

Laboratory performance and compliance with Hong Kong standards

Extensive laboratory testing has been conducted to verify the engineering performance of ASTRI’s repair mortar. The material fully complies with the Hong Kong Code of Practice for Concrete Structural Repair (PCCS-RM) and, in several aspects, exceeds the minimum requirements.

Typical Performance of ASTRI's Class 25 Repair Mortar

Notably, the air permeability result of 557 seconds indicates a dense and durable hardened mortar, significantly reducing the ingress of harmful agents such as moisture and chlorides. This performance is approximately 2.6 times better than a comparable commercial product (SIKA: 216 seconds), demonstrating superior resistance to long-term deterioration.

Construction laboratory

Meeting and exceeding statutory requirements in Hong Kong

From a regulatory perspective, ASTRI's repair mortar not only satisfies all statutory requirements for structural repair materials in Hong Kong but is further optimised beyond conventional systems.

Key technical distinctions include:

1. Mineral fibre vs. Polymer fibreThe use of mineral fibres—silicate-based and chemically compatible with cementitious materials—results in:
   
   ▪ Better bonding with the cement matrix
   ▪ Higher durability and resistance to ageing
   ▪ Reduced long-term degradation compared with polymer fibres
   
2. Optimised particle packing
   The incorporation of multiple mineral powders improves cementitious particle grading, achieving close packing and reduced porosity. This leads to:
   
   ▪ Lower air permeability
   ▪ Improved resistance to aggressive agents
   ▪ Enhanced durability under Hong Kong’s humid conditions
   
   Together, these features ensure regulatory compliance while delivering measurably improved performance for real-world applications.

Successful adoption in Hong Kong: Real cases from Yau Tsim Mong

Since July 2024, the repair mortar originally developed by NAMI and now carried forward under ASTRI has been successfully applied in 10 residential units across the Yau Tsim Mong district to address exposed steel reinforcement and severe ceiling spalling. These projects form part of the Yau Tsim Mong Scheme of Remedying Exposure of Reinforcement through Technology, implemented in collaboration with district offices, area committees, and community partners.

Successful adoption of the project

Representative case

In one pilot unit occupied by an ethnic minority family, longstanding ceiling spalling had created ongoing safety hazards. With limited access to professional repair services, the family endured falling concrete for years.

Following intervention using ASTRI’s repair mortar, the outcomes included:

• Stabilisation of the reinforced concrete slab
• Elimination of falling concrete risks
• Improved surface integrity and appearance
• Reduced moisture-related deterioration

Over the past year, beneficiaries have included:

• Elderly individuals living alone
• Low-income households and subdivided unit residents
• New immigrant families unfamiliar with support channels
• Occupants of severely deteriorated tenement buildings

These real-world applications demonstrate the material’s constructability, durability, and social impact under actual site conditions.

Practical considerations and limitations in use

While the material offers enhanced performance, proper handling and application are essential:

• Shelf Life: Dry-mixed mortar should be used within three months of manufacture
• Mixing Time: Due to fibre content, mixing should be extended by two to three minutes compared with standard repair mortars to ensure uniform fibre dispersion
• Pot Life: Once mixed, the mortar should ideally be applied within two hours
• Good Site Practice: Proper surface preparation and curing remain critical to achieving optimal performance

These considerations ensure that the material’s laboratorytested advantages are fully realised on site.

The broader meaning: Innovation serving society

ASTRI’s mineral fibre–enhanced repair mortar illustrates how engineering innovation can directly enhance public safety and wellbeing. Its success highlights the importance of:

• Translating scientific research into practical applications
• Cross-sector collaboration between government, engineers, and community organisations
• Leveraging material science to extend the service life of existing buildings

As land scarcity and urban renewal challenges persist, technologically advanced repair solutions will play an increasingly vital role in strengthening Hong Kong’s ageing building stock.

Conclusion

Hong Kong’s built environment has reached a critical juncture. With thousands of ageing buildings requiring repair, traditional methods alone are no longer sufficient. ASTRI’s advanced repair mortar shows that by integrating science, engineering, and community collaboration, it is possible to deliver solutions that are not only technically superior but socially impactful.

The Yau Tsim Mong project demonstrates how technology can meaningfully improve living conditions, restore dignity for under-resourced families, and foster a safer city for all. As ASTRI continues to advance materials research, its innovations
will play an increasingly vital role in shaping a more resilient, sustainable, and inclusive Hong Kong.