Technical validation of Grade C60 high-volume GGBS-based tremie concrete for deep foundations: A case study on innovation and sustainability

By Hong Kong Applied Science and Technology Research Institute & Hong Kong Housing Authority

In the global race toward carbon neutrality, the construction industry faces a significant challenge: the high carbon footprint of Ordinary Portland Cement (OPC), which accounts for approximately 8% of global CO2 emissions. To align with the HKSAR Government’s target of achieving carbon neutrality before 2050 and the Hong Kong Housing Authority’s (HA) commitment to reducing embodied carbon in public housing, the Hong Kong Applied Science and Technology Research Institute (ASTRI) has spearheaded a comprehensive study on Ground Granulated Blast-furnace Slag (GGBS)* based low-carbon concrete.

Fully funded by the Innovation and Technology Commission (ITC) and supported by HA, this initiative addresses a critical gap in the industry: the lack of practical experience in high-strength Grade C60 tremie concrete utilising high volumes of GGBS. Historically, Pulverized Fuel Ash (PFA) has been the primary supplementary cementitious material (SCM) in Hong Kong. However, with the decommissioning of coal-fired power plants, the local supply of PFA has become increasingly volatile. GGBS, a by-product of the steel manufacturing industry, oers a stable, high-performance alternative that significantly enhances the durability of underground structures while reducing carbon intensity.

* Ground Granulated Blast-furnace Slag (GGBS) is a by-product of the steel manufacturing industry, produced by quenching molten iron slag from a blast furnace in water or steam, resulting in a glassy, granular product that is then dried and ground into a fine powder.

The shift to Grade C60: Structural necessity

Deep foundation elements, specifically bored piles, account for approximately one-quarter to one-third of the total concrete volume in public housing projects. In recent years, Hong Kong has seen significant upgrades in rock capacity parameters, allowing for higher bearing capacity and shaft friction (Geotechnical Engineering Office, 2025). This enhancement in rock socketing capacity allows engineers to design piles that carry significantly higher axial loads.

To capitalise on these geotechnical advancements, the concrete strength must act as a complementary factor. HA identified a strategic requirement to utilise Grade C60 concrete for two dummy pile trials (TBP01 and TBP02) to be constructed at the Yuen Long South HA public housing development. This transition from the traditional Grade C45 to high-strength Grade C60 tremie concrete presents unique challenges, particularly regarding the high binder content required and the resulting management of hydration heat and rheological stability in deep boreholes.

Despite these challenges, adopting a GGBS-based high-strength concrete offers substantial long-term benefits that align with both sustainability and durability goals. Compared to conventional OPC-dominant mixes, GGBS concrete exhibits superior resistance to chloride ingress, significantly extending the service life of underground structures in Hong Kong’s aggressive marine and urban environments. Its lower hydration heat generation effectively minimises thermal cracking in thick pile caps and deep boreholes, while the refined pore structure and denser matrix reduce long-term crack development under sustained loading. Nevertheless, two practical pain points have historically hindered the widespread adoption of high-volume GGBS concrete in local construction. First, its early-age strength development is slower than that of OPC-dominant mixes, which can potentially affect construction schedules. Second, the inherent stickiness of GGBS concrete increases rheological demands, often leading to longer operation times if the mix design is not carefully optimised. Addressing these two pain points through tailored binder proportions and admixture selection is therefore critical to unlocking the full durability and sustainability potential of Grade C60 GGBS-based tremie concrete.

Fig 1. Site view and positions of TBP01 and TBP02 at Yuen Long South HA public housing development

Material science and specifications

The success of high-volume GGBS concrete hinges on the chemical and physical characteristics of the GGBS. The study emphasised the use of S95 grade GGBS or above. One critical parameter identified was the specific surface area (fineness). As shown in Table 1, a fineness exceeding 400 m2/kg is essential to ensure adequate reactivity and strength development, especially when OPC replacement levels reach 50% or higher.

Table 1. Characteristics of GGBS from different sources

For TBP02, ASTRI utilised superfine GGBS with a specific surface area of approximately 700 m2/kg. As S95 GGBS is ground to superfine, the particles tend to become more rounded with smoother surfaces because their edges are repeatedly eroded and chipped off. The relatively rounded surfaces of superfine GGBS could improve the workability of GGBS-based tremie concrete without compromising the high GGBS content. In addition, this superfine material acts as a micro-filler, refining the pore network and enhancing the density of the Interfacial Transition Zone (ITZ) between the paste and the aggregate.

Fig 2. Microstructure of (a) S95 GGBS and (b) superfine GGBS

Concrete mix design strategy

Two distinct formulations were developed for the trials. TBP01 utilised a 50% S95 GGBS replacement, aimed at achieving cost parity and broad industry applicability. TBP02 incorporated a 65% superfine GGBS replacement, which is a high replacement rate that not only enhanced operational efficiency by improving workability and accelerating early strength gain, but also contributed to a lower heat of hydration and greater cement reduction.

Table 2. Mix formulations for two piles

Rheological control and Tremie placement

Tremie concrete shall have sufficient flowability and stability to be self-levelling and self-compacting under its own weight, resistant to segregation, and capable of displacing drilling fluids. High-volume GGBS mixes are known for their high cohesiveness or high plastic viscosity, which can lead to discharge issues even if the slump value appears acceptable. To address this, the project team moved beyond the conventional in-situ slump acceptance criterion (200±50 mm) and introduced supplementary workability evaluation method.

1. Inverted cone outflow test: According to EFFC/DFI, this test measures the time required for a specific volume of concrete to flow through an inverted slump cone, using the same cone as the slump test. A range of 2–7 seconds was recommended as the acceptance criterion for tremie applications.
2. Slump-flow measurement: This assesses the horizontal spread of concrete and can be performed alongside the slump test, as it simply requires an additional measurement of the concrete spread diameter. A suggested range of 550 ± 125 mm was proposed to ensure adequate flow.

Table 3. In-situ workability results

The results indicated that despite the higher replacement levels, mix for TBP02 exhibited faster outflow times. This is likely attributable to the improved particle packing and reduced inter-particle friction provided by the superfine GGBS, whose particles tend to become more rounded with smoother surfaces as their edges are repeatedly eroded and chipped off during processing—a morphology that enhances workability.

Fig 3. Process of inverted cone outflow test

Fig 4. In-situ slump-flow recording

Table 4. Comparisons of overall concreting efficiency

Note: PBP14, DBP20, 2BP11, and 2BP07 are from different HA projects and are referred to here for reference purposes.

Table 4 list the comparison of the overall concreting efficiency of developed GGBS-based tremie concrete and conventional market tremie concrete. When evaluated against reference mixes with concreting efficiencies ranging from 4.4 to 7.4 m/h, both TBP01 and TBP02 demonstrated competitive and practicable placement performance. TBP01 achieved a concreting efficiency of 4.5 m/h, situating it well within the lower mid‑range of the reference spectrum and closely aligning with conventional mixes such as DBP20 at 4.8 m/h and 2BP11 at 4.4 m/h. TBP02 recorded 4.0 m/h, which remains within a workable onsite range despite its high GGBS replacement. Furthermore, no difficulties were encountered in extracting the bored pile casing during construction for both TBP01 and TBP02, with and without permanent liners respectively. These results confirm that the developed C60 GGBS‑based concretes impose no additional operational burden, directly addressing the concerns of concrete suppliers and contractors regarding workability and placement efficiency.

In-situ thermal measurement and hydration heat

A primary advantage of GGBS in mass concrete foundations is the reduction of peak hydration temperatures. High temperatures in bored piles can lead to thermal cracking, compromising long-term durability. In this trial, seven layers of thermocouples were installed along the depth of both piles, with two thermocouples at each level—one assigned to Series A and the other to Series B—to capture real-time thermal data during the first 28 days of curing.

Table 5. Average peak temperature and temperature rise

The maximum temperature rise in TBP01 and TBP02 peaked at 55.3°C and 47.4°C, which are both lower than the 58°C observed in conventional 25% PFA concrete (Chiu and Luk, 2024). This reduction in peak temperature, with locations of thermocouples TBP01‑A,B and TBP02‑A,B shown in Figure 7, indicates that the GGBS mix effectively mitigates heat buildup, thereby providing a greater safety margin for large‑diameter bored pile construction.

Fig 5. Details of thermocouples along the steel cage

Fig 6. Temperature evaluation of TBP01 and TBP02

Fig 7. Positions of thermocouples TBP01‑A,B and TBP02‑A,B

Structural integrity: Addressing the false alarm

Following construction, Non-Destructive Testing (NDT) via sonic logging was performed through full-length coring. This phase of the study yielded a vital finding for the industry: the sonic logging test may raise false alarm for GGBS-based tremie concrete. At 28 days, sonic logging for TBP01 indicated signal delays (Category C), which often prompt concerns regarding voids or soil inclusions. However, physical core samples from these exact depths showed perfectly intact, high-strength concrete. The observation that rock fragments remain adhered to the bottom of the bored piles provides direct, physical evidence of excellent concrete-to-rock bonds. This confirms that the rock was properly cleaned and that the concrete effectively interlocked with the rock layer, resulting in optimal load-transfer capacity and validating the structural integrity of both TBP01 and TBP02 made with GGBS-based concrete. The false alarm could be due to that the flake-like morphology of GGBS particles and the ongoing pozzolanic reaction at 28 days can scatter ultrasonic waves, mimicking the signature of a defect. When re-tested at 90 days, as the microstructure densified, the signals significantly improved, and the pile was upgraded to Category B with further investigation is not recommended. This case study serves as a critical reference for engineers to exercise caution when interpreting early-age sonic results for high-GGBS mixes, and it is advisable that the engineers should be aware of the potential false alarm phenomenon in early-age sonic logging due to pozzolanic maturation. Interpretation of test results should take into account the maturation period.

Table 6. 28-day Compressive strength of drilled core specimens

Fig 8. Drilled core specimens for full coring test

Fig 9. Interfaces between trial piles and rock layer

Fig 10. Comparison between 28-day and 90-day sonic logging test results

Environmental impact and the Construction Industry Council (CIC) carbon assessment tool

To evaluate the sustainability performance, a Life Cycle Assessment (LCA) was undertaken employing the CIC Green Product Certification (GPC) system, a collaborative framework established by the Construction Industry Council (CIC) and the Hong Kong Green Building Council (HKGBC). This system categorises the carbon emission intensity of submitted concrete mixes and assigns verified embodied carbon values through a standardised, accredited methodology. Emission data were sourced directly from the local concrete supplier under actual production conditions on the day of concreting, ensuring real-world representativeness. All carbon values were subject to independent verification by a licensed auditor and subsequently endorsed by both CIC and HKGBC. This rigorous, certified protocol renders the low‑carbon claims demonstrably verifiable and fully consonant with the HKSAR’s green procurement policy framework.

Table 7. Carbon footprint comparison

Through the incorporation of high-volumes of GGBS – specifically 50% conventional   GGBS in TBP01 and 65% superfine GGBS in TBP02 - the embodied carbon values are significantly attenuated to 216.7 kg CO₂e/m³ and 159.8 kg CO₂e/m³, respectively. These figures represent a pronounced reduction in carbon footprint, corresponding to approximately 24–29% lowering for TBP01 and 44–48% lowering for TBP02 relative to conventional concrete benchmarks. In the context of a housing project requiring 30,000 m³ of tremie concrete, adoption of the TBP01 mix would save around 2,400 tonnes of CO₂ emission - equivalent to the annual carbon sequestration capacity of 108,000 trees. The TBP02 mix would achieve an even more substantial avoidance of over 4,000 tonnes of CO₂, comparable to the yearly sequestration potential of 180,000 trees.

Economic feasibility

Material cost is often the final hurdle for the adoption of new technologies. ASTRI’s study proved that the 50% GGBS mix is cost-neutral compared to conventional PFA mixes.

Table 8. Material cost comparison (as of 2025/26)

Note: the price was provided by local concrete supplier.

While TBP02 carries a 10% premium due to the use of specialty superfine GGBS, this cost is justified by its nearly 50% reduction in embodied carbon, a meaningful trade-off that is expected to become even more favourable as economies of scale improve and more suppliers enter the market.

Conclusion and recommendations

The trials of TBP01 and TBP02 have conclusively demonstrated that Grade C60 high-volume GGBS tremie concrete is a technically robust, environmentally transformative, operatable, and economically viable solution for Hong Kong’s deep foundations.

Key recommendations for stakeholders:

1. Workmanship reference: Technical specifications and QA/QC manuals could be revised to include specification of S95 or above GGBS with a fineness exceeding 400 m²/kg. Inverted cone outflow (2-7s) and slump-flow (550±125 mm) may be adopted as supplementary reference criteria for evaluating the viscosity and flowability of GGBS tremie concrete by the industry.
2. Test interpretation: Engineers should be aware of the potential false alarm phenomenon in early-age sonic logging due to pozzolanic maturation. Interpretation of test results should take into account the maturation period.
3. Widespread adoption: Use the 50% GGBS mix for Grade C60 bored piles to achieve immediate carbon reduction at zero additional cost.
4. No operational concerns: The developed GGBS-based concretes impose no additional operational burden, with placement performance and casing extraction demonstrating practicability comparable to conventional mixes.
5. Cost parity: The cost of GGBS-based high-strength concrete is similar to that of current practice products, enabling carbon reduction without incurring additional material expenditure.

Through the successful cooperation of ASTRI’s material science and HA’s operational scale, Hong Kong is setting a new global benchmark for sustainable deep foundation engineering.

Fig 11(a). Participants and representatives of the industry in the visit for TBP01 on 22 September 2025

Fig 11(b). Participants and representatives of the industry in the visit for TBP01 on 22 October 2025

Acknowledgements

ASTRI acknowledges the 100% project funding provided by the Innovation and Technology Commission (ITC). Sincere gratitude is expressed to the Hong Kong Housing Authority (HA) for their strategic leadership and support throughout the trials. Our appreciation also goes to the main contractor, China State, for their professional collaboration and diligent work on site.

Special recognition is due to our concrete suppliers for their pivotal role in preparing the low-carbon GGBS-based concrete. Hong Kong Concrete expertly produced the developed S95 GGBS mix for the concreting of TBP01, while Quon Hing Concrete manufactured the superfine GGBS mix used in TBP02. Their commitment to quality and innovation was crucial to the project's success.

We are also grateful to Mr Calvin C K Wong, Founder of G.Seed, for providing the superfine GGBS material that enabled the high-replacement-rate mix design in TBP02.

About the authors
Dr Garrison C K CHAU, Yong FAN, Dr Boris J Q WU, Enoch N H WONG from Hong Kong Applied Science and Technology Research Institute, and
Ir Daniel H W LEUNG, Dr Sherman S L YIP, Ir Rayson W H WONG, Ir Dr Mckey C S HO, Ir Sam C K HO from Hong Kong Housing Authority are the authors of the article.

References

1. Chiu, K., & Luk, T. (2024, May 23). Sharing on trial piles with high percentage GGBS concrete on practical and technical aspects [Presentation]. GEO, CEDD, Hong Kong.
2. European Federation of Foundation Contractors (EFFC), & Deep Foundations Institute (DFI). (2018). Guide to tremie concrete for deep foundations (2nd ed.).
3. Geotechnical Engineering Office, Civil Engineering and Development Department. (2025). GEO Technical Guidance Note No. 53 (TGN 53): Supplementary guidelines for foundation design and construction (Issue No. 1, Revision A). The Government of the Hong Kong Special Administrative Region.