Preparation and Engineering Application of C60 Pumped Jacked Self-Compacting Steel Tube Concrete

Literature Overview

This paper by Chen Jing, Xu Tingbo, Xia Yuanying, and Li Donglai (China Construction Commercial Concrete Co., Ltd., 2014) addresses the preparation and engineering application of C60 pumped jacked self-compacting concrete (SCC) for steel tube concrete columns. Published in the Concrete journal, No. 6, pp. 151-154, the study focuses on solving practical challenges in the pumping and jacking construction of high-strength self-compacting concrete for super-high-rise buildings. The research was conducted in the context of the Chengdu Huazhi Square project.

Core Technical Content

The primary challenge addressed in this study is the simultaneous requirement of high strength (C60), pumpability, self-compacting properties, and long slump retention for jacking construction. In super-high-rise building construction, the jacking method involves lifting the entire upper structure while the lower floors are being constructed, which requires the concrete to maintain its workability over extended periods while also being pumpable to great heights.

The study systematically investigates the effects of sand ratio, different pumping agents, and super-absorbent polymer (SAP) on the workability properties of the C60 self-compacting concrete. The key performance indicators include flow diameter, T500 time, slump flow retention, and pumpability.

The super-absorbent polymer plays a dual role in this application. First, it acts as an internal curing agent, releasing stored water to maintain the concrete's hydration process over an extended period. Second, it helps maintain the concrete's workability by compensating for water loss due to evaporation and absorption by the formwork. This is particularly important for jacking construction, where the concrete may need to maintain its workability for several hours between pumping and placement.

Process Analysis and Technical Challenges

The preparation of C60 self-compacting concrete presents several technical challenges that must be addressed simultaneously:

1. High strength requires a low water-binder ratio, which conflicts with the high workability required for self-compacting properties.
2. Pumpability requires sufficient lubricity and low viscosity, which can be compromised by the high solid content of high-strength concrete.
3. Slump retention over extended periods requires careful selection of admixtures and the use of SAP to prevent premature stiffening.
4. Self-compacting properties require precise control of the concrete's rheological parameters to ensure proper flow and filling without segregation.

The pumping agent selection is critical for achieving the required pumpability. Different pumping agents have different effects on the concrete's rheological properties, and the optimal selection depends on the specific mix design and pumping conditions. The study found that certain pumping agents were more effective than others in maintaining pumpability over extended pumping times and heights.

The sand ratio optimization is another critical parameter. A higher sand ratio generally improves the flowability of self-compacting concrete by providing more fine particles to fill the voids between coarse aggregates. However, an excessively high sand ratio can reduce the strength and increase the risk of segregation. The optimal sand ratio for C60 SCC was found to be in the range of 38-45%, depending on the specific aggregate characteristics and admixture selection.

Engineering Application and Practice Implications

The successful application of the C60 pumped jacked self-compacting concrete in the Chengdu Huazhi Square project demonstrates the practical viability of this approach for super-high-rise construction. The jacking construction method requires that the concrete maintain its workability throughout the entire jacking cycle, which can take several hours. The use of SAP and optimized pumping agents was essential for achieving the required slump retention.

For steel pipe manufacturing and welding quality control, this study has important implications related to the steel tubes used in the concrete-filled steel tube columns:

• The surface quality and dimensional accuracy of the steel tubes affect the bond between the steel and the concrete. Any surface defects, such as rust, oil contamination, or weld spatter, can reduce the bond strength and compromise the composite action.
• The internal cleanliness of the steel tubes is critical for achieving proper concrete filling. Any residual debris, moisture, or contaminants inside the tubes can create voids or weak zones in the concrete core.
• The welding quality of the steel tube connections affects the structural integrity of the concrete-filled steel tube columns. Any weld defects can create stress concentrations that may initiate cracking under load.

The pumping and jacking construction process also places specific requirements on the steel tube geometry and connections. The tubes must be straight and true to allow smooth jacking movement, and the connections must be designed to accommodate the differential movements between the jacked structure and the fixed lower portion.

Key Questions and Reflections

The study raises the question of how the long-term performance of the C60 self-compacting concrete is affected by the jacking construction process. The extended time between mixing and placement, combined with the mechanical stresses from jacking, may affect the concrete's final properties. The internal curing provided by SAP may help mitigate this issue, but long-term monitoring data would be valuable to confirm the durability of the concrete in this application.

Another reflection is the relationship between the concrete properties and the steel tube quality. The composite action between the steel tube and the concrete core depends on the bond strength at the steel-concrete interface, which is influenced by both the concrete properties and the steel tube surface condition. Any degradation in either component can compromise the overall structural performance.

The study also highlights the importance of material compatibility in composite construction. The self-compacting concrete must be compatible with the steel tube material in terms of chemical compatibility, thermal expansion, and mechanical properties. Any incompatibility can lead to interfacial degradation over time.

Study Insights and Implications

This paper provides a practical and well-documented approach to the preparation and application of C60 self-compacting concrete for jacking construction in super-high-rise buildings. The systematic investigation of sand ratio, pumping agents, and SAP demonstrates a rigorous engineering approach to solving complex material challenges. The successful application in the Chengdu Huazhi Square project validates the technical approach and provides valuable reference data for future projects.

For steel pipe manufacturers, the key insight is that the quality of the steel tubes used in concrete-filled steel tube columns is directly linked to the performance of the composite system. The steel tubes must meet strict requirements for surface quality, dimensional accuracy, internal cleanliness, and weld integrity. Quality control procedures should include visual inspection, dimensional measurement, non-destructive testing of welds, and internal surface preparation to ensure proper concrete-steel bond.

The study reinforces the importance of integrated design and construction planning in composite structures. The concrete mix design, steel tube specifications, construction method, and quality control procedures must all be coordinated to achieve the desired structural performance. The successful application of C60 self-compacting concrete in jacking construction demonstrates that this integrated approach is not only feasible but also economically and technically advantageous for super-high-rise building construction.