Pumped and Jacked Self-Compacting Concrete-Filled Steel Tube Application Research

Literature Overview

This paper by Liu Quan, Shi Jianjun, Zhou Yang, and Li Jianxin, published in the journal Concrete (2014, No. 2), presents a comprehensive summary of twelve applications of pumped and jacked self-compacting concrete (SCC) for concrete-filled steel tube (CFST) structures. The research, conducted at the School of Urban Construction, University of South China, focuses on the practical aspects of SCC placement in CFST applications, including pumping height, mix design optimization, workability characteristics, and mechanical properties. The paper bridges the gap between laboratory research on self-compacting concrete and its actual implementation in structural applications involving steel tubes.

Core Technical Content

The primary objective of this research is to demonstrate that self-compacting concrete can be effectively pumped and jacked into steel tubes to form CFST structural elements, eliminating the need for traditional vibration compaction. This approach offers significant advantages in construction efficiency, labor reduction, and improved concrete quality, particularly for applications where access for vibration equipment is limited.

Mix Design and Workability Optimization

The authors emphasize that the success of pumped and jacked SCC placement in steel tubes depends critically on the concrete mix design. Self-compacting concrete must possess sufficient flowability to fill the tube under its own weight, yet adequate viscosity and stability to prevent segregation during pumping and placement. The following table summarizes the key parameters and their significance:

The authors report that optimizing the SCC mix design — particularly through the selection of appropriate supplementary cementitious materials, superplasticizers, and viscosity-modifying agents — is essential to achieving the required pumping performance without compromising mechanical properties.

Pumping and Jacking Process Analysis

The pumping and jacking method involves placing SCC into a steel tube through a vertical or inclined access point, with the concrete being pumped upward through the tube interior. The jacking component refers to the controlled advancement of the concrete column within the tube, which may involve sequential placement in lifts. The authors analyze twelve case studies, examining the relationship between pumping height, mix design, and placement success.

Key findings include:

• Pumping heights of up to 120 meters were achieved with properly designed SCC mixes.
• The internal diameter of the steel tube significantly affects pumping pressure requirements; smaller diameter tubes require higher pumping pressures.
• The aspect ratio (height-to-diameter ratio) of the CFST element influences the placement strategy and mix design requirements.
• Temperature control during pumping is critical to maintaining workability over extended placement durations.

Quality Control Considerations

From a quality assurance perspective, the pumped and jacked SCC method introduces several unique challenges that require careful management:

• Air voids and voids within the tube: Unlike conventionally vibrated concrete, SCC relies entirely on its own flowability to fill the tube. Incomplete filling results in voids that reduce the composite action between steel and concrete. Ultrasonic testing and radiographic examination should be employed to verify complete filling.
• Bond quality at the steel-concrete interface: The absence of vibration may affect the bond development between the concrete and the inner surface of the steel tube. Surface preparation of the tube interior, including cleaning and possible roughening, is important for ensuring adequate bond.
• Segregation and bleeding: During pumping and jacking, particularly over long distances, the risk of segregation increases. The SCC mix must have sufficient stability to maintain uniformity throughout the pumping process.
• Curing conditions: The confined environment within the steel tube creates unique curing conditions. Heat of hydration may be elevated due to limited heat dissipation, and moisture retention is generally favorable but may lead to excessive internal temperatures.

Engineering Practice Insights

The twelve case studies presented in the paper provide valuable practical data for engineers considering SCC in CFST applications. The authors demonstrate that with proper mix design and process control, pumped and jacked SCC can achieve mechanical properties comparable to conventionally placed concrete in CFST applications. The compressive strength, tensile strength, and elastic modulus of the hardened SCC within steel tubes were found to be within acceptable ranges for structural design.

For steel pipe manufacturers, this research highlights an important application scenario for steel tubes as permanent formwork for concrete columns and structural elements. The steel tube serves a dual purpose: it acts as reinforcement and as a form, while the SCC fills the interior to create a composite structural member. This approach is particularly attractive for applications requiring rapid construction, such as bridge piers, high-rise building columns, and offshore platform legs.

Key Questions and Reflections

Several important questions arise from this research. First, the long-term durability of CFST elements constructed with pumped and jacked SCC, particularly with respect to corrosion protection of the steel tube, requires further investigation. The confined concrete environment may limit oxygen access, but chloride ingress and carbonation could still occur over time. Second, the cost-effectiveness of SCC compared to conventional concrete in CFST applications depends on the balance between material costs (SCC is generally more expensive) and labor savings (elimination of vibration). Third, the scalability of this approach to very large diameter tubes or very tall structures warrants further research.

Study Insights and Implications

This paper provides a valuable practical guide for engineers considering the use of self-compacting concrete in CFST applications. The emphasis on mix design optimization and process control is well-founded, as the success of SCC placement in steel tubes depends critically on these factors. The twelve case studies provide empirical evidence that the pumped and jacked method is viable for a wide range of structural applications. For the steel pipe industry, this research opens up new application possibilities for steel tubes as permanent formwork, expanding the market beyond traditional pipeline and structural tube applications. The key to successful implementation lies in careful attention to mix design, pumping parameters, and quality verification through non-destructive testing.