Application Research on Pump-Jacking Self-Consolidating Concrete-Filled Steel Tubes

Literature Overview

This paper by Liu Quan, Shi Jianjun, Zhou Yang, and Li Jianxin from the School of Urban Construction, University of South China, published in Concrete (2014, No. 2, pp. 156–160), presents a comprehensive study on the application of pump-jacking self-consolidating concrete (SCC) for filling steel tubes in CFST structures. The research summarizes the performance of twelve structures constructed using the pump-jacking method and analyzes key parameters including jacking height, mix design, workability, and mechanical properties.

Core Technical Analysis

Pump-Jacking Method for CFST Construction

The pump-jacking method involves pumping self-consolidating concrete through the steel tube from the bottom, with the concrete rising under its own weight and pump pressure until the tube is completely filled. This method eliminates the need for internal vibration and is particularly suitable for tall, slender steel tubes where conventional pouring methods are impractical.

Parameter	Typical Range	Optimization Target
Jacking height	15–60 m	Maximize without segregation
SCC slump flow	650–800 mm	Ensure flowability through tube
V-funnel time	3–8 s	Balance workability and stability
Concrete grade	C40–C60	Match structural requirements
Steel tube diameter	400–1200 mm	Affects pumping pressure

Mix Design Optimization

The paper emphasizes that achieving successful pump-jacking of SCC requires careful optimization of the concrete mix design. Key considerations include:

Binder content: Higher cementitious content improves flowability but increases cost and heat of hydration.
Supplementary cementitious materials: Fly ash and silica fume improve workability retention and reduce segregation.
Water-reducing admixtures: High-range water reducers (HRWR) and viscosity-modifying agents (VMA) are essential for achieving the required rheological properties.
Aggregate gradation: Well-graded aggregates with appropriate maximum size reduce pumping pressure and minimize blockage risk.

Performance Analysis of Twelve Structures

The study of twelve applications revealed several important findings:

Jacking height is the primary limiting factor, with successful applications achieved up to approximately 60 meters.
SCC workability must be maintained throughout the pumping process, requiring careful control of ambient temperature and pumping duration.
Mechanical properties of the hardened concrete are generally satisfactory, with compressive strengths meeting or exceeding design requirements.
The pump-jacking method produces dense, well-compacted concrete fills with minimal voids compared to conventional methods.

Process Analysis and Quality Control

Construction Procedure

The pump-jacking process for CFST construction follows a defined sequence:

Steel tube fabrication and erection with bottom seal and top filling port
SCC mix design verification through laboratory testing (slump flow, V-funnel, L-box, flow table)
Pre-pumping checks including tube cleanliness, seal integrity, and equipment readiness
Pumping operation with continuous monitoring of pressure, flow rate, and concrete temperature
Post-pumping verification through internal inspection (camera or UT) to confirm complete filling

Common Defects and Countermeasures

Defect Type	Cause	Countermeasure
Incomplete filling	Insufficient pump pressure or seal failure	Increase pressure; improve sealing
Concrete segregation	Poor mix design or excessive pumping distance	Optimize VMA dosage; reduce pumping height
Voids or honeycombing	Air entrapment or premature stiffening	Maintain flowability; control pumping rate
Surface defects	Poor concrete-tube bond	Ensure tube cleanliness; use proper SCC rheology
Cracking	Excessive heat of hydration or restraint	Use low-heat cement; control placement temperature

Quality Assurance Measures

Quality control for pump-jacking SCC CFST construction requires:

Pre-pumping laboratory tests to verify SCC properties meet specification requirements
In-process monitoring of pumping parameters (pressure, flow rate, temperature)
Post-pumping non-destructive testing (UT or internal camera inspection) to verify complete filling
Core sampling or mechanical testing to confirm hardened concrete properties

Engineering Practice Integration

The pump-jacking method offers significant advantages for CFST construction in challenging environments:

High-rise applications: Enables filling of tall, slender steel tubes without internal access for vibration.
Complex geometries: Suitable for curved or inclined tubes where conventional pouring is difficult.
Quality consistency: SCC eliminates vibration-related defects and produces uniform concrete fills.
Construction speed: Faster placement compared to conventional methods with reduced labor requirements.

However, the method also presents challenges:

Higher material costs due to SCC admixture requirements
Limited pumping distance and height
Sensitivity to ambient conditions (temperature, wind, rain)
Requirement for specialized pumping equipment and trained personnel

Study Insights and Reflections

This paper provides valuable practical experience with the pump-jacking method for CFST construction. The analysis of twelve real-world applications demonstrates that the method is technically feasible and produces satisfactory structural performance when properly executed. The emphasis on mix design optimization as the key to successful pump-jacking is particularly important for engineers considering this construction method.

The research highlights that SCC properties must be carefully tailored to the specific application requirements. For tall structures, greater emphasis should be placed on stability (segregation resistance) rather than flowability, while for short applications, higher flowability may be beneficial. The balance between these properties is achieved through careful selection of admixture types and dosages.

For engineers planning CFST projects, the pump-jacking method should be evaluated against conventional construction methods based on project-specific factors including structure height, geometry, site conditions, and cost considerations. The method offers clear advantages for tall, slender tubes but may not be economical for shorter applications where conventional pouring is practical.