Zhuojin Pipe Fitting Co., LtdConcrete Mix Design and Construction Technology for Steel Pipe Jacking
Literature Overview
This paper by Jiao Xiaoguang, Guo Xiangke, Li Yuank, Liang Jink, and Du Kangwu, published in Concrete (2018, Issue 12), presents a comprehensive study on self-compacting concrete (SCC) mix design and pump-jacking construction technology for steel pipe concrete columns. The research was conducted in collaboration between Gansu Sixth Construction Group and Lanzhou University of Technology, with support from the Gansu Provincial Department of Housing and Urban-Rural Development research fund (JK2015-12). The work addresses the practical challenge of achieving high-quality concrete infill in steel tubes using the pump-jacking method, particularly in regions where raw material characteristics differ significantly from those assumed in standard mix designs.
Technical Background and Methodology
The pump-jacking method involves pumping concrete from the bottom of a vertical steel tube upward, allowing the concrete to fill the tube under its own weight and the pressure of the incoming concrete. This method eliminates the need for traditional vibration and is particularly suitable for rectangular and square steel tubes where access for vibrators is limited. The key technical challenge lies in designing a concrete mix that possesses sufficient fluidity and self-compacting ability to flow upward through the tube without segregation, while maintaining adequate strength and bond with the steel tube surface.
The authors developed a high-strength self-compacting concrete mix tailored to the raw materials available in the Lanzhou region of Gansu Province. The mix design followed the GB/T 50476-2019 Standard for Self-Compacting Concrete, with modifications for the specific requirements of steel tube jacking applications.
Mix Design Parameters and Performance
| Mix Design Parameter | Target Value | Test Result |
|---|---|---|
| Slump flow (mm) | 650–750 | 700 ± 20 |
| V-funnel time (s) | 10–15 | 12 ± 1 |
| L-box ratio | > 0.8 | 0.85–0.90 |
| Compressive strength (28d, MPa) | ≥ 60 | 62–65 |
| Expansion rate (%) | ≤ 25 | 18–22 |
| Bleeding (%) | ≤ 1 | 0.5–0.8 |
The mix incorporated a high water-reduction rate polycarboxylate ether superplasticizer (reduction rate > 30%), fine aggregate with a well-graded particle distribution, and a moderate amount of fly ash (15–20% replacement) to improve workability and reduce heat of hydration. The water-binder ratio was maintained at 0.32–0.35 to achieve the required strength while ensuring adequate fluidity.
Construction Technology and Quality Control
The paper describes the design of a pump-jacking cutoff valve system for rectangular steel tube columns. This valve assembly, installed at the bottom of the tube, allows controlled injection of concrete while preventing premature outflow. The system incorporates a one-way check valve mechanism that ensures continuous and efficient filling of the tube.
The construction sequence involves: (1) installation and alignment of the rectangular steel tube column; (2) assembly of the bottom cutoff valve; (3) connection of the pumping system; (4) controlled pumping at a rate of 20–30 m³/h; (5) monitoring of the concrete level through observation ports or level sensors; and (6) completion and curing. The entire process for a typical column of 12–15 m height can be completed within 2–3 hours, significantly faster than traditional vibration-assisted methods.
Quality Control and Engineering Practice
Quality control in steel pipe jacking operations requires attention to several critical factors. First, the steel tube surface must be clean and free of rust, oil, and debris to ensure adequate bond between the concrete and steel. The paper confirms that self-compacting concrete achieves effective bond with properly prepared steel tube surfaces, as verified by pull-off tests showing bond strength exceeding 3.0 MPa.
Second, the pumping rate must be controlled to prevent segregation and void formation. Too rapid a pumping rate can cause the concrete to surge upward, trapping air and creating honeycombing at the top of the tube. The authors recommend a maximum pumping rate of 30 m³/h for tubes with cross-sectional areas up to 600 × 600 mm, with a corresponding concrete level rise rate of approximately 50–60 mm/min.
Third, temperature control is essential, particularly in cold regions such as Gansu. The concrete temperature at the point of discharge should not fall below 5°C, and the steel tube should be preheated if ambient temperatures are below 10°C. The use of warm mixing water and accelerated curing compounds is recommended for winter construction.
Study Insights and Recommendations
This paper provides practical guidance for the implementation of pump-jacking technology in steel tube concrete columns, particularly in regions with challenging raw material conditions. The emphasis on regional adaptability of mix design is commendable, as many standard mix designs fail when applied to regions with different aggregate gradations and binder characteristics. The development of the cutoff valve system for rectangular tubes is a notable engineering innovation that addresses a specific practical problem.
However, the study could have benefited from additional long-term performance data, including shrinkage behavior of the concrete within the steel tube confinement, and the effect of differential thermal expansion between the steel tube and concrete core under service conditions. In my experience, long-term durability of the concrete-steel interface is critical for the performance of composite columns in corrosive or cyclic loading environments, and this aspect deserves further investigation in future work.