Jacking Construction Technology for Steel Tube Concrete Columns

Literature Overview

The paper by Tian Wei, Yang Weibo, and Zou Xiande, published in Construction Technology in 2022, presents a comprehensive technical study of the jacking construction method for steel tube concrete (CFST) columns in super-tall buildings. The authors, representing China State Construction Overseas Development Co., Ltd., China Construction Eighth Engineering Division, and CSCEC Egypt Branch, describe a construction technique in which high-performance self-compacting concrete (SCC) is pumped from the bottom of the steel tube column and jacked upward to the desired height without vibration. This method eliminates the need for high-altitude work and significantly reduces construction time. The research is supported by the China State Construction Research Project (CSCEC-2019-Z-22) and the China Construction Eighth Engineering Division Research Project (2019-2-08).

Core Technical Analysis

The jacking method for CFST columns exploits the complementary mechanical properties of steel and concrete: the steel tube provides tensile strength and ductility while the concrete provides compressive strength and fire resistance. The method involves pumping high-performance SCC through the base of the steel tube column under controlled pressure, which forces the concrete upward until it fills the column to the target elevation. The key technical challenge is to maintain sufficient pumping pressure to overcome friction losses and concrete self-weight while preventing bursting of the steel tube under internal pressure.

The bursting pressure verification is the critical safety calculation in this method. The hoop stress induced by internal concrete pressure in the steel tube must remain below the yield strength of the steel divided by an appropriate safety factor. For a thin-walled cylindrical shell under internal pressure, the hoop stress is calculated as σ_h = P × D / (2t), where P is the internal pressure, D is the outer diameter, and t is the wall thickness. The design must ensure that σ_h ≤ f_y / γ, where f_y is the yield strength and γ is the safety factor, typically 1.5 to 2.0 depending on the applicable code.

Process Workflow and Quality Control

The jacking construction process follows a systematic sequence that can be organized using the PDCA cycle to ensure quality at every stage. The process planning phase involves determining the column geometry, selecting the appropriate SCC mix design, calculating the required pumping pressure, and performing the bursting pressure verification. The execution phase involves installing the pumping manifold at the base, connecting the pressure monitoring system, and pumping the concrete under controlled conditions. The checking phase involves monitoring pressure, flow rate, and temperature in real time, and verifying that the concrete level reaches the target elevation. The acting phase involves addressing any deviations from the planned parameters and documenting the results for future projects.

Quality control for the jacked concrete is particularly challenging because the concrete is placed without vibration and without visual access during placement. The primary quality indicators are the pumping pressure profile, the volume of concrete delivered, and the post-placement integrity tests. The pumping pressure should follow a predictable curve that increases with concrete height and decreases slightly as the friction length increases. Any sudden pressure drop may indicate a void or blockage, while any sudden pressure spike may indicate a partial obstruction.

The SCC mix design is critical to the success of the jacking method. The concrete must have sufficient flowability to travel upward through the steel tube without segregation, adequate cohesiveness to maintain a homogeneous mix, and appropriate setting time to allow complete filling before the concrete begins to stiffen. Typical SCC mix designs for jacking applications include a high water-to-binder ratio of 0.35 to 0.45, a superplasticizer dosage of 2.5 to 4.0 percent by cement weight, and a fine aggregate fraction that includes a significant proportion of sand and possibly limestone powder or fly ash to improve flowability.

Structural Verification and Engineering Considerations

The bursting pressure calculation must account for the actual stress state in the steel tube during jacking, which includes the internal concrete pressure, the self-weight of the steel tube, and any external loads such as lateral earth pressure or wind load. The most critical condition typically occurs when the concrete level is near the top of the column, as the friction losses are greatest and the required pumping pressure is highest. The steel tube wall thickness must be selected to withstand this peak pressure condition with an adequate safety margin.

For super-tall buildings, the height of the CFST column can exceed 100 meters, which significantly increases the pumping pressure requirement. The hydrostatic pressure component alone, calculated as P_h = ρ × g × h, where ρ is the concrete density (approximately 2400 kg/m³), g is the gravitational acceleration (9.81 m/s²), and h is the concrete height, can reach 2.4 MPa for a 100-meter column. Adding the friction pressure component, which depends on the concrete viscosity, the tube diameter, and the pumping speed, the total required pressure can exceed 5 MPa. This necessitates the use of high-pressure concrete pumps and robust pumping manifolds.

The interface between the jacked concrete and the steel tube is a critical structural element. The bond strength at this interface depends on the surface condition of the steel tube, the concrete mix properties, and the curing conditions. For jacked concrete, the absence of vibration may result in a slightly lower bond strength compared to vibrated concrete, but the high confining pressure during pumping may partially compensate for this effect. Post-placement ultrasonic testing can be used to verify the integrity of the interface, and pull-out tests on instrumented specimens can quantify the bond strength.

Engineering Practice and Case Study Insights

The jacking method has been successfully applied in several super-tall building projects in China and the Middle East. In one notable application in Egypt, CFST columns with a diameter of 800 mm and a wall thickness of 16 mm were jacked to a height of 45 meters using C60 SCC with a slump flow of 750 mm. The pumping pressure required was approximately 3.2 MPa, and the bursting pressure verification showed a safety factor of 2.1, which was deemed acceptable. The entire column was filled in approximately 90 minutes, compared to an estimated 6 to 8 hours for conventional vibrated placement at that height.

The labor savings from the jacking method are substantial. Traditional CFST column construction requires workers to climb to the top of the column, install formwork, pour concrete through a tremie, and vibrate the concrete. This process is labor-intensive, time-consuming, and poses significant safety risks at height. The jacking method eliminates all of these activities, requiring only ground-level operations for pump setup, pressure monitoring, and quality verification. This reduction in high-altitude work is particularly valuable in regions with strict occupational safety regulations.

However, the jacking method is not without limitations. The concrete must be placed continuously without interruption, as any pause may result in a cold joint or a void at the interface between the two batches of concrete. The pumping equipment must be capable of maintaining a steady pressure over the entire filling duration, and any equipment failure during the process may result in a partial fill that is difficult to remediate. These risks necessitate a comprehensive contingency plan, including backup pumping equipment, a reserve of SCC, and a clear protocol for handling interruptions.

Standards and Code Compliance

The design and construction of CFST columns must comply with the relevant national and international standards. In China, GB 50936-2014 (Technical Code for Concrete-Filled Steel Tubular Structures) provides the primary design and construction requirements. Internationally, EN 1993-1-1 (Eurocode 3: Design of Steel Structures, Part 1-1: General Rules and Rules for Buildings and Bridges) includes provisions for composite columns, and AISC 360 (Specification for Structural Steel Buildings) provides design requirements for composite construction. The jacking method must be validated against these standards to ensure that the as-built column meets the same structural performance as a conventionally constructed column.

The SCC mix design should comply with GB/T 50476-2019 (Standard for Application of Self-Compacting Concrete) or EN 206-1 (Concrete: Specification, Performance, Production and Conformity Assessment) for SCC. The pumping pressure equipment should comply with the relevant pressure vessel standards, and the pressure monitoring instruments should be calibrated and traceable to national standards. Documentation of all quality control activities should follow the project quality management plan and be archived for the duration of the building's service life.

Key Questions and Reflections

A significant question arising from this study is the long-term durability of jacked CFST columns compared to conventionally constructed columns. The absence of vibration may result in a slightly higher air content and a less dense concrete matrix, which could affect the long-term resistance to carbonation, chloride ingress, and freeze-thaw cycling. Accelerated durability tests on jacked concrete specimens should be conducted to compare the performance with vibrated concrete of the same mix design.

Another important consideration is the thermal behavior during pumping. The friction between the concrete and the steel tube wall generates heat, which can raise the concrete temperature significantly during rapid pumping. If the temperature exceeds 40 °C, the rate of hydration accelerates, the setting time decreases, and the risk of thermal cracking increases. The pumping speed must be controlled to limit the temperature rise, and in hot climates, the SCC may need to be cooled before pumping.

Study Insights and Implications

This paper represents a significant advancement in CFST construction technology, offering a practical solution to the challenges of constructing tall composite columns in super-tall buildings. The jacking method reduces construction time, eliminates high-altitude work, and improves worker safety, while maintaining structural performance equivalent to conventional methods. The key to successful implementation lies in the careful selection of SCC mix properties, the rigorous verification of bursting pressure, and the implementation of comprehensive quality control procedures.

The method is particularly well-suited for projects in regions with high labor costs and strict safety regulations, where the economic and safety benefits of reduced high-altitude work are most pronounced. As super-tall building construction continues to grow globally, the jacking method is likely to become an increasingly important construction technique for CFST columns. Future research should focus on optimizing the SCC mix design for jacking applications, developing more accurate models for pumping pressure prediction, and conducting long-term durability studies to confirm the service life of jacked columns.

In conclusion, the jacking construction technology for steel tube concrete columns is a mature and reliable method that offers substantial benefits in terms of construction efficiency, worker safety, and project schedule. Engineers and project managers should consider this method for CFST column construction in super-tall buildings, provided that the technical requirements for SCC mix design, pressure verification, and quality control are rigorously met.