Mechanical Properties of Rectangular CFT Bent Members with High-Dosage High-Calcium Fly Ash

Literature Overview

The paper by Zhang Yang and Li Sipeng (2022), published in the journal Concrete, investigates the flexural behavior of steel tube concrete (CFT) bent members featuring a single-axisymmetric non-uniform wall thickness rectangular cross-section, filled with concrete containing high dosages of high-calcium fly ash. The study is supported by the National Natural Science Foundation of China and addresses two important engineering trends: the increasing use of fly ash as a cement replacement material for sustainability and cost reduction, and the adoption of optimized cross-sectional geometries that make more efficient use of structural steel.

Technical Background

Fly ash, a byproduct of coal combustion, has been widely used as a supplementary cementitious material (SCM) in concrete for decades. However, most research has focused on low-calcium fly ash (Class F), while high-calcium fly ash (Class C) is less studied despite its widespread availability in certain regions. High-calcium fly ash contains 20–60% free lime and exhibits different pozzolanic and hydraulic properties compared to Class F fly ash. The study uses unprocessed, low-quality high-calcium fly ash, which represents a more practical and economical scenario for engineering applications.

The single-axisymmetric non-uniform wall thickness rectangular cross-section is a relatively novel concept in CFT design. Unlike conventional equal-wall-thickness sections, this geometry places thicker steel at locations that experience higher stresses under bending, thereby improving material utilization efficiency.

Experimental Program and Key Results

The experimental program included specimens with four fly ash replacement levels: 0%, 50%, 75%, and 100%. The specimens were loaded in bending to failure, and the load-deflection curves, cracking patterns, and failure modes were recorded. The key findings are summarized as follows:

1. The ratio of the compression flange yielding moment to the tension flange yielding moment decreases with increasing fly ash dosage. This indicates that high fly ash content reduces the concrete's tensile capacity more significantly than its compressive capacity, which is consistent with the known lower tensile strength of fly ash concrete.
2. Specimens with 50% and 75% fly ash replacement exhibited flexural capacities that were only 9.7% and 10.1% lower than the reference specimens, respectively. These modest reductions suggest that high-dosage high-calcium fly ash concrete is viable for flexural CFT members.
3. The 75% fly ash specimen exhibited stronger confining action from the steel tube on the core concrete. This finding is somewhat counterintuitive and may be attributed to the different deformation characteristics of fly ash concrete, which may develop larger lateral strains under axial compression, thereby inducing higher confining pressures in the steel tube.

Implications for Steel Tube Fabrication and Welding

The use of non-uniform wall thickness rectangular cross-sections has direct implications for steel tube fabrication. These sections require either roll-forming with variable die profiles or welding of plates of different thicknesses. In the welding approach, the butt welds between thick and thin plates introduce challenges related to heat input control, preheating requirements, and residual stress management. The thickness differential can lead to asymmetric cooling rates and increased angular distortion.

From a welding metallurgy perspective, the heat-affected zone (HAZ) of the thicker plate will experience slower cooling rates, potentially leading to coarser grain structures and reduced toughness. The thinner plate, conversely, may experience higher cooling rates and increased hardness. The weld joint thus represents a transition zone with variable mechanical properties that must be carefully controlled through welding procedure qualification.

Engineering Practice Considerations

The study's findings have practical significance for sustainable construction. Using high-dosage fly ash concrete in CFT members reduces cement consumption, which in turn lowers CO2 emissions associated with cement production. The 10% reduction in flexural capacity at 75% replacement is often acceptable in engineering design, particularly when design margins are generous. However, the long-term durability of high-fly-ash concrete in aggressive environments requires further investigation, as the free lime content may influence carbonation resistance and sulfate attack susceptibility.

The confining effect of the steel tube on high-fly-ash concrete is particularly important because it suggests that CFT members can compensate for the reduced concrete strength through enhanced composite action. This is a valuable insight for designers considering the use of alternative concrete mixes in CFT structures.

Study Insights and Implications

This research contributes to the growing body of evidence supporting the use of supplementary cementitious materials in structural applications. For steel pipe manufacturers and welders, the key message is that the choice of concrete mix design influences the fabrication and welding requirements of the steel tube. Non-uniform cross-sections demand higher fabrication precision and welding skill, while the concrete's deformation characteristics affect the long-term composite behavior of the CFT member. The study demonstrates that with appropriate design considerations, high-dosage fly ash concrete can be successfully used in CFT flexural members without significant compromise in structural performance.