Axial Compression Test and Mechanical Performance Analysis of CFRP-Constrained Square Steel Tube Lithium Slag Concrete Short Columns

Literature Overview

This research investigates the axial compressive behavior of a novel composite column system that combines carbon fiber-reinforced polymer (CFRP) wrapping, square steel tube confinement, and lithium slag concrete infill. Lithium slag, a by-product of lithium-ion battery production, is used as a partial replacement for Portland cement or as a supplementary cementitious material, addressing the growing waste management challenge in the battery industry while improving concrete performance. The CFRP wrapping provides additional confinement beyond what the steel tube alone can deliver, creating a multi-layer confinement system that enhances the triaxial stress state within the concrete core. This study represents an interdisciplinary approach combining advanced polymer composites, recycled industrial waste utilization, and structural engineering principles.

Material Characterization and Properties

Lithium slag exhibits pozzolanic activity due to its amorphous silica and alumina content, reacting with calcium hydroxide produced during cement hydration to form additional calcium silicate hydrate (C-S-H) gel. The lithium slag concrete mix design incorporates 20–30% lithium slag as a replacement for ordinary Portland cement by mass.

The CFRP wrapping consists of unidirectional carbon fiber fabric (E-glass or T700 carbon fiber) with epoxy resin adhesive, applied in 2–3 layers with a total thickness of approximately 0.5–1.5 mm. The CFRP tensile strength ranges from 2500 to 3500 MPa, with an elastic modulus of 140–230 GPa.

Mechanical Performance and Confinement Mechanism

The axial compression tests on 24 specimens reveal that the CFRP-constrained square steel tube lithium slag concrete columns achieve ultimate bearing capacities 25–45% higher than equivalent columns without CFRP wrapping. The confinement pressure provided by the CFRP wrapping is calculated using the equilibrium equation p_cfrp = 2 × E_cfrp × t_cfrp × ε_cfrp / D, where ε_cfrp is the CFRP strain at failure and D is the column dimension.

The multi-layer confinement system creates a progressive confinement mechanism: the steel tube provides initial confinement through its hoop stiffness, while the CFRP wrapping engages at higher strains when the concrete dilation exceeds the steel tube's elastic deformation capacity. This sequential engagement results in a more gradual post-peak load degradation compared to steel tube-only confinement.

The failure mode involves CFRP debonding from the concrete surface at the column corners, followed by steel tube local buckling and concrete crushing. The CFRP failure strain is typically 1.5–2.5%, significantly lower than the concrete strain at peak load, indicating that CFRP debonding rather than CFRP rupture governs the ultimate capacity in most cases.

Load-Displacement Behavior and Energy Absorption

The load-displacement curves exhibit three distinct stages: a linear elastic stage up to 40% of the peak load, a non-linear hardening stage from 40% to 100% of peak load where the confinement mechanisms progressively engage, and a softening stage beyond peak load where the CFRP debonds and the steel tube buckles. The energy absorption capacity, calculated as the area under the load-displacement curve, increases by 30–50% compared to non-CFRP specimens, indicating improved ductility and damage tolerance.

Study Insights and Practical Considerations

The research demonstrates that the combination of CFRP wrapping with steel tube confinement and lithium slag concrete creates a synergistic system where each component compensates for the limitations of the others. The lithium slag concrete provides adequate strength and workability while utilizing industrial waste, the steel tube provides robust confinement and fire resistance, and the CFRP wrapping enhances confinement capacity with minimal added weight. However, engineers must address the durability of CFRP wrapping in harsh environments, as moisture ingress and UV degradation can compromise the polymer matrix over time. Protective coatings or alternative confinement materials such as basalt fiber-reinforced polymer (BFRP) may be considered for outdoor applications. The cost of CFRP materials remains a practical concern, but the potential for reduced concrete volume and improved service life may justify the additional expense in critical infrastructure applications. This study opens new possibilities for sustainable composite column design that integrates waste valorization with advanced material technology.