Effect of Pressurization Conditions on Physical and Mechanical Properties of Concrete Inside Steel Pipes

Literature Overview and Research Motivation

This research examines how internal pressurization conditions applied to concrete-filled steel pipes (CFST) influence the resulting physical and mechanical properties of the concrete core. The pressurization technique is employed to improve concrete density, reduce porosity, and enhance bonding between the steel shell and the concrete infill. From a metallurgical and materials science perspective, the interaction between hydrostatic pressure, concrete microstructure, and the steel pipe's elastic deformation constitutes a complex coupled problem with significant implications for structural performance.

Core Technical Content

Pressurization Methodology

The pressurization process involves applying internal hydraulic pressure to the concrete within the steel pipe during or after initial setting. Typical pressurization pressures range from 0.5 MPa to 5.0 MPa, applied either during the plastic stage (before initial set) or during the hardening stage (after final set). The duration of pressure application varies from 2 hours to 24 hours depending on the specific protocol. The steel pipe acts as a rigid or semi-rigid container, with its elastic deformation accommodating the volumetric changes in the concrete under pressure.

Pressurization Parameter	Range	Primary Effect
Pressure magnitude	0.5–5.0 MPa	Higher pressure improves density
Application timing	Plastic stage to 24h post-set	Plastic stage yields greater improvement
Duration	2–24 hours	Diminishing returns beyond 8 hours
Pipe material	Q235, Q345, Q460	Higher grade reduces elastic deformation
Concrete type	Ordinary, high-strength, lightweight	Lightweight concrete benefits most

Effects on Physical Properties

Pressurization significantly reduces concrete porosity, with reductions of 15%–40% observed depending on pressure level and application timing. The density of the concrete increases by 1%–5% due to the expulsion of entrapped air and water from the pore structure. The water-to-binder ratio effectively decreases as water is forced into the cement paste, resulting in a denser microstructure. The steel pipe inner surface may exhibit slight elastic deformation under high internal pressure, with radial expansion calculated from the hoop stress formula: σ_θ = p·r/t, where p is internal pressure, r is inner radius, and t is wall thickness.

Effects on Mechanical Properties

Compressive strength improvements of 10%–35% are documented across various pressure levels. The enhancement is attributed to reduced void content, improved aggregate-paste bonding, and densification of the interfacial transition zone (ITZ). Tensile strength increases are proportionally greater, typically 20%–50%, reflecting the sensitivity of tensile properties to microcrack density. The elastic modulus of the pressurized concrete increases by 8%–25%, indicating stiffer material response. The confinement effect of the steel pipe further amplifies the beneficial outcomes, as the pipe prevents lateral expansion of the concrete under internal pressure.

Process and Standards Analysis

The pressurization process must comply with relevant standards for concrete-filled steel tube fabrication. GB/T 51248 provides guidance on CFST construction, while SY/T 0413 addresses pipe testing procedures. The welding of pipe segments prior to concrete filling must meet NB/T 47013 inspection requirements, with full radiographic testing recommended for pressure-critical applications. Hydrostatic testing per GB/T 241-2008 should be performed post-filling to verify structural integrity. The metallurgical condition of the steel pipe—particularly the heat-affected zone near welded joints—must be verified to ensure that the pressurization process does not induce plastic deformation or residual stress concentrations.

Defect Analysis and Countermeasures

Defect Type	Cause	Detection Method	Countermeasure
Pipe bulging	Excessive internal pressure	Visual + UT	Limit pressure to 0.8×p_yield
Concrete segregation	Uneven pressure distribution	UT + core sampling	Use vibrator during filling
Steel-concrete debonding	Rapid pressure release	MT + acoustic emission	Gradual depressurization
Pipe wall thinning	Localized stress concentration	UT thickness measurement	Reinforce at joints

Engineering Practice Implications

The pressurization technique offers a practical pathway to enhance CFST performance without altering material compositions or increasing steel quantities. In offshore platforms and bridge piers where CFST columns are subjected to high compressive loads, pre-pressurization during fabrication can yield measurable strength gains. However, the technique introduces additional process control requirements, including pressure monitoring, temperature control, and careful depressurization protocols. Engineers must weigh the benefits of enhanced concrete properties against the increased fabrication complexity and quality control demands.

Study Insights and Reflections

The research demonstrates that controlled pressurization is a viable post-processing technique for improving CFST performance. The optimal pressure window appears to be 2.0–3.0 MPa applied during the early hardening stage, balancing property enhancement against fabrication practicality. Future work should explore automated pressure control systems and real-time monitoring of concrete densification to establish reliable process windows for industrial-scale production.