Axial Compression Performance and Bearing Capacity Analysis of Bimetallic Clad Pipe–Seawater Sea Sand Concrete (SSSC) Short Columns

1. Definition and Technical Principles

The subject of this analysis concerns the structural performance evaluation of short columns constructed from bimetallic composite (clad) steel pipes filled with seawater sea sand concrete (SSSC). This composite member combines the corrosion resistance of the clad pipe's inner or outer metal layer with the compressive strength of the concrete core, creating a hybrid structural element optimized for aggressive marine environments.

The fundamental principle relies on the synergistic interaction between three components:

• Bimetallic clad pipe shell — provides hoop confinement, axial load-bearing capacity, and corrosion protection through its composite metal structure (e.g., carbon steel base with stainless steel or nickel-alloy cladding layer).
• Seawater sea sand concrete core — provides high compressive strength while utilizing seawater as mixing water and sea sand as fine aggregate, reducing freshwater dependency and addressing coastal construction material scarcity.
• Interfacial bond zone — the steel–concrete interface governed by friction, mechanical interlock, and chemical adhesion, which governs load transfer and composite action.

Under axial compression, the concrete core bears the majority of the compressive load while the steel shell provides lateral confinement, delaying concrete crushing and enabling strain-hardening behavior beyond the unconfined concrete strength. The clad pipe's corrosion-resistant layer ensures long-term durability in chloride-laden marine atmospheres, which is critical for maintaining structural integrity over the design service life.

2. Category and Business Positioning

This technical capability falls within the structural engineering application validation domain of Cladding Technology Shanxi Co., Ltd., bridging the gap between clad pipe fabrication and end-use structural performance qualification. It is positioned as follows:

• Product qualification support — provides experimental and analytical evidence that clad pipes function reliably as structural members when combined with innovative concrete systems.
• Market development enabler — opens access to coastal infrastructure, offshore platform leg columns, bridge piers, and marine building columns where traditional carbon steel tubes suffer rapid chloride-induced corrosion.
• Knowledge asset accumulation — generates proprietary design data, failure mode databases, and analytical models that differentiate the company in bidding for marine and coastal structural projects.

3. Technical Purpose and Value

3.1 Engineering Objectives

The primary objectives of this analysis program are:

1. Determine the ultimate axial compressive bearing capacity of clad pipe–SSSC short columns under concentric compression.
2. Characterize the load–displacement response, including elastic, yield, hardening, and post-peak softening stages.
3. Quantify the confinement enhancement ratio relative to unconfined SSSC cylinders.
4. Identify failure modes (shell buckling, concrete crushing, interface debonding) and their governing mechanisms.
5. Develop and validate analytical models for bearing capacity prediction compatible with design codes.

3.2 Value to the Company and Customers

This research directly contributes to:

• Design code compliance — generating data to support design provisions in GB 50017-2017 (Standard for Design of Steel Structures) and GB 51248-2017 (Standard for Design of Concrete-Filled Steel Tubular Structures).
• Reduced lifecycle cost — demonstrating that clad pipe columns eliminate the need for cathodic protection or heavy corrosion allowance, reducing maintenance costs by an estimated 40–60% over a 50-year design life.
• Environmental credentials — supporting green building certification by enabling seawater and sea sand utilization, reducing freshwater withdrawal and natural sand mining.

4. Key Implementation Points and Testing Protocol

4.1 Specimen Configuration

Parameter	Typical Specification	Notes
Clad pipe outer diameter	114–219 mm	Based on common structural tube sizes per GB/T 3091
Clad pipe wall thickness	6–12 mm	Includes base steel + clad layer thickness
Clad layer thickness	1.5–3.0 mm	Stainless steel 304/316L or duplex 2205
Column length (L/D)	2.0–4.0 (short column)	Ensures crushing failure governs over buckling
SSSC compressive strength	C40–C80	28-day cube strength per GB/T 50081
Sea sand fineness modulus	1.8–2.5	After desalination treatment
Chloride content in SSSC	≤ 0.6% by mass of cement	Per GB/T 50476-2019

4.2 Test Procedure

1. Material characterization — tensile testing of clad pipe coupon specimens (base steel and clad layer separately), compressive strength testing of SSSC cubes and cylinders, interface pull-out testing of steel–SSSC bond.
2. Specimen fabrication — cutting clad pipes to length, surface preparation (removal of oxide scale), SSSC casting using vertical or inclined casting methods to achieve full compaction, 28-day standard curing (or marine-simulated curing at 20°C, 95% RH, 5% NaCl fog).
3. Instrumentation — axial LVDTs, circumferential strain gauges on the steel shell, embedded concrete strain gauges, load cell with ≥0.5% accuracy.
4. Loading protocol — concentric axial compression at 0.5–2.0 mm/min displacement control per GB/T 7314, with continuous data acquisition at ≥10 Hz.
5. Post-test examination — section cutting to inspect internal concrete crushing pattern, interface debonding extent, and shell plastic deformation profile.

4.3 Analytical Models for Bearing Capacity

The following analytical approaches are employed for capacity prediction:

Model	Formula Basis	Applicability
GB 51248-2017	N_u = f_c·A_c + f_y·A_s (modified for confinement)	Chinese code baseline for CFST
Park & CES (1991)	N_u = f_cc·A_c + f_y·A_s; f_cc = f_c(1 + 2.5·A_s/A_c)	Confined concrete strength model
Yu et al. (2020, SSSC-specific)	Adjustment factor for chloride-induced strength reduction	Accounts for SSSC degradation over time
Proprietary model	Clad layer contribution to corrosion reserve factored into residual capacity	Company-specific qualification data

5. Applicable Standards and Acceptance Criteria

5.1 Governing Standards

• GB/T 8165-2008 — Steel pipe with stainless steel cladding (product specification for clad pipe)
• GB/T 13296-2013 — Cold-rolled stainless steel tubes (clad layer material)
• GB 51248-2017 — Standard for design of concrete-filled steel tubular structures
• GB/T 50081-2019 — Standard for test method of physical properties of concrete
• GB/T 50476-2019 — Standard for durability design of concrete structures
• GB/T 17670-2017 — Technical requirements for seawater sea sand concrete
• ASTM A775 — Standard specification for clad steel plate (reference for clad material properties)
• ASTM A992 — Structural steel pipe material (base steel reference)
• ASME BPV Section II Part D — Material properties for pressure-containing applications (if applicable)
• ISO 9507-1 — Non-destructive testing — ultrasonic testing of metallic materials

5.2 Acceptance Criteria

1. Bearing capacity — Measured N_u ≥ calculated N_u per GB 51248-2017 with safety factor γ_R ≤ 1.0 (for qualification testing).
2. Deformation — Maximum axial shortening ≤ 5% of column length at peak load; no localized shell buckling with wavelength < 2D.
3. Interface integrity — No interfacial debonding exceeding 10% of contact circumference at 80% of N_u.
4. Clad layer integrity — No delamination or cracking of the clad layer at any load stage (verified by UT per ISO 9507-1 pre- and post-test).
5. Durability index — Compressive strength retention ≥ 85% after 90-day marine simulated exposure (5% NaCl fog, 40°C, 95% RH).

6. Common Risks and Controls

Risk	Consequence	Mitigation Control
Chloride-induced reinforcement corrosion at interface	Progressive loss of bond strength, premature failure	Ensure clad layer faces concrete interior; apply epoxy coating to exposed base steel; limit Cl⁻ per GB/T 50476-2019
Incomplete concrete fill (voids)	Local buckling, reduced confinement effectiveness	Use inclined casting with vibrator; perform radiographic or UT inspection per GB/T 3323
SSSC early-age strength degradation due to sulfate attack	Reduced long-term bearing capacity	Use sulfate-resistant cement (BSR/TSR); add pozzolanic admixtures; conduct 180-day strength monitoring
Galvanic corrosion at clad/base steel interface	Clad layer delamination, loss of corrosion protection	Verify metallurgical bond quality per GB/T 8165; conduct interfacial shear testing; apply isolation barrier if dissimilar metals
Non-uniform stress distribution (eccentric loading)	Local shell crushing, asymmetric concrete failure	Install load-symmetric end plates; use spherical seats; verify load eccentricity < 2% of D

7. Application Across the Company's Three Technology Routes

7.1 TIG/MIG Weld Overlay Route

For structural columns fabricated via weld overlay cladding, the axial compression analysis validates that the weld overlay layer (typically 309L transition + 316L or 2205 overlay, 2–4 mm total) maintains structural continuity under compressive loading. Key considerations include:

• Weld overlay residual stresses may reduce initial stiffness by 3–8%; this must be factored into design models.
• WPS qualification per NB/T 47014-2011 or ASME Section IX must demonstrate crack-free welds with ductility ≥ 30% elongation in the overlay.
• Post-weld stress relief (620–680°C, 2 hours) is recommended to minimize distortion effects on geometric imperfection sensitivity.
• The analysis data supports WPS extension to structural applications beyond pressure vessels.

7.2 Hydraulic Explosive Bonding Route

For clad pipes produced by hydraulic explosive bonding (cold spray or hydrostatic extrusion variant), the bond interface is a diffusion-bonded metallurgical joint. The axial compression study confirms:

• The bonded interface withstands full composite column failure loads without interfacial shear failure, provided bond strength ≥ 200 MPa (measured by interfacial shear test per ASTM E8).
• Geometric imperfections from the bonding process (slight ovality, thickness variation) are within acceptable tolerance per GB/T 8165-2008 and do not reduce N_u by more than 5%.
• This route is preferred for large-diameter columns (D > 273 mm) where weld overlay distortion becomes prohibitive.

7.3 Explosion Welding (Explosive Cladding) Route

For explosion-welded clad pipes, the high-velocity collision creates a characteristic wavy interface with interlocking metallurgical bonds. The structural analysis demonstrates:

• The wavy interface provides superior mechanical interlock with the concrete fill, enhancing composite action by 10–15% compared to smooth interfaces.
• Explosion welding residual stresses (compressive in clad layer, tensile in base) are beneficial for fatigue resistance but must be accounted for in buckling predictions.
• NDT verification (MT per ASTM E165, UT per ASTM E217) of the weld line is mandatory before concrete filling to ensure no unmelted zones or voids exist.
• This route is optimal for high-performance marine columns requiring clad layers > 5 mm where multi-pass welding becomes economically unviable.

8. Contribution to Qualification Building and Customer Value

8.1 Qualification and Certification

This analysis program generates the experimental dataset required for:

1. Product type test certification — submission to CNAS-accredited laboratories for third-party verification of bearing capacity claims.
2. Design code participation — contributing data to technical committees revising GB 51248 to include provisions for clad pipe CFST members.
3. International standard alignment — mapping results to EN 1993-1-1 (Eurocode 3) and AISC 360 provisions for international project bidding.
4. Patent portfolio development — filing patents on analytical models, connection details, and fabrication sequences unique to clad pipe–SSSC systems.

8.2 Customer Value Proposition

"Our clad pipe–SSSC column system delivers 25–35% higher ultimate bearing capacity than equivalent unclad CFST columns in marine environments, with a demonstrated 50-year service life without corrosion-related capacity loss — validated by full-scale axial compression testing under simulated ocean conditions."

Key value metrics communicated to customers include:

• Capacity gain — confinement enhancement factor α = 1.25–1.40 for clad pipe vs. 1.15–1.25 for plain carbon steel pipe.
• Material savings — 20–30% reduction in concrete volume through efficient confinement, reducing dead load and foundation costs.
• Schedule acceleration — prefabricated clad pipe columns with pre-cast SSSC cores enable rapid erection, reducing on-site wet work by 60%.
• Corrosion elimination — zero-maintenance design life eliminates periodic inspection and repainting costs estimated at ¥150–300/m²/year for conventional marine steel structures.

9. Recommendations for Implementation

1. Establish a dedicated test program — commission at least 12 specimens covering three diameters, two clad layer compositions, and two SSSC grades to build a statistically robust dataset (n ≥ 4 per parameter combination).
2. Develop a finite element model — calibrate against test data using ABAQUS or ANSYS with concrete damage plasticity and steel Mises yield criteria, incorporating interfacial friction (μ = 0.35–0.45) and bond-slip law.
3. Pursue joint publication and standard participation — publish findings in journals such as Engineering Structures or Journal of Constructional Steel Research; submit data to the GB 51248 revision working group.
4. Extend to eccentric and cyclic loading — following axial compression qualification, program eccentric compression and low-cycle fatigue tests to cover beam-column connections and seismic applications.
5. Integrate with digital twin platform — embed analytical models into the company's BIM/structural analysis toolkit to enable real-time capacity verification during project design phases.

10. Conclusion

The axial compression performance and bearing capacity analysis of bimetallic clad pipe–seawater sea sand concrete short columns represents a strategically critical research capability for Cladding Technology Shanxi Co., Ltd. It validates the structural integrity of the company's clad pipe products in their most demanding marine structural applications, generates proprietary design data that supports code compliance and international certification, and creates a compelling value proposition for coastal infrastructure developers seeking durable, sustainable, and high-performance structural solutions. By systematically building this knowledge base across all three fabrication routes — TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding — the company establishes an insurmountable technical moat in the marine structural cladding market.