Application Test Research of Magnesium Oxide-Based Steel Tube Carbon Composite Pile

Literature Overview and Technical Significance

The Magnesium Oxide-Based Steel Tube Carbon Composite Pile (MSCP) represents a novel composite foundation element that integrates a steel tube, magnesium oxide (MgO) cementitious matrix, and carbon fiber reinforcement to achieve enhanced mechanical performance and corrosion resistance. Traditional steel pipe piles, while offering high load-bearing capacity, are vulnerable to corrosion in aggressive environments such as marine conditions, acid mine drainage areas, and de-icing salt zones. Conventional concrete-filled steel pipe piles mitigate corrosion to some extent but suffer from heavy weight, complex construction procedures, and limited durability in severely corrosive environments. The MSCP concept addresses these limitations by replacing conventional concrete with a lightweight MgO-based composite matrix reinforced with carbon fibers, creating a pile element with improved strength-to-weight ratio, superior corrosion resistance, and simplified installation procedures.

Material Composition and Mechanical Characterization

The MSCP system comprises three primary components: a structural steel tube serving as the primary load-bearing element and formwork, an MgO cementitious matrix that provides confinement and corrosion protection, and carbon fiber reinforcement that enhances tensile capacity and crack resistance. The MgO matrix is formulated from dead-burned magnesium oxide powder, magnesium chloride activator, and water, with the addition of fly ash and silica fume to improve long-term stability. The carbon fiber reinforcement is typically incorporated as discrete short fibers with a length of 6–12 mm and a volume fraction of 1.5–3.0 percent.

The mechanical characterization presented in the study includes uniaxial compressive tests on MgO matrix specimens, tensile tests on carbon fiber-reinforced MgO composites, and full-scale axial compression tests on MSCP specimens of varying lengths and steel tube specifications. The results demonstrate that the MSCP achieves an ultimate axial capacity 35–50 percent higher than equivalent conventional steel pipe piles, attributable to the enhanced confinement provided by the MgO matrix and the crack-bridging effect of the carbon fibers. The failure mode transitions from steel tube local buckling in conventional piles to composite crushing in MSCP, indicating a more ductile and energy-absorbing failure mechanism.

Field Application Testing and Performance Evaluation

The study reports field application tests conducted at two construction sites: a marine coastal infrastructure project in southeastern China and an inland industrial facility in a moderately corrosive environment. At the marine site, MSCP specimens with outer diameters of 400 mm and lengths of 12 m were driven into sandy-silt soil at a water depth of 3.5 m. The driving records show that MSCP piles achieved target embedment depths with hammer blow counts consistent with conventional steel pipe piles, confirming that the composite construction does not significantly impede installation. Post-installation testing included low-strain integrity testing (PIT) and high-strain dynamic testing (PDA) to assess pile integrity and load-bearing capacity.

The low-strain integrity test results showed uniform wave propagation characteristics with no significant impedance variations, indicating that the MSCP maintained structural integrity throughout driving. The PDA tests confirmed axial capacities of 6.8 MN and 8.2 MN for two representative piles, exceeding the design requirements by 25 and 35 percent respectively. Long-term monitoring over a period of 18 months included periodic corrosion potential measurements and ultrasonic thickness measurements of the steel tube wall. The results showed negligible corrosion progression, with wall thickness reduction below 0.05 mm over the monitoring period, in stark contrast to the 0.3–0.5 mm annual corrosion rate typically observed for unprotected steel pipe piles in marine environments.

Key Questions and Engineering Considerations

Several practical considerations arise from the MSCP research that warrant careful attention by engineers. The MgO matrix is known to be susceptible to long-term hydrolysis degradation when exposed to prolonged water immersion, which raises durability concerns for submerged pile applications. The study addresses this through the addition of fly ash and silica fume, which react with Mg(OH)₂ to form stable C-S-H and M-S-H phases that inhibit hydrolysis. However, the long-term durability data presented covers only 18 months, and engineers should exercise caution in extrapolating these results to service lives exceeding 50 years. Additionally, the carbon fiber content must be carefully controlled; excessive fiber volume fractions above 3.5 percent can lead to fiber balling during mixing, resulting in inhomogeneous reinforcement distribution and localized weak zones.

Study Insights and Implications

The MSCP concept represents a promising advancement in composite foundation technology, particularly for corrosive environments where conventional steel pipe piles require expensive cathodic protection or heavy protective coatings. The combination of MgO matrix confinement and carbon fiber reinforcement creates a synergistic effect that enhances both compressive capacity and corrosion resistance. For engineering practice, the study provides preliminary but encouraging evidence that MSCP can be successfully installed and perform reliably in field conditions. However, before widespread adoption, additional research is needed on long-term durability under cyclic loading, seismic performance of MSCP under lateral loads, and the environmental impact of MgO production compared to conventional Portland cement. The study serves as a valuable foundation for further development of this innovative pile technology.