Restoring corrosion-damaged pile bents with epoxy injection and CFRP confinement.

The problem.

Florida operates one of the largest inventories of marine prestressed-concrete pile bents in the United States. Decades of saltwater exposure drive chloride ingress through the cover concrete to the prestressing steel, where it depassivates the strand and initiates corrosion. The resulting expansion cracks the cover, exposes more steel, and the cycle accelerates. By the time the damage is visible at low tide, the load-carrying section has already lost area, and the residual capacity of the bent is no longer the capacity that was designed.

The conventional response is invasive: pile jacketing with cast-in-place concrete cofferdams, partial pile replacement, or full bent replacement. These approaches work, but they are slow, expensive, and require lane closures and marine equipment. For owners with hundreds of bents on aging structures and finite maintenance budgets, the question worth asking is whether a less invasive intervention can restore enough lateral capacity to extend the structure's service life by another planning cycle.

The repair approach.

The repair concept tested in this study combines two well-known techniques into one compatible system. Structural-grade epoxy is injected through bored ducts to fill the corrosion-induced cracks and re-bond the cover concrete to the core. A wet lay-up carbon fiber reinforced polymer (CFRP) jacket is then applied around the perimeter to confine the section against further dilation and to add tensile capacity at the surface where it is most useful in bending.

Each technique on its own has a documented track record. Epoxy injection is standard practice for closing cracks and re-establishing monolithic behavior in distressed reinforced and prestressed concrete. CFRP wet lay-up is widely used for column confinement, beam shear retrofit, and slab strengthening. The research question was whether the two combined would restore not just stiffness but the full lateral load envelope of a pile bent, including the post-yield behavior that governs how a bent performs under wave, ship-impact, or seismic loading.

The lab program.

A one-third scale prestressed-concrete pile bent specimen was constructed at the USF structures lab under Dr. Gray Mullins. The geometry mirrored a typical Florida bridge pile bent, scaled down to fit the actuator capacity and reaction frame available. The specimen was deliberately corrosion-damaged through accelerated electrochemical exposure to simulate decades of in-service degradation in a controlled timeframe. The damage state was characterized by visual inspection, half-cell potential, and section-loss measurements before any repair was applied.

After damage characterization, the bent received the combined epoxy-and-CFRP repair. The repaired specimen was then loaded laterally to failure under a quasi-static cyclic protocol. Instrumentation captured displacement at multiple elevations along the piles, strain in the CFRP jackets, and reaction at the pile heads. The same protocol had been previously applied to undamaged-baseline and damaged-unrepaired specimens in the broader research program, providing the comparison points needed to evaluate restoration effectiveness.

What the data set out to answer.

The lateral push-to-failure test was instrumented to answer four questions:

• Stiffness recovery. Does the repaired bent return to the load-versus-displacement slope of the undamaged baseline through the elastic range?
• Capacity recovery. Does peak lateral load match or exceed the undamaged baseline?
• Ductility behavior. Does the repaired bent develop a comparable post-yield plateau, or does the CFRP confinement change the failure mode in a way that affects displacement capacity?
• Failure mode. Where does the repaired bent fail under sustained lateral load, and what does that location say about how the repair redistributes demand?

The full quantitative results, instrumentation plots, and failure-mode analysis are in the journal article. The summary version: the combined repair behaved well across all four measures, with the failure mode shifting in a way the field engineering community will recognize as a useful diagnostic on the original undamaged bent design.

Why this matters in practice.

The headline implication for owners and bridge engineers is that there is a documented, lab-validated alternative to full pile-jacket replacement for some classes of corrosion-damaged pile bents. The combined epoxy-and-CFRP system is not a universal solution. It is a tool for a specific population of bents where damage has progressed to a known degree, where the geometry permits external confinement, and where the cost-and-disruption math favors a non-invasive intervention over a major capital project.

The other implication, and one closer to how Rarefied Earth thinks about engineering work generally, is that lab work can be a real input to capital decisions when it is structured around the questions a maintenance engineer actually asks. The PCI Journal publication exists because the research program was designed from the start to produce a comparison the practicing community can use, not just an academic result. That bias toward field-usable findings is a thread that runs through everything the firm does now in the construction-technology space.

Read the full study.

Related work.

The Part 2 study built on earlier in-situ work at the Sunshine Skyway Bridge field site under Dr. Alberto Sagüés at USF, where electrochemical diagnostics were applied to post-tensioned prestressed-concrete piles in service. That undergraduate research, completed 2013–2014, characterized the field-scale corrosion behavior that the lab specimens were calibrated against. The thread from in-situ diagnostics to lab repair to peer-reviewed publication is the methodology Rarefied Earth applies to civil-infrastructure problems generally.

The same posture, structuring deliverables around the questions practitioners actually ask, runs through the firm's construction-technology work. See how AI takeoffs actually work for general contractors for the field-guide companion piece.