A new study published in Case Studies in Construction Materials by Seon Yeol Lee, Seoung Ik Choi, Taek Hee Han, and Jun Kil Park tackles this gap by asking a deceptively simple question: what happens when polyethylene fiber-reinforced mortar is exposed to real seawater while it is still curing? The researchers focused on ultra-high-molecular-weight polyethylene fibers, which, unlike steel fibers, are immune to corrosion and chemically robust in marine conditions. Polymer fibers of this kind are the backbone of engineered cementitious composites, materials famous for their ability to hold cracks below roughly 60 micrometers in width, a scale fine enough to slow chloride ingress dramatically. Recent variants have even embraced seawater and sea sand as mixing ingredients to conserve freshwater and terrestrial aggregates, making the question of seawater’s influence on mechanical development all the more urgent.

What distinguishes this work from earlier research is its attention to timing. Previous studies had either mixed seawater directly into the concrete, submerged specimens immediately after demolding, or exposed them to seawater only after 28 days of standard curing. Each approach interacts with cement hydration at a different stage, and prior work on cement pastes had shown that the age of first seawater exposure significantly alters which hydration products form. The Korean team designed their experiment around the practical scenario facing precast marine elements: mortar cured in tap water for 0, 3, 7, or 14 days, then transferred to natural seawater collected from Busan, with a total curing period of 28 days for every condition. They compared these against control specimens cured entirely in tap water or entirely in seawater.

The material itself was engineered for high performance. The mortar used Type I ordinary Portland cement, Australian silica sand, a water-to-cement ratio of just 0.25, and a polycarboxylate superplasticizer to keep the mix workable at that low water content. The polyethylene fibers were remarkably slender, with an average diameter of 23.5 micrometers and a length of 12 millimeters, giving an aspect ratio above 500, an elastic modulus of 100 gigapascals, and a tensile strength of 3000 megapascals. Three fiber contents were tested: zero, one, and two percent by volume. The team targeted a compressive strength of at least 80 megapascals, placing the material at the lower bound of the high-strength engineered cementitious composite range considered suitable for harsh marine loading including earthquakes, typhoons, and tsunamis.

The results, drawn from flexural, compressive, and length-change tests on hundreds of specimens, revealed a strikingly nuanced picture. Every series showed improved first-crack behavior, quantified as the Limit of Proportionality, as seawater exposure lengthened. For the unreinforced mortar, the first-crack toughness rose by up to 132.8 percent under full seawater curing compared with tap-water curing. But the peak flexural performance, the Modulus of Rupture, split the fiber-reinforced series in opposite directions. The one-percent fiber mortar grew stronger and tougher with seawater, while the two-percent fiber mortar grew weaker, with its peak toughness falling by 34.9 percent under full seawater exposure relative to its tap-water counterpart.

The best overall performer was the one-percent fiber mortar cured in tap water for seven days and then exposed to seawater for 21 days. Compared with the same mix cured entirely in tap water, its first-crack strength, deflection, and toughness increased by 94.2, 66.7, and 174.2 percent respectively, while its peak strength, deflection, and toughness rose by 5.6, 23, and 42.8 percent. This mix also achieved the highest compressive strength of the entire study, 98.3 megapascals. The findings suggest that a brief freshwater pre-cure followed by seawater exposure may actually be an ideal sequence for precast marine elements, turning an unavoidable logistical constraint into a performance advantage.

The mechanism behind these gains lies in cement chemistry. When chloride ions from seawater react with tricalcium aluminate phases and calcium hydroxide in the cement paste, they form Friedel’s salt, a chloride-bearing aluminate phase that fills capillary pores and accelerates hydration. Seawater also promotes the formation of a high-surface-area calcium silicate hydrate gel, the glue of concrete, refining the pore structure. Earlier studies cited by the authors reported that seawater-mixed pastes showed a 12 percent reduction in 28-day porosity and finer pore networks. The result is a denser microstructure and, crucially, a strengthened interfacial transition zone, the porous region surrounding each fiber where cracks preferentially begin. Because flexural and tensile behavior are far more sensitive to this zone than compressive behavior, the improvements showed up most dramatically in the bending tests, and the measured expansion of the specimens tracked neatly with the strength gains.

So why did the two-percent fiber mix falter? The answer lies in a phenomenon known as the group effect. When fibers are packed too closely together, they interfere with one another, concentrating stress in the matrix and reducing the reinforcing efficiency of each individual fiber. Using a constitutive model developed for steel fiber-reinforced concrete, the team calculated that fiber spacing in the two-percent mix, 171.2 micrometers, sat uncomfortably close to the fiber effect distance of 141 micrometers, the threshold at which the group effect activates. Seawater exposure strengthens the fiber-matrix bond, which extends that effect distance, pushing the crowded two-percent mix over the edge. Image analysis of polished cross-sections confirmed the story: while the overall fiber distribution appeared more uniform at higher dosage, the count of distinct fiber objects dropped by about 14 percent, indicating local clustering, and crack surfaces of the two-percent specimens showed visible clumps of fibers where uniform dispersion was expected.

The practical implications are considerable. For engineers designing precast elements for offshore wind platforms, submerged tunnels, and coastal defenses, the study suggests that one percent polyethylene fiber offers the sweet spot, and that seven days of tap-water curing before seawater exposure yields the best flexural performance under the tested conditions. It also adds momentum to the broader movement toward seawater and sea-sand concrete, which could relieve pressure on freshwater supplies and river aggregates in coastal construction booms. Because polymer fibers cannot corrode, they sidestep the classic failure mode of steel-reinforced marine concrete entirely, and the microcrack-control ability of these composites further delays chloride penetration.

The authors are careful to note the limits of their work. The study covers only the first 28 days of strength development, and the formation of Friedel’s salt and high-surface-area calcium silicate hydrate was inferred from the literature and from expansion measurements rather than directly verified with microscopy or spectroscopy. Fiber orientation was not quantitatively evaluated, the seawater’s chemistry was not analyzed, and seasonal temperature variations were not considered. Future work involving X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, three-dimensional fiber imaging, and pull-out tests should pin down the mechanisms more firmly. Long-term exposure studies will be essential before these findings translate into design codes. Still, the central message stands: the ocean, so often cast as concrete’s enemy, can under the right curing schedule and fiber dosage become an unexpected ally in building tougher, more durable marine infrastructure.

Subject of Research: Effects of seawater exposure timing during curing on the flexural behavior of polyethylene fiber-reinforced mortar for marine precast concrete

Article Title: Effects of seawater exposure during curing on the flexural behavior of polyethylene fiber-reinforced mortar

Article References: Lee, S. Y., Choi, S. I., Han, T. H., & Park, J. K. (2026). Effects of seawater exposure during curing on the flexural behavior of polyethylene fiber-reinforced mortar. Case Studies in Construction Materials, 25, Article e06591. https://doi.org/10.1016/j.cscm.2026.e06591

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06591

Keywords: polyethylene fibers, seawater curing, fiber-reinforced mortar, marine concrete, precast concrete, Friedel's salt, flexural behavior, engineered cementitious composites, chloride ingress, fiber-matrix bond, group effect, durability

UHPFRC is an extraordinary material by any measure. Its ultra-dense matrix dramatically restricts chloride penetration, while embedded steel fibers give it compressive strengths above 150 megapascals and tensile strengths far beyond those of conventional concrete. Earlier experiments have shown that a 60-millimeter UHPFRC layer can more than triple the load capacity of a severely corroded beam, and that U-shaped jackets encasing a beam on three sides can dramatically boost stiffness. Yet a critical complication has gone largely unexamined: not all UHPFRC behaves the same way after it cracks. The French standard NF P18-470 formally separates the family into strain-softening, low strain-hardening, and high strain-hardening grades. Strain-hardening UHPFRC keeps gaining stress after first cracking, spawning networks of fine, distributed micro-cracks and a pseudo-ductile plateau. Strain-softening UHPFRC, though still mechanically superior to ordinary concrete, sheds stress progressively once past its peak, relying on fewer, wider cracks.

Which of these behaviors a given mix displays depends on fiber type, mix proportions, admixtures, and curing conditions, and the consequences for structural rehabilitation are far from academic. Prior experimental campaigns produced apparently contradictory results: one study found that strain-hardening UHPFRC enhanced the ductility of corroded beams, while another reported that strain-softening layers actually reduced it. Because most existing research on this strengthening technique has been experimental, and because numerical modeling of corrosion-induced degradation remains notoriously difficult—with researchers split over how to represent rust expansion, bond loss, and interfacial behavior—the field lacked a reliable computational tool to systematically separate these effects. The new study, published in Case Studies in Construction Materials, was designed to close that gap.

Lead author Zhukai Tang, together with Wai-Meng Quach, Ran Feng, and Zhiyuan Chen, built a nonlinear finite element framework in ABAQUS that tackles the three hardest problems head-on. First, nonlinear spring elements spaced at 50-millimeter intervals connect reinforcement nodes to the surrounding concrete, capturing the bond-slip deterioration that corrosion causes; the springs follow calibrated constitutive laws in which maximum bond stress falls with corrosion ratio according to established models for corroded reinforcement. Second, the interface between the UHPFRC jacket and the substrate concrete is governed by a traction-separation law, whose normal and shear strengths of 19.2 and 5.9 megapascals were extracted directly from slant-shear and double-shear interface tests performed by the team, with damage initiating at a plastic displacement of 0.241 millimeters. Third, the researchers implemented two distinct post-peak tensile constitutive models for UHPFRC, one strain-hardening and one strain-softening, carefully tuned so that both materials shared an identical tensile strength of 11.4 megapascas under the same steel fiber conditions, 15-millimeter-long fibers at 0.2 millimeters diameter and a 2 percent volume fraction. This clever control variable isolates the pure effect of post-cracking behavior on structural response.

The model itself was grounded in real experiments. The team validated it against 24 corroded, UHPFRC-strengthened beams from two published four-point bending campaigns. In the first, six strengthened beams and three unstrengthened controls were exposed to 360 days of wet-dry cycling in artificial seawater, producing 2 to 3 percent corrosion of the bottom reinforcement and stirrups before being retrofitted with 15, 30, or 45 millimeter layers of strain-hardening UHPFRC with a compressive strength of 153.7 megapascals. In the second, 18 beams were corroded by impressed current to controlled ratios of 10 to 23 percent in the bottom bars, then strengthened with strain-softening UHPFRC in bottom-face or U-shape configurations at thicknesses from 20 to 60 millimeters. Across all 24 specimens, the simulated-to-experimental load ratios ranged from 0.90 to 1.08, averaging 0.994 with a standard deviation of just 0.0504, an accuracy rare in corrosion modeling, where stochastic deterioration usually wrecks convergence.

With the model validated, the researchers ran 48 simulations of simply supported beams, sweeping UHPFRC thicknesses from 10 to 60 millimeters in 10-millimeter increments, comparing bottom-face and U-shape jacket configurations, varying corrosion ratios from 5 to 30 percent, and switching between the two tensile constitutive laws. The results are striking. For uncorroded beams, increasing the strain-hardening UHPFRC thickness from 10 to 60 millimeters raised peak load capacity by 8.2 to 79.6 percent with bottom-face strengthening, and by a remarkable 27.8 to 273 percent with U-shape jackets. The strain-softening variant followed similar trends, gaining 4.5 to 59.2 percent and 26.3 to 222.2 percent respectively. In the most extreme case, a 60-millimeter strain-hardening U-shape jacket boosted flexural capacity by 273 percent, yield load by 200.1 percent, and lifted the cracking load from 19.7 kilonewtons to 210.2 kilonewtons—more than a tenfold improvement in the load at which the beam first cracks.

The mechanism behind these gains is partly geometric and partly material. Thicker jackets increase the section depth and moment of inertia, while UHPFRC’s superior tensile capacity carries stress across cracked zones and unloads the corroded steel. But the fiber-bridging effect proved especially potent for crack control: fibers mechanically anchor micro-cracks, and energy dissipated during fiber pull-out delays the coalescence of macro-cracks. Crack resistance improved even faster with thickness than raw strength did. Failure modes also evolved with the jacket. Bottom-face strengthened beams shifted from tightly concentrated midspan flexural cracking toward a more uniform flexural-shear crack pattern as thickness grew, while U-shape beams tended toward a single dominant flexural crack at peak load, a signature of UHPFRC’s crack resistance concentrating deformation once fibers began pulling out. Interfacial contact stress analysis showed the highest stresses at the jacket ends, flagging those zones as debonding initiation sites, but the U-shape configuration’s side arms relieved end-of-span stress concentrations and made debonding significantly less likely than in bottom-face strengthening.

The most consequential finding, however, concerns the two tensile behaviors. Although both materials shared the same tensile strength, the performance gap widened with thickness: peak load differences between strain-hardening and strain-softening versions grew from 5.6 to 25.2 kilonewtons for bottom-face beams, and from 2.0 to 68.5 kilonewtons for U-shape beams, as thickness increased from 10 to 60 millimeters. Yield strength gaps followed the same trajectory. The strain-hardening material’s extended hardening phase means fibers slip over a longer strain range before pull-out or fracture, sustaining load even as micro-cracks accumulate. Corrosion amplified the contrast further: as corrosion ratios climbed to 30 percent, ductility of strain-softening-strengthened beams degraded much more sharply, because those beams lean heavily on the steel reinforcement for deformation capacity, precisely the component corrosion destroys. Strain-hardening jackets preserved ductility far better. Interestingly, strain-softening beams showed marginally higher initial stiffness, attributed to their stiffer pre-cracking tensile response—small consolation given their deficits elsewhere.

Beyond the simulations, the team distilled a closed-form theoretical model for predicting moment resistance of strengthened beams, accounting for the different stress blocks that strain-hardening and strain-softening UHPFRC produce across the cracked section. Validated against the 48 simulated beams plus test data from four independent experimental studies, the formulation achieved a mean prediction-to-test ratio of 1.012 with a standard deviation of 0.097, giving designers a practical hand-calculation tool for the first time that explicitly distinguishes post-peak material classes. The authors are candid about limitations: corrosion was modeled as uniform rather than pitting, stirrup corrosion was excluded, the flexural-shear mechanism transition was not captured, and the traction-separation law awaits full sensitivity analysis. The model is therefore best suited to moderate corrosion and monotonic loading, with non-uniform corrosion, coupled degradation, and long-term performance flagged for future work.

The practical message for infrastructure owners is clear and actionable. Bottom-face UHPFRC strengthening is the more economical option and still delivers large capacity gains, making it attractive for routine rehabilitation budgets. But when maximum strength, long-term corrosion resistance after retrofit, and preserved ductility are the priorities—as they usually are for critical marine structures—the U-shape jacket in a strain-hardening UHPFRC is the superior choice, and the study shows that specifying the material’s post-cracking class is not a detail but a decisive engineering parameter. As coastal concrete assets worldwide age into their corrosion-prone decades, tools like this model give engineers a quantitative basis for choosing not just whether to strengthen, but exactly how.

Subject of Research: Finite element analysis of corroded reinforced concrete beams strengthened with strain-hardening and strain-softening UHPFRC layers

Article Title: Flexural behavior of corrosion-damaged RC beams strengthened with strain-hardening and strain-softening UHPFRC: Numerical study

Article References: Tang, Z., Quach, W.-M., Feng, R., & Chen, Z. (2026). Flexural behavior of corrosion-damaged RC beams strengthened with strain-hardening and strain-softening UHPFRC: Numerical study. Case Studies in Construction Materials, 25, Article e06486. https://doi.org/10.1016/j.cscm.2026.e06486

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06486

Keywords: UHPFRC, reinforced concrete, corrosion, finite element analysis, structural strengthening, flexural behavior, marine infrastructure, bond-slip, strain hardening, ductility, concrete durability, rehabilitation