LECA is a manufactured aggregate produced by expanding clay at high temperature, yielding hard, spherical granules riddled with internal voids. Because it weighs far less than natural sand and gravel, it is attractive for lightweight concrete, which reduces structural dead load and improves thermal and acoustic insulation while lowering cost. The catch is well documented: previous studies found that replacing natural aggregate with LECA can significantly weaken concrete. Corinaldesi and Moriconi reported compressive strength losses of roughly 18 to 28 percent when natural sand was replaced with LECA in self-compacting concrete, and Youm and colleagues found that coarse LECA reduced splitting tensile strength, compressive strength, and elastic modulus by up to nearly 48 percent. Polypropylene fibers, meanwhile, are known to stitch microcracks together and improve toughness, but above roughly 0.3 percent by weight of cement they tend to clump and ball during mixing, degrading the mix rather than improving it.

The Iranian research team set out to explore the underexplored middle ground: what happens when a modest 10 percent of sand is replaced by volume with LECA and a modest 0.2 percent by volume of polypropylene fibers is added, individually and in combination? They prepared four mortar mixes with a constant cement content of 670 kilograms per cubic meter and a water content of 302 kilograms per cubic meter: a plain control mix (P0L0), a mix with 10 percent LECA (P0L10), a mix with 0.2 percent fibers (P0.2L0), and a hybrid containing both (P0.2L10). The LECA had dry and saturated surface-dry densities of 1542 and 1704 kilograms per cubic meter with 5.0 percent water absorption, while the fibers were 12 millimeters long, 19 micrometers in diameter, with a tensile strength of 400 megapascals. Fibers were added gradually in several steps to ensure uniform dispersion, and cube and prismatic specimens were water-cured and tested at 3, 7, 14, and 28 days.

The compressive strength results reveal a striking early-age reversal. Replacing sand with LECA alone cut the 3-day strength from 19.20 to 15.87 megapascals, a drop of about 17.4 percent, as expected from the porous, weaker aggregate. Yet by 14 and 28 days the LECA mix had overtaken the control, with gains of 23.8 and 18.2 percent respectively. The authors attribute this to LECA’s internal water reservoir effect: the porous aggregate absorbs mixing water early and gradually releases it during hydration, densifying the cement matrix and strengthening the bond between paste and aggregate over time. Fibers alone had little effect at 3 days but raised 7-day strength by about 26.9 percent, from 24.27 to 30.80 megapascals, by bridging and restraining early microcracks during setting.

The hybrid mix delivered the best of both worlds early on. Combining LECA and fibers lifted 3-day compressive strength by roughly 23 percent, to 23.60 megapascals, and 7-day strength by about 25 percent, to 30.27 megapascals, compared with the control. At 28 days, however, the benefit largely evaporated: the hybrid reached only 32.80 megapascals, a marginal 2 percent gain over the control and slightly below the LECA-only mix. The researchers explain that by later ages the fiber-induced voids and weak fiber–matrix interfaces begin to offset LECA’s hydration benefits. Interestingly, the standard logarithmic strength-versus-time model fit the fiber-containing mixes poorly, with a bi-linear relationship describing their development better.

Flexural strength told an even more compelling story. LECA alone reduced early flexural strength, dropping the 3-day value from 6.43 to 4.70 megapascals and the 7-day value by about 16.5 percent, because its porous structure weakens the aggregate–paste interface. Fibers alone slightly hurt 3-day flexural strength but boosted it substantially from 7 to 28 days, reaching 11.69 megapascals at 28 days, thanks to their crack-bridging action under bending stresses. The hybrid mix, however, produced the highest flexural strength at every single age tested: 6.74, 7.95, 9.12, and 10.75 megapascals at 3, 7, 14, and 28 days respectively. Continued hydration refined the matrix while the fibers delayed crack propagation, a synergy that proved most valuable precisely where concrete is most vulnerable—under tension at early ages.

Durability was probed with two complementary techniques. In the Rapid Chloride Migration Test, performed according to AASHTO TP64 on 28-day cylinders, a 60-volt direct field drives chloride ions from a sodium chloride solution into the specimen for about 18 hours, after which silver nitrate spraying reveals the penetration depth. The control mix showed a migration rate of 0.0123 millimeters per volt-hour. Adding 10 percent LECA reduced this by about 4.9 percent to 0.0117, an improvement the authors link to increased tortuosity of transport pathways. Fibers alone left migration essentially unchanged, and the hybrid mix measured 0.0125, only marginally above the control. Electrical conductivity, measured with an alternating-current system to avoid polarization errors, dropped noticeably in the LECA mixes, from 0.0370 to 0.0339 per ohm-meter, confirming that the lightweight aggregate interrupts the continuous capillary pore network that normally serves as the highway for ion movement.

Scanning electron microscopy helped explain why apparent porosity and actual transport do not always go hand in hand. Images of LECA-containing specimens revealed abundant spherical voids up to 188 micrometers across, and in the hybrid mix up to 395 micrometers, yet these belong largely to the sealed interior of the LECA particles themselves, whose impervious outer shell and dense interfacial transition zone keep them out of the connected pore network. Fluid and ion transport occurs mainly through the fine capillary pores of the cement paste, and LECA particles interrupt that network, forcing ions onto longer, more winding paths. The microscopy also exposed a subtle trade-off with the fibers: hydrophobic polypropylene bonds poorly to the hydrophilic cement matrix, creating micro-gaps of roughly 15 to 30 micrometers around the filaments and at the LECA interface. These gaps do not connect into continuous channels, but under the sustained 60-volt field of the migration test they can act as local weak zones, which is why the hybrid mix showed slightly higher chloride migration despite lower overall conductivity.

The broader significance lies in the balance achieved. Previous studies in which large amounts of lightweight aggregate weakened the matrix, or in which high fiber dosages caused clustering and non-uniform transport, suggest that more is not better. By capping LECA at 10 percent sand replacement and fibers at 0.2 percent by volume, the researchers produced a composite that reaches 32.80 megapascals in compression and 10.75 megapascals in flexure at 28 days, resists bulk ion transport by disrupting continuous capillary pathways, and weighs measurably less than the control, at 2.03 versus 1.92 grams per cubic centimeter for the LECA mixes. For infrastructure exposed to chlorides—marine structures, bridge decks, and pavements subjected to de-icing salts—such a material could extend service life while reducing structural weight. The study also delivers a methodological caution: mechanical strength alone does not capture durability, and pairing migration testing with conductivity measurements, as this team did, reveals transport behavior that strength data alone would hide. As lightweight and fiber-reinforced concretes move toward wider structural use, this kind of coupled mechanical–transport–microstructural framework may become the standard by which new mixes are judged.

Subject of Research: Mechanical and durability characterization of polypropylene fiber-reinforced mortar containing lightweight expanded clay aggregate

Article Title: RCMT, electrical conductivity, and mechanical characterization of polypropylene fiber-reinforced LECA mortar

Article References: Taghvaei, D., Behzad, P., Ghaderan, S., & Dastan Diznab, M. A. (2026). RCMT, electrical conductivity, and mechanical characterization of polypropylene fiber-reinforced LECA mortar. Results in Engineering, 32, Article 113256. https://doi.org/10.1016/j.rineng.2026.113256

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113256

Keywords: LECA, polypropylene fibers, lightweight concrete, chloride migration, electrical conductivity, compressive strength, flexural strength, durability, pore structure, SEM, cement mortar, RCMT

The research team, led by Hui Huang and Cong Zhang, prepared three mixtures under an identical binder–aggregate–water framework: a plain mortar control, a PP-fiber-reinforced composite, and a PVA-fiber-reinforced composite. Both fiber types were 6 millimeters long and added at a volume fraction of 1.0 percent, so that fiber dosage would not confound the comparison. The fibers themselves, however, are chemically and mechanically very different. The PP fiber, with a diameter of 20 micrometers, has a tensile strength of 800 megapascals and an elastic modulus of 5 gigapascals, and its surface is hydrophobic. The finer PVA fiber, at 14 micrometers, is roughly twice as strong at 1,800 megapascals and nearly three times stiffer at 14 gigapascals, and its hydrophilic chemistry bonds more readily with cement hydration products. Those differences turned out to matter enormously.

The specimens, 70.7-millimeter cubes, were cured for 28 days and then subjected to a rapid water-freezing and water-thawing regime in which temperatures swung between −18 °C and 5 °C every four hours, with the samples fully immersed. Measurements were taken after 0, 25, 50, 75, 100, 125, and 150 cycles. Before exposure, the plain mortar was actually the strongest material, at 65.03 megapascals of compressive strength, compared with 61.92 megapascals for the PVA composite and 54.61 megapascals for the PP composite. But strength at the start is not the same as strength that lasts.

After 150 freeze–thaw cycles, the ranking had reversed in the way that matters for durability. The fiber-reinforced composites retained 82.29 percent (PP) and 84.50 percent (PVA) of their initial compressive strength, while the unreinforced mortar retained only 66.45 percent. Mass loss told a parallel story: the mortar reached 5 percent mass loss by 125 cycles through continuous surface scaling, whereas both fiber mixtures degraded more slowly in a three-stage pattern—slow loss in the first 50 cycles, acceleration between 50 and 100 cycles as cracks connected, and a slowdown thereafter. Even after 150 cycles, the fibers still reduced surface material loss by more than 59 percent relative to plain mortar, with PVA consistently outperforming PP.

Perhaps the most striking finding was that stiffness, not strength, is the more sensitive fingerprint of freeze–thaw damage. The apparent elastic modulus, derived from the slope of the compressive stress–strain curve between 10 and 30 percent of peak stress, fell by 32.9 percent for PP-FRCC and 29.2 percent for PVA-FRCC over 150 cycles—nearly double the corresponding strength losses. Because the modulus reflects the load-transfer continuity of the internal microstructure, it registers the accumulation of pores, microcracks, and debonded interfaces long before catastrophic strength loss appears. For engineers assessing aging infrastructure, this suggests that stiffness-based or non-destructive measurements could serve as an early-warning system for frost damage.

Scanning electron microscopy revealed why the two fibers behaved differently. Using calibrated image analysis with two semi-quantitative descriptors—a visible defect area ratio and an interfacial deterioration ratio—the researchers quantified what they saw in the micrographs. The visible defect area ratio climbed from 1.8 to 8.2 percent in mortar, from lower starting values to 6.8 percent in PP-FRCC and 5.6 percent in PVA-FRCC. More telling was the interface: the fraction of visibly deteriorated fiber–matrix boundary rose from 3.0 to 21.2 percent for PP-FRCC but only from 2.7 to 17.4 percent for PVA-FRCC. The hydrophobic PP surface, chemically stable and inexpensive, gradually separated from the matrix under repeated hydraulic and crystallization pressures, while the hydrophilic PVA fiber maintained more continuous contact with surrounding hydration products.

Those microscopic differences echoed upward into failure behavior. Under splitting tensile loading, PVA-FRCC specimens displayed primary or deflected splitting cracks in 17 of 21 observations (81.0 percent), whereas PP-FRCC showed localized or branched damage features in 15 of 21 (71.4 percent). The tensile-to-compressive strength ratio, a marker of balanced degradation, stayed remarkably stable for PVA-FRCC—fluctuating within a coefficient of variation of just 0.83 percent—while mortar’s ratio swung the most. In compression, fiber-bridged cracking kept the reinforced cubes intact even after 150 cycles, while the plain mortar failed brittly with severe spalling and a conical crushing zone at its core.

To connect these observations mechanically, the team built a three-dimensional mesoscale model in ABAQUS, representing the composite as a cementitious matrix, randomly distributed equivalent fibers, and an equivalent pore phase whose volumetric expansion stands in for freeze–thaw damage. Calibrated against the measured compressive response at each cycle stage, the model reproduced the monotonic strength decline well at early and intermediate stages, with discrepancies under about 4 percent through 75 cycles. By 150 cycles the gap widened to roughly 17 percent for both composites—a limitation the authors attribute to the simplified pore-expansion representation and the absence of explicit fiber–matrix bond-slip and pull-out. Still, the simulated damage fields were revealing: equivalent strain, stiffness degradation, and compressive damage, initially scattered around isolated weak regions, progressively coalesced into connected damage paths as freeze–thaw pre-damage accumulated, mirroring the experimentally observed shift from distributed fine cracking to localized through-cracks.

Two-way analysis of variance confirmed that material type, cycle number, and their interaction significantly shaped mass loss, compressive strength retention, and stiffness retention, with strikingly large effect sizes—partial eta squared above 0.9 for the cycle effect on mass loss. For splitting tensile strength retention, the interaction was not significant, indicating that tensile degradation followed overall material and cycle effects rather than a material-specific trajectory. Together, the statistics reinforce the central message: fiber reinforcement does not merely slow damage, it changes the pathway by which deterioration proceeds.

The practical implications reach cold-region roads, bridges, repair overlays, and any concrete that may sit water-saturated through winter. Adding just 1 percent by volume of either fiber substantially curbed surface scaling and strength loss, with PVA offering slightly better residual strength, stiffness retention, and interfacial stability under this particular mixture design, while PP delivered comparable protection at lower cost. The authors are careful to note that the comparison is bounded by their specific mix proportions, cube geometry, and exposure regime, and that dosage optimization, salt frost exposure, flexural testing, and quantitative porosity measurements remain for future work. Even so, the study delivers a rare, unified account of frost damage—from the swelling pore to the debonding fiber to the cracking cube—and a compelling case that the humble fiber–matrix interface may be the most important square micrometers in frozen concrete.

Subject of Research: Freeze–thaw damage evolution and durability of polypropylene- and polyvinyl alcohol-fiber-reinforced cementitious composites

Article Title: Freeze–thaw damage evolution of PP- and PVA-fiber-reinforced cementitious composites: Mechanical degradation, interfacial deterioration and mesoscale modeling

Article References: Huang, H., Zhang, C., Liu, X., Xu, X., Jia, B., Gu, S., & Wang, W. (2026). Freeze–thaw damage evolution of PP- and PVA-fiber-reinforced cementitious composites: Mechanical degradation, interfacial deterioration and mesoscale modeling. Case Studies in Construction Materials, 25, Article e06594. https://doi.org/10.1016/j.cscm.2026.e06594

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06594

Keywords: freeze–thaw cycles, fiber-reinforced cementitious composites, polypropylene fibers, PVA fibers, concrete durability, interfacial deterioration, mesoscale modeling, elastic modulus degradation, scanning electron microscopy, compressive strength retention, cold-region infrastructure, damage localization