A new study published in Case Studies in Construction Materials by Yuki Kobayashi, Taku Koyama, Keiichi Nakamura, and Mitsuo Ozawa tackles this problem with an elegant twist. Instead of combining steel fibers for strength with polypropylene fibers for fire protection, the team developed a UHPFRC reinforced exclusively with polyvinyl alcohol, or PVA, fibers. Their material achieved a compressive strength of at least 150 megapascals while carrying 1.7 percent by volume of PVA fibers, and their experiments reveal in quantitative detail how these hydrophilic synthetic fibers suppress explosive spalling through a two-stage mechanism that shifts as temperatures climb past the fibers’ melting point.

The physics of spalling in dense concrete is unforgiving. When a fire heats the surface, thermal stress builds at the heated face while water trapped in the pores near the surface turns to vapor. Because UHPFRC has an extremely low permeability, that vapor cannot escape quickly, and pressure builds rapidly in the near-surface pores. At the same time, the material’s high elastic modulus amplifies thermal stresses generated by steep internal temperature gradients. The combination can crack the surface layer and then blast it away. Once cover concrete is lost, embedded reinforcement heats up faster and the load-bearing cross-section shrinks, so preventing spalling is essential for structural fire safety.

The conventional defense has been to add low doses of polypropylene fibers, which melt at around 170 degrees Celsius and leave behind channels that vent vapor pressure. Steel fibers, meanwhile, boost tensile performance at ambient temperature but are considered ineffective against spalling on their own, and some studies suggest they may even increase spalling risk. PVA fibers occupy a unique middle ground: they are highly hydrophilic and form strong chemical bonds with the cement matrix, which improves crack dispersion and flexural toughness at room temperature while also contributing to fire-spalling mitigation. They are also immune to the corrosion that plagues steel fibers, opening applications in highly corrosive environments or structures requiring non-magnetic properties.

To test their material under conditions closer to real structures, the researchers used the ring-restraint heating test, a method standardized by the Japan Concrete Institute and recommended by RILEM. Rather than heating free-standing cylinders, they cast the UHPFRC into steel rings 300 millimeters in diameter that confine the specimen’s outer circumference, mimicking the restraint that surrounding structure imposes on a heated member. After preheating a small furnace to roughly 1050 degrees Celsius, they exposed the specimen’s surface to one-sided heating for 30 minutes, a deliberately severe thermal shock harsher than standard tunnel-fire curves such as ISO 834. Throughout each test they recorded internal temperature, internal water vapor pressure, and the restraint stress acting on the concrete at depths of 5, 10, 25, and 40 millimeters, using strain gauges on the steel ring corrected for thermal expansion and a silicone-oil-filled steel pipe calibrated to separate true vapor pressure from oil-expansion artifacts.

The experimental campaign covered four fiber volume fractions: zero, 0.5, 1.0, and 1.7 percent, with fibers replacing an equivalent volume of sand in a mix with a water-to-binder ratio of just 16 percent. Preliminary testing showed that anything above 1.7 percent destroyed workability, making that the practical ceiling. The PVA fibers themselves were slender, 0.2 millimeters in diameter and 12 millimeters long, with tensile strengths between 1092 and 1716 megapascals. Thermogravimetric analysis showed the fibers begin losing weight near 80 degrees Celsius, melt at 237 degrees Celsius, and thermally decompose between 270 and 460 degrees Celsius, a thermal fingerprint that proved central to interpreting everything that followed.

The results were striking. Spalling depth and spalling volume both decreased steadily as fiber content rose. The fiber-free control developed deep, mortar-shaped defects at the center of the heated surface, and rapid temperature surges repeatedly rippled inward as each spalling event peeled the surface away and shifted the heated front deeper. In the fiber-reinforced specimens, the damage took a different form: layered cracking parallel to the heated surface rather than explosive detachment. Because the surface concrete stayed in place, heat transfer into the interior was limited, and internal temperatures at depth remained markedly lower than in the control. Restraint stress also rose more gradually in fiber-containing specimens, reaching roughly 3 megapascals in the 1.7 percent mix within five minutes of heating, compared with 4.5 megapascals in the fiber-free specimen over the same interval.

One finding defied expectations. Peak internal vapor pressures were higher in the fiber-reinforced specimens, up to 5.7 megapascals at 0.5 percent fiber content, than in the fiber-free control, which peaked at 2.3 megapascals. The researchers attribute this counterintuitive trend to a synergy between the PVA fibers’ high melting point, their strong chemical bonding to the matrix, and the powerful crack-bridging effect of the high fiber dosage. Instead of venting early through cracks, as low-melting, weakly bonded polypropylene fibers allow, the vapor remained trapped because PVA fibers held crack surfaces together until temperatures approached the melting point. The pressure peaked in the range immediately before rapid fiber mass loss, then declined as the fibers melted and vanished, opening dissipation pathways through the voids they left behind.

Complementing the heating tests, residual strength experiments on small cylinders heated slowly to 200, 300, and 400 degrees Celsius mapped how the material degrades. Compressive strength fell to about 55 percent of its unheated value by 200 degrees Celsius, a sharper drop than seen in lower-strength PVA materials or in Eurocode 4 predictions for normal-weight concrete, likely because the dense, ultra-strong matrix generates larger thermal stresses that initiate fine map-like cracks early. The static modulus of elasticity proved even more sensitive, falling to just 18 percent of its initial value at 400 degrees Celsius, reflecting both the loss of fibers and the dehydration and decomposition of the cement hydrates that bind the matrix together.

Finally, the team applied a tensile strain failure model to connect the measurements to a coherent mechanism. When heated concrete expands but is restrained, compressive stress develops in the plane of the surface while tensile strain builds perpendicular to it through the Poisson effect; cracking occurs when that strain exceeds a critical value. Using experimentally derived residual modulus values and restraint stresses, the model reproduced initial spalling progression within about 10 millimeters of the heated surface when the tensile failure strain was set between 50 and 100 microstrain. Notably, for fiber contents of 1.0 percent and above, the model predicted earlier spalling than actually occurred, precisely because it cannot capture the fiber bridging and vapor dissipation that suppress spalling in reality. The authors propose that PVA fibers act in two stages: before melting, crack-bridging holds fractured fragments in place and prevents them scattering; after melting, the void network they leave behind dissipates accumulated vapor pressure. Direct microscopic confirmation of that void network remains future work, but the indirect evidence, from pressure curves that peak near the melting point and then fall, from enhanced air permeability reported in prior studies, and from notched-beam tests showing fibers intact at 200 degrees Celsius but gone at 300, makes a compelling case. If confirmed in full-scale beam tests under standard fire curves, this single-fiber strategy could simplify the design of fire-safe, corrosion-proof ultra-high-performance concrete for the infrastructure of the coming decades.

Subject of Research: Spalling suppression in ultra-high-performance fiber-reinforced concrete using polyvinyl alcohol fibers

Article Title: Fire-induced spalling suppression by polyvinyl alcohol fibers in ultra-high-performance fiber-reinforced concrete

Article References: Fire-induced spalling suppression by polyvinyl alcohol fibers in ultra-high-performance fiber-reinforced concrete. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: UHPFRC, polyvinyl alcohol fibers, explosive spalling, fire safety, concrete, vapor pressure, thermal stress, ring-restraint heating test, construction materials, fiber reinforcement, residual strength, high-performance concrete

The study, published in Case Studies in Construction Materials, tackles a bottleneck that has constrained protective design for years. Existing C120-grade high-strength concrete typically exhibits densities between 2,400 and 2,500 kilograms per cubic meter, which inflates the self-weight of protective structures, raises construction costs and limits use in weight-sensitive applications. Conventional C80 concrete is lighter, but its strength and anti-penetration performance fall short of the demands of high-end protective engineering. The researchers set out to achieve what they describe as a synergistic optimization of protective efficiency and structural economy: ultra-high compressive strength to withstand high-velocity projectile impact, combined with a substantially reduced density.

The key to the new material lies in its mix design, which was guided by particle close-packing theory. The team systematically analyzed how varying amounts of cement, silica fume and fly ash microspheres affect the packing density of particles in the cementitious system, then tuned the aggregate ratio, sand-binder ratio and fiber content to optimize apparent density, mechanical properties and workability. The resulting C120 lightweight high-performance concrete contains 682.5 kilograms of cement, 210 kilograms of fly ash and 157 kilograms of silica fume per cubic meter, achieving a 28-day compressive strength of 121.3 megapascals and a flexural strength of 17.2 megapascals at a density of just 2,050 kilograms per cubic meter.

Two ingredient choices stand out. First, all natural aggregates were replaced with ceramsite sand, a lightweight ceramic aggregate with particle sizes between 0.5 and 5 millimeters, a bulk density of 1,450 kilograms per cubic meter and a cylinder compressive strength of 8.5 megapascals. Its rough, porous surface forms a strong bond with the cementitious matrix, while its moderate particle strength preserves load-bearing capacity at a much lower density. Second, steel fibers were added at a volume fraction of about 2.0 percent. Each fiber is 13 millimeters long, 200 micrometers in diameter and has a tensile strength of 2,000 megapascals, providing the tensile reinforcement and impact toughness needed to restrain crack propagation during penetration. For comparison, the conventional C80 mix used haydite aggregate with natural sand and basalt fibers, reaching 82.6 megapascals of compressive strength at 2,450 kilograms per cubic meter.

To test the material under realistic ballistic conditions, the team cast cylindrical concrete targets measuring 900 millimeters in diameter and 600 millimeters thick, each enclosed in a 5-millimeter steel plate that provided lateral restraint while minimizing boundary effects. The dimensions were chosen so that the radial size exceeded 30 projectile diameters and the thickness exceeded 1.5 times the expected maximum penetration depth of 360 millimeters, ensuring that neither lateral boundaries nor the rear free surface would distort the results. After 28 days of standard curing at 20 degrees Celsius and at least 95 percent relative humidity, the targets were struck with pointed ogive-nose penetrators made of 35CrMnSiA steel, each 120 millimeters long, 24 millimeters in diameter and weighing 352 grams, launched from a 30-millimeter gun at approximately 700 meters per second at normal incidence.

The results were striking. Against the conventional C80 target, struck at 699 meters per second, the projectile penetrated 0.251 meters and opened a large frontal crater with a maximum diameter of 0.421 meters and a minimum of 0.348 meters, a nearly circular shape ratio of 1.21 characteristic of dispersed brittle failure. High-speed photography at 100,000 frames per second captured the concrete at the impact point being pulverized, radial cracks spreading outward and large chunks shearing away as the projectile head entered the material. The C80 matrix has low compactness and a weak transition zone between aggregate and paste, so stress waves rapidly induced interfacial cracking that extended irregularly, dissipating energy through large-scale crushing and spalling while severely damaging the specimen’s integrity.

The lightweight C120 target, struck even faster at 726 meters per second, performed dramatically better. Penetration depth was only 0.158 meters, and the frontal crater shrank to a maximum diameter of 0.263 meters and a minimum of 0.157 meters, a shape ratio of 1.68 indicating localized, concentrated compressive failure rather than widespread destruction. The steel fibers raised tensile strength and impact toughness, increasing the energy absorbed during penetration, while the tightly bound lightweight aggregate interface suppressed crack initiation and radial expansion. Impact energy was dissipated through plastic compression within a small contact zone, leaving the overall structure intact. Notably, recovered projectiles showed more extensive surface abrasion on those that had penetrated the C120 target than on those that hit the C80, evidence that the lighter material actually imposed greater axial and sidewall frictional resistance.

A cross-literature comparison reinforced the finding. The team benchmarked its results against a published test of C120 ultra-high performance concrete with a density of 2,450 kilograms per cubic meter and a cube compressive strength of 117.9 megapascals, impacted at 728 meters per second by a smaller 15-millimeter, 101.8-gram projectile. Because both projectiles remained essentially rigid, the impact energies and penetration resistances could be compared quantitatively. The new lightweight concrete faced roughly 3.44 times the impact kinetic energy and a 34.3 percent higher sectional energy density, yet its penetration depth was 10.7 percent shallower. Its average penetration resistance reached 1.30 gigapascals, about 51.2 percent higher than the 0.86 gigapascals of the denser UHPC, and the strength-normalized resistance index rose from 7.3 to 10.7, all while the target density was 16.3 percent lower.

The data also revealed an unexpected mathematical pattern. The researchers defined a penetration depth-density coupling coefficient K, the ratio of penetration reduction to density reduction, and found K equals 2.54: every 1 percent decrease in density corresponded to a roughly 2.54 percent decrease in penetration depth. This superlinear inverse coupling suggests the material offsets the loss of inertial resistance from being lighter through mix optimization and improved interface performance, shifting anti-penetration behavior from density-dominated inertia toward domination by intrinsic material properties. Marginal-efficiency analysis told a similar story: moving from C80 to the new C120 raised compressive strength by 50 percent, but basic anti-penetration efficiency, the reciprocal of penetration depth, rose 58.86 percent, and density-normalized intrinsic efficiency rose 85.96 percent, gains that far exceed what strength increase alone would predict and that break the conventional trend of diminishing returns in traditional high-strength concrete.

Finally, the team built a finite element model using the K&C concrete constitutive model with a quarter-symmetry mesh of hexahedral elements, and validated it against the experiments. The simulations reproduced the full penetration process, including radial cracks from constrained lateral expansion, circumferential cracks from unloading tensile waves after the projectile’s compressive effect vanished, the funnel-shaped spallation crater, and the steady-state cylindrical tunnel phase, with projectile deceleration curves and instantaneous penetration depths matching the measured data closely. The authors note that their tests covered a single projectile size at roughly 700 meters per second under normal incidence, so velocity dependence, scale effects and oblique or repeated impacts remain for future study. Even so, the validated model and the material itself point toward practical use as external protective layers for nuclear power plants, substations and oil storage facilities, where a shield that is both lighter and harder to penetrate could reshape how critical infrastructure is defended.

Subject of Research: Anti-penetration performance of a new lightweight high-performance C120 concrete

Article Title: Research on the anti-penetration performance of a new type of lightweight high-performance concrete

Article References: Liu, J., Zhang, C., Wang, G., Cao, S., Li, L., Dang, A., & Liu, X. (2026). Research on the anti-penetration performance of a new type of lightweight high-performance concrete. Case Studies in Construction Materials, 25, Article e06592. https://doi.org/10.1016/j.cscm.2026.e06592

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06592

Keywords: lightweight concrete, high-performance concrete, anti-penetration, projectile impact, protective engineering, ceramsite aggregate, steel fibers, finite element simulation, penetration depth, compressive strength, critical infrastructure, materials science