Glass Fibers Turn Brittle Tunnel Liners Into Energy-Absorbing Shields

Deep underground engineering is pushing into territory where conventional support materials are failing. As tunnels in China extend into deeper zones marked by high in-situ stress, intense excavation disturbance, and large deformation of surrounding rock, traditional shotcrete has revealed a fatal weakness: it is brittle. When squeezed and flexed by deforming rock masses, rigid concrete linings are prone to sudden fracture and spalling, releasing accumulated elastic energy in violent, unpredictable bursts. A new study published in Case Studies in Construction Materials offers a detailed answer to this problem, showing how chopped glass fibers can transform a thin spray-on liner from an elastic-brittle material into a ductile, energy-dissipating shield for deep tunnels.

The material at the center of the research is the thin spray-on liner, or TSL, a flexible support layer sprayed onto rock surfaces in layers typically 5 to 20 millimeters thick. Unlike rigid shotcrete, a well-designed TSL penetrates micro-fractures in the rock during application, binds fragmented rock into a continuous load-bearing composite, and forms a shallow load-bearing arch in partnership with the surrounding ground. Commercial TSLs fall into two families: latex-based products, which are highly ductile but prohibitively expensive, and polymer-modified cementitious liners, which are cost-effective but historically too stiff and brittle for the demands of deep tunnels with large deformation. The research team, led by Xing Gao and Jinlong Teng, set out to close that performance gap with fiber reinforcement.

The researchers formulated a two-component cementitious TSL from white cement, fine quartz sand, vinyl acetate-ethylene copolymer powder, and various admixtures, then added chopped glass fibers at four dosage levels: 0, 0.5, 1.0, and 1.5 percent of the powder mass. The fibers themselves were slender, 6-millimeter strands with a tensile strength of 555 megapascals and an aspect ratio of nearly 484. Specimens were cast, cured for seven days under controlled temperature and humidity, and then subjected to uniaxial compression and three-point bending tests, each group run with six parallel samples to ensure statistical reliability.

What distinguishes this study from earlier work is its insistence on measuring toughness across the entire loading-to-failure process, not just at the peak. Using an energy-based framework rooted in the first law of thermodynamics, the team partitioned the compressive response of each specimen into four stages: compaction, elastic deformation, stable crack expansion, and post-peak accelerated crack growth. By integrating the stress-strain curves, they separated the total input energy into stored elastic energy and dissipated energy, the latter consumed by crack closure, crack propagation, and frictional sliding on fracture surfaces. From this accounting they derived a pre-peak toughness index, a post-peak toughness index normalized against the unreinforced material, and a coupled comprehensive compressive toughness index formed by their product.

The results traced a clear arc of improvement followed by decline. Compressive strength rose from 12.64 megapascals in the unreinforced liner to a peak of 14.75 megapascals at 1 percent fiber content, then fell back to 12.85 megapascals at 1.5 percent. Energy absorption followed the same pattern, peaking at 317.9 kilojoules per cubic meter at the 1 percent dosage. Failure modes evolved in parallel: the unreinforced material split with two longitudinal tensile cracks and shed fragments, the 0.5 percent mix developed a combined tension-shear crack pattern, the 1 percent mix failed through a single diagonal shear band while remaining intact, and the 1.5 percent mix displayed a complex tri-directional shear crack network with pronounced ductile dilation. The comprehensive compressive toughness index nearly doubled, climbing from about 0.48 to 0.93 across the fiber range.

Flexural testing told an equally striking story. Flexural strength peaked at 8.47 megapascals at 1 percent fiber content, a 77.2 percent gain over the unreinforced matrix, while fracture energy surged more than fivefold, from 408.53 newtons per meter to 2189.69 newtons per meter at the highest fiber dosage. The flexural toughness ratio climbed steadily from 0.22 to nearly 0.68. Crack lengths in bending shrank progressively from 37.5 millimeters in the unreinforced beams to 27 millimeters in the most heavily reinforced ones, and the post-peak load response shifted from a cliff-like brittle drop to a gentle stress-tailing curve sustained by the dense fiber network bridging the main crack.

To unify these findings, the team constructed a comprehensive toughness index R that multiplies the pre-peak compressive index, the post-peak compressive index, and the flexural toughness ratio. The multiplicative, non-compensatory design ensures that weakness in any single dimension drags down the overall score. The index rose from roughly 0.10 to 0.63 as fiber content increased, and unlike the flexural-to-compressive strength ratio, which peaked at 1 percent and then declined, R kept rising because it captured the continuously improving post-peak energy dissipation. Independent validation against failure modes and crack patterns confirmed that R tracked actual toughness evolution more faithfully than strength-based measures alone.

Beneath the macroscopic numbers lies a more complicated microscopic story, one the authors describe as a competition between positive toughening and negative damage. Microscopy of fracture surfaces and helium pycnometry revealed that fibers hinder the migration and escape of air bubbles during mixing and compaction, a cofferdam effect that traps pores near fiber intersections and, at high dosages, produces fiber agglomeration and continuous defect zones. Porosity fluctuated in a rise-fall-rise pattern, reaching its lowest value of 10.54 percent at the 1 percent dosage, where the fiber network achieved its optimal spatial arrangement. This pore evolution explained the ladder-like drop in elastic modulus and the sharper strength penalty under bending, which is far more defect-sensitive than compression: flexural strength fell 25.9 percent at 1.5 percent fiber content, against only 13.2 percent for compressive strength.

Field-emission scanning electron microscopy exposed the three-stage energy dissipation mechanism that gives the material its resilience. First, hydration products, including calcium silicate hydrate gel, calcium hydroxide platelets, and polymer films, tightly encapsulate the fiber surface, forming a dense interfacial transition zone that debonds under load and converts mechanical energy into debonding work. Second, the debonded fiber slides against rough matrix pore walls, dissipating energy as frictional heat during crack opening. Third, when the stress on a bridging fiber exceeds its ultimate tensile strength, the fiber fractures in a flush, brittle break, completing the final stage of energy absorption. Crucially, the team found that this positive toughening mechanism dominates over the pore-related damage, which mainly erodes strength rather than toughness.

The practical upshot is a clear design recommendation: 1 percent glass fiber content delivers the best overall balance of strength, toughness, cost, and pumpability, since higher dosages raise the risk of pipe clogging during spraying. The authors caution that their results come from laboratory-cast specimens, and that fiber orientation, moisture migration, and substrate constraint in real sprayed layers may shift performance, so field validation on rock surfaces remains necessary. Even so, the study delivers something the field has lacked: a quantitative, multi-scale framework that links fiber dosage, pore structure, interfacial micromechanics, and full-process energy dissipation. For engineers contending with squeezing ground and violent rockbursts in ever-deeper tunnels, that framework turns a promising flexible liner into a rationally designed one.

Subject of Research: Multi-scale toughness evaluation and fiber toughening mechanisms in cementitious thin spray-on liners for deep tunnel support

Article Title: Study on multi-scale toughness evaluation and toughening mechanism of fiber-reinforced cementitious thin spray-on liner

Article References: Gao, X., Teng, J., Feng, J., Liu, Y., Ma, L., Bu, X., & Cheng, L. (2026). Study on multi-scale toughness evaluation and toughening mechanism of fiber-reinforced cementitious thin spray-on liner. Case Studies in Construction Materials, 25, Article e06593. https://doi.org/10.1016/j.cscm.2026.e06593

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06593

Keywords: thin spray-on liner, glass fiber reinforcement, cementitious composites, tunnel support, toughness evaluation, energy dissipation, fracture energy, porosity, fiber-matrix interface, deep underground engineering, polymer-modified cement, flexural strength