Glass Meets Plastic: Hybrid Fibres Crack the Code for Low-Carbon Bendable Concrete

Concrete is famously strong in compression and famously terrible in tension. For decades, engineers have compensated by drowning brittle cement matrices in steel reinforcement, but a new generation of thin, lightweight building components—façade panels, cladding boards and deck slabs—demands something subtler: materials that resist the first crack, then keep stretching and bending long after that crack appears. A study published in Case Studies in Construction Materials by researchers including Iqra, Khin Soe, Richard (Chunhui) Yang and Y.X. Zhang reports a promising recipe that blends two very different fibres into a low-clinker binder, and pairs the mechanical results with an imaging breakthrough that finally makes the invisible fibres visible in three dimensions.

The team’s starting point was the engineered cementitious composite, or ECC, a class of material designed to bend without shattering. Instead of failing with a single catastrophic crack, a well-designed ECC strain-hardens: once the matrix cracks, embedded fibres bridge the gap and the load keeps rising as a network of fine, tightly controlled microcracks spreads through the material. Polyethylene (PE) fibres are the stars of this show. Previous studies have reported tensile strain capacities of six to eight percent in PE-reinforced ECCs—hundreds of times the stretch of ordinary concrete—along with crack widths fine enough to keep water and corrosive agents out. But PE fibres are slippery and relatively compliant, which means they do little to raise the stress at which the very first crack forms.

That is where the hybrid strategy comes in. The researchers combined PE fibres with alkali-resistant (AR) glass fibres, which are stiffer, bond more strongly to the matrix, and are durable in the alkaline chemical environment of cement. The logic is complementary: glass fibres stiffen the composite and delay crack initiation, while PE fibres carry the load through the long post-cracking phase. Both fibre types were 12 millimetres long, with the glass fibres slightly thinner at 18 micrometres versus 24 micrometres for PE, and the glass fibres carrying more than 1000 megapascals of tensile strength against 3000 megapascals for PE. Crucially, the total fibre volume was held constant at two percent across all mixes, so any change in behaviour could be attributed purely to the ratio between the two fibre types.

The binder itself was chosen with the climate in mind. Rather than ordinary Portland cement, the mixes used a blend of 30 percent OPC, 20 percent reactive magnesium oxide and 50 percent ground granulated blast-furnace slag, a by-product of steelmaking. The high slag content slashes clinker consumption, while the reactive MgO promotes the formation of brucite and magnesium silicate hydrate phases that densify the matrix and improve strength retention. Water-to-binder and sand-to-binder ratios were fixed at 0.25 and 0.36. Five mixes were tested: an unreinforced control, a conventional 2 percent PE mix, and three hybrids in which 0.5, 0.75 and 1.0 percent of the PE was swapped for glass fibre.

The mechanical results reveal a clear trade-off. In compression, the plain control reached 71.90 megapascals, the highest of all mixes, thanks to its superior flow and compaction. Adding fibres reduced compressive strength modestly for the PE-only mix (67.31 megapascals, a 6.4 percent drop) but progressively more as glass content rose: 64.02, 60.11 and 54.47 megapascals for the three hybrids. The researchers attribute this to fibre agglomeration and trapped air, which increase porosity and weaken the fibre-matrix interface. Still, two of the three hybrids remained above the 55-megapascal threshold that American Concrete Institute guidelines use to define high-strength concrete.

In tension, the picture flipped. The unreinforced matrix cracked at about 3.1 megapascals and immediately failed, with a negligible strain capacity of 0.014 percent. Every fibre-reinforced mix, by contrast, satisfied the ECC ductility criterion of exceeding 0.5 percent tensile strain. The PE-only mix delivered the greatest stretch at 5.91 percent strain with a peak stress of 6.55 megapascals, but its first-cracking strength of 2.80 megapascals was the lowest among the fibre mixes. Replacing a quarter of the PE with glass fibre produced the most balanced tensile response: the 1.25P0.75G mix combined the highest first-cracking strength of 4.13 megapascals with a peak stress of 6.12 megapascals and a still-respectable 2.84 percent strain capacity. Beyond that ratio, ductility fell steadily, dropping to 2.08 percent when the fibres were split evenly.

Flexural testing told a similar story with a twist. The plain beam snapped at 2.99 megapascals after just 0.35 millimetres of deflection. All fibre mixes showed deflection-hardening, but the sweet spot was the 1.5P0.5G mix, which paired a first-cracking strength of 7.81 megapascals and a peak flexural stress of 13.78 megapascals with the largest ultimate deflection of any mix, 21.66 millimetres—actually exceeding the PE-only mix’s 19.95 millimetres. The small dose of glass fibre delayed crack localisation while the remaining PE fibres sustained large post-cracking deformation. Pushing glass content higher raised first-cracking resistance further, up to 10.68 megapascals in the 1P1G mix, but shrank deflection capacity to 14.82 millimetres, confirming that PE fibres are the indispensable ingredient for bending ductility.

Perhaps the most technically impressive part of the study is the imaging. X-ray micro-computed tomography is a powerful non-destructive tool, but PE fibres attenuate X-rays so weakly that they are nearly invisible against a cementitious matrix, which is why most micro-CT studies have focused on steel fibres. The researchers solved this with a dedicated workflow on a SkyScan 1273 scanner: 100 kilovolts, an aluminium filter, 2500-millisecond exposures and a voxel size of about 6 micrometres, followed by segmentation that combined grey-value thresholds with an aspect-ratio filter to pick out elongated PE-fibre-like features geometrically. The result was full three-dimensional maps of pores, glass fibres and PE fibres. These showed fibre clustering worsening as glass content increased, and detectable porosity climbing from 3.47 percent in the plain matrix to 5.71 percent in the 1P1G mix, with the pore-size distribution broadening toward larger voids. A Pearson correlation analysis reinforced the trends: glass fibre content correlated strongly and positively with porosity and flexural first-cracking strength, but negatively with compressive strength and flowability, while PE content tracked tensile strain capacity and deflection with correlation coefficients around 0.98 and 0.94.

The sustainability accounting adds an honest caveat. The binder dominated the carbon footprint, with reactive MgO the largest single contributor, and fibres added further burden: embodied carbon rose from 0.864 tonnes of CO2-equivalent per cubic metre for the plain mix to 0.963 for the best hybrid, while primary energy climbed from 4.462 to 6.854 gigajoules per cubic metre, driven largely by the energy-intensive PE fibre. Even so, the 1.5P0.5G mix emerged as the best overall compromise, delivering high flexural strength, exceptional deflection capacity and acceptable compressive performance at a moderate environmental cost. For serviceability-critical elements such as thin façade panels, where crack control and deformation tolerance matter more than raw strength, hybrid PE-glass fibre reactive magnesia-slag composites offer a credible path toward low-clinker infrastructure that bends instead of breaking—and, thanks to the adapted micro-CT method, engineers can now see exactly what is happening inside.

Subject of Research: Hybrid polyethylene and glass fibre reinforcement of reactive magnesia-slag cementitious composites

Article Title: Effect of hybrid fibres on reactive magnesia-slag composites: mechanical behaviour, 3D microstructure by micro-CT and sustainability

Article References: Iqra, Soe, K., Yang, R. C., & Zhang, Y. (2026). Effect of hybrid fibres on reactive magnesia-slag composites: mechanical behaviour, 3D microstructure by micro-CT and sustainability. Case Studies in Construction Materials, 25, Article e06576. https://doi.org/10.1016/j.cscm.2026.e06576

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06576

Keywords: engineered cementitious composites, polyethylene fibres, alkali-resistant glass fibres, reactive magnesia, ground granulated blast-furnace slag, strain-hardening, micro-CT, porosity, flexural behaviour, embodied carbon, low-clinker binder, fibre dispersion