The material at the center of the campaign is a PET fiber mortar. The binder was a ternary blend of ordinary Portland cement, rapid-hardening sulfoaluminate cement and Class F fly ash, with undensified silica fume as a densifying admixture. The fiber was a virgin polyester monofilament 6 mm long, with a tensile strength of 616 MPa, an elastic modulus of 5620 MPa and an elongation at break of 23 percent. Hydroxypropyl methylcellulose regulated water retention, a polycarboxylate-ether superplasticizer controlled dispersion, and natural medium sand served as fine aggregate at a binder-to-sand ratio of roughly 1:2.5 and a water-to-binder ratio of 0.33. Each cubic meter of the base mix contained 395 kg of Portland cement, 44 kg of sulfoaluminate cement, 132 kg of fly ash and 1460 kg of sand.

Four admixture-level factors were varied in a four-factor, three-level Box-Behnken design of 27 runs: PET fiber dosage from 0.9 to 1.5 kg per cubic meter, superplasticizer from 3 to 7 per mille of binder mass, cellulose ether from 0.3 to 0.7 per mille, and silica fume from 13 to 31 kg per cubic meter. Six responses were measured per run: 28-day compressive strength, 28-day flexural strength, 7-day and 28-day drying shrinkage, 28-day water loss rate, and fresh flowability assessed with a truncated-cone flow-table test. The maximum measured compressive strength reached 56.76 MPa. The placement target for the manufacturing process was a flow spread near 180 mm, enough to let the steel truss be pressed into position without sinking to the bottom of the panel.

The original campaign used the classic Derringer-Suich desirability construction, mapping each predicted response onto an individual desirability score between zero and one and aggregating them through a weighted geometric mean. The reported optimum lay at a PET dosage of 1.500 kg per cubic meter, a superplasticizer dose of 6.881 per mille, a cellulose ether dose of 0.700 per mille and a silica fume dosage of 27.176 kg per cubic meter, with an overall desirability of 0.768. A three-specimen validation batch confirmed all six point predictions within 5 percent of the response-surface predictions. That deterministic reference establishes local agreement between predictions and one measured batch, but it does not quantify the probability that all six responses will meet their criteria simultaneously in a future mix.

To address that gap, the team built a multi-output Gaussian process with intrinsic coregionalization, coupling the six standardized response functions through a task-covariance matrix and a shared Matérn-5/2 kernel with one lengthscale per coded factor. At any candidate mix the model returns a posterior mean vector and covariance matrix, and adding modeled observation noise gives the predictive distribution of a future run-summary vector. Integrating that distribution over the stated six-response event produces a probability field over the entire four-factor design box rather than a single value at the desirability optimum. Because the six events are correlated through the joint covariance, the satisfaction probability has no closed form and is estimated by Monte Carlo simulation with hundreds of thousands of common Gaussian draws.

The fitted ARD lengthscales of approximately 8.89, 6.32, 18.66 and 9.68 coded units exceed the side length of the design box, so the latent response surfaces vary smoothly over the experimental domain. Structural sensitivity comparisons show how the joint assessment depends on the covariance representation. Under the same Matérn-5/2 kernel and a lengthscale upper bound of 30 coded units, the future-observation probability at the desirability optimum was 0.809 for rank 1, 0.927 for rank 2, 0.931 for rank 3 and 0.916 for rank 6. Changing the kernel to Matérn-3/2 gave 0.890, an RBF kernel gave 0.858, and restricting the lengthscale upper bound to 3 coded units dropped the probability to 0.629. These comparisons describe structural sensitivity rather than an empirical selection rule.

The team then compared nine surrogate families and two ensembles under a common outer-excluded error rule. Each of the 27 outer folds removes the same test run for every entry, with scaling, variable selection, likelihood fitting and prediction using only the remaining 26 rows. The pool spans quadratic response surfaces, regularized polynomials, support vector regression, random forest, XGBoost, independent Gaussian processes, the multi-output coregionalization model and sparse polynomial chaos expansion, together with pseudo-Bayesian model averaging and stacking. Different families performed best on different responses: support vector regression had the lowest outer leave-one-out RMSE for compressive strength and water loss, the multi-output Gaussian process for flexural strength and both shrinkage responses, and stepwise response surface methodology for flowability. Full quadratic RSM remained competitive for compressive strength, water loss and flowability, with coefficients of determination of 0.957, 0.921 and 0.974 respectively.

Under the fixed one-sided 5 percent relative event, the eight prediction-qualified entries placed the original robust candidate F01 among the best locations found over the entire four-factor box, with an empirical scenario attainment of 16 out of 27. The desirability optimum F02 and the model-form candidate F03 gave 15 and 10 out of 27 respectively. With all eleven entries included, the best found minimum dropped to only 3 out of 27, showing how a worst-case recommendation depends on the predictive quality and diversity of the models in the pool. Joint prediction-set diagnostics using a foldwise maximum-score jackknife+ construction included 25 of 27 joint response vectors at alpha 0.10 and 26 of 27 at alpha 0.05, with theoretical lower bounds of 0.80 and 0.90 under exchangeability. Split-mean and split-conformalized quantile regression methods gave comparable inclusion counts but wider intervals, particularly for the quantile approach, which produced a mean compressive-strength interval width of 17.8 MPa.

The 21 historical independently mixed batches at 12 formulations provided measured outcomes for comparing candidates. Under the fixed six-response benchmark, the repeated F01, F02 and F03 series achieved 3 of 3, 5 of 6 and 1 of 3 joint successes. Twenty of the 21 records satisfied the compressive-strength threshold, but only 14 satisfied all six thresholds simultaneously. The single-batch comparison at F09 illustrates the trade-off: its measured compressive strength of 54.33 MPa exceeded the values in the F01 series, but its 28-day shrinkage of 0.0490 percent and flow of 171 mm both fell outside the corresponding limits of the fixed relative event. A high-strength location can therefore expose a drying or fresh-state compromise that is less apparent in the aggregate desirability score.

The continuous-domain integration of the frozen future-observation field estimated the region with attainment at least 0.9 at 0.000659 percent of the design-box volume, with an at-least-95 percent simultaneous numerical interval of 0.000593 to 0.000763 percent. This narrow window quantifies spatial-sampling and inner-integration uncertainty conditional on the fitted model, not model-estimation or physical-process uncertainty. The team also examined published confirmation cases from grout, 3D-printing mortar and coal-mine backfill studies, with mean absolute percentage errors of 2.72 to 8.32 percent. In the 3D-printing case, favorable point predictions and successful buildability were reported, but the measured flow of 147.5 mm fell below the selected 150 to 180 mm optimization window, illustrating that a workflow checking both margins would flag flow for confirmation before adopting the formulation. In the backfill case, strength predictions agreed closely with confirmation measurements, yet the source found the strength insufficient for the intended application and adjusted the binder proportions. Accurate prediction of an optimum therefore remains distinct from meeting application requirements.

At the author-supplied prices, the historically supported robust candidate F01 has a listed product cost of approximately 69.21 yuan per square meter, close to the 69.24 yuan per square meter for the desirability choice F02. The calculated material-cost difference and break-even condition relate the multiresponse screening decision to the additional expenditure and quality outcomes needed to justify implementation. Over 10,000 square meters, the material-cost difference between F01 and F02 amounts to 276.84 yuan, so expenditure above that amount would require an independently supported benefit in quality or decision making. The recorded single-thread implementation takes approximately 139 seconds for nested fitting of the eleven entries, 84 seconds for full-domain searches and 141 seconds for continuous volume integration. Computation takes minutes; specimen preparation and 28-day testing determine the experimental schedule.

The study concludes that joint-response analysis complements desirability optimization by distinguishing favorable mean responses from simultaneous attainment under stated uncertainty assumptions. The 27-run PET-mortar campaign provides a common experimental basis for comparing these two aspects of formulation performance, and the 21 historical batches show practical trade-offs across 12 formulations. Observation-noise treatment, model structure and response dependence materially affect predicted joint attainment. Three published confirmation cases extend the quantitative comparison to other mixture applications, and the placement target near 180 mm relates the present study to steel-truss floor-panel manufacture. The workflow informs mixture selection and confirmation, while product conformity requires evidence from the corresponding product tests.

Subject of Research: Probabilistic reliability assessment and robust design of PET fiber mortar

Article Title: Probabilistic reliability assessment and robust design of PET fiber mortar

Article References: Probabilistic reliability assessment and robust design of PET fiber mortar. (n.d.). https://doi.org/10.1016/j.rineng.2026.113414

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113414

Keywords: Probabilistic, reliability, assessment, robust, design, fiber, mortar, scientific research

The motivation for the work is grounded in one of the largest waste streams on the planet. The global textile sector generates more than 92 million tons of waste annually, and denim, composed primarily of cotton cellulose fibers, makes up a significant share of both post-consumer and post-industrial textile refuse. While recycling initiatives have improved diversion rates in recent years, a substantial fraction of this material is still landfilled, contributing to soil contamination, groundwater pollution, and greenhouse gas emissions. Recycling textile waste is technically difficult because fabrics are heterogeneous, often blended, and laced with chemical additives from processing. Yet embedding waste textile fibers into polymer matrices has emerged as a promising strategy to transform low-value waste into functional materials for structural, thermal, and acoustic applications.

Denim has particular appeal as a reinforcement. Its woven structure, high fiber content, and residual mechanical integrity allow recycled denim composites to achieve competitive performance compared with some conventional glass-fiber-reinforced systems, while offering clear sustainability advantages. Jute, meanwhile, is one of the most widely available and cost-effective natural fibers in the world, prized for its favorable stiffness and tensile strength among bast fibers. Hybridizing the two waste streams seemed like an obvious win. But there was a catch the researchers deliberately chose to confront: untreated jute fibers carry a surface burden of waxes, lignin, hemicellulose, and pectins that inhibit effective wetting and bonding with hydrophobic epoxy matrices. Chemical treatments such as alkali treatment, silane coupling, and acetylation can fix this problem, but they add cost, processing complexity, and chemical usage that erode the environmental logic of natural fiber composites in the first place.

The experimental design was elegantly simple. The team fabricated two laminates by hand lay-up, each containing identical amounts of epoxy resin, hardener, and recycled denim. The denim, a heavyweight 3/1 twill cotton fabric with an areal density of 300 to 400 grams per square meter, came from pre-consumer offcuts at jeans factories and was used as received, without washing or treatment. The jute, by contrast, was recovered from used and deteriorated jute bags, washed, solar-dried for two hours, and cut into short fibers roughly five to ten millimeters long. Five denim layers were stacked in each mold; in the hybrid version, 5.80 grams of untreated jute, about 1.7 percent of the laminate by weight, was first mixed into the epoxy resin and brushed between the denim plies. The laminates were compressed under a 30-kilogram load, cured for 72 hours at ambient conditions, and post-cured at 60 degrees Celsius for one hour, yielding final thicknesses of approximately 4.1 millimeters.

The mechanical results tell a story of good intentions colliding with interfacial chemistry. In tensile testing according to ASTM D3039, the denim-only composite achieved an ultimate tensile strength of 31.4 plus or minus 0.2 megapascals at a strain of 10.5 percent, exhibiting continuous strain hardening as the woven cotton yarns progressively straightened and reoriented under load. The jute-containing hybrid plateaued at just 25.2 plus or minus 0.3 megapascals, fracturing at less than half the strain. That represents a 19.5 percent reduction in strength and a 59 percent reduction in strain at maximum stress. The energy penalty was even more dramatic: numerical integration of the stress-strain curves showed the denim composite absorbed 2.69 megajoules per cubic meter up to peak stress, while the hybrid managed only 0.80, a roughly 70 percent loss in tensile energy absorption. Initial stiffness, by contrast, was nearly identical between the two materials, at around 1.3 gigapascals, indicating the jute only revealed its destructive influence once significant deformation began.

Flexural and impact testing confirmed the same pattern. In three-point bending, flexural strength fell from 67.8 plus or minus 3.4 megapascals for the denim composite to 55.5 plus or minus 4.6 megapascals for the hybrid, an 18 percent drop, while flexural modulus declined 13 percent, from 2,843 to 2,482 megapascals. Unnotched Charpy impact tests told the most sobering story: impact strength plummeted 33 percent, from 8.30 to 5.60 kilojoules per square meter. Poor adhesion between the untreated jute and the epoxy creates weak zones that act as stress concentrators under sudden loading, facilitating crack initiation, fiber pull-out, and premature fracture. The researchers note that in bending, the compressive half of the section and the denim layup partially mask the weak interface, whereas in pure tension the poorly bonded jute phase governs failure, which explains why the energy loss was so much larger in tensile loading.

Scanning electron microscopy of the fractured surfaces provided direct, visually compelling evidence for the mechanism. The denim-epoxy composite showed fibers well embedded in the matrix with good interfacial interaction, with matrix residue clinging to the cotton yarns and indicating effective load transfer. The hybrid composite told a different story: inadequate fiber wetting, extensive fiber pull-out, microvoids, and clean jute fiber surfaces with visible gaps at the interface, the microscopic fingerprints of debonding at low stress. The pulled-out jute appeared as split technical-fiber bundles, a phenomenon known as fibrillation, in which failure proceeds by separating elementary fibers held together by a pectin- and lignin-rich middle lamella rather than by fracturing the fiber itself. Because bundle splitting and pull-out dissipate little energy, these morphological features explain the early plateau in the tensile curves, the severe loss of energy absorption, and the depressed impact resistance of the hybrid material.

Interestingly, the thermal and moisture results were not uniformly negative for the jute hybrid. Thermogravimetric analysis showed both composites exhibited comparable degradation profiles, with the principal mass-loss region between 300 and 450 degrees Celsius and maximum decomposition rates at approximately 365 degrees Celsius for both. Differential scanning calorimetry revealed a main endothermic decomposition peak at 388.1 degrees Celsius for the denim composite and a slightly lower peak at 381.3 degrees Celsius for the hybrid, consistent with the earlier onset of hemicellulose decomposition in the jute phase. Notably, the hybrid retained 8.3 percent char residue at 600 degrees Celsius, while the denim-only composite was almost fully volatilized. Even more surprising, the hybrid absorbed less water, 5.60 percent versus 6.39 percent, and took longer to saturate, 384 hours versus 288 hours. The researchers attribute this to the jute-filled resin occupying the inter-yarn channels that otherwise act as wicking pathways in the denim-only laminate, rather than to the jute surface chemistry itself.

The practical upshot is a clear engineering directive rather than a dead end. The denim-epoxy composite, with its balanced property set, is well suited for low-to-moderate load-bearing, non-safety-critical applications such as furniture panels, interior partitions, automotive trim, packaging inserts, and equipment casings. The jute hybrid, with lower strength but slower water uptake and reduced porosity, may find a niche in indoor panels for humid environments where mechanical demands are modest. But the study’s most important contribution may be its cautionary message for the green materials movement: untreated jute fibers behaved as defect sites rather than reinforcements, and the literature the authors compiled shows that alkali, silane, or acetylation treatments can substantially improve jute-epoxy performance. Until surface treatment is incorporated, the authors conclude, the promise of denim-jute hybrid composites will remain unfulfilled, a reminder that in sustainable materials engineering, chemistry at the interface matters as much as the sustainability of the supply chain.

Subject of Research: The effect of untreated jute fiber incorporation on the mechanical, thermal, and hygroscopic properties of recycled denim-epoxy hybrid composites

Article Title: Effect of untreated jute fiber incorporation on the properties of recycled denim–epoxy hybrid composites

Article References: Hossen, R., Apu, M. U., Neha, T. R., Ahasan, E., Islam, M. R., & Mim, J. J. (2026). Effect of untreated jute fiber incorporation on the properties of recycled denim–epoxy hybrid composites. Results in Engineering, 32, Article 112828. https://doi.org/10.1016/j.rineng.2026.112828

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.112828

Keywords: Effect, untreated, jute, fiber, incorporation, properties, recycled, denim, epoxy, hybrid, composites, scientific research