A team of researchers led by Jie Wang and Kunfeng Kong collected mixed demolition waste from a recycling source in Zhengzhou, Henan Province, China. The material, derived from demolished brick-concrete buildings, was crushed, screened, and cleaned of impurities before testing. Particle-by-particle analysis showed that recycled crushed stone, recycled concrete, and recycled clay brick together accounted for more than 95 percent of the mass in every size fraction examined, with impurities below 3.5 percent. Because demolition waste from brick-concrete structures naturally contains all three components mixed together rather than separated, the researchers argue this composition is representative of what recycling plants actually supply, making large-scale reuse more practical than processes demanding careful component sorting.

The material’s physical properties reveal why it behaves so differently from natural aggregates. Its water absorption of 8.5 to 9.8 percent and crushing value of 29.5 percent are far higher than those of conventional fill materials, reflecting the porous, weaker nature of recycled concrete and clay brick particles. Its optimum moisture content of roughly 13.5 percent is considerably higher than that of natural granular fills of similar gradation, while its maximum dry density of 1.827 grams per cubic centimeter is markedly lower. In practical terms, this means field crews must control moisture carefully during compaction, allow time for water to redistribute within the absorbent particles, and accept a lighter but more void-rich skeleton that is more sensitive to drainage conditions.

To characterize the material’s static strength, the team prepared cylindrical specimens 150 millimeters in diameter and 300 millimeters tall, compacted at three degrees of compaction representing well-compacted, insufficiently compacted, and relatively loose states, and tested them in a large-scale triaxial apparatus under confining pressures of 50, 100, and 150 kilopascals. The saturated drained shear tests showed clear strain-softening behavior: specimens reached a peak deviatoric stress and then weakened as shearing continued. Both peak strength and residual strength rose substantially with confining pressure and compaction. When confining pressure increased from 50 to 150 kilopascals, residual strength climbed by 106.8 percent, 87.3 percent, and 121.0 percent for specimens compacted to 96, 92, and 88 percent, respectively, demonstrating that even loosely packed waste fill gains considerable post-peak capacity when laterally confined.

The stiffness results carried a subtler message. The secant modulus at 50 percent of peak stress, a standard measure of deformation stiffness, was far more sensitive to compaction at low confining pressure than at high. At 50 kilopascals, the modulus of 96-percent-compacted specimens exceeded that of 88-percent-compacted specimens by more than 111 percent. Yet raising confining pressure from 50 to 150 kilopascals improved the modulus by only 6.8 percent for the densest specimens, compared with 77.3 percent for those at 92 percent compaction. The implication is that loosely compacted waste fill benefits enormously from lateral confinement, making it best suited to subgrade zones where surrounding material can provide strong restraint, such as well-compacted internal fill layers or track-supporting layers.

Volumetric behavior added another dimension. Under shear, the specimens first contracted slightly and then dilated, with the maximum dilation exceeding that of conventional natural subgrade fills. Denser specimens dilated more because their tight initial contacts left little room for further compression, forcing deformation to proceed through particle rotation and interlocking. Looser specimens contracted instead, densifying progressively. The researchers note that this contraction-dilation response matters for railway engineering: contraction signals ongoing settlement accumulation, while dilation indicates a well-interlocked skeleton but, near critical stress levels, may disturb interfaces with overlying ballast or track layers.

The heart of the study, however, lay in its long-term cyclic triaxial tests, which subjected specimens to 10,000 loading cycles at 1 hertz under saturated drained conditions. Here the team introduced the shear stress ratio, or SSR, defined as the applied cyclic deviatoric stress divided by the material’s static peak strength measured under identical compaction and confinement. This strength-normalized measure proved decisive. When SSR remained below roughly 0.5, cumulative plastic deformation grew slowly and stabilized as the particle skeleton compacted and rearranged. Once SSR exceeded about 0.5, the fitted curves steepened sharply and deformation accelerated toward incremental failure. Grey relational analysis confirmed SSR as the single most influential variable, ranking ahead of confining pressure and degree of compaction.

Perhaps the most striking finding concerned densely compacted specimens under high stress. At an SSR of 0.8, the final cumulative deformation of the 96-percent-compacted specimens was about 3.7 times that of the 92-percent specimens, the largest relative increase among all compaction states. The explanation lies in the dense skeleton’s limited remaining void space: once cyclic stress approaches the mobilized strength capacity, particle sliding, abrasion, and breakage are repeatedly activated, and with no room left for gradual compaction, the structure responds through dilation and abrupt loss of interlocking. Looser fills, by contrast, can still absorb cyclic deformation through progressive densification. In other words, compaction alone does not guarantee stability if the traffic-induced stress level creeps too close to the material’s strength reserve.

The resilient modulus, which quantifies the recoverable elastic stiffness under repeated loading, generally increased with compaction, confining pressure, and cyclic deviatoric stress. Dropping compaction from 96 to 88 percent reduced the modulus by roughly 13 to 25 percent across the tested stress states. But the researchers caution against a tempting misreading: a rising resilient modulus does not necessarily indicate a healthy structure. Correlation analysis across 21 matched loading conditions found only a moderate linear association between modulus evolution and final cumulative plastic deformation, meaning the recoverable and irreversible components of deformation can evolve independently. A subgrade may appear stiff while quietly accumulating permanent settlement.

From these results the authors distill a preliminary design framework: compaction near 96 percent is preferred when cyclic stresses stay within the stable range; an SSR at or below 0.30 offers a conservative reference for long-term service; the range between 0.30 and 0.50 warrants caution; and SSR at or above 0.50, associated with accelerated deformation, should be avoided under conditions comparable to those tested. The team stresses that these thresholds are laboratory observations specific to the tested gradation, moisture state, and loading regime, not universal design limits, and that field-scale and in-situ verification remain necessary. Unresolved questions include the effects of wetting-drying cycles, freeze-thaw weathering, particle breakage quantification, and varying train speeds. Still, the study offers railway engineers a concrete, stress-based recipe for turning demolition debris into dependable subgrade, a step that could simultaneously ease landfill pressure and relieve the aggregate shortage confronting infrastructure construction worldwide.

Subject of Research: Strength and deformation behavior of brick-concrete construction waste used as railway subgrade fill

Article Title: Analysis of the strength and deformation characteristics of brick-concrete construction waste used as subgrade fill

Article References: Wang, J., Kong, K., Li, Y., Lin, L., Wang, Z., Chen, F., Yao, J., Wang, M., & Xiao, Y. (2026). Analysis of the strength and deformation characteristics of brick-concrete construction waste used as subgrade fill. Case Studies in Construction Materials, 25, Article e06565. https://doi.org/10.1016/j.cscm.2026.e06565

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06565

Keywords: construction waste, railway subgrade, recycled aggregates, triaxial testing, shear stress ratio, cumulative deformation, resilient modulus, strain softening, compaction, cyclic loading, geotechnical engineering, sustainable materials

The work matters because most existing material stock and flow analysis, the field that tracks the resources accumulated in our buildings and infrastructure, operates at coarse administrative scales. National or prefectural averages can tell you roughly how much concrete a country holds, but they blur out the dramatic differences between a thriving station district and a hollowing-out hillside neighborhood. That blindness has real consequences: planners cannot target recycling infrastructure, anticipate waste flows, or identify emerging vacancy hotspots when their data is smeared across entire municipalities.

The research team, led by Masatoshi Hasegawa of Nagoya University together with Hiroaki Shirakawa, Marianne Faith Martinico-Perez, Osamu Higashi, and Hiroki Tanikawa, attacked this problem with four-dimensional geographic information systems, essentially three-dimensional city models extended through time. By overlaying building footprint datasets from 2010 and 2018 and matching records across the two snapshots, they identified which structures had vanished. Every unmatched building was flagged as demolished, creating a demolition record for thousands of individual structures in a shrinking industrial city.

Onto that spatiotemporal foundation, the researchers layered a statistical machine borrowed from reliability engineering: a Weibull accelerated failure time model. This survival analysis framework, routinely used to predict when machine parts fail or patients relapse, treats building demolition as an event whose timing depends on measurable covariates. Each building was assigned structural attributes such as construction type and use, spatial attributes including land use zone and slope angle, and demographic attributes capturing the aging rate of the surrounding population mesh.

The model’s parameters yield intuitive and sometimes counterintuitive insights. Apartment buildings in Kitakyushu are expected to last roughly thirty percent longer than the reference category, with median lifespans around 66 years, while detached houses cluster between roughly 51 and 53 years. Steel structures showed shorter expected lifespans than reinforced concrete in this dataset, a result the authors caution may partly reflect locational and redevelopment pressures rather than material durability alone. Commercial and industrial zone buildings outlived their residential-zone counterparts by more than twenty percent, echoing earlier findings that zoning shapes building longevity.

The most striking result concerns demographic aging. Neighborhoods with a higher share of residents aged 65 and older were associated with longer expected building lifespans, an effect that at first sounds benign but carries a warning. When populations shrink and age, buildings are replaced less often, not because they are wanted but because demand has collapsed. The researchers interpret this as a growing risk of vacancy: structures persist on the map even as they empty out, inflating apparent housing stock while the city’s population, which peaked at about 1.068 million in 1979, is projected to fall to roughly 729,000 by 2050.

Translating demolition probabilities into material tonnage, the team multiplied projected demolished floor area by material intensity factors derived from a Japanese government construction survey. Aggregate dominates the flows in every zone, followed by cement, with steel contributing a larger share in industrial areas and bitumen barely registering. A Monte Carlo simulation placed cumulative material outflow between 2018 and 2040 at 38.23 million tons, with a confidence interval spanning 37.63 to 38.95 million tons, a narrow 3.5 percent spread that speaks to the framework’s statistical stability.

The spatial forecasts reveal a city divided. Areas within 500 meters of railway stations are projected to generate an average of 55.2 thousand tons of demolition material per grid mesh from 2022 to 2040, more than double the 22.3 thousand tons expected beyond that radius. Inside the city’s Residential Induction Zones, designated for compact urban consolidation, material output is dominated by apartment buildings and peaks between 2041 and 2045 at up to 1.143 million tons annually. Outside those zones, detached houses and industrial facilities dominate, with output peaking earlier, between 2036 and 2040, and detached house demolition alone cresting at 233 thousand tons per year in the late 2020s.

Perhaps the most practically powerful feature of the approach is its flexibility. Because probabilities are estimated for individual buildings, results can be aggregated to any geography the user chooses: school districts, census meshes, station catchments, or administrative boundaries, without the sample-size collapse that afflicts methods that subdivide observations into ever finer categories. The authors argue this makes the framework ideal for small-start planning, where municipalities begin with a limited pilot district and expand incrementally, a mode well suited to cities with constrained data and budgets. It also positions buildings as urban mines, letting recyclers and policymakers anticipate where streams of recoverable steel, cement, and timber will surface decades ahead.

The researchers are candid about limitations. The dataset does not distinguish occupied homes from vacant ones, and with Kitakyushu’s vacancy rate climbing from 16.8 percent in 2013 to 19.1 percent in 2023, some surviving buildings are surely empty shells that overestimate true service life. Year-of-construction data, essential to the model, remains scarce in many Japanese municipalities due to privacy restrictions, and the team hopes their work will encourage broader data sharing. Still, the message resonates far beyond one Japanese city: as populations age and shrink across the developed world, the buildings left behind are not merely a planning problem but a vast, forecastable store of materials waiting to re-enter the economy, and the tools to map that future are now proven.

Subject of Research: Building-level demolition forecasting and construction material output prediction using 4d-GIS in a shrinking Japanese city

Article Title: Building-level demolition and material output forecasting using 4d-GIS: a case study of Kitakyushu City, Japan

Article References: Building-level demolition and material output forecasting using 4d-GIS: a case study of Kitakyushu City, Japan. (n.d.). https://doi.org/10.1007/s44498-026-00178-x

Image Credits: AI Generated

DOI: 10.1007/s44498-026-00178-x

Keywords: 4d-GIS, material stock and flow analysis, Weibull AFT model, urban shrinkage, building lifespan, demolition probability, circular economy, construction waste, Kitakyushu, Japan, survival analysis, urban planning