Search Results (60)

The Ningxia Yellow River Urban Agglomeration, located in the ecologically fragile arid and semi-arid zone of the upper Yellow River, serves as a critical spatial carrier for maintaining the ecological security of the Yellow River Basin and supporting the regional economy and population agglomeration in Ningxia. Driven by rapid urbanization, intensified human–land conflicts have induced widespread ecological degradation and unbalanced water–soil resource allocation across the region. Based on land use data from 2010, 2015, 2020 and 2023, we applied the land use transition matrix, land use dynamic degree and standard deviational ellipse to characterize the spatiotemporal patterns of spatial expansion of the Ningxia Yellow River Urban Agglomeration over the past decade. The Patch-generating Land Use Simulation (PLUS) model was further employed to predict the land use demand and spatial distribution of the study area under diverse scenarios in 2035. The research results reveal three key findings. First, grassland, cropland and unused land constitute the dominant land use types across the study region, jointly occupying more than 90% of the total territorial area. Over the past decade, regional land use has undergone noticeable changes: grassland area has continuously declined, cropland and built-up land have sustained steady expansion, and water areas have experienced a mild reduction. Land use conversions mainly occur among grassland, cropland and built-up land. Second, driving factors vary substantially in their spatial contributions to the expansion of different land use types. The spatial growth of cropland and built-up land is comprehensively shaped by terrain conditions, economic development and transportation location superiority. In comparison, the distribution and dynamic changes in forestland, grassland and water areas are predominantly restricted by natural elements, including precipitation, temperature and soil characteristics. Third, multi-scenario simulation results verify that differentiated territorial spatial planning and regulatory policies profoundly affect the evolutionary trajectory of regional territorial patterns. The natural development scenario experienced the most intensive expansion of built-up land, with a newly increased area of 181.11 km². The ecological protection scenario can effectively curb the loss of ecological land and minimize the shrinkage of grassland resources. The cropland protection scenario is conducive to stabilizing cropland scale to the greatest extent and restraining the disorderly sprawl of urban land. The sustainable development scenario realizes coordinated and balanced changes in all land use types and delivers mutually beneficial progress between regional ecological conservation and socioeconomic development. Full article

Lightweight aggregates have a porous structure and high water absorption, which may lead to underestimation of the powder content in conventional mix design methods for lightweight ultra-high-performance concrete (LUHPC). To address this issue, this study used ceramsite sand as the lightweight aggregate and combined the excess paste theory with the particle packing method to design and evaluate ceramsite-sand-based LUHPC mixtures based on the modified Andreasen packing model (APM) and the compressible packing model (CPM). By optimizing the particle size distribution of ceramsite sand and the binder composition, a mix design method suitable for ceramsite-sand-based LUHPC was developed. The workability, apparent density, mechanical properties, elastic modulus, and shrinkage behavior of the material with different steel fiber contents were systematically investigated. The results showed that the total binder content, water-to-binder ratio, and paste volume of the mixtures designed using the two models differed only slightly. However, the aggregate skeleton formed by CPM was denser, and its skeleton packing volume was approximately 3.5% lower than that obtained using APM. At the same steel fiber content, the main mechanical properties of the CPM-designed LUHPC were generally superior to those of the APM-designed mixtures. Specifically, the 28-day cube compressive strength increased by 5.0–7.6%, the axial compressive strength by 8.8–12.2%, the axial tensile strength by 6.4–25.8%, the flexural strength by 14.1–17.2%, and the shear strength by 3.1–6.5%. The elastic modulus was also slightly higher, while the shrinkage remained consistently lower. The CPM-2.0 LUHPC mixture achieved a 28-day cube compressive strength of 124.6 MPa and an apparent density of approximately 1982 kg/m³, realizing a compressive strength above 120 MPa at a density below 2000 kg/m³. The 28-day cube compressive strength of the CPM-3.0 mixture further increased to 131.7 MPa. As the steel fiber content increased from 1.5% to 3.0%, the workability of LUHPC decreased, whereas its compressive, tensile, flexural, and shear properties generally improved, and the elastic modulus increased slightly. Steel fibers effectively restrained shrinkage deformation, but the improvement showed diminishing marginal benefits with increasing fiber content. Considering the mechanical performance, shrinkage control, and material economy, a steel fiber content of approximately 2.0% is recommended as a reference range for ceramsite-sand-based LUHPC. Overall, CPM is more suitable than APM for the mix design of ceramsite-sand-based LUHPC and can provide guidance for mix optimization and performance regulation of lightweight ultra-high-performance concrete. Full article

Expansive soil slopes are highly susceptible to rainfall-induced shallow failures due to cyclic swelling–shrinkage behavior governed by matric suction variation. This study proposes a composite frame–geosynthetic system (CFGS), comprising a rigid frame integrated with high-performance turf reinforcement mats (HPTRMs), for expansive soil slope protection. The performance of the CFGS was evaluated through geometrically scaled, materially representative physical model tests under repeated wetting–drying cycles and further examined using coupled hydro-mechanical numerical simulations in COMSOL Multiphysics. A bare slope and an HPTRM-protected slope were used for comparison. Under identical laboratory conditions, CFGS reduced cumulative erosion to approximately 13% of that of the bare slope. It also moderated the internal hydraulic response, reducing pore-water pressure fluctuation by approximately 26%, and restrained swelling–shrinkage deformation, with an average deformation attenuation of up to 61%. The numerical simulations showed consistent response trends with the physical model tests, supporting the proposed mechanism of hydraulic regulation, deformation restraint, and stress redistribution. Overall, the results demonstrate the comparative effectiveness of CFGS in mitigating wetting–drying-induced deterioration of expansive soil slopes. Full article

Drying shrinkage is a critical durability issue in concrete structures, particularly in high-strength concrete (HSC), which is more susceptible to early-age cracking due to its low water–cement ratio and dense microstructure. This study experimentally evaluates the restrained drying shrinkage behavior of fiber-reinforced concretes with compressive strengths ranging from 23 to 84 MPa, employing a total of 84 ASTM C1581 ring specimens exposed to three exposure conditions: outdoor climate, indoor laboratory conditions (25 °C, 50% RH), and a controlled chamber (50 °C, 30% RH). Plain concretes exhibited increasing shrinkage with both strength and environmental severity. Under indoor exposure, 90-day shrinkage reached approximately 660 × 10⁻⁶ (23 MPa), 291 × 10⁻⁶ (40 MPa), 753 × 10⁻⁶ (60 MPa), and 338 × 10⁻⁶ (84 MPa), with high-strength mixes showing greater cracking susceptibility. Fiber incorporation significantly mitigated both strain and cracking in a dosage-dependent manner. Steel fibers at 1.0–1.5% reduced shrinkage by up to 75% in 40–60 MPa concretes, while polypropylene fibers at 0.25–0.5% achieved reductions up to 66% and eliminated cracking in several cases. Results demonstrate that concrete strength, exposure condition, fiber type, and dosage collectively govern shrinkage and cracking resistance. Full article

This study investigated the effects of varying water–binder (w/b) ratios and internal curing materials—superabsorbent polymer (SAP) and ceramsite—on the shrinkage behavior and crack resistance of ultra-high-performance concrete (UHPC). Although internal curing has been extensively studied, the comparative effectiveness of different internal curing materials on early-age shrinkage and restrained cracking behavior of UHPC under consistent mixture proportions remains unclear. To address this gap, a systematic experimental comparison of SAP and ceramsite was conducted. The influences of w/b ratio and different amounts and addition methods (dry and pre-absorbed addition) of SAP and ceramsite on the flowability, mechanical properties, early autogenous shrinkage, drying shrinkage, and early crack resistance of UHPC were discussed. Findings indicate that increasing the w/b ratio reduces autogenous shrinkage but compromises mechanical properties, altering the cracking mode from primary microcracks to a few wider cracks. Pre-saturated ceramsite (less than 10% volume) and SAP effectively mitigate autogenous and drying shrinkage, enhancing crack resistance without significantly reducing mechanical properties. However, exceeding a ceramsite volume dosage of 10% or using the dry addition method increased the flowability of UHPC, while decreasing crack resistance. Microstructural analysis reveals that internal curing materials facilitate hydration and enhance structural density through the formation of ettringite and calcium silicate hydrate. To optimize shrinkage reduction while maintaining mechanical properties, SAP should be incorporated in a dry state, with a dosage limited to 0.4% of the mass of the cementitious material; ceramsite needs to be pre-saturated and limited to 5% of the total volume. Full article

Calcium sulfoaluminate–calcium oxide expansive agents (HCSA) are commonly used in mass concrete to compensate for thermal shrinkage. However, the ettringite (AFt) formed by HCSA hydration decomposes when temperatures exceed 70 °C. This study examines the synergistic effects of curing temperature (20 °C to 80 °C), fly ash (FA) content (0%, 40%), and water–binder ratio (w/b, 0.3, 0.4, 0.5) on the expansion behaviour and microstructure of HCSA–cement systems. A critical temperature threshold was identified at 60 °C. Below this limit, elevated temperatures accelerate hydration and enhance expansion, with the restrained expansion ratio peaking at 9.2 × 10⁻⁴ mm/mm under 60 °C curing. Beyond 60 °C, expansion capacity significantly diminishes due to the thermal decomposition of AFt into monosulfoaluminate (AFm), as confirmed by XRD and SEM analysis. Calculations of expansive stress reveal a critical mismatch at temperatures ≥ 40 °C, where the expansive stress exceeds the early-age tensile strength, causing microstructural damage. Furthermore, subsequent cooling to standard curing conditions triggers the reformation of AFt from AFm, leading to Delayed Ettringite Formation (DEF), which poses potential risks for late-stage cracking. AFt morphology shifted from needle-like (2–5 μm) to prismatic (5–8 μm). The incorporation of FA suppressed early-stage expansion but improved expansion stability. above 40 °C, although excessive temperatures (>70 °C) led to pore coarsening and reduced mechanical strength. These findings provide a theoretical basis for optimizing the curing regimes of HCSA-admixed mass concrete to ensure structural integrity. Full article

Early-age cracking limits the structural use of steel fiber-reinforced lightweight aggregate concrete (SFLWAC), and robust experimental evaluation methods are still needed. This study examines the influence of steel fiber volume fractions (i.e., 0%, 0.5%, 1.0%, and 2.0%) on the cracking performance of SFLWAC through mechanical testing, autogenous shrinkage measurements, and two types of partially restrained ring tests, with and without a clapboard. The performance of three crack resistance indices is compared: the strain-based ASTM C1581 index, a stress-based area index, and a newly proposed energy-based index defined as the strain energy accumulation degree (SEAD), i.e., the ratio between the accumulated and critical strain energy density. The 28-day splitting tensile strength was improved by 77.9% and autogenous shrinkage was diminished by 30.7% as steel fiber volume content increased from 0 to 2.0%, thereby improving the resistance to shrinkage-induced cracking. In the partially restrained ring tests, SEAD decreased with increasing fiber content, and crack initiation occurred when SEAD reached an approximately constant threshold, whereas ASTM C1581 and the area index could not consistently rank mixtures when some rings cracked and others remained intact. These results demonstrate that SEAD provides a physically meaningful and unified measure of cracking risk for SFLWAC under partially restrained shrinkage and has the potential to be extended to other fiber-reinforced concretes and shrinkage-related cracking problems. Full article

This study elucidates the synergistic effects of polypropylene fiber and cement (physical–chemical) on stabilized expansive soil slurry. A comparative analysis was conducted on the fluidity, 28-day mechanical strength, and shrinkage properties (autogenous and drying) of slurries with different modifications. The underlying mechanisms were further investigated through Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) analysis. Results demonstrate that the cement addition substantially enhanced fluidity, mechanical strength, and early-age volume stability through hydration. However, it was insufficient to mitigate long-term drying shrinkage at low dosages. Conversely, incorporating 0.5% polypropylene fiber reduced slurry fluidity but markedly improved flexural strength. Crucially, a pronounced synergistic effect was observed in the co-modified slurry; the specimen with 20% cement and 0.5% fiber exhibited a 28-day drying shrinkage of only 0.57%, a performance comparable to the specimen with 60% cement and no fibers. Microstructural analysis revealed that cement hydration products created a robust fiber-matrix interfacial transition zone, evidenced by C-S-H gel enrichment. This enhanced interface enabled the fibers to effectively bridge microcracks and restrain both autogenous and drying shrinkage. This research validates that the combined cement–fiber approach is a highly effective strategy for improving expansive soil slurry, yielding critical enhancements in flexural performance and long-term dimensional stability while allowing for a significant reduction in cement content. Full article

Full-section interval casting technology was adopted for the integral immersed tube of the Chebei Immersed Tunnel. Field tests (Chebei Immersed Tunnel) were conducted to establish the time-dependent development of the concrete shrinkage strain of the full-section casting segments. And laboratory experiments were then carried out to investigate the influence of factors such as the reinforcement ratio and stress, expansive agent content and composition, fly ash content, and curing temperature and humidity on the expansive effect of calcium–magnesium composite expansive agents. Field tests revealed that casting segments exhibit initial expansion followed by shrinkage, reaching a final strain of 348 με (microstrain). Laboratory investigations demonstrated that reinforcement (20–30 MPa stress) in post-casting belts effectively restrains segments without compromising the performance of calcium–magnesium composite expansive agents. The optimal 5:3:2 ratio of CaO, MgO 90s, and MgO 200s agents controlled shrinkage strain within 80 με by combining CaO’s rapid early expansion with MgO’s sustained effect. Field validation confirmed the mix’s effectiveness in preventing cracking, with key findings: (1) fly ash content and curing conditions significantly influence expansive behavior, and (2) shrinkage development can be precisely regulated through agent composition adjustments. Full article

In order to improve early-age strength, steam curing is mostly used for railway prefabricated components, which consumes a lot of energy and affects the durability of concrete. Synthetic calcium silicate hydrate (C-S-H) has an excellent early-age strength effect, which can improve the early-age strength of concrete and help to reduce the energy consumption of steam curing, but C-S-H will increase the shrinkage of concrete and affect the durability of concrete. In this work, C-S-H/SRPCA was synthesized using a shrinkage-reducing polycarboxylate superplasticizer (SRPCA) in order to increase the early-age strength and decrease the shrinkage of concrete. The effects of 0.5%, 4.0%, and 8.0% C-S-H/SRPCA on the shrinkage and strength of concrete were studied. Meanwhile, the internal mechanism was also explored through cement hydration, the physical aggregation morphology of hydration products, pore structure and classification, and the chemical properties of pore solution. The results suggest that C-S-H/SRPCA can shorten the setting time and accelerate cement hydration. Specifically, when the dosage of C-S-H/SRPCA is 4.0%, the initial setting time of concrete is shortened by 2.5 h and the final setting time is shortened by 6.2 h compared with the control group. As a result, the 1-day compressive strength is effectively increased by 29.5%, and the plastic shrinkage is reduced. In the stage of plastic shrinkage, the plastic shrinkage time of the concrete with 4.0% C-S-H/SRPCA is 4.1 h, which is 6.1 h shorter than that of the control group. In addition, C-S-H/SRPCA decreases the porosity. When the dosage is 4.0%, the porosity of the hardened cement paste at 28 days is reduced by 15% compared with the control group. It lessens the content of the capillary pores at 10–50 nm. At 24 h, the content of 10–50 nm capillary pores in the paste with 4.0% C-S-H/SRPCA is 40% lower than that of the control group. It also reduces the surface tension of the pore solution. The surface tension of the simulated pore solution with 4.0% C-S-H/SRPCA is 34 mN/m, which is 53% of that of the control group, and it inhibits the volatilization of the pore solution. At 28 days, the evaporation rate of the pore solution in the paste with 4.0% C-S-H/SRPCA is 40% lower than that of the control group. Thus, the drying shrinkage of concrete is inhibited. Given the above, at the optimum content of 4.0%, C-S-H/SRPCA improves the 1-day compressive strength of concrete by 29.5%, reduces the 28-day total shrinkage by 21.7%, and restrains the development of microcracks. Full article

This study investigates the development of two sustainable cements, CEM II/B-LL and CEM VI, in accordance with the UNI EN 197-1 and 197-5 standards. CEM II/B-LL was produced by replacing Portland cement with limestone (LS) at varying dosages (0%, 15%, 25%, and 35% by mass), and CEM VI was made by substituting blast furnace slag with limestone at different levels (0%, 10%, 20%, 30%, and 40% by mass). The results show that both binders are classified as structural cements. LS substitution increases the setting time of CEM II/B-LL but does not significantly affect the setting time of CEM VI. When cured at low temperatures (10 °C), CEM VI mortars retain their mechanical properties even at high LS levels, making them particularly suitable for cold climates. Mortar properties such as total porosity and capillary water absorption increase with LS content, with CEM VI exhibiting lower sensitivity to LS additions. Free shrinkage in CEM II/B-LL mortars decreases with LS content, whereas in CEM VI mortars, it initially increases with up to 20% LS and then decreases at higher LS levels (30–40%). Restrained shrinkage is also lower in CEM VI than in CEM II/B-LL. The Global Warming Potential (GWP) of CEM II/B-LL decreases significantly with increased LS content, whereas in CEM VI, it remains almost constant up to a 40% substitution. However, CEM VI demonstrates a 50% lower environmental impact compared to CEM II/B-LL, underscoring its superior sustainability. Full article

The purpose of this study is to find appropriate mixtures for self-leveling mortar that meet the fluidity requirements without displaying segregation by using a combination of two types of cement (Type I Portland cement and sulfoaluminate cement (SAC)) with reactive ultra-fine fly ash (RUFA). Unlike the fly ash, RUFA has a strong strength activity index and exhibits a significant pattern of amorphous phase in XRD. Appropriate mix proportions of raw materials, including the superplasticizer, require investigation in depth. A fixed water-to-binder ratio of 0.6 was selected, with varying proportions of the two cementitious materials considered (the SAC volume percentages were 0%, 10%, 20%, and 30%) and different RUFA contents (the RUFA volume percentages were 5%, 10%, and 15%). Twelve experiments were conducted to examine the properties of the self-leveling mortars. We found that a higher RUFA volume percentage results in lower porosity, higher compressive strength, and better resistance to drying shrinkage, abrasion, and restrained shrinkage cracking. Increasing the SAC volume percentage increases the porosity of self-leveling mortar and its early compressive strength but decreases late-stage strength. At a 10% volume percentage level, SAC achieves an ideal balance among drying shrinkage, brasion, and shrinkage cracking. Full article

Adding expansion agents to compensate for concrete shrinkage is a common crack resistance technique, but excessive expansion can also increase the porosity of concrete and reduce its strength. The addition of fibers can reduce expansion and improve the compactness of concrete. However, too little fiber will not be effective in inhibition, while too much fiber will cause aggregation. In this study, steel fiber and MgO expansive agent were used at the same time, and the effect of fiber on the mechanical properties of MgO concrete was studied. The results showed that the appropriate amount of MgO (8%) could compensate for the shrinkage of concrete and slightly improve the strength of concrete. When the content reached 10%, MgO produced excessive expansion under free conditions, which reduced the strength of the concrete. After using MgO and steel fiber at the same time, steel fiber could restrain the expansion of MgO, improve the compactness of concrete, produce a “super superposition” benefit, and increase the strength of concrete by 20%. In addition, the reinforcing effect of steel fiber on MgO was closely related to its distribution. In the composite system, steel fiber not only played a “bridge role” but also needed steel fiber to effectively restrain the expansion of MgO and produce self-stress. Only when the steel fibers were evenly distributed could reliable bonding be ensured between the fibers and the matrix, and at this time, the fibers could restrain the expansion of MgO. Considering the uniformity of steel fiber distribution and construction cost, adding 8% MgO and 1% steel fiber has the maximum benefit. Full article

A precise prediction of the cure-induced shrinkage of an epoxy resin is performed using a finite element simulation procedure for the material behaviour. A series of experiments investigating the cure shrinkage of the resin system has shown a variation in the measured cure-induced strains. The observed variation results from the thermal history during the pre-cure. A proposed complex thermal expansion model and a conventional chemical shrinkage model are utilised to predict the cure shrinkage observed with finite element simulations. The thermal expansion model is fitted to measured data and considers material effects such as the glass transition temperature and the evolution of the expansion with the degree of cure. The simulations accurately capture the exothermal heat release from the resin and the cure-induced strains across various temperature profiles. The simulations follow the experimentally observed behaviour. The simulation predictions achieve good accuracy with 2–6% discrepancy compared with the experimentally measured shrinkage over a wide range of cure profiles. Demonstrating that the proposed complex thermal expansion model affects the potential to minimise the shrinkage of the studied epoxy resin. A recommendation of material parameters necessary to accurately determine cure shrinkage is listed. These parameters are required to predict cure shrinkage, allow for possible minimisation, and optimise cure profiles for the investigated resin system. Furthermore, in a study where the resin movement is restrained and therefore able to build up residual stresses, these parameters can describe the cure contribution of the residual stresses in a component. Full article

With the development of salt cavern gas storage technology, the construction of large-scale salt cavern gas storage using sediment voids is expected to solve the problems of low effective volume formation rate and poor construction economy of high-impurity salt mines. At present, there are few studies on the long-term operational mechanical behavior of salt cavern gas storage under the influence of sediment accumulation. The present paper studies the influence of sediment height, particle gradation, and operating pressure on the stability of salt caverns by constructing a coupling model of sediment particle discontinuous medium and surrounding rock continuous medium. The continuous–discontinuous coupling algorithm is suitable for analyzing the influence of sediment height and particle gradation on the creep shrinkage of salt caverns. The increase in sediment height slows down the creep shrinkage of the cavern bottom, which strengthens the restraining effect on the surrounding rock of the cavern. As a result, the position of the maximum displacement of the surrounding rock moves to the upper part of the cavern. The sediment particle gradation has little effect on the cavern volume shrinkage rate. The greater the coarse particle content, the smaller the cavern volume shrinkage rate. The greater the operating pressure, the more conducive to maintaining the stability of the cavern. This situation slows down the upward movement of the sediment accumulation and increases the gas storage space in the upper part of the cavern. The obtained results can provide a reference for evaluating the long-term operational stability of sediment-type high-impurity salt cavern gas storage. Full article