Wet-dry Cycles Research Articles

Deep-Ultraviolet Photoexcitation of Aqueous Urea Forms Carbamic Acid/Carbamate in Less Than One Picosecond.

Urea is believed to have been essential to the synthesis of prebiotic nucleotides and thereby the RNA or DNA of the first lifeforms. Models suggesting that life began in wet-dry cycles around shallow aquatic ponds imply that reactants such as urea were exposed to deep ultraviolet irradiation from the young sun. Detrimental photodissociation of urea induced by deep UV excitation potentially challenges these models. We here follow the primary deep ultraviolet photochemistry of aqueous urea. The data show that urea is barely excited at 200 nm due to weak ultraviolet absorption. The likelihood of photodissociation is further reduced by strong intra-molecular coupling of the CN and CO stretch vibrations accompanied by an efficient dissipation of the excitation energy to the surrounding water molecules mitigated by urea-water hydrogen bonds. We find that 54±5 % of the excited urea molecules dissociate. Reactions between the photoproducts and surrounding solvent molecules form carbamic acid or the carbamate anions within 0.6 ps. The molecules that do not dissociate return to the electronic ground state in 2 ps. Interestingly, the photodissociation processes of urea in the aqueous phase is different from earlier reported reactions observed following the VUV photolysis of urea in noble gas matrices and highlight the potential influence of water on the prebiotic photochemistry.

The use of natural non-expansive soil cover to prevent water loss and crack formation in underlying expansive soil slope

ABSTRACT This study was conducted to explore the use of non-expansive soil as protective cover for expansive soil slopes. Laboratory model experiments were carried out on expansive soil systems with varying thickness of non-expansive soil cover. The models were subjected to three wet-dry cycles. Variation in soil moisture content was monitored using moisture probes. Surface and internal cracking of soil was observed using cameras. Variation of infiltration rate of the cover with wet-dry cycles was measured in-situ. Results of the study show correlation between cover thickness and evaporation rate and crack formation in the expansive soil. Crack size, quantity, depth, and interconnectivity in the expansive soil increased with decreasing cover thickness. Even the thinnest cover significantly reduced the the number and depth of cracks. The infiltration rate of the cover remained unchanged after three cycles wet-dry cycles. The final water content, after the third drying, in the expansive soil increased with increasing cover thickness.

Pore characteristics and microscopic damage mechanism of disintegrated carbonaceous mudstone exposed to dry-wet cycles

Disintegrated carbonaceous mudstone (DCM), derived from pre-disintegration treatments of waste carbonaceous mudstone, is utilized as filler for highway embankments with the aim of conserving resources. However, the mechanical behavior of DCM is greatly influenced by its microstructure, particularly pore characteristics. The aim of this study is to investigate the pore characteristics and microscopic damage mechanisms of DCM subjected to dry-wet cycles. Triaxial tests, along with microscopic observations using scanning electron microscopy and mercury intrusion porosimetry, were performed. Three-dimensional pore models were reconstructed from the microscopic test results via the Markov chain-Monte Carlo method. Subsequently, a finite element method that accounts for the anisotropy of porosity was proposed based on these pore models. Finally, numerical triaxial tests were performed to analyze stress changes in DCM under triaxial loading using the proposed method. The results show that the shear strength and cohesion of the DCM decrease with an increase in dry-wet cycles, stabilizing notably after four cycles. Meanwhile, the porosity and the number of slender pores in the specimen increase, leading to intricate alterations in pore shape and distribution. A comparison between numerical and laboratory triaxial tests demonstrates closely aligned results, with distinct stress concentration behavior related to porosity anisotropy. It is revealed that dry-wet cycles cause an increase in porosity and the occurrence of more irregular-shaped pores, subsequently leading to a decrease in the cohesion. This microscopic mechanism is responsible for the shear strength damage of DCM.

Destabilization Mechanism and Stability Study of Collapsible Loess Canal Slopes in Cold and Arid Regions

The combination of seasonal shutdowns, water conveyance, cold, and drought can easily lead to the deterioration of the anti-seepage system and loess foundation of the canal, which contributes to the destruction of the slope. To reveal the failure mechanism of the collapsible loess canal slope, this paper is based on the results of laboratory tests and adopts numerical simulations to analyze the stability of the canal slope under different conditions. The results show that the shear strength indexes and elastic modulus E of loess decrease following an exponential pattern with the increase in wetting-drying and freezing-thawing (WD-FT) cycles. The height of the seepage overflow point yields little effect on the water level behind the impermeable membrane, whereas the height of the water level has a significant effect. In the operation period, the slope under any working conditions is in a relatively stable state. However, the slope with a water level of 4.5 m behind the impermeable membrane tends to be unstable after three WD-FT cycles during the shutdown period. By replacing the surface-degraded loess with sand gravel and picking a depth of 0.9–1.2 m, the slope will maintain a long-term stable state.

Durability of Prestressed Piles in a Leachate Environment.

Prestressed pipe piles are common concrete components characterized by dense concrete structures and favorable mechanical properties, and thus, extensively used as coastal soft soil foundations. However, their durability in harsh environments has not been fully clarified. In this study, leachate from an actual landfill site was collected from the east coast of China as the corrosive medium, and the corrosion process was accelerated by electrifying prestressed pipe piles. The results demonstrated that the concentration of chloride ions in the concrete of the prestressed pile increased with the increase in corrosion time. Moreover, the experimental corrosion of these prestressed piles in the drying-wetting cycle proved to be the most severe. However, a protective layer of epoxy resin coating can effectively inhibit the diffusion of chloride ions into the interior of the piles. The final theoretical corrosion amounts of the piles were 1.55 kg, 1.20 kg, and 1.64 kg under immersion, epoxy resin protection, and a drying-wetting cycle environment. The application of epoxy resin reduced chloride penetration by 22.6%, and the drying-wetting cycle increased chloride penetration by 5.8%, respectively, with corresponding corrosion potentials following similar patterns. The actual corrosion depth of the welding seam was 3.20 mm, and there was a large corrosion allowance compared with the requirement (6.53 mm) for the ultimate bending moment. In summary, these prestressed piles exhibited good durability in a leachate environment.

A comprehensive evaluation on particles breakage characteristics of waste argillaceous gangue as subgrade filler

Reutilising waste argillaceous gangue (AG) in road engineering is crucial to reducing its environmental pollution. To effectively control settlement and long-term performance of subgrade filled with AG, the effects of humidification and loading on the particle crushing evolution of AG were investigated. The effects of dry-wet cycles, compaction work, and initial coarse grain content (CGC) on the breakage characteristics were evaluated. The relationship among fractal dimension D and compaction work, initial CGC, and breakage index Bg was established. The results show that the initial CGC and compactness are essential factors affecting the breakage characteristics of AG. It is recommended that the CGC be controlled at 50% when AG is used as subgrade filler. The fractal dimension increases logarithmically with the increase of compaction work and decreases linearly with the rise of CGC. Fractal dimensions D and Bg can effectively evaluate the breaking process of AG and the gradation after compaction.

Revealing degradation mechanisms of archaeological flax textiles through the evolution of fibres’ parietal polymers by synchrotron deep-UV fluorescence

Autofluorescence generated by plant cells and tissues provides a label-free method for investigating them without altering their structure and biochemistry. While in the literature, research mainly focuses on investigating fresh plant tissues, this study examines flax yarns from five archaeological artefacts to understand how cell wall fibres degrade and how their biochemical composition is modified after natural ageing over millennia. Using Synchrotron-based deep UV fluorescence (S-DUV), we aim to correlate fibre autofluorescence with the environmental conditions of discovery (clothes of votive statuettes buried in a sanctuary or shrouds and bandages for mummification rituals found in tombs) or their intended purposes (fishing net, which underwent wet-dry cycles during its use). Out of the five archaeological objects studied, the two yarns sampled from the buried fabrics were found encrusted with minerals, but with significant differences in the fibres’ degradation. The yarn sampled from the fishing net displayed a great variability of the dataset, with some fibres biochemically similar to the buried samples while others were closer to the funerary linen shrouds. Lignin's predominant presence at 480 nm in the fishing net suggests extensive cellulose-enriched cell wall layer degradation while the lignin-enriched layer is still preserved. Additionally, second harmonic generation imaging microscopy (SHG) and 2-photon excitation fluorescence (2PEF) were used alongside S-DUV analyses to evaluate the flax fibres’ ultrastructure. SHG confirmed S-DUV findings and indicated poor crystalline cellulose preservation in one linen shroud, explaining the yarn's brittleness.

Insights into saline soil cracking subjected to drying-wetting cycles

Insights into saline soil cracking subjected to drying-wetting cycles

Improved Design and Economic Estimation of Cold-End Systems for Inland Nuclear Power Plants

Improved Design and Economic Estimation of Cold-End Systems for Inland Nuclear Power Plants

Viscoelastic–plastic rheological model and its application to tunnels

Tunnels exhibit obvious continuous deformation during excavation and operation. This behaviour is closely associated with the time-dependent behaviour of rocks, which is induced by groundwater-level fluctuation and prolonged periodic rainfall infiltration. This paper proposes a rheological model consisting of a Hooke elastomer, Kelvin body and novel plastic element in series (called the HKP model) to describe the creep response of rocks considering the characteristics of dry–wet cycles. First, dry–wet cycle creep tests were carried out to investigate the time-dependent behaviour: that is, the creep behaviour of sandstone. Then the creep equation of the viscoelastic–plastic model was derived, and the damage coefficients under the effect of dry–wet cycles and time were obtained. Finally, the HKP model was established to investigate continuous deformation during tunnel excavation. The results reveal that dry–wet cycles have obvious effects on the physical properties and creep behaviour of sandstone. The creep behaviour of sandstone undergoes three stages – namely, the decaying, steady and accelerated stages – that can be reasonably described by the proposed HKP model. In practice, the proposed model can accurately predict the creep behaviour of rocks in tunnels owing to excavation. Thus, the HKP model can help in establishing tunnel maintenance strategies to ensure long-term safety.

Air permeability variation of compacted loess: effects of soil basic properties and dry-wet cycles

Air permeability variation of compacted loess: effects of soil basic properties and dry-wet cycles

Effects of seawater corrosion on compression-induced buckling performance of FRP-CFST columns

This study experimentally investigated axial compression-induced buckling performances of FRP confined concrete filled steel tube (FRP-CFST) columns after exposure to seawater corrosion environment. Sixty-three stub FRP-CFST are manufactured, and sixty of them were exposed to three simulated seawater corrosion conditions with 120, 240, 360 days. Four-stage failure mode of the axial compression-induced buckling tests was observed and analyzed. Comparative analysis indicated that FRP-CFST can effectively resist normal seawater corrosion, while weaker durability is observed from columns in triple-salinity seawater environments, particularly with wet-dry cycles conditions. Under the same corrosion conditions, CFST columns wrapped with Carbon Fiber reinforced polymer (CFRP) exhibited preferable buckling resistance performance than those with Basalt Fiber reinforced polymer (BFRP). Based on mechanism analysis of the axial compression-induced buckling tests, a theoretical evaluate model for compression-induced buckling performances of FRP-CFST after seawater corrosion is proposed and verified. The outcomes of this research can provide experimental and theoretical basis for evaluating the durability of FRP-CFST under seawater corrosion environment.

Morphology characterization of unsaturated soils under drying-wetting cycles: crack opening and closure

AbstractThe volumetric and hydrological responses of clayey soils subjected to drying-wetting (D-W) cycles are of paramount importance for the integrity of geoenvironmental infrastructures. The study aimed to investigate the cracking behavior of clayey soils under D-W cycles by using advanced 2D imaging and 3D scanning techniques to capture the initiation and propagation of desiccation cracks within a soil specimen. The temporal variation in the soil water content and the corresponding 2D digital photography and 3D morphology of cracks were simultaneously monitored, and the cracking characteristics were interpreted. It was found that the time-dependent evaporation process was independent of the D-W cycles. Both 2D and 3D characterization showed the cracking hysteresis phenomenon in the unsaturated soil, which indicates the dependency of the crack opening and closure on the degree of saturation. D-W cycles led to the formation of subcracks and the increase in the total crack length, reflecting the soil degradation. Additionally, it was demonstrated that the 3D characterization exhibited the advantage of capturing the volumetric change and the subtle change in the macroporosity of the cracked soil over the 2D visualization. The current study provides a perspective of combining 2D and 3D characterization for interpreting the volumetric change of cracked soils and enhancing the understanding of the hydromechanical responses and the soil-atmosphere interactions.

Effect of biochar on microplastics penetration treatment within soil porous medium under the wetting-drying cycles and optimisation of soil-biochar mixing format

Plant-based biochar was demonstrated promising capability in adsorbing microplastic particles (MPs) within soil porous mediums. However, biochar's function in mitigating MPs' vertical penetration during wetting-drying cycles, typical of seasonal precipitation and evaporation, remains uncertain. Furthermore, few studies have investigated the structures of how biochar combines with soil. This study conducted column tests to assess the MPs retention capabilities of soil-biochar porous media under saturated and wetting-drying conditions. The water retention and hydrophilic properties were investigated to elucidate the impact of wetting-drying cycles. Additionally, different biochar-soil structures were compared to optimise the structural design. Without biochar, wetting-drying cycles resulted in 8.74 % more MPs escaping from samples. However, incorporating 15 % biochar led to only around 2 % more MPs in effluent. Biochar significantly enhanced soil's MP absorption capacity and mitigated the negative effects of wetting-drying cycles. Biochar's alveolate morphology provides ample adsorption sites and creates complex flow paths. The hydrophilic groups of biochar and capillarity by micropores facilitated slower water release during drying, preventing crack propagation and flush on MP particles. This effect was more pronounced with higher biochar content and lower porosity. Moreover, layer structure was found to improve MPs removal, benefiting the long-term performance and management of the biochar functional layer.

Effect of dry-wet cycling on NOx degradation and sterilization properties of cement mortar with g-C3N4/CoAl-LDH

The influencing mechanism of dry-wet cycling on the NOx degradation and sterilization properties of photocatalytic cement mortar (PCM) with g-C3N4/CoAl-LDH was investigated through the optical properties, mineral composition of cement hydration products, pore characteristics and nitrogen (N) content. The results indicate that as the cycle number increases, the N content, band gap, Urbach energy, NOx degradation rate and sterilization rate exhibit periodic variations, while the peak intensity of CH (a cement hydration product) decreases gradually. After 10 cycles, PCM demonstrates the maximum Urbach energy, N content, NOx degradation and sterilization rate, as well as the minimum band gap. At this stage, the Urbach energy, N content, NOx degradation and sterilization rate of PCM with 1.0 % g-C3N4/CoAl-LDH increase by 16.2 %, 24.9 %, 15.4 % and 40.8 %, while the band gap and the CH peak intensity decrease by 5.4 % and 23.4 %, respectively. Although the NOx degradation and sterilization rates of PCM exhibit periodic fluctuations during the dry-wet cycling, both of them surpass those before the cycles, which shows the excellent photocatalytic durability of PCM. It is expected that this study not only presents a new perspective on evaluating the durability of PCM’s photocatalytic property, but also provides an alternative approach for the design of new multifunctional nanocomposites in cementitious materials for environmental pollution control.

Chloride Corrosion Process of Concrete with Different Water-Binder Ratios under Variable Temperature Drying-Wetting Cycles.

In this paper, four water-binder ratios (w/b) of 0.29, 0.33, 0.39, and 0.46 were designed. A variable test temperature was implemented in the drying-wetting cycle test according to the temperature fluctuations in the actual service environment, and the constant temperature test was established as the control group. The mechanical properties and chloride corrosion resistance of concrete with different w/b ratios under variable temperature drying-wetting cycles, as well as the microstructure changes, phase composition, and damage mechanism inside the concrete, were investigated. The results showed that the mechanical properties of concrete increased first and then decreased with drying-wetting cycles increasing, whereas the chloride corrosion resistance continued to decline. A higher w/b exacerbated the deterioration of the concrete performance. A higher w/b increased the porosity, chloride diffusion depth, and chloride content, thus reducing the resistance of chloride corrosion. Compared with w/b = 0.29, the compressive strength, splitting tensile strength, mass, and relative dynamic elasticity modulus of w/b = 0.46 exposed to 60 drying-wetting cycles decreased by 54.50%, 52.44%, 0.96%, and 6.50%, respectively, while the porosity, peak chloride content, and erosion depth increased by 45.12%, 70.45%, and 45.00%. Compared with the drying-wetting cycle with a constant temperature, the cumulative damage caused by the drying-wetting cycle with a variable temperature was greater, resulting in more severe deterioration of concrete performance. The increase in the test temperature significantly accelerated the diffusion rate, penetration depth, and chemical binding capacity of chloride ions. After 60 drying-wetting cycles, the peak chlorine content and erosion depth of w/b = 0.46 under variable temperature cycles were 15.38% and 10.32% higher than those under a constant temperature, while the compressive strength, splitting tensile strength, mass, and relative dynamic elastic modulus were reduced by 7.76%, 14.81%, 0.33%, and 2.40%, respectively. Microscopic analysis confirmed that higher w/b and variable temperature cycles accelerated the decay of mechanical properties and the decline of chloride corrosion resistance. According to the numerical fitting analysis, the w/b should be 0.29~0.39 under the condition that the mechanical properties and chloride corrosion resistance of concrete are met.

Effect of Moisture Content and Wet–Dry Cycles on the Strength Properties of Unsaturated Clayey Sand

Based on the actual situation of the project on the Weihai–Yanhai Expressway section of Rongwu Expressway, the effects of water content change and the dry–wet cycle on the mechanical behavior of unsaturated clayey sandy soil were analyzed in this study. In this study, ventilated undrained triaxial shear tests were carried out on unsaturated clayey sandy soils with different water contents (6%, 8%, 10%, 12%, 14% and 16%). Concurrently, the soil samples were subjected to three distinct wet and dry cycle pathways (2~22%, 2~12%, and 12~22%) to gain an understanding of how the mechanical features of the soil changed under the different conditions. The test findings demonstrate that when the water content increases, the unsaturated clayey sandy soil’s cohesiveness and shear strength diminish. The strength of shear decline exhibits a pattern of first being quick, followed by sluggish. The strength of shear and cohesiveness of clayey sandy soil declined under the influence of the dry and wet cycles, with the first cycle primarily affecting variations in cohesiveness and strength of shear. Furthermore, the strength of shear and cohesiveness of clayey sandy soil diminish more with increasing wet and dry cycle amplitude and upper water content limits. Lastly, the drying shrinkage and hygroscopic expansion of clay particles in clayey sandy soils during wet and dry cycles are not significant, resulting in less structural damage and deterioration of the mechanical properties of the soils. The study’s findings have a significant impact on the durability of roadbeds made of unsaturated clayey sandy soil in both wet and dry situations.

Non-Destructive Assessment of Stone Heritage Weathering Types Based on Machine Learning Method Using Hyperspectral Data

Abstract. Stone cultural heritage is exposed to various environments, resulting in a diverse range of weathering types. The identification of these weathering types is vital for targeted conservation efforts. In this paper, a weathering type classification method based on hyperspectral imaging technology is proposed. Firstly, fresh sandstones are collected from Yungang Grottoes to carry out the simulated weathering experiments, including freeze-thaw cycles and wet-dry cycles with acid, alkali and salt solutions. Subsequently, the hyperspectral imaging system was used to collect the visible-near-infrared (VNIR) and short-wave infrared (SWIR) images of the sandstone samples with different weathering types and degrees. The surface spectral reflectance of sandstone samples with different weathering types were used as training data, with weathering types serving as the labels. Support vector machine (SVM), K-nearest neighbour (KNN), linear discriminant analysis (LDA) and random forest (RF) were used to establish weathering type classification models. The results show that the SVM model and LDA model based on both VNIR and SWIR spectra exhibit outstanding performance, with a best accuracy of 0.994. The framework proposed in this paper facilitates rapid and non-contact assessment of the weathering types of the superficial layers of stone cultural heritage, thereby supporting more targeted conservation work.

An Experimental Study on the Mechanical Properties and Microstructure of the Cemented Paste Backfill Made by Coal-Based Solid Wastes and Nanocomposite Fibers under Dry-Wet Cycling.

The mechanical properties and microstructure of the cemented paste backfill (CPB) in dry-wet cycle environments are particularly critical in backfill mining. In this study, coal gangue, fly ash, cement, glass fiber, and nano-SiO2 were used to prepare CPB, and dry-wet cycle tests on CPB specimens with different curing ages were conducted. The compressive, tensile, and shear strength of CPB specimens with different curing ages under different dry-wet cycles were analyzed, and the microstructural damage of the specimens was observed by scanning electron microscopy (SEM). The results show that compared with the specimens without dry-wet cycles, the uniaxial compressive strength, tensile strength, and shear strength of the specimens with a curing age of 7 d after seven dry-wet cycles were the smallest, being reduced by 40.22%, 58.25%, and 66.8%, respectively. After seven dry-wet cycles, the compressive, tensile, and shear strength of the specimens with the curing age of 28 d decreased slightly. The SEM results show that with the increasing number of dry-wet cycles, the internal structure of the specimen becomes more and more loose and fragile, and the damage degree of the structural skeleton gradually increases, leading to the poor mechanical properties of CPB specimens. The number of cracks and pores on the specimen surface is relatively limited after a curing age of 28 d, while the occurrence of internal structural damage within the specimen remains insignificant. Therefore, the dry-wet cycle has an important influence on the both mechanical properties and microstructure of CPB. This study provides a reference for the treatment of coal-based solid waste and facilitates the understanding of the mechanical properties of backfill materials under dry-wet cycling conditions.

Modelling soil-water retention curves subject to multiple wetting-drying cycles: An approach for expansive soils

This study proposes an innovative and simple method to quantify the effect of wetting–drying history on soil–water retention curve (SWRC) of expansive soils. This method is based on the increment relationship between degree of saturation and initial void ratio corresponding to irreversible swelling or shrinkage after each wetting–drying cycle, following the double-structure scheme for three-phase reactive porous media. The approach satisfies the intrinsic constraints for partially saturated porous media, and the incremental relationship can be applied in any existing SWRC equations for future water retention capacity prediction. In this respect, only one new rate function is proposed, which could be easily calibrated by relevant experimental data. The prediction of the model is verified through the comparison with relevant experimental data of two expansive clayey soils. To evaluate the general applicability of the proposed method, two typical SWRC equations proposed in literature were used. The results showed a very good agreement with experimental data subject to multiple wetting–drying cycles, indicating its potential as an effective tool for estimating preliminary SWRCs of expansive soils.