Free Radicals Produced from the Oxidation of Ferrous Sulfides Promote the Remobilization of Cadmium in Paddy Soils During Drainage.

Most of the cadmium (Cd) accumulated in rice grains is derived from its remobilization in soils during the grain filling period when paddy water is drained. The factors affecting Cd remobilization upon drainage remain poorly understood. Here, we show that the free radical effect produced from the oxidation of ferrous sulfides is an important mechanism affecting the oxidative remobilization of Cd during soil drainage. When soils were flooded, microbial sulfate reduction results in the formation of various metal sulfides including CdS and FeS. Upon soil drainage, the oxidation of FeS produced considerable amounts of hydroxyl free radicals (OH•), which could oxidize CdS directly and thereby promote the oxidative dissolution of CdS and increase Cd mobilization in soils. FeS and CdS could also form a within-sulfide voltaic cell, with FeS protecting the oxidative dissolution of CdS due to the lower electrochemical potential of the former. However, this voltaic effect was short-lived and was surpassed by the free radical effect. The amounts and composition of metal sulfides formed during soil flooding vary with soils, and the oxidative dissolution of CdS is affected by both the free radical and voltaic effects offered by different metal sulfides. These effects are also applicable to the biogeochemistry of other chalcophile trace elements coupled with sulfur and iron redox cycles during the anoxic-oxic transition in many environments.

Application of Ag/AgCl Sensor for Chloride Monitoring of Mortar under Dry-Wet Cycles.

An Ag/AgCl electrode used as a corrosion sensor in a reinforced concrete structure isconsidered as having good application prospect. However, its performance under complexconditions, such as dry-wet cycle condition, is not affirmed. In the current study, the performanceof Ag/AgCl as chloride selective electrode in mortar exposed to dry-wet cycle condition wasinvestigated. A simple Ag/AgCl electrode was prepared and fabricated by electrochemicalanodization. These Ag/AgCl electrodes were embedded into a mortar specimen with temperaturesensors, humidity sensors and anode ladder monitoring system (ALS). After 28 d curing time, theupper surface of mortar specimen was wetted (with 5% NaCl solution) and dried regularly. Theobtained results indicate that Ag/AgCl electrode responds to the ingress of chloride ion, sensitively.The chloride ion concentration variation can be reflected by the potential trend. Furthermore, thebalance potential of Ag/AgCl electrodes is influenced by dry-wet cycles. Compared with ALS, itdemonstrates that Ag/AgCl electrodes are more sensitive to chloride. The research provides the keyelement for the specific application of Ag/AgCl electrode for corrosion monitoring in the future.

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.

Investigating the impact of the quantity of wet and dry cycles on the mechanical characteristics and fracture variations of sandstones.

The sandstone is in a state of dry-wet cycle under the repeated action of rainfall, and its mechanical properties are deteriorated to varying degrees, which causes cracks in the sandstone. Therefore, it is of great significance to study the mechanical properties and fracture propagation of sandstone under the action of dry-wet cycles. Currently, there are limited studies using numerical simulation methods to study the fracture extension of rocks under various dry and wet cycling conditions.Therefore, in this paper, the effects of different amounts of dry and wet cycling on the mechanical properties and fracture behavior of sandstone are investigated through uniaxial compression tests and numerical simulations of fracture extension. The findings indicate that the deformation stage of sandstone remains unchanged by the dry-wet cycle. The uniaxial compressive potency and coefficient of restitution gradually diminish as the quantity of cycles rises, while the Poisson's ratio exhibits the opposite trend, and the impact on the mechanical performance of sandstone wanes with cycle increments, and the correlation coefficient surpasses 0.93, signifying a substantial influence of the dry-wet cycle on sandstone's mechanical performances. The discrepancy between the numerical simulation and experimental results is minimal, with a maximum error of only 3.1%, demonstrating the congruence of the simulation and experimental outcomes.The mesoscopic examination of the simulations indicates that the quantity of fractures in the sandstone specimens rises with the escalation of dry-wet cycles, and the steps of analysis linked to crack inception and fracture propagation are accelerated, and the analysis steps from fracture initiation to penetration are also reduced.

Rice straw biochar and lime regulate the availability of heavy metals by managing colloid-associated- but dissolved-heavy metals.

Heavy metal (HM) pollution has extensively spread in agricultural soils, posing potential threats to food safety and human health. Biochar and lime are two amendments used to remediate the soils contaminated with HMs. However, colloids have been shown to increase the mobility of HMs in paddy soils. Nevertheless, limited investigations have been made into the impact of biochar and lime on the formation of colloid-associated (colloidal) HMs in paddy soils. In this study, column and microcosm incubation experiments were conducted to examine how biochar and lime affected the availability of HMs (arsenic, cadmium, copper, iron, manganese, lead, and zinc) in different layers of paddy soils. The results revealed that biochar significantly inhibited the formation of colloidal HMs in the soil flooding phase, whereas the lime increased the colloidal HMs. These colloids containing HMs were identified as poorly dissolved metal sulfides. When the soil was drained, colloidal HMs transformed into dissolved forms, thereby improving the availability of HMs. Biochar decreased HM availability by reducing colloidal- but dissolved- HMs, whereas lime had the opposite effect. Hence, biochar demonstrated a stable and reliable remediation ability to decrease HM availability in paddy soil during flooding and drainage processes. In conclusion, this study highlighted that biochar efficiently reduced HM availability by mitigating the formation of colloidal HMs during flooding and their transformation into dissolved HMs during drainage in paddy soils.

Drying-wetting cycle enhances stress resistance of Escherichia coli O157:H7 in a model soil.

Outbreaks of Escherichia coli (E. coli) O157:H7 in farms are often triggered by heavy rains and flooding. Most cells die with the decreasing of soil moisture, while few cells enter a dormant state and then resuscitate after rewetting. The resistance of dormant cells to stress has been extensively studied, whereas the molecular mechanisms of the cross-resistance development of the resuscitated cells are poorly known. We performed a comparative proteomic analysis on O157:H7 before and after undergoing soil dry-wet alternation. A differential expression of 820 proteins was identified in resuscitated cells compared to exponential-phase cells, as determined by proteomics analysis. The GO and KEGG pathway enrichment analyses revealed that up-regulated proteins were associated with oxidative phosphorylation, glycolysis/gluconeogenesis, the citrate cycle (TCA cycle), aminoacyl-tRNA biosynthesis, ribosome activity, and transmembrane transporters, indicating increased energy production and protein synthesis in resuscitated O157:H7. Moreover, proteins related to acid, osmotic, heat, oxidative, antibiotic stress and horizontal gene transfer efficiency were up-regulated, suggesting a potential improvement in stress resistance. Subsequent validation experiments demonstrated that the survival rates of the resuscitated cells were 476.54 and 7786.34 times higher than the exponential-phase cells, with pH levels of 1.5 and 2.5, respectively. Similarly, resuscitated cells showed higher survival rates under osmotic stress, with 7.5%, 15%, and 30% NaCl resulting in survival rates that were 460.58, 1974.55, and 3475.31 times higher. Resuscitated cells also exhibited increased resistance to heat stress, with survival rates 69.64 and 139.72 times higher at 55 °C and 90 °C, respectively. Furthermore, the horizontal gene transfer (HGT) efficiency of resuscitated cells was significantly higher (153.12-fold) compared to exponential phase cells. This study provides new insights into bacteria behavior under changing soil moisture and this may explain O157:H7 outbreaks following rainfall and flooding, as the dry-wet cycle promotes stress cross-resistance development.

Research on Performance Deterioration of Multi-Walled Carbon Nanotube-Lithium Slag Concrete under the Coupling Effect of Sulfate Attack and Dry-Wet Cycles.

Sulfate attack is one of the main factors affecting the durability of concrete structures. In recent years, multi-walled carbon nanotubes (MWCNTs) have attracted the attention of scholars for their excellent mechanical properties and durability performance. In this paper, the influence of sulfate attack and dry-wet cycles on the performance of multi-walled carbon nanotube-lithium slag concrete (MWCNT-LSC) with varied MWCNT content (0 wt.%, 0.05 wt.%, 0.10 wt.%, and 0.15 wt.%) and varied water-cement ratios (0.35, 0.40, and 0.45) were investigated. In addition, scanning electron microscopy (SEM) and X-ray computed tomography (CT) tests were conducted to analyze the microstructure and pore structure of the concrete. The results showed that concrete incorporated with MWCNTs could effectively mitigate sulfate attack. The resistance to sulfate attack of concrete was negatively related to the water-cement ratio when the dry-wet cycle was fixed. The MWCNT-LSC showed the best compressive strength at the water-cement ratio of 0.35 and 0.10 wt.% MWCNTs. The SEM test results showed that the MWCNTs filled the pores and cracks within the specimen and formed bridges between the cracks, enhancing the resistance to sulfate attack. The CT test results also showed that the addition of MWCNTs could reduce the porosity of concrete, refine the pore size and inhibit the generation and development of cracks, thus optimizing the internal structure of concrete and improving its resistance to sulfate attack.

Mechanical Properties and Microstructure of Rice Husk Ash-Rubber-Fiber Concrete under Hygrothermal Environment.

In order to study the mechanical properties of rice husk ash-rubber-fiber concrete (RRFC) under hygrothermal environment, the optimal group was selected by orthogonal test. The mass loss, relative dynamic elastic modulus analysis, strength analysis, degradation degree analysis after cyclic loading and internal microstructure analysis of the optimal group of RRFC samples after dry-wet cycles under different environments and temperatures were compared and analyzed. The results show that the large specific surface area of rice husk ash optimizes the particle size distribution of RRFC specimens, reacts to form C-S-H gel, enhances the compactness of concrete, and forms a dense structure as a whole. The presence of rubber particles and PVA fibers effectively improves the mechanical properties and fatigue resistance of RRFC. The comprehensive mechanical properties of RRFC with rubber particle size of 1-3 mm, PVA fiber content of 1.2 kg·m-3 and rice husk ash content of 15% are the best. The compressive strength of the specimens after dry-wet cycles in different environments generally increased first and then decreased, reaching a peak at the seventh dry-wet cycle, and the compressive strength of the specimens under chloride salt solution decreased more than that under clear water solution. Thes provided new concrete materials for the construction of highways and tunnels in coastal areas. Under the premise of ensuring the strength and durability of concrete, it is of great practical significance to explore new roads for energy conservation and emission reduction.

Damage Evolution of Polypropylene-Basalt Hybrid Fiber Ceramsite Concrete under Chloride Erosion and Dry-Wet Cycle.

To investigate the influence of polypropylene-basalt hybrid fibers (PBHFCC) on the durability of ceramsite concrete, this study determined the appearance change, mass loss rate, relative dynamic elastic modulus, compressive strength and splitting tensile strength of ceramsite concrete with four kinds of hybrid fibers volume admixture under chloride erosion and dry-wet cycles. The results reveal that under this effect, the apparent damage of each group of specimens increased with the growth of the erosion time. The quality, compressive strength and splitting tensile strength of the specimens all increased gradually during the erosion age period of the first 72 d and gradually decreased after 72 d. The relative dynamic elastic modulus was similarly mutated in 48 d. When the hybrid fiber content of the specimens is 0.15 vol %, the enhancement effect of ceramsite concrete is better than that of the other three amounts. The relative dynamic elastic modulus value is used as a damage variable to establish the damage equation, and the damage evolution equation of PBHFCC considering the volume of hybrid fiber under chloride erosion and dry-wet cycle is derived. The conclusions can be used as a reference for the durability design and construction of PBHFCC.

Research on Resilient Modulus Prediction Model and Equivalence Analysis for Polymer Reinforced Subgrade Soil under Dry-Wet Cycle.

The subgrade soil of asphalt pavement is significantly susceptible to changes in moisture content, and therefore many projects introduce polymer-based reinforcement to ensure soil performance. This paper aims to incorporate a variable representing the dry-wet cycle into the prediction model of resilient modulus of polymer reinforced soil. The polymer adopted is a self-developed subgrade soil solidification material consisting of sodium dodecyl sulfate and polyvinyl oxide. The current resilient modulus prediction model is improved, notably involving the effects of the dry-wet cycle. Combined with finite element method (FEM) analysis, the actual stress state of pavement and the coupling effect of dry-wet cycle and vehicle load on the resilient modulus are studied. The deterioration in resilient modulus with the variation in seasonal climate and load response is also investigated. Results show that the deviator stress is negatively correlated with the resilient modulus while the bulk stress has a linearly positive relation. The decreasing rate at low deviator stress is larger than that at the high level. Moreover, the dry-wet cycle can reduce the resilient modulus and the reducing amplitude is the largest at the first dry-wet cycle. FEM analysis shows that the middle position of the subgrade slope has the largest initial resilient modulus with decreasing amplitude in the first year of dry-wet cycles, while the upper position shows a smaller change. The variation in resilient modulus is closely related to the changes in cumulative volumetric water content. Considering that different positions of subgrade bear the external vehicle load, the equivalent resilient modulus is more realistic for guiding the subgrade design.

Protective Performance of Coated Reinforcement in Coral Concrete under Dry/Wet Cycling.

The actual protective performance of the coated reinforcement in coral concrete was investigated by measuring the chloride ion diffusion coefficient, electrochemical analysis, and numerical simulation. The test results show that the corrosion rate of coated reinforcement in coral concrete under the action of wet and dry cycles is kept at a low level, and the Rp value is always greater than 250 kΩ·cm2 during the test period, which is in the uncorroded state and has good protection performance. Moreover, the chloride ion diffusion coefficient D is in accordance with the power function relationship with the wet and dry cycle time, and a time-varying model of chloride ion concentration on the surface of coral concrete is established. The surface chloride ion concentration of coral concrete reinforcement was modeled as a time-varying model; the cathodic zone of coral concrete members was the most active, increasing from 0 V to 0.14 V from 0 to 20 years, with a large increase in potential difference before the 7th year, and a significant decrease in the increase after the 7th year.

Experimental Investigation of the Performance of Corn Straw Fiber Cement-Stabilized Macadam.

Recently, the application of plant fibers to improve the cementitious mix performance has attracted interest in the field of road materials owing to advantages of environmental protection and cost-effectiveness. As a planting crop, corn exhibits the advantages of being a more abundant resource with a wider distribution than those of other plant fibers. In this study, the effect of corn straw fiber on the properties of cement-stabilized macadam (5% cement) was investigated with the fiber length and content as variables. The test results revealed that the addition of a small amount of fiber marginally affects the compression density of cement-stabilized macadam. At a fiber length of 10 mm and a fiber content of 1%, the maximum increase in the compressive strength was 18.8%, and the maximum increase in the splitting strength was 35.4%. Moreover, at a fiber length of 15 mm and a fiber content of 1%, the shrinkage coefficient was reduced by 29%, and the crack resistance of cement-stabilized macadam was enhanced. In addition, the dry-wet cycle durability of cement-stabilized macadam was improved.

Experimental Study on Chloride Ion Diffusion in Concrete Affected by Exposure Conditions.

The transport mechanism of chloride ions in concrete is relatively complicated since the erosion process is influenced by many factors. To investigate the effect of exposure conditions on the chloride ion diffusion property, three exposure conditions (long-term immersion in static sodium chloride solution, long-term immersion in circulating sodium chloride solution and dry-wet cycles in circulating sodium chloride solution) were considered in chloride ion diffusion experiments. Experimental results indicated that the chloride ion content at a certain depth increased with erosion age. The chloride ions in static sodium chloride solution transported more rapidly than those under dry-wet cycle conditions. Moreover, the chloride ion content of concrete under dry-wet cycles of the circulating sodium chloride solution was slightly higher than that under long-term immersion in the circulating solution. Based on Fick's second law, empirical equations for the chloride diffusion coefficient and chloride content at the surface of concrete were proposed by fitting experimental data, and the values of correlation coefficients of different exposure conditions were suggested. By comparison with the experiment results, it was verified that the calculation formula had better applicability. This method could be used to predict and analyze the chloride ion content under different exposure conditions.

Research on freeze-thaw and dry-wet durability of enzymatic calcification for surface protection.

The enzymatically induced carbonate precipitation (EICP) technique is currently studied for dust control because of the formation of cemented crust layer. In the present study, polyvinyl acetate (PVAc) was used with EICP together as the EICP-PVAc treatment to solidify dust soils. In addition, several treated dust soil areas always experience repeated freeze-thaw (FT) or dry-wet (DW) cycles, both of which result in the damage of structure. Therefore, the FT cycle test and the DW cycle test were conducted to study the durability of EICP-PVAc treatment. Results showed that both FT cycles and DW cycles affected the EICP-PVAc-treated dust soils. The wind-erosion resistance and rainfall-erosion resistance were impaired, and the surface strength decreased. However, the decreasing range resulted from the FT cycle was smaller than the decreasing range resulted from the DW cycle. It indicated the EICP-PVAc-treated dust soils had better FT durability, but the DW durability was worse. Moreover, a field test was used to study the durability of application of EICP-PVAc treatment in practical field test site. Based on the surface pattern observation after 9 months, the grasses in the treated area are in good growth condition; however, few grasses grew in the untreated area. The field test demonstrated that the combined EICP-PVAc and grass seeds treatment can ensure the long-term solidification effect and durability. The results lay a solid foundation for the applications of EICP-PVAc treatment to solidify dust soils for dust control.

Study the effects of dry-wet cycles and cadmium pollution on the mechanical properties and microstructure of red clay.

In order to study the effect of cadmium ions on the mechanical properties and micro-structure characteristics of the red clay in Guilin, we have conducted triaxial test and the scanning electron microscope tests to analyze the effects of cadmium ion concentration and the number of dry and wet cycles on the mechanical properties and micro-structure changes of the red clay. The results showed the effects of cadmium ions and dry-wet cycles destroy the structure of red clay. The cohesive force of red clay decreases with the increase of cadmium ion concentration, and the internal friction angle first increases and then decreases. With the rise in the number of dry and wet cycles, the cohesive force of cadmium-contaminated red clay first increases and then decreases, and the angle of internal friction rises gradually. Under the action of different cadmium ion concentrations, the stress-strain curve is strain hardening. With the concentration of cadmium ions increases, the strain hardening becomes more apparent; the peak value reached faster. and the axial strain corresponding to the peak value of the line decreases. With the increase in the number of wet and dry cycles, the volume of cadmium-contaminated red clay shrinks and its compactness increases; it gets the peak shear strength faster during the shearing process, and its peak value becomes larger and larger. The main reason for the phenomenon is that cadmium ions destroy the cementation between the particles. The soil particles are mainly in point contact which loosens the structure of the soil; on the other hand, the thickness of the surface diffusion layer of the clay particles increases through chemical action, The exchange of cations increases the porosity of the soil and weakens its strength. The dry-wet cycle shrinks the volume of the red clay, and the soil particles are mainly in surface contact; as the number of dry-wet cycles increases, the soil particles connection is closer, the soil porosity decreases and the strength increases.

Performance degradation and damage model of rice husk ash concrete under dry-wet cycles of sulfate environment.

Rice husk ash concrete (RHAC) is a new type of concrete that has been rapidly gaining acceptance in recent years. In this paper, the improvement effect of rice husk ash (RHA) on the sulfate erosion performance of concrete was confirmed. The ratio of rice husk ash concrete (RHAC) was optimized and compared with ordinary concrete (OC). The performance degradation of 9%RHAC (rice husk ash at 9% by weight of cement) and OC within 135 times erosion dry-wet cycles solution with Na2SO4 at 5% by weight of solution were studied, including the change of apparent phenomena, compressive strength, tensile strength, effective porosity, and dynamic elastic modulus. The microstructure changes of samples before and after sulfate dry-wet cycle were observed by using a scanning electron microscope (SEM). The results show that with the increase of sulfate dry-wet cycle times, the concrete specimen gradually peels off and expands in volume. The compressive strength and tensile strength increase first and then drop sharply, the effective porosity decreases first and then increases, and the relative dynamic elastic modulus increases and then decreases. The reason is that the ettringite and gypsum are formed by the reaction of sulfate intrusion and hydration products under wetting treatment. After drying treatment, ettringite and free water combine to form sodium sulfate. In the early of circulation, ettringite, gypsum, and sodium sulfate fill the internal pores of the concrete and improve the density. As the number of sulfate dry-wet cycles increases, expansion products accumulate, causing structural expansion damage and deterioration of mechanical performance. However, the hydrated calcium silicate hydrate gel was produced by mixing rice husk ash with concrete to improve the material strength and corrosion resistance. The deterioration degree of the 9%RHAC is better than that of OC at all stages. Finally, the damage constitutive models were established, and the accuracy is higher compared with the measured value.

Study on the Durability of Acid Rain Erosion-Resistant Asphalt Mixtures.

Acid erosion can accelerate the process of early damage of asphalt pavement and decrease the durability of asphalt pavement. However, there are limited research results for asphalt mixtures that can resist acid rain erosion. To systematically evaluate the impact and action law of acid rain erosion on the durability of asphalt mixtures, three gradation schemes were used: periodic dry-wet cycle immersion test, contact angle test and road performance test. The acid rain erosion resistance of epoxy asphalt mixture, SBS-modified asphalt mixture and 70# matrix asphalt mixture were tested from three aspects of anti-aging performance, freeze-thaw cycle performance and fatigue performance. The results show that the erosion of acid rain can significantly decrease the adhesion between asphalt and aggregate, and affects the road performance of the asphalt mixture. Acid rain erosion can significantly decrease the mechanical properties, adhesion and durability of asphalt mixtures. Epoxy asphalt has better physical properties, adhesion and acid rain erosion resistance than 70# matrix asphalt and SBS-modified asphalt. Epoxy asphalt has excellent adhesion due to its polar group, high cohesion and thermosetting resin with low shrinkage, which can effectively resist moisture erosion, spalling and temperature stress cracking, thereby effectively resisting the erosion of acid rain. Epoxy asphalt mixture has the strongest acid rain erosion resistance, which can be further enhanced when used together with waste rubber powder and modified bamboo fiber. On the whole, asphalt mixture with high-density structure and thicker asphalt film can effectively resist acid rain erosion. The durability of asphalt concrete (AC)-type gradation mixture and stone mastic asphalt (SMA)-type gradation mixture are equivalent, and both are superior to open-graded friction courses (OGFC)-type gradation mixture. The gradation of asphalt mixtures and the type of asphalt binder have great influence on their acid rain erosion resistance and durability. In order to realize the directional control of the acid rain erosion resistance and durability of different asphalt mixtures, a multi-parameter comprehensive assessment indicator system between the type and property of asphalt, the gradation of asphalt mixture, and the acid rain resistance and durability of the mixture need to be established in the future.

Corrosion Performance of Nano-TiO2-Modified Concrete under a Dry-Wet Sulfate Environment.

This study compared the effects of the sulfate dry-wet cycle on the properties of ordinary concrete and nano-TiO2-modified concrete, including the mass loss rate, ultrasonic wave velocity, compressive strength, and XRD characteristics. In addition, a series of compression simulations carried out using the PFC2D software are also presented for comparison. The results show the following: (1) with an increase in dry-wet cycles, the damage to the concrete gradually increased, and adding nano-TiO2 into ordinary concrete can improve the material's sulfate resistance; (2) after 50 sulfate dry-wet cycles, the mass loss rate of ordinary concrete was -3.744%, while that of nano-TiO2-modified concrete was -1.363%; (3) the compressive strength of ordinary concrete was reduced from 41.53 to 25.12 MPa (a reduction of 39.51%), but the compressive strength of nano-TiO2-modified concrete was reduced from 49.91 to 32.12 MPa (a reduction of 35.64%); (4) after a sulfate dry-wet cycle, the nano-TiO2-modified concrete surface produced white crystalline products, considered to be ettringite based on the XRD analysis; (5) when considering the peak stress and strain of the concrete samples, the numerical results agreed well with the test results, indicating the reliability of the method.

Experimental Study on Mechanical Properties of High-Ductility Concrete against Combined Sulfate Attack and Dry-Wet Cycles.

Concrete will deteriorate and damage under sulfate attack.In order to study the degradation characteristics of HDC under sulfate attack, the mechanical properties of high-ductility concrete (HDC) were investigated using the uniaxial compressive strength test of HDC specimens soaked in different concentrations of sulfate solution and subjected to different times of dry-wet cycles. The variations in the compressive strength, loss rate of compressive strength, and the max compressive strength under the action of sulfate attack and dry-wet cycles were analyzed. The analytical expressions of damage variables were given. SEM was used to observe the microstructure of the sample, and the microdamage mechanism of the HDC was explored. The deterioration of the HDC was found to be the result of the combined action of sulfate attack and dry-wet cycles and was caused by physical attack and chemical attack. PVA prevented the rapid development of deterioration. On the basis of the change of compressive strength, the damage variable was established to quantitatively describe the degree of damage to HDC. The experimental results showed that with the increase in the number of dry-wet cycles, the compressive strength of HDC generally increased first and then decreased. As the concentration of the sulfate solution increased, the loss rate of the compressive strength of HDC generally increased and the max compressive strength gradually decreased. With the increase inthe number of dry-wet cycles, HDC first showed self-compacting characteristics and then gradually became destroyed. Compared with ordinary concrete (OC), HDC is superior to OC in sulfate resistance and dry-wet cycles. This study provided a test basis for the engineering application of HDC in sulfate attack and dry-wet cycles environment.

Study on the Deterioration of Concrete under Dry-Wet Cycle and Sulfate Attack.

In order to study the deterioration and mechanism of dry-wet cycles and sulfate attack on the performance of concrete in seaside and saline areas, the deterioration of compressive strength of concrete with different water cement ratios under different erosion environments (sodium sulfate soaking at room temperature and coupling of dry-wet cycling and sodium sulfate) was studied here. At the same time, ICT (industrial computed tomography) and NMR (nuclear magnetic resonance) techniques were used to analyze the internal pore structure of concrete under different erosion environments. The results show that the compressive strength under different erosion environments increases first and then decreases, and the dry-wet cycle accelerates the sulfate erosion. With the increase of dry and wet cycles, larger pores are filled with erosion products and developed into small pores in the early stage of erosion; in the later stage of erosion, the proportion of larger pores increases, and cracks occur inside the sample. In the process of sulfate soaking and erosion, the smaller pores in the concrete account for the majority. As the sulfate erosion continues, the T2 spectrum distribution curve gradually moves right, and the signal intensity of the larger pores increases.