Abstract:To realize the multi-objective performance of concrete proportion design,the optimal ranges for slag content,fly ash content,and water/binder ratio in C40 ready-mixed concrete were studied though single-factor experiments.The response surface methodology was employed to construct quadratic polynomial regression models,with which the effects of different slag content,fly ash content,and water-binder ratio on the slump and compressive strength of concrete were systematically investigated.Furthermore,the non-dominated sorting genetic algorithm(NSGA-Ⅱ)combined with the technique for order preference by similar to ideal solution(TOPSIS)comprehensive evaluation method was applied to achieve the multi-objective optimization design of concrete mix proportion.The results demonstrate that the regression models of concrete slump and 28 d compressive strength established by response surface methodology have correlation coefficients of 0.944 7 and 0.960 4 respectively,indicating good prediction accuracy.The fly ash content has a significant influence on the slump,while the compressive strength is mainly affected by the water-binder ratio.After optimization,the optimal mix proportion scheme is obtained as follows:slag content of 8.66%,fly ash content of 25.00%,and water-binder ratio of 0.34.The relative error between the predicted values and the experimental values is less than 5%.

Abstract:This study employed the pore network modeling(PNM)method to describe the pore structure of cementitious materials,while also considering the moisture distribution within the pore network.Further,a 2D numerical model for chloride diffusion based on the pore network was developed.The feasibility of the model was verified by comparing it with the third-party tests.By considering pore structure characteristics including porosity,pore size distribution,connectivity,and tortuosity,this model studied the influence of the microstructure on the chloride diffusion process.Results show that the diffusion performance of chloride highly depends on the saturation degree and the characteristics of pore structure.The decrease in porosity and the reduction in average pore size can lead to a decrease in the chloride diffusion coefficient of cementitious materials.Moreover,there is an inverse relationship between tortuosity and connectivity.When connectivity is below 0.7 and tortuosity exceeds 1.5,the relative diffusion coefficient of chloride is less than 0.2,indicating a significant improvement in the chloride erosion resistance of the material.

Abstract:A discrete element numerical model was established based on the volume expansion theory,and the damage and failure process of the rock-concrete interface under the coupled action of freeze-thaw cycles and dynamic three-point bending load was systematically simulated.The results show that the crack propagation of the rock-concrete interface undergoes three stages:slow growth,rapid growth,and unstable failure;the fracture toughness has a significant dependence on the loading rate.At low loading rates(v=0.5-1.0 m/s),the fracture toughness exhibits the characteristic of"slight increase-sharp decrease".When the loading rate increases to 1.3-1.7 m/s,the fracture toughness exhibits a"sharp increase-sharp decrease"trend.The growth rate of the stress intensity factor shows a general downward trend with increasing number of freeze-thaw cycles.

Abstract:It is easy to induce calcium corrosion for cement paste in a carbonated water-rich environment,which becomes a severe threat to its service performance.In order to properly reflect the dissolution-diffusion process of CO2 in water and its interaction with cement minerals,a reactive transport model(RTM)was therefore developed,which was then utilized to study the spatial and temporal distribution of cement minerals,solutes and pore structure during groundwater erosion.In this way,the deterioration behavior of hardened cement paste can be thus assessed.The results show that generated calcite precipitation will fill in available pore space when the content of dissolved CO2 in water is low,which prevents further loss of calcium ions.Once its concentration is larger than 14.00 mmol/L,the calcite begins to decompose due to an increased acidity of pore solution,resulting in an accelerated deterioration progress of cement paste.

Abstract:The funding characteristics and research evolution of the National Natural Science Foundation of China(NSFC)in the field of concrete structure durability over the past 40 years were systematically analyzed.An analysis of more than 1 300 approved projects was conducted.The results show that the funding scale in this field has experienced a leap through four stages:basic cultivation,demand enhancement,scale expansion,and innovation leadership.The research in this field has the characteristics of environmental responsiveness and regional relevance:the Yangtze River Delta,Pearl River Delta,and northern coastal areas have formed innovation hubs in the fields of chloride ion erosion and steel bar corrosion,while the central and western regions and Northeast China have achieved breakthroughs in aspects such as freeze-thaw/salt corrosion coupling.The funded projects are divided into two categories:material science and structural engineering.The material science category focuses on the performance evolution and deterioration mechanism of cement-based materials,and the structural engineering category focuses on the degradation mechanism of the service performance of concrete structures throughout their whole life cycle—both provide theoretical support for the construction of major projects.In the future,low-carbon,durable,and high-performance materials and structural systems will be developed,the application of artificial intelligence will be promoted,zoned planning and collaborative innovation will be deepened,and the exploration of extreme service conditions will be expanded.

Abstract:Rice fermentation residue and liquid were used in desulfurization gypsum blocks as admixtures.The results indicate that these admixtures effectively increase the standard consistency water requirement,retard setting time and enhance water resistance of desulfurization gypsum.Specifically,after adding 0.9% rice fermentation residue and liquid,the softening coefficients increase by 41.9% and 35.4%,respectively,while the water absorption rates decrease by 57.9% and 57.0% compared to the blank control group.Although rice fermentation residue and liquid negatively affect the mechanical properties of desulfurization gypsum,the degree of mechanical property reduction is comparable to that of common salt based admixtures.Additionally,rice fermentation products influence the growth of the(002)plane of calcium sulfate dihydrate without forming covalent bonds with the crystals,and the morphology of dihydrate gypsum crystals remains largely unchanged.

Abstract:Based on an actual project in Qingdao,basalt fiber reinforced polymer(BFRP)anti-floating anchors were applied to the anti-floating project of coastal underground structures,and on-site creep performance tests on the BFRP anti-floating anchors were carried out.Through real-time testing of anchor bar and anchor soil displacement,the evolution characteristics of load-displacement of BFRP anti-floating anchor bar were clarified,and the spatial and temporal distribution law of the internal force of anchor bar of BFRP anti-floating anchor bar was revealed.The results show that the displacement of anchor bar in BFRP anti-floating anchor bar is affected by the load level and loading time,and the displacement of anchor bar and anchor soil grows with the increase of loading time showing two stages of initial creep and steady state creep.At the maximum load level,the axial force of the anchor bar shows a non-linear distribution along the depth,which decays gradually with the increase of depth,and tends to zero at the deepest point.At the same depth,the axial force decreases gradually with loading time.The interfacial shear stress between the anchor bar and anchor soil first increases and then decreases with the increase of depth,and the peak stress appears at approximately 0.75 m from the orifice.The peak stress decreases with the increase of loading time,and the shear stress attenuation is concentrated in the range of 0.7-1.7 m.

Abstract:To address the rapid stiffness degradation and significant residual deformation of concrete-filled steel tube columns under seismic loading,a novel composite column,the ultra-high performance concrete-filled steel tube(UHPCFST)reinforced with steel-FRP composite bars(SFCB),was proposed.A series of cyclic loading tests was conducted to evaluate its seismic performance.The results indicate that,compared to UHPCFST columns reinforced with either steel bars or basalt fiber-reinforced composite bar,those columns reinforced with SFCB exhibit intermediate levels of load-bearing capacity,ductility,energy dissipation,and residual deformation.Both methods of increasing the SFCB reinforcement ratio and replacing conventional concrete with ultra-high porformance contrece(UHPC)in the steel tube can enhance the seismic performance of the composite column.Moreover,a high axial compression ratio benifits the load-bearing capacity,initial stiffness,and energy dissipations,but accelerates stiffness degradation and reduces ductility.When the SFCB reinforcement ratio increases from 3%to 4%,the comprehensive seismic performance of the composite column improves by 21%,while material costs rise by only 5%.Therefore,increasing the SFCB reinforcement ratio is the most cost-effective approach to enhancing the seismic performance of composite columns.

Abstract:To address the demand for building exterior wall self-cleaning coatings to simultaneously possess dual functions of photocatalytic degradation of pollutants and hydrophobicity,a core-shell structured ZnCdS@SiO₂-CF composite was developed.Zn0.5Cd0.5S semiconductor was synthesized at room temperature via a stepwise ion exchange-co-precipitation method.Subsequently,a SiO₂ shell was constructed through an in-situ hydrolysis coating technology,and long carbon-fluorine chains were grafted onto the shell,forming a micro-nano rough core-shell composite with low surface energy modification.The results show that Zn0.5Cd0.5S exhibits a large specific surface area;after 6 h of hydrolysis coating with tetraethyl orthosilicate,the composite exhibits the highest roughness and can effectively suppress the recombination of photogenerated carriers.Spray test results show that a coating amount of 16.33 mg/cm² endows the cement-based self-cleaning coating with superhydrophobicity,acid-base corrosion resistance,and mechanical stability.Under visible light irradiation,the coating still maintains a degradation rate of 79.52%for methylene blue,which is 2.52 times that of titanium dioxide.

Abstract:A numerical model of ITZ was developed based on the corpuscular nature,the continuous slicing method and the cement hydration theory and was verified by third-party experimental data.The effects of the non-uniform distribution of cement particles and local differences in water-to-cement ratios due to the wall effect were comprehensively taken into account.Based on the proposed model,the water-to-cement ratios,cement particle size distribution,and aggregate surface roughness were analyzed.The results show that,though the hydration degree of the cement in ITZ is high,its hydration product is significantly lower than that of the cement paste matrix.The larger the water-to-cement ratio is,the higher the thickness and porosity of ITZ are.Finer cement particles can fill the gaps between the large particles,thus reducing the ITZ thickness and porosity.Rough aggregate surfaces,on the other hand,can enhance the wall effect and increase the thickness and porosity of ITZ.

Abstract:To investigate the early hydration characteristics of magnesium-doped tricalcium aluminate(C3A)in the absence of gypsum,C3A samples incorporating 16.67%and 28.57%MgO were prepared by calcination,and the evolution of phase composition and hydration degree during hydration were analyzed.The results demonstrate that MgO significantly promotes the hydration of C3A,with magnesium-aluminum layered double hydroxides(Mg-Al LDH)forming within 1 h of hydration.The content of Mg-Al LDH increases markedly with higher MgO incorporation in the samples.The sample with 28.57%MgO achieves a C3A hydration degree of 95%after 2 days,representing a 20%-30%improvement compared to the sample with 16.67%MgO.Mg(OH)2 cannot be observed during the formation process of Mg-Al LDH,indicating that the formation of Mg-Al LDH does not transfer from an intermediate product of Mg(OH)2 but rather through direct reaction between Mg2+(from MgO hydrolysis)and Al3+(from C3A hydrolysis)under alkaline conditions.Consequently,the formation of Mg-Al LDH also accelerates the hydrolysis of MgO.

Abstract:A double-layer placed-fill method was utilized to introduce aggregates in two layers during the concrete pouring process,with the aim of optimizing the aggregate skeleton structure of pumice lightweight aggregate concrete.This method improved the concrete strength and established a quantitative evaluation system for the particle size range and placed-fill rate of the aggregates.The results show that when the maximum particle sizes of the aggregates are 19 mm and 31.5 mm,and the placed-fill rates are 10%and 20%,the compressive strength of the concrete increases by 16.45%and 8.98%compared to the control group,respectively,and the splitting tensile strength also improves concurrently.The aggregates optimized the microstructure of the matrix.As the throwing-in rate increases,the number of harmful pores decreases,and the width of interfacial microcracks narrow.Dimensionless fitting analysis of the stress-strain curve shows that the variance reaches up to 99%,indicating a good fitting accuracy.Furthermore,the established toughness analysis system indicates that when the particle size range of the aggregates is 4.75-19 mm and the placed-fill rate is 20%,the toughness index increases by 51.9%compared to the control group,while the brittleness index decreases by 47.2%.The aggregate concrete achieves a brittle-to-tough transition.This research provides a process optimization scheme and theoretical support for the engineering application of lightweight aggregate concrete.

Abstract:To identify an effective restoration material for masonry structures,lime was separately modified by three organic additives such as pre-gelatinized corn starch,anionic polyacrylamide(APAM)and alkalized straw powder in combination with NaHCO₃.The effects of three organic additives on the physical properties,compressive strength and microscopic mechanism of lime were systematically investigated.The results indicate that 7%pre-gelatinized corn starch exhibits superior performance compared to two organic additives,effectively decelerating water evaporation and prolonging the carbonation period of lime.Compared to the control group,the lime modified by 7%pre-gelatinized corn starch exhibits reduction of 96.81%,75.73%,85.80%and 30.91%in 28 d capillary water absorption,drying rate,water absorption and porosity.Concurrently,its softening coefficient and compressive strength increase by 78.05%and 985.23%,with a slight increase in its apparent density.—OH groups in pre-gelatinized corn starch can modulate the crystallization of CaCO₃,and the structure of the lime after carbonation is denser.The lime modified by 7%pre-gelatinized corn starch achieves optimal performance in masonry repair.

Abstract:Molecular dynamics methods were employed to conduct an in-depth investigation into the interfacial interaction between graphene oxide and calcium silicate hydrate(GO/C-S-H)and the corresponding mechanism was analyzed.The results show that the carboxyl group on the GO surface synergizes with the Ca—O ionic bond network through strong C=OOH…Hw hydrogen bonds,enhancing the interfacial binding energy by 61.1%-65.2%compared to the epoxy group/hydroxyl group system.Under the combined influence of temperature and water content,the interfacial bonding performance of the GO/C-S-H interface exhibits significant changes.An increase in temperature leads to a decline in the interfacial debonding tensile force and peeling work,while excessive hydration at the interface further weakens the bonding strength.The bonding performance of the multi-layer GO system can be improved with an increase in the number of GO layers after optimizing the stacking layer number to avoid defects.Based on the simulation results,the rule that GO functional groups regulate the structure of interfacial water molecules and dominate the bonding performance is revealed,and the coupling mechanism for the multiple factors,including temperature,water content,GO layer number is elucidated,which provides a quantitative framework for the design of high durability nano concrete.

Abstract:To investigate the influence of steel fiber content on the mechanical properties of glass fiber reinforced polymer(GFRP)tube confined steel fiber reinforced concrete(SFRC)short columns under axial compression,acoustic emission(AE)technology was employed to monitor the damage evolution of the core concretes.The results show that as the steel fiber content increases from 0%to 1.8%,the peak load and displacement ductility of specimens increase,with the maximum increments reaching 31.55%and 15.60%respectively.The axial compression process of the specimens can be divided into three distinct stages based on AE energy evolution trends.AE energy is the most active in the crack development stage,and the Kaiser effect appears.With the progress of the loading process and the increase of steel fiber content,the proportion of shear cracks gradually increases,and the rise time/amplitude-count/duration(RA-AF)can be used to quantitatively characterize the failure characteristics of the core concretes.The evolution characteristics of b value can reflect the fracture process of concretes.The downward trend of the b value in the crack development stage can be used as important precursor information for the fracture and instability of the specimens.When the b value reaches the minimum and tends to be stable,the specimens undergo ultimate failure.

Abstract:Electrochemical modification treatment was carried out on carbon fiber prepreg waste(CFPW).The changes in performance of the modified CFPW and its impacts on the fluidity,mechanical,and electrical conductivity properties of cement-based materials were studied,and its electrical conduction mechanism was analyzed in combination with microscopic testing techniques.The results show that when the current density is 80 mA/cm²,the surface resin removal effect of the modified CFPW is better than that at 60 mA/cm².At this time,the wettability,surface roughness,and graphitization degree of CFPW are all improved.Besides,the density is increased,and oxygen-containing functional groups are introduced into CFPW.After incorporating the modified CFPW into cement-based materials,the maximum increases in compressive strength and flexural strength reach 31.05%and 77.63%,respectively.Meanwhile,the resistivity is significantly reduced,and the effect becomes more pronounced with the extension of the curing age.At a fiber content of 0.3%,the resistivity decreases by as much as 57.94%.

Abstract:The effects of basalt fiber bar(BFB)diameter and content on the mechanical and permeability properties of cement-based materials were investigated.The regulatory effect of BFB on the pore structure of cement-based materials was explored by combining computed tomography(CT)technology.The results indicate that BFB with small diameters can effectively reduce the porosity,improve the impermeability and flexural toughness of the cement-based materials.Meanwhile,it can slightly weaken the compressive strength.With the increase of BFB content,both toughness and impermeability of the cement-based materials are significantly improved.When the BFB diameter is 0.2 mm and the content is 3.0%,the comprehensive performance of the specimen B0.2-3.0 is optimal.Compared with the specimen without BFB,the flexural strength of specimen B0.2-3.0 is increased by about 1.6 times,and the porosity is reduced by 36.49%.The specimen B0.2-3.0 exhibits multi-crack propagation and flexural hardening characteristics.When the diameter of BFB is 0.4 mm,the overall performance of the specimens is lower than that of the specimens without BFB.

Abstract:To investigate the mechanical properties and damage evolution process of early-age concrete under axial compression loading,axial compression tests were conducted on cylindrical concrete specimens with ages of 1,3,5,7,14,28 days.Additionally,a mesoscale finite element model based on three-dimensional random polyhedral aggregates and zero-thickness cohesive elements was generated in Abaqus software using Python programming.The results show that the compressive stiffness,compressive strength,and peak strain of concrete all increase with the extension of age.The established model can accurately predict the axial compression performance of early-age concrete.Based on the above results,the damage state,crack distribution,and force transfer mechanism of early-age concrete during axial compression failure are further analyzed.It further reveals the mechanism to the influence of age.

Abstract:Chloride ion diffusion behavior of recycled fine aggregate(RFA)mortar under freeze-thaw cycle was studied through chloride rapid migration test(RCM)and COMSOL simulation,and the effects of recycled fine aggregate and fly ash on the chloride ion diffusion performance of recycled fine aggregate mortar were discussed.The results show that the chloride ion migration coefficient of RFA mortar determined by RCM method increases with the increase of RFA substitution rate and decreases with the increase of fly ash substitution rate;The chloride diffusion coefficient of in RFA mortar under freeze-thaw cycle increases with the increase of RFA substitution rate,and first decreases and then increases with the increase of fly ash substitution rate,and the chloride ion diffusion resistance in 3%NaCl solution is better than that in tap water.The numerical simulation further reveals the diffusion law of chloride ions during the phase change process of freeze-thaw cycles.

Abstract:To address the increasing challenges posed by shield soil dreg(SSD)accumulation and the shortage of river sand,unscreened SSD is used as a replacement for river sand,and fly ash(FA)is used as supplementary cementitious material.The response surface methodology(RSM)is employed to optimize the mix proportions,with SSD replacement rates,FA replacement rates,water-to-binder ratio,and water content as factors.The evaluation metrics include slump,compressive strength,and life cycle assessment(LCA),complemented by microstructural analysis to elucidate the mechanisms of SSD in concrete.The results indicate that:(1)Under the conditions of meeting basic workability and mechanical properties,incorporating FA and utilizing RSM to optimize the concrete proportions can enable the successful utilization of SSD as sand in the production of concrete.Compared with ordinary concrete of similar performance,the ecological impact of SSD concrete with optimized mix proportions can be reduced by up to 11%.(2)The microscopic results indicate that the influence of SSD on compressive strength is primarily attributed to adhesion effects,ion adsorption and exchange mechanisms,internal curing effects,and its ability to alter the interfacial transition zone(ITZ)width and the effective water-to-binder ratio.