| With the implementation of "the Belt and Road" and "Western Development" strategies,as well as the construction of "Sichuan-Tibet Railway"," New Western Land-Sea Corridor"and "West to East Gas Transmission" projects,the west region has become the main plot of China’s foundation engineering construction.However,saline soil is widely distributed in the western region,where severe "cold-drought-salt" characteristics may cause serious damage to concrete structures such as roads,bridges,tunnels,buildings,substantial infrastructure etc..In extreme cases,the infrastructures may face "pulverization in one year and collapse in three years" risk.On the other hand,the sulfate ions in the western saline soil with higher content enter the cement-based materials and generate corrosion products,leading to the cracking and damage of cement-based materials.The process of sulfate attack involves interdisciplinary(such as physics,chemistry,mechanics,and other disciplines),temperature-humidity-salt multi-factor coupling,which finally leads to the complex damage.Thus,the study of sulfate corrosion of cement-based materials in the harsh western environment is still not thorough.Therefore,the performance evolution,mechanisms,and corrosion model of cement-based materials exposed to saline soil with sulfate are studied.The sulfate saline soils with different concentrations of sulfate ions and types of cations were first prepared based on the investigation and survey.According to the main characteristics of concrete structures corroded by saline soils in the field,the methods of accelerated corrosion for cementitious materials exposed to indoor saline soils are proposed.The typical corrosion methods of saline soils were selected to corrode cement paste,mortar and concrete.The macroscopic properties such as appearance morphology,corrosion depth,strength,stress-strain,sulfate ion content and ultrasound were tested at specified corrosion ages to obtain the evolution of the properties of cementitious materials corroded by saline soils.The micro measurement and analysis methods such as CT(computed tomography),XRD(X-ray diffraction),TG(thermogravimetric analysis)and SEM/BSE(electron microscopy)were used to obtain the corrosion microstructure of cementitious materials and to investigate the mechanisms of corrosion.An analytical model of sulfate corrosion of cementitious materials was established based on the corrosion mechanisms and was verified with the corrosion depth of mortar subjected to sulfate attack.Considering the different indoor and outdoor corrosion conditions of saline soil,the experiments of concrete exposed to field saline soil were conducted in typical western environments to support model revision,standard development and corrosion database establishment,etc.The results of the paper are as follows:(1)As the testing of sulfate content in concrete involves complex characteristics such as turbid liquid,multiple ions,comprehensive content,and less powder,the adaptability of the traditional method in testing the sulfate content inside the concrete in the early stage of sulfate corrosion is inevitably decreased.An automatic titration device and method for testing the content of sulfate ions in cementitious materials are developed based on the principle of the sudden change in conductivity caused by ion concentration change and the IoT(Internet of Things technology).This method has a measurement accuracy of 0.8%and a minimum resolution of 0.1%.The endpoint of the titration is easy to be determined and the titration process is not affected by the color and turbidity of the measured solution.Besides,it is also simple to operate during the titration process.Those characteristics of the method provide a new technology for accurate and efficient measurement of sulfate content in cement-based materials.(2)According to the actual situation of concrete structure corroded by saline soil,four typical indoor methods for accelerated corrosion of saline soil to cementitious materials are established based on the different types of burial and moisture supply.It can be listed as fully buried corrosion without supplement of moisture,fully buried corrosion with the top supplement of moisture,partial buried corrosion with the top supplement of moisture,partial buried corrosion with the bottom supplement of moisture.The relevant test devices are also developed.These methods directly use saline soil as the corrosion medium,fully consider the exchange of external moisture and soil,and are closer to the actual engineering,which provides an effective test means for scientific research on the degradation of concrete performance under saline soil corrosion.Full burial test results show that the difference ability of corrosion between sulfate saline soil and sulfate solution to the cementitious material is not from the internal corrosion mechanism of cementitious material but from the difference of the salt ion concentration between the saline soil due to soil particle gradation,microstructure,charged characteristics and the corresponding salt solution.The above difference results in a stronger corrosion ability of sodium sulfate saline soil than the corresponding salt solution under fully buried corrosion without supplement of moisture.However,the result in magnesium sulfate saline soil is the opposite.When considering water evaporation,the ability of corrosion of saline soil and salt solution to concrete is successively weakened by partially buried corrosion with the bottom supplement of moisture,partial immersion corrosion and partial buried corrosion with the top supplement of moisture.The evolution laws of the strength,stress-strain properties,and ultrasonic characteristics of cement-based materials with the corrosion time are obtained under the corrosion conditions of sulfate saline soil with no water supply completely buried and with top water supply partially buried.Based on the evolution of properties and microscopic analysis,it was found that cementitious materials subjected to sulfate corrosion can be temporally divided into three stages:enhanced rising period,enhanced falling period and deterioration period.The space distribution of corroded cementitious materials during the enhanced rising period spatially consists of the corrosion interface area and uncorroded area,while it consists of the corrosion area,corrosion interface area and uncorroded area during the enhanced falling and deterioration period.At the early stage of corrosion,the pores present in the surface layer with several hundred microns wide corrosion interface area are filled with a certain amount of ettringite to improve compactness characteristics of the corrosion interface area,which leads to the emergence of the enhancement of the rising period.With the increase in corrosion age,the growing expansive ettringite in the pores leads to debonding of the corrosion interface into the corrosion area.The gradually wider corrosion area due to the characteristics of reduced adhesion,where the debonding cracks filled by gypsum with the same spacing and width are generated and perpendicular to the corrosion direction,offsets the enhanced corrosion interface area and leads to the emergence of enhanced decline.With the further increase in corrosion age,the deterioration of the corrosion area with low adhesion exceeds the enhancement of the corrosion interface area with high density,leading to the deterioration period’s emergence.At the same time,the increasing width of debonding cracks filled by the expansion of gypsum in the corrosion area leads to the emergence of macroscopic deformation.In addition,aggregates also influence the corrosion process by changing the development direction of debonding cracks in the corrosion interface area.(4)The debonding of the corrosion interface area of cement-based materials due to the densely filling then swelling by ettringite is the basis of microscopic experiments to establish the sulfate corrosion mechanism model.Assuming that the sulfate ion diffusion coefficient has the time-exponential decay of the transient characteristics to consider the influence of the corrosion zone on slowing down the corrosion rate,the time index is obtained by fitting the sulfate ion diffusion coefficient for different corrosion ages.The influence of sulfate corrosion rising period on slowing down the corrosion rate is considered by setting the minimum pore filling degree of ettringite in the corrosion interface area.The minimum filling degree of pore is determined by the measured thickness of the corrosion interface area.Finally,an analytical model of corrosion rate(?)for cement-based materials subjected to sulfate attack is established by combining corrosion reactions controlled by diffusion,the constitutive relation of the corrosion interface zone,and debonding fracture criterion in the corrosion interface zone.The parameters in the model are related to the composition of the cementitious material(maximum content of sulfate ion per unit volume consumed CE concerning the cement component and mix proportion),the mechanical parameters of the corrosion interface zone(modulus of elasticity Ec,Poisson’s ratio vc,strain energy storage fraction f,fracture interface energy γf and roughness of fracture surface a),expansion chemical reaction parameters(reaction partial molar volume of sulfate corrosion β),and transport characteristics(diffusion coefficient related to time D0ta),as well as the structural characteristics of pores in the uncorroded area(porosity Φ and minimum filling degree λ)and the external corrosive environment(corrosion boundary concentration CS).Combined with experimental data,it is confirmed that the prediction error of the model is less than 10%.(5)Given the different indoor and outdoor saline soil corrosion conditions,field exposure experiments of concrete were conducted in Gansu,China,and relevant sensors are embedded in the exposed concrete and saline soil.A remote real-time monitoring system for the exposed environmental parameters of saline soil and long-term performance indexes of concrete is established based on IoT.The exposed environmental parameters mainly include wind speed,air temperature,and humidity,ultraviolet rays,evaporation,snow water equivalent,soil temperature and humidity,conductivity and pH.The long-term performance indexes of concrete mainly include temperature and humidity. |
