Microscopic Damage Evolution And Numerical Analysis Of Concrete With Freeze-thaw Exposure

The freezing-thaw action is one of the main reasons that cause concrete deterioration and damage in cold regions.In extremely cold weather,concrete material is subjected to the alternating actions of temperature raise and drop,resulting in internal expansion pressure and osmotic pressure.Under the freezethaw induced fatigue stress,the interior of concrete would eventually lead to the development of micro-cracks and micro-pores damage,and gradually forming macro-cracks and rebar erosion and matrix spalling.These continue to result in some reductions in concrete properties,loss of serviceability and other potential diseases.The mass reduction and relative dynamic modulus are used to characteristic the F-T durability of concrete.These two methods focus on marcoscale experimental result to describe the F-T resistence.Based on the CT imaging technique,this study analyzes the microscopic damage evolution of concrete under the cyclic freeze-thaw actions,with the combined help of the digital image processing technique and finite elements method.An optimized probability function is proposed to represent the important index of concrete freeze-thaw performance: the internal air-void system parameter “spacing factor”.In addition,the numerical simulation analysis is conducted to investigate the mechanical properties of freeze-thaw conditioned concrete from the perspectives of microstructure,damage mechanics,experimental mechanics and numerical simulation.Based on the experimental and theoratical work conducted in this study,a few specific conclusions can be drawn as follows:(1)CT image technique is used to scan the ordinary concrete(PC)and ultrahigh performance concrete(UHPC)under different freeze-thaw cycles with different resolution,to obtain the internal structural images.Based on these CT images,the internal damage evolution is characterized qualitatively and quantitatively by means of digital image analysis.To propose a novel volumetrics thresholding algorithm for CT image segementation.Used the volumetric properties as the main criterion for estabilish grey-scale shresholds for the boundaries between air-matrix and matrix-aggregates raising the accurate of the results.The results show that F-T damage developed from the top to the center of the concrete,the damage ratio caused by F-T is greater at the beginning and then slow down after certain cycles.New pores are developed as a cluster during F-T action.When PC is subjected to freeze-thaw cycles,the interfacial transition zone between aggregate and cement and the initial micro-cracks first break down.With the continuous freeze-thaw cyclic loading,these micro-cracks gradually develop and expand,and eventually form macroscopic cracks.Freeze-thaw actions also have a significant effect on the low-degregation aggregate in the PC.On the other hand,owing to the dense microstructure of UHPC,no significant effect has been observed at the early age of freeze-thaw cycles(about 0-600 cycles).As the freeze-thaw cyclic loading continues to 1500 times,the cement matrix sightly fell off at the bottom and sides of the concrete beam,and cracks and damages also appeared in the interfacial transition zone of “steel fiber-cement matrix”.(2)Based on the information provided by CT two-dimensional images,a two-point probability function is proposed to solve the three-dimensional pore size distribution of concrete pores.The freeze-thaw damage characterization is carried out based on the changes of pore structure parameters.This method can directly reveal the real pore size distribution in concrete through CT images,instead of solving relevant data through assumed pore parameters.The probability-based image analysis method plays an important role in the optimization of the air-void structure.In particular,it involves the interaction of quantity density,air content and spacing factor.The results show that the spacing factor of UHPC is generally smaller than those of PC.As the air content increases with the help of air-entraining agent,the spacing factor decreases and the durability factorf increases.Because UHPC has a very dense microstructure,and there is no coarse aggregate,so the initial pores and micro-cracks are less than those of PC.When the same dosage of the air-entraining agent is added,the air content of UHPC is smaller than that of PC.(3)CT imaging technique is used to obtain the internal structure of UHPC and an effective numerical homogenization model is established in this study.Based on CT images,the representative volume element(RVE)is determined for the numerical modeling,including dimensional size,location and statistical quantity,mesh size,the direction of steel fibers,CT image resolution,filter,and pore distribution.The relationship between the internal microstructure and macroscopic performance of UHPC is established,which provides an effective technical means for the evaluation of concrete's elastic properties.It is concluded that the numerical model based on CT images can be used to characterize the macroscopic mechanical properties by homogenizing the RVE,but the RVE are affected by dimensional size,location and statistical quantity.In general,the larger the porosity,the smaller the volume fraction of steel fiber,and the smaller the elastic modulus of the homogenized model.In some special cases,the resolution of the CT image can be reduced to save the computational time,but it is not recommended to use a filter to process the images.The larger the pore size,the greater the influence on the elastic modulus of the homogenized model,especially significant when the pore diameter is greater than 1mm.(4)The wave elastic modulus of concrete measured using the ultrasonic technique is significantly higher than that of static elastic modulus,plus to study the relationship of F-T and wave elastic modulus of concrete.Combined with the micromechanics and finite element method,the associated factors and reasons are deeply analyzed.The results show that the irregular shape of concrete aggregate is not the main reason that affects the wave elastic modulus of concrete.The volume fraction of concrete aggregate has a certain influence on its wave elastic modulus of concrete.The orientation of aggregates has little impact on the static elastic modulus but great impact on wave modulus.The existence of a large amount of aggregate leads to the refraction of wave in the process of irregular aggregate propagation,which increases the propagation path and prolongs the time,resulting in low wave elastic modulus.The regular shape hole has a slight influence on the wave elastic modulus of concrete.The crack-like hole in concrete,that is,the hole with length to width ratio less than a certain value,is the most important factor affecting the wave elastic modulus of concrete.Meanwhile,when the F-T cycles comes to 1500,the sphericity under 0.45 arrived 55% of the whole porosity,which means the cracks and crack like pores are the mainly factors that cause wave modulus drop during F-T cycles.