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Concrete structures in cold regions are exposed to cyclic freezing and thawing environment, leading to degraded mechanical and fracture properties of concrete due to microstructural damage. While the X-ray micro-/nano-computed tomography technology has been implemented to directly observe concrete microstructure and characterize local damage in rec...
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Under cold environments, the freezing and thawing cycles of water in concrete reduce the lifetime and durability of concrete structures. For enhanced freezing and thawing resistance, entrained air voids are generally required, but malfunctioning of air entrainment is sometimes reported in the field. To evaluate the quality of air entrainment, this...
Cold weather concrete construction under sub-zero temperatures presents challenging problems for the professionals involved in construction industry and are the main reason for poor development of such regions. Freezing of concrete before it gains required minimum strength at early age together with considerable retardation in setting time due to f...
In this research, the effect of minimum freezing temperature, freezing inhibitor concentration (NaCl), proportioning (water cement ratio, sand cement ratio) on salt scaling using the small piece test method were examined. The following results were obtained.
Scaling did not occur in freshwater, but scaling did occur in a wide range of concentrati...
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... After 100 FTC, the water absorption of concrete increased by 65.85-270.73% due to increased microcracks generated within the concrete as a result of cyclic freeze-thaw (Gonen et al. 2014;Yijia et al. 2018). After 10 FTC, the ice pressure within the soil pores causes an increase in the degree of separation of the soil particles, resulting in more macropores with more water storage space, and an increased ability to absorb water (Hotineanu et al. 2015). ...
In this paper, the effects of freeze-thaw cycle (FTC), freezing temperature (FT) and initial moisture content (IMC) on Cunninghamialanceolata’s water absorption behavior were investigated. The porosity was measured with a gas permeability-porosity analyzer, and its tracheid structure changes were obtained using an optical microscope to explore microstructural deformation under varied freeze-thaw conditions. The results showed a maximum increase in water absorption and mean absorption rate of 25.2% and 24.8%, after 30 FTC experiments. The FTC experiments revealed a positive correlation between FTC and both water absorption and mean absorption rate. The FT experiments showed that Cunninghamia lanceolata’s water absorption and mean absorption rate initially increased, then decreased, and increased again with decreasing temperature, due to the tracheid volume change through icing ratio and ice crystal volume alteration. IMC experiments show water absorption and mean absorption rate decrease for cCunninghamia lanceolata with IMC 0-45% and increase for IMC 60-110% post freeze-thaw. The water absorption and mean absorption rate data were fitted to tracheid volume to derive the predictive models. Correlation analysis identifies the strongest impact of IMC on water absorption and mean absorption rate. FTC primarily influenced tracheal volume, while FT had the weakest effect on tracheal volume, water absorption, and mean absorption rate.
... Recent studies [11][12][13][14][15] have also shown that UHPC is an excellent 55 repair material. UHPC was first used in repairing NC in bridge deck systems [16], with 56 experiments confirming its ability to improve repair interface performance. Feng et al. 57 [17] compared NC-UHPC's repair effectiveness using circumferential confinement and 58 discovered that UHPC can create a denser and stronger OTZ, enhancing adhesive 59 performance. ...
In the harsh environment of extreme salt-freezing conditions, concrete structures face secondary durability issues after undergoing repairs, making it a critical yet challenging topic. This paper employs a novel ex-situ X-ray computed tomography approach to explore the mechanism of damage evolution in NC-UHPC composites under salt-freezing conditions. For the first time, we consider evolution in both external and internal pores to quantitatively assess the damage. Additionally, slant shear tests, Mercury Intrusion Porosimetry (MIP) were also employed to validate the damage mechanism of NC-UHPC. Finally, empirical formulas summarizing the bond strength and pore changes in NC-UHPC were derived. The results reveal a crucial factor influencing the progression of damage in the NC-UHPC composite: the presence of external pores that directly interact with the salty solution. Damage initiation primarily occurs within the NC-UHPC composite due to these external pores located in the NC region, subsequently extending into the OTZ and UHPC sections. Remarkably, the resistance of the OTZ, previously identified as the weakest zone in the NC-UHPC composite, surpasses that of the NC. This exceptional performance can be attributed to the higher porosity of the OTZ, offering additional space for the dissipation of pressure caused by freezing. What's even more important is that this highlights the consistent origin of damage in the NC-UHPC composite, emphasizing that it always begins within the NC. Considering this perspective prompts the question of whether the exceptionally high strength and durability of UHPC may lead to an excess of repair capabilities?
... Sulfate attack has been identified as a prominent factor contributing to the reduced durability of concrete [33,34], with documented cases of damage due to sulfate attack in coastal regions. Degradation of the mechanical properties of concrete due to freezethaw damage in civil engineering practice in cold climates is recognized as the primary factor [35,36]. Therefore, the development of high-performance concrete is of great importance in various engineering applications. ...
In this paper, firstly, the effects of graphene oxide on the mechanical properties of concrete were investigated. Secondly, the degradation and mechanism of the mechanical properties of graphene oxide concrete (GOC) under sulfate attack and a freeze–thaw environment were investigated. In addition, the dynamic modulus of elasticity (MOEdy) and uniaxial compressive strength (UCS) of the GOC were measured under different environmental conditions. According to the test results, the incorporation of graphene oxide in appropriate admixtures could improve the mechanical properties of concrete in these two working environments. It is worth noting that this effect is most pronounced when 0.05 wt% graphene oxide is incorporated. In the sulfate attack environment, the MOEdy and UTS of the GOC0.05% specimen at 120 cycles decreased by 22.28% and 24.23%, respectively, compared with the normal concrete specimens. In the freeze–thaw environment, the MOEdy and UTS of the GOC0.05% specimen at 90 cycles decreased by 13.96% and 7.58%, respectively, compared with the normal concrete specimens. The scanning electron microscope (SEM) analysis showed that graphene oxide could adjust the aggregation state of cement hydration products and its own reaction with some cement hydration crystals to form strong covalent bonds, thereby improving and enhancing the microstructure density.
... In cold regions, concrete structures are exposed to freeze-thaw cycle (FTC) environment, thereby degrading concrete's mechanical and fracture properties due to microstructural damage [27]. Freeze-thaw damage has a severe impact on the load-carrying capacity and durability of concrete structures. ...
To obtain the shear capacity calculation method and competition failure mechanism of headed stud connectors of steel-concrete composite structures under the influence of freeze-thaw cycles (FTCs) and artificial corrosion (AC), the material performance of concrete specimens after the FTCs and push-out tests of 36 headed stud connectors specimens were tested under the influence of FTCs and AC. The cubic compressive strength, dynamic elastic modulus, and mass loss rate of C50 concrete were obtained under different FTCs. Failure modes, load-slip curve, shear capacity, and shear stiffness of stud connectors were investigated based on push-out tests of miniaturized specimens. Based on the nonlinear fitting of test data, a formula for calculating the shear capacity reduction factor is proposed. Results show that the shear capacity and stiffness of stud connectors will deteriorate due to the deterioration of concrete performance caused by FTCs or the weakening of stud section caused by AC. The two failure modes of stud breakage or concrete cracking will change with the aggravation of FTCs and AC effect.
... In this section, the proposed DeepFEM is utilized to analyze the cohesive fracture process of notched beams in the three-point bending test. The cohesive zone model (Borg et al. 2004;Dong et al. 2018Dong et al. , 2021 has been widely used to study the fracture behavior of brittle or quasi-brittle materials, such as rock and concrete. Thus, the cohesive element is introduced in DeepFEM to capture the crack initiation and propagation process, in which the crack criterion is characterized by the strength, fracture energy, and softening law. ...
In this paper, an element-based deep learning approach named DeepFEM for solving nonlinear partial differential equations (PDEs) in solid mechanics is developed to reduce the number of sampling points required for training the deep neural network. Shape functions are introduced into deep learning to approximate the displacement field within the element. A general scheme for training the deep neural network based on derivatives computed from the shape functions is proposed. For the sake of demonstrations, the nonlinear vibration, nonlinear bending, and cohesive fracture problems are solved, and the results are compared with those from the existing methods to evaluate the performance of the present method. The results demonstrate that DeepFEM can effectively approximate the solution of the nonlinear mechanics problems with high accuracy, while the shape functions can significantly improve the computational efficiency. Moreover, with the trained DeepFEM model, the solutions of nonlinear problems with different geometric or material properties can be obtained instantly without retraining. Finally, the proposed DeepFEM is employed in the identification of material parameters of composite plate. The results show that the longitudinal and transverse elastic moduli of the ply in the composite plates can be accurately predicted based on the nonlinear mechanical response of plates.
... Therefore, obtaining the damage patterns of the microstructures during the deterioration process is crucial for a comprehensive understanding of the mechanical deterioration mechanisms. Many studies have investigated the damage evolution during FTC deterioration using either destructive or nondestructive test methods (Dong et al. 2018, Sokhansefat et al. 2020, Pilehvar et al. 2019. However, because the measurement of expansion or mechanical deterioration was not combined with that of the microstructural changes, the microscale damage and macroscale deterioration were not quantitatively related in these studies. ...
... The critical saturation value of concrete was 85% [142]. Liu [143] and Dong [144,145] used CT technology to find that the increase in microcracks after F-T cycles changes the bending degree and macroscopic crack length of the concrete, which causes the fracture path to change from a straight line to a bending path, and therefore, the crack resistance of concrete decreases. Luo et al. [146], through nano-CT scanning, found that under the initial conditions, the surface of the concrete was intact, and the overall defects were few. ...
The deterioration of concrete microstructures in freeze–thaw (F-T) cycles is the primary reason for the reduction in the service life of concrete. This paper reviews recent progress in the theory of damage mechanisms and damage models of concrete in F-T cycles. It is a detailed review of the salt-freeze coupling condition, microstructure testing, and models for the evolution of concrete properties that are subjected to F-T damage. Summarized in this paper are the deterioration theory of water phase transition; the mechanism of chloride-F-T and sulfate-F-T damage; the microstructure testing of hydration products, pore structure, microcracks, and interfacial transition zones (ITZ). Furthermore, F-T damage models for the macrostructure are presented. Finally, the issues that are existing in the research and outlook of concrete F-T damage are highlighted and discussed. This paper is helpful in understanding the evolution of F-T damage, and also provides a comprehensive insight into possible future challenges for the sustainable design and specifications of concrete in cold environments.
... Furthermore, nondestructive testing techniques, including computed tomography (CT) and scanning electron microscopy (SEM), are used to analyze the microstructural damage evolution of concrete subjected to FTCs [12][13][14][15]. Dong et al. [16] quantitatively studied the microstructural damage evolution and its effect on fracture behavior in a three-point bending test of concrete under FTCs by CT and micro-scale cohesive zone model. The results revealed that microcracks caused by FTCs were the primary reason for the degradation of concrete mechanical properties. ...
An improved ordinary state-based peridynamic (OSBPD) model is proposed to study the damage evolution of shotcrete lining under freeze-thaw cycles (FTCs) in cold region tunnels. The fracture criterion of bond in PD model is modified by introducing freeze-thaw damage function and by considering the difference of mechanical behaviors of shotcrete under tension and compression. In combination with material point dormancy method, a PD model is constructed to simulate the excavating and supporting process of tunnels in cold regions. In the numerical implementation, a global convergence criterion is employed to accelerate the solution convergence. For comparison, a finite difference method (FDM) model also is established to simulate the same excavating and supporting process. It shows that the results of the improved OSBPD model are consistent with that of the FDM, and the improved OSBPD model is more efficient. With the proposed model, some influence factors, such as tunnel buried depth, surrounding rock grade and section shape, on damage evolution of the lining structure under FTCs are originally investigated. The results demonstrate that accumulated freeze-thaw damage at the arch foot is more severe while buried depth increasing or rock elastic modulus decreasing.
... In cold regions, freeze-thaw is often one of the main causes of the destruction of concrete; therefore, its resistance has become an important indicator of the durability of concrete in these regions, and it has gradually become the top priority of concrete durability research [19][20][21]. ...
To study the effects of graphene oxide (GO), fly ash, and steel fiber on the mechanical properties and durability of concrete, the mechanical properties, frost resistance, and internal pore structure of modified concrete are investigated by compression tests, freeze–thaw cycle tests, and industrial computed tomography (CT) tests. The test results show that the compressive strength of concrete with GO is better than that of mixed concrete, concrete mixed with only steel fiber, and ordinary concrete. Further, it is strongest at all ages when the GO content is 0.03%; the compressive strength of mixed concrete with 30% of fly ash is generally better than that with 15% and 45% of fly ash. In general, the frost resistance of concrete with only GO is better than that of ordinary concrete. With the increase in fly ash content, the internal porosity of concrete decreases, and its compressive strength increases accordingly; as GO increases, the porosity decreases and then increases, with the lowest porosity and the highest compressive strength of concrete at 0.03% of GO. With an increase in porosity, the mass loss and relative dynamic elastic modulus of concrete increase after 100 freeze–thaw cycles, which indicates that porosity directly affects the frost resistance of concrete.
... Therefore, freeze-thaw damage can be viewed as a complex fracture propagation process [21]. According to Dong Y (2018), microcracks, caused by freeze-thaw cycles, are the primary reason for the degradation of mechanical properties of concrete [22]. Goszczyńska (2012) reported that fracture parameters are sensitive to the microstructural changes caused by the accumulation of damage within the material, as a result of repetitive actions [23]. ...
This research will help to improve our understanding of the fracture properties of ECC at low temperatures (long-term low temperatures, freeze–thaw) and evaluate the safety properties of ECC under low-temperature conditions. Three levels of saturation (saturated, semi-saturated, and dry), four target temperatures (20, 0, −20, and −60 °C), and the effect of the coupled of the two on the mode I fracture properties of ECC were investigated. Then, we compared and analyzed the fracture properties of ECC loaded at 20 and −20 °C, after different freeze–thaw cycles (25, 50, 100 cycles), which were compared with saturated specimens without freeze–thaw at the four target temperatures to analyze the differences in low-temperature and freeze–thaw failure mechanisms. Temperatures and saturation have a significant effect on the fracture properties. Low temperatures and freeze–thaw treatments both decreased the nominal fracture energy of ECC. Distinct differences in matrix and fiber-matrix interface damage mechanisms have been discovered. Low temperatures treatment transforms ECC from a ductile to a brittle fracture mode. However, even after 100 freeze–thaw cycles, it remains ductile fractured. This study complements the deficiencies of ECC in low-temperature theoretical and experimental applications, and it sets the stage for a broad range of ECC applications.
