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The study of the constitutive model under stress loading for rock after undergoing freeze-thaw weathering cycles (FTWCs) treatment has important theoretical significance for the site operation and numerical calculation of rock mass engineering in cold regions. In this work, a series of FTWCs treatment tests of sandstone were carried out, and the evolution characteristics of the T 2 spectrum distribution curve for sandstone before and after FTWCs treatment were analyzed utilizing nuclear magnetic resonance (NMR) technology. The schematic diagram of freeze-thaw damage evolution for sandstone was drawn, and the damage evolution mechanism was analyzed. Evolution laws of the stress-strain curves, peak and residual strength, peak and residual strain, and Young's modulus of sandstone under different FTWCs and confining pressures were analyzed by conventional triaxial compression tests. A piecewise constitutive model under triaxial stress loading for sandstone after undergoing FTWCs treatment was established, and the model considered the significant influence of FTWCs on the nonlinear deformation in the compaction stage and residual strength. By introducing error analysis indexes, the proposed model and published models were compared with the experimental data, it was found that the proposed model's performance is better than the published models, which indicated that the proposed model has good performance and strong universality. [ABSTRACT FROM AUTHOR]

To study the dynamic response of granite to different levels of microwave power, an intelligent microwave rock-breaking instrument is used to irradiate different power from three directions. The servo universal testing machine is used to carry out a uniaxial compression test on the granite after microwave damage to analyze the strength damage characteristics and the degree of pore damage. Pore fractal characteristics are analyzed based on nuclear magnetic resonance to establish the microwave damage degradation model. In parallel, the energy evolution process of granite under the influence of various power levels is analyzed using the theory of energy dissipation. Simultaneously, based on the energy dissipation theory, we analyze the energy evolution process of granite under the action of different powers. The results show that with higher microwave power, the peak strength and modulus of elasticity show a linear decreasing law. The degree of fragmentation is more obvious, showing the damage characteristics with two big ends and little in the middle. The higher the power, the greater the porosity and the more sensitive the micropore becomes to microwaves. Additionally, the damage degradation model established to evaluate the microwave damage of the rock showed that it was feasible. The higher the power, the lower the total energy, elastic energy, and dissipation energy, and the granite is gradually transformed from elastic deformation to plastic deformation. The elastic energy ratio decreases, the dissipation energy ratio increases, and the degree of damage becomes more and more serious. This study provides theoretical support for exploring the mechanical behavior and mechanism of microwave-assisted rock breaking and is of great practical significance. [ABSTRACT FROM AUTHOR]

It has been demonstrated that microwave pretreatment can weaken rocks, reduce tool wear, and improve mechanical rock breaking efficiency. The dielectric properties of rock minerals are a major factor affecting the ability of rocks to absorb microwaves, so it is important to conduct experimental research on rock modification to improve the microwave absorption capacity of rocks. The concept of "microwave-assisted absorbing reagents" has been proposed to increase the microwave absorption capacity of rocks. Sandstone specimens are first pretreated with strong dielectric substances before being exposed to microwave radiation to increase their microwave absorption capacity, then they are exposed to microwave fields and finally conducted to uniaxial compression tests. Enhancement in microwave absorption capacity of sandstone specimens is showed by P-wave velocity, nuclear magnetic resonance, and mechanical tests before and after microwave treatment. The results showed that barium titanate suspension and water are both effective microwave-assisted absorbing agents. The specimens treated with barium titanate suspension exhibited better microwave absorption capacity; in 5 kW microwave power irradiation, its p-wave velocity falls by 14.04%, porosity rises by 25.43%, and uniaxial compressive strength falls by 24.98%. Additionally, the failure form of sandstone specimens changes from brittle to plastic once it has fully absorbed microwave energy. [ABSTRACT FROM AUTHOR]

Predicting the strength evolution of fiber-reinforced cement mortar under freeze-thaw cycles plays an important role in engineering stability evaluation. In this study, the microscopic pore distribution characteristics of fiber-reinforced cement mortar were obtained by using nuclear magnetic resonance (NMR) technology. The change trend of T2 spectrum curve and porosity cumulative distribution curve showed that the freeze-thaw resistance of cement mortar increased first and then decreased with the fiber content. The optimal fiber content was approximately 0.5%. By conducting mechanical experiments, it is found that the uniaxial compressive strength (UCS) of the samples exhibited the 'upward convex' evolution trends with freeze-thaw cycles due to cement hydration, and based on fractal theory, the negative correlation between UCS and Dmin was established. Eventually, a freeze-thaw strength prediction model considering both porosity and pore distribution was proposed, which could accurately predict the strength deterioration law of cement-based materials under freeze-thaw conditions. [ABSTRACT FROM AUTHOR]

Freeze-and-thaw (F&T) weathering cycles induced by day–night and seasonal temperature changes cause a large number of rock mass engineering disasters in cold areas. Investigating the impact of F&T weathering process on the strength and deformation characteristics of frozen–thawed rocks is therefore of critical scientific importance for evaluating the stability and optimizing the design of rock mass engineering in these areas. In this research, the evolution characteristics of F&T damage were analyzed based on T2 spectrum distribution curves of sandstone specimens before and after F&T weathering cycles. The coupling impact of the quantity of F&T weathering cycles and confining pressure on pre-peak and post-peak deformation behaviors of sandstone specimens were analyzed in detail. By introducing the confining pressure increase factor (CPIF), the impact of confining pressure on the triaxial compressive strength (TCS) of sandstone specimens after undergoing different quantities of F&T weathering cycles was further investigated. A novel strength evolution model was proposed that could effectively describe the coupling impact of the quantity of F&T weathering cycles and confining pressure on TCS of rocks after undergoing the F&T weathering process. The proposed strength evolution model was cross-verified with experimental data from the published literature and all correlation coefficients were above 0.95, which proved that the strength evolution model proposed in this paper was reasonable; in addition, this model has strong applicability. [ABSTRACT FROM AUTHOR]

The nuclear magnetic resonance (NMR) technique was used to study freeze–thaw damage degradation of sandstone with initial damage. The initial damage was fabricated by preloading, and the pore structure was detected using the NMR technique. Porosity, T2 spectrum distribution, and T2 spectral area were obtained. The magnetic resonance imaging (MRI) technique was used to obtain two-dimensional pore distribution images. Combined with uniaxial compressive strength tests, the effects of initial damage and freeze–thaw cycles on the mechanical properties of rock were studied and the failure mode of rock was analyzed. The results show that in the first 20 cycles, the numbers of micropores (d ≤ 1 μm) and mesopores (1 μm < d ≤ 10 μm) both increased. As the number of cycles further increased, the porosity of the sample was mainly affected by micropores. When the initial damage level is 50% or lower, as the number of freeze–thaw cycles increases, the ability of sandstone to accommodate permanent strain before failure gradually weakens and then increases again after 40 cycles. When the initial damage level reaches 70% or higher, the ability to accommodate permanent strain continues to weaken. The strain produced by sandstone before elastic deformation is roughly positively correlated with the number of mesopores and macropores in the sandstone. Under loading, the sandstone presents four failure modes: splitting failure, single shear failure, cone failure, and multi-crack failure. [ABSTRACT FROM AUTHOR]

To study the evolutionary characteristics of mesostructural damage to saturated limestone under different loading and unloading paths, three types of loading and unloading tests involving three different loading rates and initial peak stresses were performed. Nuclear magnetic resonance technology and scanning electron microscopy were used to investigate the evolutionary characteristics of pore water during the loading and unloading of the limestone. The results indicated that, with an increase in the initial peak stress, the rock viscoplasticity gradually decreased, and the variation of pore radius and the reduction of bound water decreased. With an increasing loading rate, the mesostructure evolution law under disturbance-increasing amplitude (DIA) cycling was opposite to those under increasing amplitude (IA) and repeated-increasing amplitude (RIA) cycling. With the increasing loading stress level, the porosity decreased and then increased. Under increasing amplitude cycling, a larger initial porosity resulted in higher pore compaction and expansion limits. Reducing the initial peak stress inhibited the pore expansion, whereas it had the opposite effect under RIA and DIA cycling. During loading and unloading, bound water exists in pores of organic matter and mesopores, and free water exists in macropores of intergranular and transgranular fractures. These changes indicate certain laws under different loading and unloading paths. The results of this study indicate that the mesostructure characteristics of rock depend on the loading and unloading paths. [ABSTRACT FROM AUTHOR]

The evolution of the rock pore structure is an important factor influencing rock mechanical properties in cold regions. To study the mesoscopic evolution law of the rock pore structure under freeze-thaw weathering cycles, a freeze-thaw weathering cycle experiment was performed on red sandstone from the cold region of western China with temperatures ranging from -20°C to +20°C. The porosity, T2 spectral distribution, and magnetic resonance imaging (MRI) characteristics of the red sandstone after 0, 20, 40, 60, 80, 100, and 120 freeze-thaw weathering cycles were measured by the nondestructive detection technique nuclear magnetic resonance (NMR). The results show that the porosity of sandstone decreases first and then increases with the increase of the freeze-thaw weathering cycles and reaches the minimum at 60 of freeze-thaw weathering cycles. The evolution characteristics of porosity can be divided into three stages, namely, the abrupt decrease in porosity, the slow decrease in porosity, and the steady increase in porosity. The evolution characteristics of the T2 spectrum distribution, movable fluid porosity (MFP), and MRI images in response to the freeze-thaw weathering process are positively correlated with the porosity. Analysis of the experimental data reveals that the decrease in the porosity of the red sandstone is mainly governed by mesopores, which is related to the water swelling phenomenon of montmorillonite. Hence, the pore connectivity decreases. As the number of freeze-thaw cycles increases, the effect of the hydrophysical reaction on the porosity gradually disappears, and the frost heaving effect caused by the water-ice phase transition gradually dominates the pore evolution law of red sandstone. [ABSTRACT FROM AUTHOR]

The mechanical properties of rocks are significantly affected by chemical corrosion. To explore the influence of chemical corrosion on the weakening laws of sandstone mechanical properties, the porosity and pore size distribution (PSD) of sandstone samples immersed in different chemical solutions was measured by the nuclear magnetic resonance (NMR) technique. The damage variable based on the change of porosity was proposed to analyse the chemical damage to the sandstone samples. Moreover, both compressive and Brazilian tensile tests under static and dynamic conditions were carried out using a conventional servo-controlled testing machine and a split Hopkinson pressure bar (SHPB) system. The results showed that the porosity and proportion of macropores of the sandstone increase after chemical corrosion. The weakening laws of compressive and tensile strength of the sandstone under static and dynamic states are similar, and the relations among them and the damage variable are exponential. The dynamic tensile strength is most sensitive to the effects of chemical corrosion. The order of the degree of damage of chemical solutions on mechanical properties of sandstone is: DH2SO4 > DNaOH > DDistilledwater. Based on the experimental data, the relationships between the mechanical properties and chemical damage variable can be described as exponential equations. Additionally, the variations of dynamic increase factors versus chemical damage variable, the relationship between PSD and the strength of the chemically corroded sandstone, and the corrosion mechanism are also investigated. [ABSTRACT FROM AUTHOR]

Chemical corrosion has significant impact on the properties of rock materials. To investigate the effect of chemical corrosion on the porosity and mechanical properties of sandstones, the nuclear magnetic resonance (NMR) technique was used for the measurement of porosity. Uniaxial compression tests were then conducted for rock specimens treated with chemical corrosions. The test results showed that, compared with the rock specimens in their natural state, after chemical corrosions, the porosity increased, the uniaxial compressive strength and elastic modulus of sandstone both decreased, but the corresponding peak strain increased. A chemical damage variable derived from the change of porosity and the effective bearing area of rock samples was proposed. Based on the chemical damage variable, the corrosion order of different chemical solutions on sandstone was obtained as H2SO4 > NaOH > distilled water. The mechanism of chemical corrosion was also explored based on water-rock reactions. Finally, by introducing the compaction coefficient, an improved statistical damage constitutive model was established to describe the damage evolution of the sandstones treated with different chemical corrosions. [ABSTRACT FROM AUTHOR]

Low-field nuclear magnetic resonance (NMR) technology has the characteristics of nondestructive, rapid, and accurate. In the present paper, the mechanical properties and the size and distribution of pores of cement mortar during freeze-thaw cycles were studied by using the NMR technology for the first time. The change law of surface and quality, compressive strength, splitting tensile strength, and elastic modulus of cement mortar under 0, 25, 50, 75, and 100 freeze-thaw cycles were studied. And the changes of T2 spectra of cement mortar under different freeze-thaw environments were analyzed; the change rule between freeze-thaw cycles and the size of the pore within the cement mortar were also obtained. Moreover, the relationship between the mechanical properties and the pore structure of cement mortar was studied. [ABSTRACT FROM AUTHOR]

Chemical corrosion plays a significant role in affecting the properties of rock materials. To understand the effects of chemical corrosion on the pore structure and mechanical properties of sandstones, porosity, T2 spectrum distribution, and NMR images of sandstone specimens were measured after every 10 days of immersion in chemical solutions using the nuclear magnetic resonance (NMR) technique. Static uniaxial compressive tests and dynamic compressive tests were conducted using a conventional servo-controlled testing machine and a split Hopkinson pressure bar (SHPB) system for specimens treated with chemical corrosion. The test results showed that after being treated with chemical corrosion, the porosity of a specimen increased, the T2 spectrum distribution would successively shift towards the right, and the distribution of pores tended to become more irregular. Additionally, all of the compressive strength and elastic modulus of sandstone treated with chemical corrosion under static and dynamic loads decreased, and the peak strain increased. The effect order of a chemical solution on the pore structure and mechanical properties of sandstone was H2SO4>NaOH>distilled water, which would be related to the different mechanisms of a water-rock reaction. According to the experimental results, the correlations between the mechanical properties and porosity were established. The results can serve as a reference for research in related fields. [ABSTRACT FROM AUTHOR]

The issue of rock deterioration in chemical environments has drawn much attention in recent years in the rock engineering community. In this study, a series of 30 freeze-thaw cycling tests are conducted on sandstone samples soaked in H2SO4 solution and in pure water, prior to the application of nuclear magnetic resonance (NMR) on the rock specimens. The porosity of the sandstone, the distribution of transverse relaxation time T2, and the NMR images are acquired after each freeze-thaw cycle. The pore size distribution curves of the sandstone after freeze-thaw cycles, four categories of pore scale, and the features of freeze-thaw deterioration for pores of different sizes in H2SO4 solution and pure water are established. The result shows that, with the influence of the acid environment and the freeze-thaw cycles, the mass of the samples largely deteriorates. As the freeze-thaw cycles increase, the porosity of rocks increases approximately linearly. The distribution of the NMR T2 develops gradually from 4 peaks to 5 or even to 6. Magnetic resonance imaging (MRI) dynamically displays the process of the freeze-thaw deterioration of the microstructure inside the sandstones under acid conditions. The results also show pore expansion in rocks under the coupling effects of chemistry and the freeze-thaw cycles, which differ largely from the freeze-thaw deterioration of the rock specimens placed in pure water. [ABSTRACT FROM AUTHOR]

Abstract Multi-scheme lateral unloading confining pressure tests on marble specimens were conducted by using an MTS 815 electrohydraulic servo testing machine, and nuclear magnetic resonance (NMR) tests were performed on the unloaded specimens for the first time to investigate the mesodamage evolution characteristics of the unloading rock. The change laws of stress-strain curves, rock porosity, and NMR parameters during the failure process were obtained. The results indicate that the rock sample exhibits evident plastic strain softening and shear dilation characteristics during the unloading process, and that the deformation modulus, Poisson's ratio, and axial compression strength decrease continuously with a decrease in the confining pressure. Given the different levels of unloading points, there are differences in the failure characteristics. When the unloading intensities were lower than 90%, the microfissures inside the sample extended uniformly, and the internal damage to the marble was mainly caused by the increases in the number of small pores. When the unloading intensities exceeded 90%, interpenetrating fissures developed and extended rapidly, and rock damage was mainly caused by the increase in the number and size of the large pores. The spectrum peaks area of large pores play a dominant role in the development of marble porosity. Furthermore, the development process of microcracks and fractures in rock specimens is dynamically indicated by NMR imaging. Highlights • NMR tests were conducted to investigate the mesodamage evolution characteristics of the unloading rock. • The change laws of stress-strain curves, rock porosity, and NMR parameters during the failure process were obtained. • The spectrum peaks area of large pores plays a dominant role in the development of marble porosity. • The development process of micro cracks and fractures in the rock specimens is dynamically indicated by NMR imaging. [ABSTRACT FROM AUTHOR]

For a deeper understanding of the freeze-thaw weathering effects on the microstructure evolution and deterioration of dynamic mechanical properties of rock, the present paper conducted the nuclear magnetic resonance (NMR) tests and impact loading experiments on sandstone under different freeze-thaw cycles. The results of NMR test show that, with the increase of freeze-thaw cycles, the pores expand and pores size tends to be uniform. The experimental results show that the stress-strain curves all go through four stages, namely, densification, elasticity, yielding, and failure. The densification curve is shorter, and the slope of elasticity curve decreases as the freeze-thaw cycles increase. With increasing freeze-thaw cycles, the dynamic peak stress decreases and energy absorption of sandstone increases. The dynamic failure form is an axial splitting failure, and the fragments increase and the size diminishes with increasing freeze-thaw cycles. The higher the porosity is, the more severe the degradation of dynamic characteristics is. An increase model for the relationships between the porosity or energy absorption and freeze-thaw cycles number was built to reveal the increasing trend with the freeze-thaw cycles increase; meanwhile, a decay model was built to predict the dynamic compressive strength degradation of rock after repeated freeze-thaw cycles. [ABSTRACT FROM AUTHOR]

The effect of freeze-thaw weathering cycles caused by diurnal and seasonal temperature changes is a significant disadvantage in the stability of rock engineering in cold regions. To investigate the evolution rule of rock strength and infrared radiation characteristics of igneous, metamorphic, and sedimentary rock subjected to freeze-thaw weathering cycles, freeze-thaw weathering cycle treatment was conducted on coarse-grained granite, fine-grained marble and soft red sandstone from cold regions in Western China. The nondestructive detection technique of nuclear magnetic resonance (NMR) was used for rock damage detection. Infrared thermography was used to monitor the rock debris movement, crack propagation and average infrared radiation temperature (AIRT) during unconfined compressive strength (UCS) testing. The mechanisms of rock damage and change in AIRT induced by freeze-thaw weathering cycles were carefully analyzed. According to the UCS evolution rule, the ranking order (from high to low) of the results of rock resistance to freeze-thaw weathering processes is as follows: marble, granite and sandstone. An indicator called the infrared radiation rate was proposed to measure the rate of increase in infrared radiation temperature with increasing stress during the UCS tests. The rank order (from high to low) of the infrared radiation rate results is as follows: sandstone, marble and granite. Based on the infrared images and photographs of the failed rock specimens, the failure patterns and processes of the different rock types are investigated. • The ranking order (from high to low) of rock resistance to freeze-thaw weathering is marble, granite and sandstone. • Infrared radiation characteristics for three main types of rock subjected to freeze-thaw weathering cycles were presented. • Infrared radiation rate is proposed to measure the rate of infrared radiation temperature increase with increasing stress. • The failure pattern and process of different rock types are investigated based on infrared images. [ABSTRACT FROM AUTHOR]

This research combines a centrifugal test and nuclear magnetic resonance (NMR) technology to study the water retention capacity of the cemented paste backfill. Backfill samples with cement–tailings ratios of 1:4, 1:8, and 1:12, and solid concentrations of 71%, 74%, 77%, 80%, and 83% respectively, were prepared for the test. The relative centrifugal force ( RCF ) required for accurate testing and the T2 cutoff value that characterizes the water retention capacity were obtained through an NMR test on the backfill samples after centrifugation in saturated conditions. Based on the soil–water characteristic curve (SWCC), the NMR pore water characteristic distribution model was established, and the pore size distribution and effective water retention characteristics were analyzed. This study shows that when the rotating speed is between 1500 and 4000 rpm, the R C F of the backfill ranges from 125.8 to 894.4 g/min , and the T2 cutoff value will vary from 3 to 10 ms. With an increase in solid concentration of the backfill, both the RCF and T2 cutoff value decline. The Scanning Electron Microscope (SEM) analysis confirms that an increase in the solid concentration and cement–tailings ratio will lead to obvious bimodal characteristics of the pore size distribution curve of the backfill. In addition, the porosity will decrease, the critical pore value, which represents a value to distinguish pores with different movable fluid retention capabilities and characterizes the pore size classification, will become smaller, and the pore size distribution will become more diverse. These changes indicate that a high-concentration backfill can effectively reduce the flow of a fine-grained matrix with large pores. [ABSTRACT FROM AUTHOR]

The issue of rock pore structure deterioration and dynamical mechanical properties has drawn much attention in recent years in the rock engineering community. In this study, a series of 140 freeze-thaw cycling tests are conducted on sandstone samples. The sandstone pore structure after freeze-thaw cycles was detected by Nuclear Magnetic Resonance (NMR), and the dynamic load test was carried out by Split Hopkinson Pressure Bar system (SHPB). The results are: with the increase of freeze-thaw cycles, the saturated mass and porosity of sandstone increase, the dynamic peak stress of sandstone decreases; while the peak strain and total strain increase gradually, and the macroscopic damages of rocks increase. The results of T 2 distribution and pore size changes are that the pore sizes of sandstones increase, especially that of micro-pores and macro-pores increase obviously after 140 freeze-thaw cycles. Under the effect of freeze-thaw cycles, the sandstone pores become connected with each other, some of the micro-pores and mini-pores are changed into meso-pores and macro-pores. The number of marco-pores increases evidently, and the largest increase is 197.23%. The rock pore structures have changed, which would lead to the change of mechanic properties. As the number of freeze-thaw cycles increases, the rock dynamical peak stress gradually decreases and the peak strain and the overall strain increase gradually. [ABSTRACT FROM AUTHOR]

Taking sandstone as a research object, this study investigated the damage evolution law of rock mass with different macroscopic and microscopic defects under freeze–thaw cycles. For the first time, macroscopic defects were simulated by prefabricating 0°, 45°, and 90° fractures in real rock materials. Initial microscopic damage was caused by applying prestressing forces equal to 0%, 30%, 50%, and 70% of the ultimate compressive strength of the specimen. The microstructure changes of samples during freeze–thaw cycles were tracked by nuclear magnetic resonance (NMR) technology, and the characteristic parameters such as porosity, aperture distribution, and T 2 spectral distribution were obtained. Through freeze–thaw cycle tests and uniaxial compression tests on specimens undergoing 0, 20, 40, and 60 freeze–thaw cycles, the influence of different damage levels on the changes in the surface, mass, volume, compressive strength, and elastic modulus of rock mass was investigated. The experimental results show that the porosity of the intact specimen is greater than that of the fractured specimen during freeze-thaw cycles. The mechanical properties of the intact specimen and the 90° fractured specimen are controlled by the pore distribution, and the strength drop increases first and then decreases during freeze-thaw cycles, while the mechanical properties of the 0° fractured specimen and the 45° fractured specimen are affected by the prefabricated fracture, and the strength drop increases with the increase of the fracture damage. The initial damage can accelerate the damage degradation of the rock mass, and when the damage level exceeds a certain threshold, which is between 30% and 50%, the deterioration effect will be more significant. • Freeze-thaw damage evolution of rock mass was studied with macro and micro defects considered. • The microstructure changes of samples during freeze-thaw cycles were tracked by NMR technology. • The influence of crack inclination on the mechanical properties of the specimen was analyzed. • The initial damage has an increasing effect on the degradation of the rock mass with damage level. [ABSTRACT FROM AUTHOR]