Ultrasonic Wave Velocity Research Articles

Combining ultrasonic testing, infrared thermography, and stereoscopy to unveil characteristics of the adhesion capacity of metakaolin plastering mortars

In the present work, characteristics of the adhesion capacity of metakaolin plastering mortars are unveiled by combining ultrasonic testing, and infrared thermography (IRT), associated with stereoscopy. The investigation was carried out for plastering mortars with metakaolin incorporated by adding and partially replacing the Portland cement, in which incorporations of 5%, 10%, and 15% were employed. Scanning Electron Microscope (SEM), and Energy Dispersive Spectroscopy (EDS) were employed to investigate the morphological characteristics of the metakaolin mortars' surface. In addition, flexural strength tests were also performed to evaluate mechanical behavior. The SEM, and SEM/EDS results revealed aspects of the incorporation of metakaolin in the plastering mortars and revealed the formation of Portlandite, Ettringite, and the CSH phases. Considering the flexural strength behavior, only the plastering mortar with 10% incorporated metakaolin has a better mechanical performance than the reference mortar. Further, the propagation velocity of incident ultrasonic waves was useful in calculating Young's modulus, which is associated with the adhesion capacity of the plastering mortars, due to its mechanical response to the test. Furthermore, the IRT results revealed a significant water absorption by the substrate when a layer of roughcast was applied before the metakaolin plastering mortar. When the plastering mortars are directly applied to the concrete substrate, a smaller ability to transfer water to the solid surface is verified. These adhesion behaviors are associated with the capacity of the system to transfer water to the substrate. Finally, stereoscopy results showed heterogeneity in the interface area between the mortar directly applied to the concrete surface and homogeneity in the interface between the mortar and roughcast/concrete surface, corroborating previous results. In conclusion, the ability of metakaolin plastering mortars to transfer water molecules to solid substrates can be unveiled by combining ultrasonic testing, IRT, and stereoscopy, and this is particularly useful in explaining it’s adhesion property, as shown throughout the work.

Experimental study on the influence of ultrasonic excitation duration on the pore and fracture structure and permeability of coal body

The study aims to explore the effect of ultrasonic duration on the pore structure and seepage characteristics of coal. Herein, we adopted a self-developed ultrasonic excitation experiment system using low-field nuclear magnetic resonance (NMR) technology and NM-4B non-metallic ultrasonic wave velocity detector. The study investigated NMR relaxation time T2 patterns and the number of pores of each size, internal structural damage, and seepage characteristics of the coal body before and after ultrasonic fracturing. Based on the Warren-Root model, the permeability model of coal under ultrasonic fracturing was established, revealing the changing law of ultrasonic fracturing at different times on the change of pore scale, connectivity, and nuclear magnetic permeability of the coal body. The mechanism of ultrasonic fractured coal body evolution at different times was clarified. The results showed that the ultrasonic fracturing at different times improved the number and volume of all size pores in the coal body, expanded primary fractures, and developed newborn fractures. The rate of change in pore size, porosity, wave velocity, and coal permeability had a linear relationship with the fracturing time. The growth rate of seepage pore volume proportion was positively correlated to the decreased rate of adsorption pore volume proportion. The fracturing duration leads to the interconnection of micropores within the coal body, forming medium and large pore fractures. This transformation between pores and fractures enhances the pore structure and seepage characteristics of the coal body. The coal rock permeability model can be used to calculate the permeability of a coal body, which follows the same change law as nuclear magnetic permeability. When evaluating the index using absolute and relative errors, the range of relative error is 0.97%–7.91%. The error variation range is small, indicating that the established model can effectively reflect the change law of porosity and permeability of the coal body responding to ultrasonic fracturing. The ultrasonic fracturing rendered the ultrasonic cavitation takes micropores as fracturing units, developed pores of all sizes, connected closed pores, increased free fluid space proportion, and improved the coal permeability.

Stress Measurements of GIS Epoxy Composite with Transverse Waves Based on the Ultrasonic Pulse-Echo Method

The mechanical reliability of the gas-insulated metal-enclosed switchgear (GIS) is critical to the safe and stable operation of a power system. The internal stress induced in GIS insulators, which typically lead to cracking, can degrade the mechanical properties and compromise the reliability of the GIS. In this work, ultrasonic nondestructive measurements of the internal stress in a GIS 126 kV epoxy composite with the transverse waves are demonstrated for the first time. Our results show that the velocities of ultrasonic transverse waves increase linearly with the compressive stress and decrease linearly with the tensile stress. The acoustoelastic coefficients are −1.18 × 10−4/MPa and −1.96 × 10−4/MPa for measurements with transverse waves polarized vertically and parallelly to stress, respectively. Based on these acoustoelastic coefficients, the internal stresses are calculated, and the maximum absolute values of relative errors are 35.4% and 26.3% for measurements with the transverse wave polarized vertically and parallelly to stress, respectively.

Stress-Dependent Petrophysical Properties of the Bakken Unconventional Petroleum System: Insights from Elastic Wave Velocities and Permeability Measurements

The net-effective stress is a fundamental physical property that undergoes dynamic changes in response to variations in pore pressure during production and injection activities. Petrophysical properties, including porosity, permeability, and wave velocities, play a critical role and exhibit strong dependence on the mechanical stress state of the formation. The Williston basin’s Bakken Formation represents a significant reservoir of hydrocarbons within the United States. To investigate this formation, we extracted core plugs from three distinct Bakken members, namely Upper Bakken, Middle Bakken, and Lower Bakken. Subsequently, we conducted a series of measurements of ultrasonic compressional and shear wave velocities, as well as pulse decay permeabilities using nitrogen, under various confining pressures employing the Autolab-1500 apparatus. Our experimental observations revealed that the ultrasonic wave velocities and permeability display a significant sensitivity to stress changes. We investigated existing empirical relationships on velocity-effective stress, compressional-shear wave velocities, and permeability-effective stress, and proposed the best models and associated fitting parameters applicable to the current datasets. In conjunction with the acquired datasets, these models have considerable potential for use in time-lapse seismic monitoring and the study of production decline behavior. The best fitting models can be used to forecast the petrophysical and geomechanical property changes as the reservoir pore pressure is depleted due to the production, which is critical to the production forecast for unconventional reservoirs.

Microseismic Signal Characteristics of the Coal Failure Process under Weak-Energy and Low-Frequency Disturbance

In deep mining, “critical static stress + slight disturbance” is an important inducing form of coal mine rockburst disasters. In previous studies, the critical static stress has been shown to be consistent with the loading direction of a slight disturbance but cannot reflect all types of rockbursts. In addition, the research that uses microseismic (MS) signals to reflect the overall process and critical stages of coal failure and instability under weak-energy and low-frequency disturbance conditions is immature, and more information, such as the critical state, has not been fully revealed. The aims of this paper are to further elucidate the important role of weak-energy and low-frequency disturbances in the occurrence of rockburst disasters. First, briquette samples were prepared from the Tashan Coal Mine, which is severely affected by rockbursts, and their homogeneity was verified using ultrasonic longitudinal wave velocity. Second, the natural frequency of the coal sample specimens was measured using a testing system. Then, based on the self-developed static pressure loading system, dynamic and static combined loading test system and MS signal monitoring device, the MS signal characteristics during the process of coal body failure and instability were comprehensively analysed. Finally, a comparison was made between weak-energy and low-frequency disturbances and impact disturbances. The results are summarized as follows. (1) The longitudinal wave velocity test results reflect that the briquette samples prepared in the experiment have high homogeneity. The smaller the particle size is, the higher the density and moulding pressure, and the denser the sample. (2) The natural frequency of the briquette samples is between 30.79 Hz and 43.34 Hz, and most of them fluctuate at approximately 35 Hz. (3) During the static loading stage, the occurrence of more than three MS signals of larger magnitude in a continuous cluster is an important criterion for the critical failure of the samples. (4) The weak-energy and low-frequency disturbance actually leads to fatigue damage, and the briquette sample experiences three stages: the near-threshold stage, the high-speed expansion stage and the final fracture stage. The smaller the particle size of the coal sample, the denser the specimen, the stronger the amplitude and energy of the single effective MS signal formed during the destruction process, the longer the time duration of crack expansion from the near-threshold stage to the high-speed expansion stage, and the stronger the ability of the coal sample to resist weak-energy and low-frequency disturbances. This study may contribute to a more comprehensive understanding of the destabilization mechanism of coal bodies and MS signal characteristics under weak-energy and low-frequency disturbances and provide a reference for further research and discussion.

Effects of CO2 on creep deformation in sandstones at carbon sequestration reservoir conditions: An experimental study

We conducted four hydrostatic creep/flow-through experiments to investigate the evolution of rock hydraulic and mechanical properties due to fluid-rock interactions in the context of underground CO2 utilization and storage. The experiments were performed on the macroporosity-dominated lithofacies of the Morrow B sandstone. The Morrow B is an early Pennsylvanian fluvial/marine deposit, and the target reservoir of an ongoing CO2-enhanced oil recovery project at the Farnsworth Unit oilfield, Ochiltree County, Texas. Core samples were held at 38.5 MPa effective stress and 100 °C while flowing through reservoir brackish solution (control) or CO2-enriched reservoir brackish solution at different flow rates (0.01, 0.02, and 0.1 ml/min). We monitored the evolution of the outlet fluid chemistry, sample axial strain, permeability, and bulk modulus, and measured pre- and post-experiment porosities and ultrasonic wave velocities. Results showed mineral dissolution, but no enhanced creep deformation in the presence of CO2. Samples experienced fluctuations in the bulk modulus of elasticity with up to 18% and 3% increases for flow-through with reservoir brackish solution and CO2-enriched reservoir solution, respectively. Permeability decreased monotonically during flow-through with brackish solution, but fluctuated with CO2-rich brackish solution. All samples experienced a decrease in the post-test ultrasonic wave velocities, where the decrease was proportional to the test duration. Chemical reactions were not rate limited, and ion concentrations varied as the fluid volume through the core for all experiments irrespective of flow rate, duration, or experiment type. We believe that the observed fluctuation in sample bulk modulus was due to dissolution/microcracking leading to rock softening versus rock consolidation/pressure solution leading to rock stiffening, which is supported by petrographic observations. CO2-driven dissolution of disseminated ankerite cements did not cause enhanced compaction because of their low occurrence and the presence of the more abundant unreactive pre-compaction quartz cements forming long contacts with the framework grains. Porous sandstone reservoirs whose framework grains are supported by early diagenetic (pre-compaction) quartz cements make good targets for the long-term safe underground storage of CO2.

BARIK: laboratory programme within the framework of the development of an extended Hoek–Brown-based anisotropic constitutive model for fractured crystalline rock

Abstract. The disposal of heat-generating radioactive waste in deep geologic formations is a global concern. Numerical methods play a key role in understanding and assessing the disposal scenarios of radioactive waste in deep geological repositories. However, the complexities of the thermal, hydrological, mechanical, chemical, and biological processes associated with the disposal of radioactive waste in porous and fractured materials constitute significant challenges. One of the most challenging issues in this field is the complex material behaviour of fractured crystalline rock. The presence of fractures makes the rock anisotropic, nonlinear, and dependent on loading paths. Additionally, the Biot coefficient cannot be considered constant throughout the critical and subcritical fracture development regions. These factors make the development of an accurate constitutive model for fractured crystalline hard rock a critical component of any deep geological disposal project. Furthermore, to demonstrate the integrity of the “containment effective rock area” in crystalline host rock, the qualitative integrity criteria must be quantified so that numerical simulation can be performed with concrete numerical values. Part of this assessment for a crystalline host rock is a dilatancy criterion, which is currently based on the Hoek–Brown constitutive model. This contribution provides an overview and first results of the laboratory programme, which was performed as a part of the research project. The aim was to generate a fundamental and extensive dataset for an anisotropic material used for verification and validation purposes of the newly developed constitutive model. The rock material for the test programme was “Freiberger gneiss” because of its pronounced anisotropic properties regarding deformation and strength as well as the good access to obtain a larger amount of sample material. A wide range of basic tests have been performed, e.g. determination of density and porosity, measurement of ultrasonic wave velocities to get dynamic elastic properties, Brazilian tests, and uniaxial compression tests to obtain strength data. Additionally, a large number of more complex multi-stage triaxial compression tests with examination of the post-failure region and hydromechanical coupled triaxial compression tests have been conducted or are in progress. The hydromechanical part, in particular, plays an important role in examining and quantifying the evolution of the micromechanical damage process, which changes the permeability and Biot coefficient and therefore the effective stresses inside a saturated rock material. Tests were conducted with different orientations of the structural planes in relation to the loading directions.

Evolution law of ultrasonic characteristics and its relationship with coal-measure sandstone mechanical properties during saturation and desaturation

Underground buildings are easily destabilised by water and there is a strong link between their mechanical properties and ultrasonic characteristics. To grasp the above features and the relationship among them, the mechanism of sandstone degradation under water erosion was briefly reviewed. Using the coal-measure sandstone as the object, experimental studies on the dynamic evolution of water content, ultrasonic wave velocity, waveform, uniaxial compressive strength (UCS), elastic modulus (E), and damage mode during saturation-desaturation were carried out. The findings demonstrate that coal-measure sandstones usually exhibit more mechanical parameter deterioration following saturation than non-coal-measure sandstones. During saturation, the water absorption velocity slows down; the P-wave velocity (VP) first fluctuates down, then fluctuates up and finally decreases slightly (VP increases after saturation), and the wave velocity anisotropy coefficient (δi) decreases; the waveform shows a trend of unequal time and unequal amplitude → equal time and equal amplitude → unequal time and equal amplitude, and the first wave reaches a progressively shorter time. During desaturation, the dehydration rate slows down, VP first drops rapidly and then fluctuates slightly and rises slightly near drying (VP decreases after desaturation), δi increases, and the waveform changes in the opposite trend to saturation. Saturation treatment significantly weakened the mechanical properties, while desaturation restored some of the strength lost due to immersion. The sandstone mechanical properties and ultrasonic features are more closely related during saturation than desaturation.

The Influence of the Hardness of the Tested Material and the Surface Preparation Method on the Results of Ultrasonic Testing

Non-destructive ultrasonic testing can be used to assess the properties and condition of real machine elements during their operation, with limited (one-sided) access to these elements. A methodological question then arises concerning the influence of the material properties of such elements and the condition of their surfaces on the result of ultrasonic testing. This paper attempts to estimate the influence of material hardness and surface roughness on the result of such testing study area testing machine or plant components of unknown exact thickness. Ultrasonic testing was carried out on specially prepared steel samples. These samples had varying surface roughness (Ra from 0.34 to 250.73 µm) of the reflection surface of the longitudinal ultrasonic wave (the so-called reflectors) and hardness (32 and 57 HRC). The ultrasonic measures were the attenuation of the wave, estimated by the decibel drop in the gain of its pulses, and the propagation velocity of the longitudinal ultrasonic wave. Ultrasonic transducers (probes) of varying frequencies (from 2 to 20 MHz), excited by a laboratory and industrial defectoscope were used as the source of such a wave. The results of our research provide a basis for the recommendation of two considered ultrasonic quantities for assessing the material properties of the tested element. This is of particular importance when testing machines or plant components of unknown exact thickness and unknown roughness of inaccessible surfaces, which are the reflectors of the longitudinal ultrasonic wave used for testing. It has been demonstrated that by using the ultrasonic echo technique, it is possible to evaluate the roughness and hardness of the tested elements.

The structure perfection studies of LiCaAlF6 crystals by laser acoustics technique

The acoustic properties of LiCaAlF6 crystals and the annealing procedure impact on them were studied. The velocities of longitudinal and shear ultrasonic waves along the direction of the optical c-axis of the crystals were measured by laser acoustics, and the mechanical constants were determined: modules of elasticity, all-round compression, shear, and Poisson's ratio. It has been established that the attenuation coefficient of ultrasonic waves in the 1-15 MHz frequency range can serve as a characteristic of the optical perfection of the crystals. The effect of ultrasonic annealing of LiCaAlF6 crystal samples under prolonged exposure by pulsed ultrasound was discovered.

Characterization and Degradation of Ancient Architectural Red Sandstone in a Natural Erosion Environment

The properties and appearance of ancient architectural red sandstone will be damaged after being eroded by the natural environment for a long time. In order to investigate the weathering and erosion characteristics of the red sandstone structure of an existing ancient building, ultrasonic testing techniques, combined with scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray computed tomography (X-CT), were used to analyze a building in Ganzhou. The variation in chemical substances contained in the red sandstone specimens according to phenology was analyzed by X-ray diffraction (XRD). The characteristic parameters of the CT grayscale images of the red sandstone were extracted and combined with the ultrasonic wave velocity values to comprehensively analyze the degradation characteristics of the red sandstone specimens, and a method to characterize the degradation degree of the red sandstone as a whole plane is proposed. We use the gray model (GM (1, 1)) to predict the surface degradation degree of red sandstone specimens, and gray relation analysis (GRA) to further analyze the correlation between the characteristic parameters of CT grayscale images of red sandstone and its degradation degree. The results show that in the natural erosion environment, dolomite and chlorite are generated on the exposed surface of the red sandstone, which can protect the internal sandstone to a certain extent. The degradation degree of the red sandstone specimens in the horizontal X and Y directions varies, and the proposed method of calculating the overall plane degradation degree of the red sandstone is feasible. The minimum average relative error of the surface degradation degree obtained from the gray prediction GM (1, 1) model is 1.4591%. There is a good correlation between the characteristic parameters of the red sandstone CT grayscale images and the degradation degree.

Understanding the interactions of NaClO oxidant with coal for intensified hydraulic fracturing effectiveness

The efficacy of hydraulic fracturing coalbeds to enhance methane production suffers from a reduction of the effective permeability due to formation damage. To address this practical challenge, this paper explores the feasibility of applying the oxidative treatment to coals in the post-hydraulic fracturing period. Experimental methodologies are utilized to reveal the fundamental interactions of coal with the oxidative in the context of a combined treatment of hydraulic fracturing and NaClO solution injection. Anthracite and bituminous coals are experimented to account for the effect of coal rank. A comprehensive package of techniques, i.e., ultrasonic wave velocity and hardness (for mechanical properties), scanning electron microscopy (for microstructure and morphology), contact angle (for wettability), and FTIR spectroscopy (for functional groups) were employed to comprehensively investigate the impacts of fracturing fluid and NaClO solution on the chemical and physical properties of the coal samples. The results show that the fracturing fluid exerts varying degrees of harm to the permeability of the two coals, while the effects of oxidative treatment are rank specific in dissolving the blockage and smoothening the fracture surface. It is further unveiled that NaClO solution promotes aromatic structure decomposition at the coal surface to form oxygen-containing functional groups, and its effects on coal permeability evolve in multiple stages. The findings of this study shed light upon implementing oxidative treatment to aid hydraulic fracturing performance in CBM for clean coal production in a carbon-constrained world.

Results of the COOMET 706/RU-A/16 Pilot Comparison of National Standards of the Unit of Propagation Velocity of Longitudinal Ultrasonic Waves in Solids

Results of the COOMET 706/RU-A/16 Pilot Comparison of National Standards of the Unit of Propagation Velocity of Longitudinal Ultrasonic Waves in Solids

A Review on Acoustics of Wood as a Tool for Quality Assessment

Acoustics is a field with significant application in wood science and technology for the classification and grading, through non-destructive tests, of a large variety of products from standing trees to building structural elements and musical instruments. In this review article the following aspects are treated: (1) The theoretical background related to acoustical characterization of wood as an orthotropic material. We refer to the wave propagation in anisotropic media, to the wood anatomic structure and propagation phenomena, to the velocity of ultrasonic waves and the elastic constants of an orthotropic solid. The acoustic methods for the determination of the elastic constants of wood range from the low frequency domain to the ultrasonic domain using direct contact techniques or ultrasonic spectroscopy. (2) The acoustic and ultrasonic methods for quality assessment of trees, logs, lumber and structural timber products. Scattering-based techniques and ultrasonic tomography are used for quality assessment of standing trees and green logs. The methods are based on scanning stress waves using dry-point-contact ultrasound or air-coupled ultrasound and are discussed for quality assessment of structural composite timber products and for delamination detection in wood-based composite boards. (3) The high-power ultrasound as a field with important potential for industrial applications such as wood drying and other applications. (4) The methods for the characterization of acoustical properties of the wood species used for musical instrument manufacturing, wood anisotropy, the quality of wood for musical instruments and the factors of influence related to the environmental conditions, the natural aging of wood and the effects of long-term loading by static or dynamic regimes on wood properties. Today, the acoustics of wood is a branch of wood science with huge applications in industry.

Exploring the physio-elastic properties and optical band gap energies of boro-telluro-dolomite glasses infused with Nd2O3 dopants

In search for the innovative optical glass host, structural stability and optical band gap engineering of glasses infused with rare earth ions dopant is prerequisite. Inspired by this standpoint, a new series of boro-telluro-dolomite glass system activated with Nd2O3 (BTDN) has been developed via classic melt-quenching process. The amorphous status of the as made glasses was validated by XRD spectra whereas its physical features were exposed by inspecting the samples density and related parameters. Further, elastic moduli and its derivatives of these glasses were assessed from the measured ultrasonic wave velocities recorded at 5 MHz frequency. By using UV–Vis absorption spectrum data, the band gap energies and refractive index of our produced BTDN glasses are evaluated through Tauc's, absorption spectrum fitting (ASF) and differential absorption spectrum fitting (DASF) wherein similar results have been obtained by these tripartite models. Our verdicts herein affirmed that Nd3+dopant acts as a glass modifier, which meaningfully augment the physio-elastic performance of the BTDN glasses.

Determination of mechanical and damping properties of hazelnut shell powder reinforced biocomposites by ultrasonic method

AbstractThis research was carried out to figure out the effect of chemical treatments of hazelnut shell powders (HSPs) on the elastic properties, ultrasonic wave velocities, and damping properties of bio‐based epoxy resin (BER) biocomposites. Natural hazelnut shells (HSs) were chemically treated using sodium hydroxide (NaOH), and acetic anhydride (AA). Then, HSs that were chemically treated with NaOH and AA, and HSs that were not subjected to chemical treatment were ground to obtain HSPs. The treated HSPs (HSP‐NaOH and HSP‐AA), and untreated HSPs were contributed to the neat BER in varied compositions such as 10–50 wt% to obtain the BER/HSP, BER/HSP‐NaOH, and BER/HSP‐AA biocomposites. The effect of untreated, and treated HSP ratios on the density, ultrasonic wave velocities, Young's modulus, Bulk modulus, Shear modulus, Poisson ratio, microhardness, and damping characteristics (attenuation coefficient, loss tangent and quality factor) of the novel HSP‐based biocomposites, was investigated by the ultrasonic pulse‐echo overlap method (PEOM). A significant increase in the density, ultrasonic wave velocities, and elastic modulus values of the biocomposites was seen compared to the neat BER. Based on the obtained elastic modulus values, the most appropriate combination ratio between the neat BER, and HSP‐NaOH was determined as 50:50.

Acoustic Studies of the Melting and Crystallization of Eutectic Gallium–Silver Alloys in Porous Glasses

The paper presents the results of acoustic studies of the melting and crystallization of Ag–Ga alloys with a silver content of 1.5 and 3 at % embedded into porous glasses with an average pore size of 13 nm. The temperature dependences of the velocity of longitudinal ultrasonic waves are measured by a modified pulse-phase method at a frequency of 7 MHz in the 200–325 K range for complete and partial cooling–heating cycles. The temperature dependences of the ultrasonic velocity showed regions corresponding to phase transitions. Significant changes in the phase diagram of the bulk alloy due to nanostructuring have been revealed. It is shown that segregates with different crystal structures are formed in the pores for alloy of different compositions.

Properties and Mechanical Strength Analysis of Concrete Using Fly Ash, Ground Granulated Blast Furnace Slag and Various Superplasticizers

Supplementary cementitious materials (SCMs) have been widely used to replace cement in recent years in order to reduce the burden of cement on the environment. In this study, fly ash (FA) and ground-granulated blast furnace slag (GGBFS) were used as long-term 40%, 50% and 60% replacement cement in order to explore the mechanical strength of different superplasticizers (SPs) under high substitution amounts. The results of the study showed that, in terms of the nature of work, when 60% of cement was replaced with SCM, the initial setting time was increased by 40–70 min. The values of the ratio of the initial to final setting time (I/F ratio) are equivalent when the I/F values of PCE and SNF are at W/B = 0.27 and 0.35, and at the lowest W/B (0.21) in this study, the I/F calculation result was the difference between PCE and MLS. The I/F value is equal, which means that when the W/B is low, PCE and MLS have the same impact on workability, and as W/B increases, the impact of PCE and SNF is similar. In terms of compressive strength, W/B = 0.21. The 1-day curing age of PCE was compared with the 91-day curing age, and it was found that at high volumes of replacement, increasing GGBFS by 10% can increase the strength by 37%. Using the ultrasonic wave velocity as the input value and the compressive strength result as the output value, the MATLAB back propagation neural network prediction model was carried out. The best correlation coefficient R value of MLS was 0.97, and the mean squared error was 2.21, which has good prediction ability.

Durability and damage model of polyacrylonitrile fiber reinforced concrete under freeze–thaw and erosion

In an alpine saline soil area, the combined effect of freeze–thaw and erosion ions causes the imbalance of concrete durability. In studying the macro–meso deterioration law and failure mechanism of polyacrylonitrile fiber-reinforced concrete (PANFC) under this action, PANFC specimens formed by open-air curing were prepared, and a composite salt solution containing 5% MgSO4, 5% Na2SO4, and 3.5% NaCl was used as the freeze–thaw medium for salt-freezing test. Results demonstrate that the degradation of PANFC under salt-frost conditions can be divided into the micro-damage stage and damage acceleration stage, with 50 cycles as the limit. When the content of PAN fiber is 1.5–1.8 kg/m3, the salt frost resistance of concrete is advantageous. The macro-scale results show that the relative dynamic elastic modulus and ultrasonic wave velocity of PANFC decrease with the increase of salt-freezing cycles, whereas the weight of PANFC increases before 25 salt-freezing cycles. The meso-scale results show that after 50 cycles, the evolution of micro-pores to macro-pores and micro-cracks in PANFC is accelerated. The pore size ratio of micro-pores (0–0.1 μm) decreases with the increase of cycles, and the pore size ratio of micro-cracks (greater than10 μm) increases with the increase of cycles. Failure mechanism analysis shows that the failure of PANFC results from the combined effect of surface damage and internal damage. Finally, the salt-frost damage model of PANFC was established on the basis of the damage mechanics and classical Aas-Jakobsen fatigue theory, and the damage determination value Dn was between 0.2527 and 0.2781. Using Dn, the failure criteria of concrete damaged under salt-frost conditions can be re-evaluated.

Effect of High Temperature on the Expansion and Durability of SSRSC

This study explores the potential of using stainless steel slag, an industrial by-product of the stainless steel refining process, as a substitute for cement in concrete to promote material reuse and ecological sustainability. The research involves preparing concrete a cylindrical specimen with varying levels of substitution, ranging from 0 to 20%, and curing them for different ages (1, 3, 7, 28, and 56 days) to evaluate the engineering durability of the resulting stainless steel reducing slag concrete (SSRSC). The study found that the compressive strength of the SSRSC at 28 days was 27.44 MPa, with a splitting strength ranging from 12.81 MPa to 15.34 MPa. As the substitution amount increased, the strength decreased, but there was a positive correlation between the compressive and splitting strength. The ultrasonic wave velocity growth also increased with each substitution amount, showing that the compactness and growth of the samples improved. The surface resistance of all the samples was lower than 20 kΩ-cm, indicating that the porosity and change in porosity caused by substitution were minimal. Regarding durability, the study found that high-temperature fire damage at 200 °C catalyzed the quality, compressive strength, and resistance, but the ultrasonic wave velocity decreased. After fire damage at 600 °C and 800 °C, the compressive strength of the samples decreased by 48–57% and 76–85%, respectively, indicating that higher temperatures have a greater effect on concrete and resistance to early aging. In terms of sulfate corrosion resistance, a higher substitution amount reduced the likelihood of spalling during the early stages of the cycle, and the cumulative weight after the fifth cycle was higher than that of the control group. The autoclave expansion test revealed that the later curing age of the sample, the greater the expansion and the amount of substitution. The porosity of the samples also increased with higher temperatures and substitution amounts.