Uniaxial Compressive Stress Strain Research Articles

Study on mechanical properties and meso-damage mechanism of carbon-polyvinyl alcohol hybrid fiber reinforced recycled coarse aggregate concrete under the coupling action of sodium sulfate and dry-wet cycles

In order to investigate the influence of the coupling effect of sodium sulfate dry-wet (DW) cycles on the mechanical properties of carbon-polyvinyl alcohol hybrid fiber reinforced recycled coarse aggregate concrete (HFRAC), HFRAC specimens were subjected to uniaxial compression tests. The total volume fraction of fibers was kept constant at 0.4%, and various volume fractions of carbon fibers (CF) were added to the recycled coarse aggregate concrete (RAC), namely 0%, 0.1%, 0.2%, 0.3%, and 0.4%, with polyvinyl alcohol (PVA) fibers replacing a portion of CF in equivalent volume. Uniaxial compressive stress–strain curves were obtained for HFRAC specimens subjected to different numbers of DW cycles (0, 30, 60, and 120 cycles). Microscopic testing methods including acoustic emission (AE), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR) were employed to analyze the microstructure, erosion product composition, and pore structure changes of the specimens. The results revealed that, for RAC specimens, the peak stress (σp) and elastic modulus (E) initially increased and then decreased, whereas the peak strain (εp) initially decreased and then increased with an increase in the number of DW cycles. After adding hybrid fibers, the σp values of CF, Hybrid 3:1, Hybrid 1:1, Hybrid 1:3, and PVA specimens increased (or decreased) by 12.0%, 11.43%, 0.9%, −9.5%, and −8.5%, respectively, compared to RAC. The Hybrid 1:3 specimen exhibited a negative hybrid effect. With an increase in the erosion age, the Hybrid 3:1 specimen demonstrated enhanced resistance to sulfate attack and achieved the highest σp value after 120 cycles. The influence of sulfate DW cycles coupling on the microscopic damage mechanism of HFRAC was analyzed using a statistical damage constitutive model. The microscopic damage evolution process was characterized by five characteristic parameters: E0, εa, εh, εb, and H. The analysis results indicated a clear correlation between the variation of these characteristic parameters and the number of DW cycles. An effective connection was established between the microscopic damage mechanism and the macroscopic nonlinear constitutive behavior. The phenomenon of relative lagging of acoustic emission signals during the uniaxial compression process was explained from the perspective of effective stress. This study provides a theoretical basis for the promotion and application of RAC under sulfate DW cycle environments.

Study on the Self–Bearing Mechanism and Mechanical Properties of Gangue Slurry under Overburden Loading

The mining of coal resources is accompanied by a large amount of solid waste such as gangue, which seriously affects the ecological environment. The gangue grouting backfilling technique can achieve the dual goals of gangue disposal and surface deformation control by injecting gangue slurry into the underground. The bearing mechanical characteristics of gangue slurry directly affect the surface deformation control effect of the grouting backfilling technique. In this study, a loading simulation system of grouting backfilling materials was designed, uniaxial confined compression tests were conducted, and the self–bearing mechanism of large particle–sized gangue slurry with different fluidities under instantaneous and creep loading modes was investigated. Additionally, the mechanical characteristics of the compacted body (i.e., the gangue slurry after creep loading) were analyzed. The results indicate that the self–bearing process of gangue slurry can be divided into three stages: the rapid compression and drainage stage, the pore compaction and water bleeding stage, and the particle crushing and elastic–plastic deformation stage. The uniaxial compressive stress–strain curve of a compacted body can be classified into four stages: elastic stage, yield stage, reinforcement stage, and crushing stage, and the strength of the compacted body is affected by the loading time and fluidity of the slurry. When the slurry with a fluidity of 240 mm is subjected to constant pressure for 3 h, the compressive strength of the slurry reaches the maximum value of 4.98 MPa, and 13.1% stress damage occurs when the constant pressure reaches 4 h. This research provides a theoretical basis for the improvement of the proportion and bearing characteristics of gangue grouting materials.

Parametric Study on Mechanical Properties of Basalt Fiber-Reinforced Pea Gravel Concrete

Basalt fiber-reinforced pea gravel concrete (BFRPGC) has remarkable potential for use as the retrofitting covers for masonry walls. However, a quantitative understanding of the mechanical properties of the BFRPGC material is still a perceived gap in the current literature. In this study, the role of basalt fibers in pea gravel concrete was evaluated by a comprehensive experimental investigation involving compressive strength tests and splitting tensile tests. Fiber length and volume fraction were selected as the key parameters. Two fiber lengths of 6 mm and 12 mm were considered, while the volume fraction corresponding to each of the fiber lengths was increased from 0.3% to 0.8%, with a step of 0.1%. The measured strengths were not simply proportional to the fiber volume fraction. The reason behind this phenomenon, i.e., the coupling effect of the bridging role of basalt fibers on concrete microcracks and the fiber agglomeration in concrete, was analyzed. The best performance of the BFRPGC material was achieved by incorporating 12-millimeter-length fibers with a volume fraction of 0.4%. Compared to that of the reference pea gravel concrete, a significant increase of up to 44.5% in compressive strength was recorded in this case. Furthermore, the failure mechanism of basalt fibers in pea gravel concrete was disclosed via the scanning electron microscope observations. In addition, the uniaxial compressive stress–strain model of the BFRPGC material was established.

Experimental and analytical investigation of a model towards predicting the compressive stress–strain behavior of Graded Glass Fiber Reinforced Concrete (GGFRC) using fiber reinforcing index

AbstractThe stress–strain behavior of graded glass fiber reinforced concrete (GGFRC) is a crucial factor in its performance and appropriateness for diverse applications. In the present study, experimental and analytical methods were used to develop a model for the stress–strain behavior of GGFRC under uniaxial loading. The experimental program is designed to investigate the impact of mono glass fibers (3, 6, 12, and 20 mm) with varying volume fractions (0.1%–0.5%) and graded glass fibers (combinations of 3 + 6 + 12 + 20 mm) on the behavior of concrete of M50 grade. By grading glass fiber lengths in the concrete, GGFRC's pre‐peak strength and post‐peak deformation have increased, allowing the composite to control the various scales of cracking. A uniaxial compressive stress–strain model has been developed utilizing the fiber reinforcing index to predict the stress–strain curves of GGFRC in compression. The fiber reinforcing index, which is a measure of the quantity of fiber reinforcement in the material, is used as a variable in the current model to observe how it impacts the material's behavior. This would help evaluate the material's behavior under uniaxial compressive loading conditions and then use that data to develop a mathematical model that can predict the material's response under other conditions. Finally, it can be concluded that there is a significant correlation between the experimental results and the proposed analytical model.

Uniaxial compressive mechanical properties and stress–strain model for roller-compacted concrete with initial damage subjected to freeze–thaw cycles

Roller compacted concrete (RCC) often remains in a damage state as a result of improper vibrating compaction control, which makes it more susceptible to the action of freezethaw (FT) cycles in cold regions and thus results in the degradation of mechanical behaviors. In this paper, the influences of initial damage and FT cycles on the physical and mechanical behaviors of RCC were systematically examined through physical tests, including FT cycle tests, pore structure tests and uniaxial compressive tests. An in situ vibrating compaction procedure was employed in the experiment to accurately replicate the field compaction process for RCC under laboratory conditions, and the dynamic elasticity modulus (DEM) was adopted as the non-destructive testing (NDT) index to quantify the initial damage caused by improper RCC construction control. The results indicated that the higher values of initial damage and FT cycles exhibited a greater influence on the frost resistance and pore structure distribution. Remarkable attenuation of compressive strength and elastic modulus and an increase in peak strain were observed to occur in RCC when the initial damage degree exceeded 0.1 and the number of FT cycles exceeded 75. Finally, the uniaxial compressive stressstrain model was established through segmented linear regression analysis based on the Weibull-Lognormal model. The proposed model was validated to be effective and reasonable in predicting the mechanical properties of RCC under the combined effects of initial damage and FT cycles.

A Constitutive Model for Circular and Square Cross-Section Concrete Confined with Aramid FRP Laminates

Fiber-reinforced polymer (FRP) has been used for seismic retrofitting and structural reinforcement over recent decades. Numerous researchers have created stress–strain models based on experimental data to predict the mechanical properties of FRP-confined concrete. In this study, circular and square cross-section specimens with different design concrete strength were prepared, and the compressive strength of the specimens confined with different layers of aramid FRP (AFRP) were measured in compressive tests. A constitutive model was proposed to simulate the uniaxial compressive stress–strain relationship of the AFRP-confined concrete, which was derived from the Mohr–Coulomb failure envelope theory, and the corresponding axial strain was determined from the regression analysis. The internal friction angle of the proposed constitutive model was determined for the cylindrical concrete specimens confined with one and two layers of AFRP. The compressive strength of one and two layers of AFRP-confined concrete specimens were used to obtain the parameters of the constitutive model; the absolute average error between experimental and predicted compressive strength was 7.01%. Then, the constitutive model was used to predict the strength of a three-layer AFRP-confined concrete specimen, and the absolute average error was 4.95%. The cross-sectional shape coefficient of the square concrete specimen was obtained analytically. Substituting the cross-sectional shape coefficient into the proposed constitutive model, the average absolute error of the square cross-section concrete specimen was about 3.84%. The results indicated that the proposed constitutive model can predict the compressive strength of circular and square cross-section concrete specimens confined with AFRP.

Mechanical properties and microscopic characteristics of fly ash–slag composite backfill

The strength of the backfill is crucial for safe and efficient mining, and the cementing material is the key to obtaining a strong backfill. In this paper, industrial waste was used as a raw material, into which basis basalt fibres of different lengths and dosages were incorporated. First, the influencing factors were determined through orthogonal test designs. The optimal ratio of solid–liquid ratio of 2 and basalt fiber content of 2% and length of 6 mm was obtained, and thereby the complete proportion of the backfill material was obtained. The compressive strength and slump of the backfill material mixed with basalt fibres were investigated, and the interfacial mechanism of the backfill material was analysed at the microscopic level using scanning electron microscopy and X-ray diffraction. The results indicated that the degrees of influence of the different factors on the mechanical properties of the backfill material decreased in the following order: Solid–liquid ratio > fibre dosage > fibre length. Furthermore, there were different degrees of interaction among them. Under the best test proportion, that is, when the Solid–liquid ratio was 2, the fibre dosage was 2%, and the fibre length was 6 mm, the slump was 210 mm, and the compressive strength was 5.03 MPa. The mechanical properties were the best at this time. Through data regression, the relationship between the fibre dosage and the average peak stress was established, and the full curve expression for the uniaxial compressive stress–strain of the BFGF specimen was obtained. According to a microscopic characterisation analysis, the slag formed C–S–H gel, which contributed to the early-stage strength of the filling material; fly ash and coal gangue contained a large amount of aluminium, which formed hydrated aluminosilicate with a framework structure, and this contributed significantly to the later-stage strength of the backfill material. The addition of basalt fibres reduces the internal defects of the backfill, allowing the internal pore structure of the backfill to be optimised and the compressive strength to be increased. The results of this study have been successfully implemented in actual projects. Additionally, they provide a reference for similar mine management projects and have considerable promotion and application value.

Attack resistance mechanism, uniaxial compressive mechanical properties and meso-simulation of high-performance concrete exposed to brine in salt lake for 10 years

The study investigated the corrosion resistance, corrosion micro-mechanisms, and the uniaxial compression stress–strain constitutive relationship of high-performance concrete (HPC) specimens subjected to long-term brine corrosion in an underground salt lake environment (Xining, China), which possesses a salt content of 180894.5mg/L. A three-dimensional random aggregate concrete meso-model was established considering the mesostructural characteristics of concrete. The mechanical behavior of HPC was assessed using the LS-DYNA software platform. The findings indicated that both the compressive strength and the dynamic elastic modulus of HPC demonstrated a pattern of initial increase, followed by a decrease, with extended brine exposure. Nevertheless, after 10 years of exposure to salt lake brine, the compressive strength of the HPC samples was still considerably greater than their original strength, with minimal evident corrosion damage to the concrete. Physical and chemical corrosion products formed exclusively in the spherical pores created by the air-entraining agent within the shallow layer of HPC, without notable micro-cracks or spalling. The HPC specimens exposed to brine for ten years under uniaxial compression load exhibited typical characteristics of splitting failure. In addition, the results from the mesoscopic numerical simulations aligned well with the experimental findings. The study shows that the three-dimensional random aggregate concrete mesoscopic model can simulate the uniaxial compressive mechanical behavior of HPC specimens under long-term corrosion conditions.

Rapid inverse design of metamaterials based on prescribed mechanical behavior through machine learning

Designing and printing metamaterials with customizable architectures enables the realization of unprecedented mechanical behaviors that transcend those of their constituent materials. These behaviors are recorded in the form of response curves, with stress-strain curves describing their quasi-static footprint. However, existing inverse design approaches are yet matured to capture the full desired behaviors due to challenges stemmed from multiple design objectives, nonlinear behavior, and process-dependent manufacturing errors. Here, we report a rapid inverse design methodology, leveraging generative machine learning and desktop additive manufacturing, which enables the creation of nearly all possible uniaxial compressive stress‒strain curve cases while accounting for process-dependent errors from printing. Results show that mechanical behavior with full tailorability can be achieved with nearly 90% fidelity between target and experimentally measured results. Our approach represents a starting point to inverse design materials that meet prescribed yet complex behaviors and potentially bypasses iterative design-manufacturing cycles.

Constitutive relationships and acoustic emission behavior of plant-fiber/magnesium oxychloride cement composites under uniaxial compressive load

Plant-fiber/magnesium oxychloride cement composites (PF-MOC) are bio-composite building materials that are produced by mixing magnesium oxychloride cement (MOC) with plant fibers. The application of PF-MOC in construction engineering can not only promote the reuse of bio-waste but also compensate for the shortage of wood. In this study, 33 prismatic specimens of PF-MOC were prepared using the extrusion molding method for the uniaxial compression test. The compression characteristics of PF-MOC under uniaxial compression load were studied by combining the test with acoustic emission (AE). The results illustrated that the apparent density of the PF-MOC ranged from 771 kg/m3 to 1806 kg/m3, and the compressive strength ranged from 4.7 MPa to 44 MPa. PF-MOC uniaxial compressive stress–strain relationship model was established. The compressive strength of PF-MOC with different types and content of plant fiber is closely related to the apparent density, and thus a compressive strength prediction model was developed. The PFC-MOC with bamboo scrap exhibited the highest AE energy near the peak load, indicating greater brittleness. Conversely, the PF-MOC with wheat straw demonstrated a well-distributed energy release throughout the compression process, suggesting lower brittleness in the straw specimens. The utilization of the b-value proved effective in characterizing the crack development and size transformation process of PF-MOC. Furthermore, SEM revealed that straw fibers exhibited the weakest bonding with the MOC matrix, while wood sawdust fibers and bamboo scrap showed superior interactions with the hydration products of MOC.

Experimental investigation on the mechanical properties of concrete utilizing pre-coated Brick-Concrete recycled coarse aggregates

In this study, low water-cement ratio composite cementitious materials were utilized to pre-coat the brick-concrete recycled coarse aggregate (BCRA), and a two-stage mixing method was proposed to pre-coat BCRA and produce the recycled concrete together. The compressive and tensile strength, fracture behavior, and uniaxial compressive stress–strain properties of five concrete groups, including untreated or pre-coated BCRA and natural aggregate concrete, were experimentally investigated. The results show that the 28 days compressive strength fcu and split tensile strength fst of pre-coated BCRA concrete were 9.9% and 7.7% higher, respectively, than those of untreated BCRA concrete, while using the same amount of total cementitious material. The notched beam three-point bending test revealed that utilizing untreated BCRA tends to deteriorate the brittle fracture behavior of concrete. The two-stage mixing method demonstrated the most significant improvement in enhancing the anti-crack initiation ability of BCRA concrete, while pre-coated BCRA exhibited greater effectiveness in improving BCRA concrete's unstable fracture toughness. Additionally, uniaxial stress–strain curves of five test groups were discussed and the backscattered electron analyses were conducted to gain a better understanding of their mechanical behaviors.

Investigation of Uniaxial Compression Stress–Strain Relationship of Early Age Manufactured Sand Concrete and Its Application

To improve the construction efficiency of the manufactured sand concrete engineering at an early age, the uniaxial compressive stress–strain relationship of C50 manufactured sand concrete is investigated starting at 2 days to 28 days. With the characteristics of the stress–strain relationship, the uniaxial compression constitutive model is determined for the C50 manufactured sand concrete at early age. The influence of age, water-to-binder ratio, and fly ash admixture on the peak stress and peak strain of manufactured sand concrete is analyzed for the parameters of the constitutive model. Results show that the stress–strain curve of manufactured sand concrete is essentially similar to that of ordinary concrete. Via six typical constitutive models, the Sargin model produced the best fitting: its R2 mean is 0.9775, MAE mean is 0.1335, and MSE mean is 0.0175. Considering the influence of different factors, the early age uniaxial compressive constitutive models of manufactured sand concrete were proposed based on the Sargin model. Combined with the on-site construction process of the high pier formwork climb, the finite element analysis was carried out using the proposed early age uniaxial compressive constitutive model. Compared with the measured results of strain near the climbing cone, the error was less than 10% from the simulated value. The findings confirm that the proposed early age uniaxial compressive constitutive model presents great reasonableness for the manufactured sand concrete construction at an early age.

Comparative study on mechanical and environmental properties of coal gangue sand concrete

To reduce the adverse impact of coal gangue heaping and save natural resource, artificial coal gangue sands are utilized as fine aggregates in the concrete preparation. The mechanical and environmental properties of coal gangue sand concrete were studied experimentally and theoretically by considering concrete strength grades (C30, C50) and replacement ratios (0, 25%, 50%, 75%, 100%). The results show that coal gangue sand will weaken the adhesion performance of the transition zone between cement mortar and coarse aggregate. The elastic modulus and axial compressive strength of coal gangue sand concrete were decreased by increasing the replacement ratio regardless of concrete grade. When the replacement ratio is 100%, the elastic modulus and axial compressive strength of C30-grade concrete were reduced by 2.9% and 12.6% respectively whilst the values of C50-grade concrete were 5.0% and 21.5% respectively. Based on the tested results, the prediction formulas of the mechanical properties and the uniaxial compressive stress–strain model of the coal gangue sand concrete under different replacement ratio are proposed. Through life cycle assessment, it is found that the replacement ratio of the coal gangue sand is negatively correlated with environmental impact and willingness to pay (considering transport cost), and positively correlated with willingness to pay (without considering transport cost). The balance point is defined by the environmental influence of coal gangue sand concrete when the willingness to pay (considering transport cost) of coal gangue sand concrete under different replacement ratio is equal. According to the tested results, the prediction formula of the balance point under different transport distance is proposed. When the consistent transportation distance of gangue sand and natural sand exceeds 140 km, the utilization of coal gangue sand can produce good environmental benefits.

Finite element analysis of stress–strain relationship and failure pattern of concrete under uniaxial compression considering freeze–thaw action

In order to study the uniaxial compression behavior of concrete before and after freeze–thaw cycles, the uniaxial compression test of 100 mm × 100 mm × 300 mm prism was carried out by comparing the experimental and numerical simulation results. A three-dimensional mesoscopic model considering the random shape and size of aggregate was established. The failure mechanism, failure pattern and stress–strain curve of concrete under uniaxial compression before and after freeze–thaw cycles were compared to verify the reliability of the three-dimensional modeling method and corresponding calculation parameters of concrete. Finally, a reliable unified constitutive model of concrete stress–strain curve relationship considering freeze–thaw damage variables is established, which can better reflect the continuous change law of concrete uniaxial compressive stress–strain relationship during freeze–thaw process. More importantly, this work provides a reference for future study of stress–strain intrinsic relationship of concrete freeze–thaw damage using numerical simulation through a small amount of experiments.

Experimental Study on Steel-Fiber-Reinforced Scoria Aggregate Concrete Subjected to Freeze-Thaw Cycles and Then Exposure to Elevated Temperatures

Steel-fiber-reinforced scoria aggregate concrete (SFSAC), which contains scoria aggregate and steel fiber, was developed to reduce the environmental impacts and improve the energy efficiency of buildings. Experimental studies were performed. The test variables included steel fiber volume contents (0%, 0.5%, 1.0%, and 1.5%), freeze-thaw cycles (0 and 25 times), and temperature (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C). Mass loss, relative dynamic elastic modulus, mechanical properties, and the variation pattern of the complete stress–strain curves were analyzed through rapid freeze-thaw, high-temperature, and mechanical tests. The test results showed that after 25 freeze-thaw cycles and then exposure to high temperatures, the surfaces of SFSAC specimens showed aggregate spalling accompanied by dense cracks. Moreover, the residual mechanical properties of steel-fiber-reinforced natural aggregate concrete (SFNAC) were better than those of natural aggregate concrete (NAC). Although the incorporation of steel fiber cannot significantly improve the anti-freezing performance of SFSAC, it can improve the residual mechanical properties of SFSAC, and the optimal amount of incorporation is 1%, considering the economic cost factors. The stress–strain curves of both SFSAC and SFNAC showed the same trend after freeze-thaw cycles and then high temperatures, i.e., the peak stress decreased, the peak strain increased, and the descending section tended to level off. Finally, based on the concrete damage mechanics theory, considering the role of steel fibers in the uniaxial compression process of scoria aggregate concrete (SAC) and the effect of freeze-thaw and high-temperature tests on the SFSAC, the mechanical damage model and the uniaxial compression stress–strain constitutive model were proposed as being able to highly accurately reflect the overall process damage characteristics of SFSAC after freeze-thaw and then high-temperature tests, and also provided a theoretical basis for the high-temperature resistance assessment of SFSAC structures in cold regions.

On the utilization of rice husk ash in high-performance fiber reinforced concrete (HPFRC) to reduce silica fume content

The HPFRC refers to a category of fiber-reinforced cement-based materials that have the remarkable capability to flex and strengthen prior to shattering. At present, research is being conducted with the intention of producing a worldwide guideline for the development of structures using HPFRC. However, due to its high initial price and constrained availability, its implementation is challenging, particularly in developing countries. In this study, the effects of fly ash (FA) and rice husk ash (RHA) were examined, with 10%, 20%, and 30% of the cement replaced with FA. Furthermore, the mix providing maximum compressive strength was then taken to replace the silica fume content at 10, 20, 30, and 40% by RHA, steel fiber was also added to optimize the compressive and flexural ductility performance of the specimens. An extensive evaluation of fresh, mechanical, microstructural, and durability of HPFRCs were carried out. In addition, the eco-mechanical properties of fiber-reinforced concrete are studied by taking into account the post-peak behavior of the manufactured specimens and associated CO2 emissions. Test results show that the maximum improvement in compressive, tensile, and flexural strengths was 6.49%, 12.85%, and 5.27%, respectively, at 10% RHA replacement. In addition, as the concentration of RHA increased, the flexural bending toughness increased between 7.4% and 9.2%, with good agreement between the analytical models and the experimental results of the uniaxial compressive stress–strain. Moreover, the gradual increase in RHA concentration improved the durability of the HPFRCs, as evidenced by a maximum reduction in sorptivity coefficient of up to 48 percent for 30% RHA replacement. Finally, the investigation shows how the eco-mechanical index (EMI) can be used to evaluate material design options for HPFRCs.

Uniaxial Compressive Stress-Strain Model of Jujube Nucleus Concrete following Exposure to Elevated Temperatures.

Aiming to provide a solution for natural resource consumption and agricultural waste pollution, jujube nucleus is utilized as a substitute for coarse aggregate in the preparation of lightweight aggregate concrete. The effect of the jujube nucleus (JN) replacement ratio and the elevated temperature on the uniaxial compressive stress-strain curves of jujube nucleus concrete (JNC) are experimentally studied. The results show that the failure of the JNC prisms became more serious with the increase in the JN replacement ratio. The linear proportion in ascending branch and the descending slope of the stress-strain curves for JNC increased gradually with the increase in the JN replacement ratio and elevated temperature, which is probably owing to the higher porosity and lower stiffness of the jujube nucleus, compared to natural aggregate. Moreover, as the JN replacement ratio and the elevated temperature increase, the peak stress and elastic modulus in the stress-strain curves of JNC decrease gradually, whilst an increase in the peak strain shows up, which is possibly due to the growth of hydrate calcium silicate and calcium hydroxide hampered by sucrose molecules. Based on the test results, a series of theoretical formulas are proposed to predict the compressive performance of JNC. A material constitutive model is developed for describing the stress-strain relationship of JNC by considering the JN replacement ratio and elevated temperature.

Synthetic salt rock prepared by molten salt crystallization and its physical and mechanical properties

Physical simulation is an important research method for salt cavern energy storage construction technology. However, its progress is constrained by lack of effective model salt. Exploratory experiments were conducted to prepare salt rock by molten salt crystallization. A well-made synthetic salt rock was obtained from repeated adjustment of staged cooling conditions and controlled crystal growth procedures. Samples were taken for microscopic, physical, and mechanical tests, and compared with natural salt rock. SEM, CT and p-wave velocity results show that synthetic salt rock achieved crystallization. Its crystal structure, grain size and intercrystalline relationship are consistent with those of natural salt rock. Its density distribution is uniform and compaction is good. X-ray results prove a high crystallinity and stable mineral composition. Porosity of synthetic specimens ranges from 2.3% to 5.9%, permeability is in the range of E−13 to E−16 m2. Uniaxial compression stress-strain curves, strengths, deformation modulus and poisson's ratios are close to those of natural salt rock. It has obvious ductility and damage occurs after dilation deformation. We initially illustrate energy storage and heat storage-transfer principles of molten salt. The mechanism of layered growth in molten salt crystallization is revealed. The findings provide new insight on salt rock preparation, crystallography, molten salt and salt cavern study.

Carbonation Resistance and Pore Structure of Mixed-Fiber-Reinforced Concrete Containing Fine Aggregates of Iron Ore Tailings

The disposal of industrial by-product tailings has become an important issue in solving environmental pollution. In this study, 15%, 30%, 50%, and 70% iron tailings were used to replace the natural sand in concrete, and 1.5% steel fiber and 0-0.75% PVA fibers were added to the iron tailings concrete. The effects of the iron tailings replacement rate and the fiber content on the mechanical properties, carbonization depth, and concrete porosity were studied in a carbonization environment. The results demonstrated that the compressive and splitting tensile strengths of concrete first increased and subsequently decreased with an increase in the iron tailings replacement rate, while the carbonation depth and porosity initially decreased and subsequently increased. When the replacement rate of iron tailings was 30%, the compressive strength and split tensile strength were increased by 7.6% and 17.7%, respectively, and the porosity was reduced by 8.9%. The compressive strength, carbonation depth and porosity of single-doped steel-fiber concrete were superior to those of ordinary iron tailings concrete. However, compared with single-doped steel fiber, the performance of steel-PVA fiber was further improved. Based on the mechanical properties, the carbonation depth test results of the three aforementioned types of concrete, the mathematical expression of the uniaxial compression stress-strain curve of iron tailings concrete, and the prediction equation of the carbonation depth of mixed-fiber iron tailings concrete were proposed. This study provides a reference for the application and popularization of fiber-reinforced iron tailings concrete in carbonization environments.

Predicting stress-strain behavior of normal weight and lightweight aggregate concrete exposed to high temperature using LSTM recurrent neural network

Lightweight aggregate is commonly used to reduce the self-weight of concrete elements which can efficiently mitigate the environmental impact. This study investigates the mechanical performance of normal weight aggregate concrete (NWAC) and Lightweight aggregate concrete (LWAC) exposed to high temperature, focusing on developing a predictive model for the stress-strain relationship of the LWAC based on the property of cement, silica fume, light weight aggregate and water/cement ratio. Mechanical properties of 30 different mixtures, including uniaxial compressive stress–strain relationship, compressive strength, modulus of elasticity, and failure strain are determined before and after high temperature exposures at 250, 500, and 750 °C. Results show that the modulus of elasticity and compressive strength of LWAC increased with exposure temperature. In particular, mixture S4 was found to outperform other mixtures, which retained 96, 75, and 46 % of compressive strength following exposures to 250, 500, and 750 °C, respectively. At all the temperatures investigated, the specimens of this mixture were able to achieve satisfactory modulus of elasticity and maximum strain. The specimens prepared with mixtures containing 75 % LECA (S23) showed a higher retention compressive strength at 750 °C. This study also utilizes the long short-term memory (LSTM) neural network to predict the stress-strain relationship of NWAC and LWAC mixtures after exposure to high temperatures. The results show that the LSTM model could adequately predict the stress-strain relationship of both LWAC and NWAC mixtures in terms of compressive strength, modulus of elasticity, and failure strain at elevated temperatures.