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[Display omitted] • A mesostructure model of lightweight coral aggregates was constructed from CT scanning images. • The fracture mechanics parameters of the cohesive element of the model were calibrated adopting uniaxial compression tests of coral aggregates. • A combined finite discrete element meso-mechanical modeling algorithm for coral coarse aggregate concrete was developed. • A uniaxial compression test of coral coarse aggregate concrete was designed to verify the rationality of the model. • Based on the cohesive model, two types of damage variables, microcrack density and fracture angle, were defined. • The crack propagation and cracking mechanism of compressed coral coarse aggregate concrete were revealed by damage variables. Coral aggregate concrete (CAC) is one of the typical quasi-brittle materials, which is of great significance to explore the damage process of CAC under load from the perspective of fracture mechanics. Therefore, the Cohesive Zone Model (CZM) was introduced to simulate the physical mechanism of crack propagation in CAC materials at the mesoscale adopting the discrete crack method. The main work included test and simulation parts. The tests included CT scanning and mechanical tests, which were used to calibrate the model parameters of CZM and verify the rationality of the meso-mechanical model. Numerical simulation was used to reveal the internal damage and fracture mechanism of CAC. According to the defined damage variables, it was found that the crack growth rate inside the CAC material evolves regularly with the axial strain during compression failure. The crack of CAC with higher strength grade showed oblique sliding shear failure and was more likely to form fracture surfaces, and the aggregate damage was more significant. The revealing of the cracking modes in each strain stage of CAC provides a theoretical basis for guiding the corresponding crack prevention measures. For example, in the stage of significant shear failure, targeted improvement of shear strength and toughness will bring more significant gains. [ABSTRACT FROM AUTHOR]

• The polyhedral gravity compaction algorithm was introduced to improve the existing aggregate placement algorithm to increase the volume ratio of aggregate. • The finite element, combined finite discrete element and cohesive finite element modeling algorithms were proposed on the meso -scale. • Compared with the finite element model, the embedding of the cohesion model had the following advantages: • A good representation of the residual stages of concrete in tension and compression. • Capable of capturing evenly distributed microcracks in concrete under tensile loading. • The crack propagation of the fictitious crack model was more consistent with the fracture mechanics theory, and the failure fattern is also more consistent with the test. • The cracking mode in tension concrete was mainly opening-mode, while the cracking mode in compression concrete was mainly sliding-mode. The application of the Cohesive Zone Model (CZM) in concrete meso -mechanics solves the nonlinear problem of material crack tip well. At present, the application methods of CZM in concrete mesoscopic models include the bond mechanics characterization of the aggregate-mortar interface transition zone (ITZ) and the characterization of fracture characteristics between material micro-elements. From this, two different numerical models were derived—the combined Finite Discrete Element (FDEM) model and the Cohesive Finite Element (CFEM) model. To evaluate the difference between the two numerical models and the traditional Finite Element (FEM) model in simulating concrete mechanical behavior and crack propagation, a 3D concrete mesostructure modeling algorithm was developed in this study, including innovations in gravitational self-compacting of polyhedral aggregates. After that, the FDEM model and the CFEM model were successfully constructed based on the FEM model. Through uniaxial compression and tensile numerical simulations, it was found that the CFEM model is more in line with the theoretical and experimental mechanisms in the characterization of the static mechanical behavior and fracture process of concrete. The fracture failure angle variable defined by the CFEM model revealed that the compressed concrete material exhibited better strength and toughness due to a higher damage degree and more energy-consuming cracking mechanism. The above findings to a certain extent promote the future development of CZM in the field of concrete meso -mechanics. [ABSTRACT FROM AUTHOR]

Magnesium oxysulfate (MOS) cement is a promising low-carbon and high-performance cement. Basic research on the MgO–MgSO 4 –H 2 O system is of great significance for further optimizing the properties of MOS cement. In this study, through the qualitative hydrolysis test of the Mg2+(MgSO 4)–OH-–H 2 O system and the test and characterization of the composition of the hydration products and microstructure of the MgO–MgSO 4 –H 2 O system, the relationship between the composition of the hydration products of the MgO–MgSO 4 –H 2 O system and the molar ratio of raw materials and the mechanism of chemical additives were investigated. The results show polynuclear complexes [Mg P (OH) q (H 2 O) r ]2p-q in the MgO–MgSO 4 –H 2 O system. The main group of hydration products of MOS cement without chemical additives, which is the 3·1·8 phase, is formed by the transformation of the unstable 1·1·5 phase through the 5·1·3 phase. However, chemical additives can change the hydration reaction processes of MOS cement through buffering and complexation, which causes the main hydration products of MOS cement to change to the 5·1·7 phase. The 5·1·7 phase is a metastable phase (or stable phase) formed by the transformation of the unstable 1·1·5 phase through the 3·1·8 phase. • There is indeed the formation of polynuclear complexes [Mg P (OH) q (H 2 O)r]2p-q in the MgO–MgSO 4 –H 2 O system. • The 5·1·7 phase is formed by the transformation of the unstable 1·1·5 phase through the 3·1·8 phase. • The phase diagrams of the MgO–MgSO 4 –H 2 O system are drawn. • The mechanism of chemical additives is analyzed from the perspective of its buffering effect. [ABSTRACT FROM AUTHOR]

• The equivalent quantization method of corrosion damage of concrete beam was proposed and verified. • Effects of four factors with twelve levels on durability of concrete beam were studied. • Three types of self-produced protective materials were tested. • The relation between the linear polarization resistance and electrochemical impedance spectroscopy methods was established. The electrochemical measurement technique was used to conduct a non-destructive test the corrosion degree of reinforcement cage for concrete beam in tidal environment of island and reef region in real-time. Apart from this, the corrosion resistance of concrete beam with different concrete strength grades, steel bar, darex and migrating corrosion inhibitors, including three types of self-produced protective materials, was studied. Furthermore, based on electrochemical parameters, the relation of electrochemical measurement techniques was established. The results showed that the proposed equivalent quantization method of corrosion damage could be available for the reinforcement cage in concrete beam. In addition, the durability of concrete beam in tidal environment of island and reef region can be effectively improved by improving the concrete strength grade and the corrosion resistance of steel bar and using darex and migrating corrosion inhibitors. The polarization resistance obtained by the linear polarization resistance method and charge transfer resistance obtained by the electrochemical impedance spectroscopy method were linear. This study provides a key technology for actual reinforcement corrosion in concrete structure and some protective measures. [ABSTRACT FROM AUTHOR]

• A research on the acquisition of macro-mechanical data of the ITZ in the meso-mechanical calculation of concrete. • The mechanical properties of the ITZ with different mix ratios were measured, the relationship between the bond strength and the microhardness of the ITZ in concrete is established. • A two-parameter linear relationship between the splitting tensile strength of the interface bond and the age and the W/B ratio was established. • The priority order of the calculated value of the interface strength in the numerical analysis of concrete meso-mechanics is proposed. The problem of taking macro-mechanical data of the interface between aggregates and slurry in the calculation of meso-mechanics of concrete was studied in this paper. Through a systematic standard interface test study where the interfacial bond splitting tensile strength method and the interfacial transition zone (ITZ) microhardness method were used, mechanical properties were investigated at macro-scale (bond strength), and followed by ITZ microhardness at meso-scale, moreover the relationships between macro- and meso-mechanical properties were established. The results show that interfacial bond splitting strength of aggregates-slurry gradually increases with increasing hydration age and decreases with increasing water-binder ratio; the ITZ thickness decreases and the ITZ microhardness valley value increases with the extension of curing age. There is a certain correlation between macro-scale mechanical strength of interface and meso-scale mechanics of ITZ. There is a significant linear relationship between the splitting tensile strength of the interface bond and the splitting tensile strength of the cement slurry, so the interface strength can be calculated according to the splitting tensile strength of the slurry substrate. The interfacial bond splitting tensile strength and ITZ microhardness valley have different degrees of correlation. The interfacial bond splitting tensile strength has a particularly significant linear relationship with the microhardness value of the cement paste substrate. Therefore, the calculation value of the interface strength in the numerical analysis of concrete micromechanics should preferably adopt the linear relationship formula with the microhardness of the cement paste substrate. [ABSTRACT FROM AUTHOR]

• Basic magnesium sulfate coral aggregate concrete (BMSC-CAC) is a new type of concrete. • Continuous reduction in mass of the specimens with upsurge in temperature. • Relationships between results from destructive and temperature tests. • Pulse velocity and elevated temperature can predict the concrete residual strength. Different proportions of coral aggregate, coral sand and basic magnesium sulfate cement (BMSC) were used to prepare the basic magnesium sulfate cement-coral aggregate concrete (BMSC-CAC), which was applied in the island and marine engineering as a brand new type of concrete. The specimens of BMSC-CAC exposed to elevated temperature (100°C, 300°C, 500°C, 700°C, and 900°C) were tested by the non-destructive method to obtain the surface characteristics at different temperatures. A new parameter, elevated temperature damage (D), derived from continuous damage mechanics (CDM) was applied to indicate the degree of concrete damage after being exposed to elevated temperature. The surface characteristics of the specimen were changed significantly at different temperatures, especially the aggregate colours. The fitting relationship between ultrasonic velocity and temperature was established, which can be well used to fit different types of concrete. The elevated temperature damage degree (D) was defined by damage mechanics, and the mathematical relationship between D and the residual compressive strength was discussed. [ABSTRACT FROM AUTHOR]

• The ice pressure of airport deicers was measured. • The ice pressure impulse of airport deicers was calculated. • The microcrack density of cement concrete was calculated. • The evolution mechanism of freeze-thaw microcracks was further analyzed. The rapid freeze–thaw test about ordinary Portland cement concrete (PCC) was performed in the deicers of water, 3.5% ethylene glycol, 3.5% NaCl and 3.5% calcium magnesium acetate (CMA). The ice pressure (I p) of deicer was tested, and the concept of the ice pressure impulse (IPI) was proposed. A crack density model was employed to describe the freeze–thaw damage process of concrete. Results indicated that the freeze–thaw damage of PCC in CMA deicer was the mildest. The I p value of water was the highest but its IPI value was the lowest. The freeze–thaw microcracks inside concrete was induced by the I p of deicer restrained in concrete pores. The freeze–thaw crack density decreased with the relative depth of specimen. The value of superficial critical crack density was 0.85 ∼ 0.88, when the exterior mortar of concrete began spalling. The value of central critical crack density was 0.32 ∼ 0.34, when the interior freeze–thaw failure of concrete occurred. [ABSTRACT FROM AUTHOR]

• The ice pressure and icing volume expansion rate of acetate-based deicers were measured. • The heat of liquid–solid phase transformation was discussed. • The cooling process and ice crystal growing process of acetate-based deicers were divided into five stages. • The impulse of ice pressure was proposed to evaluate the frost damage induced by deicers. Acetate-based deicers are frequently used as airport pavement de-icing fluids in cold regions. In this study, the ice pressure (I p), icing volume expansion rate (Δ V P), freezing point (T 0), and undercooling degree of acetate-based deicers were investigated. The above mentioned indices were also compared with that of water and NaCl deicers. The results showed that the cooling process and ice crystal growing process of acetate-based deicers could be divided into five stages. Additionally, the stabilised values of I p and Δ V P decreased with the deicer concentration increases. Furthermore, the impulse concept of ice pressure (IPI) was proposed to evaluate the frost damage of concrete caused by deicers. The I p , Δ V P , and T 0 values of calcium magnesium acetate (CMA) deicer with different Ca/Mg mole ratios (i.e. 1/1, 0.91, 7/3, and 3/7) were tested in the ice pressure test. The value of CMA deicer with 3/7 Ca/Mg mole ratio was the lowest. However, the I p values of CMA deicers with 0.91 Ca/Mg and 1/1 Ca/Mg mole ratios were the highest. Among all the tested acetate-based deicers, CMA deicers had the lowest values of I p , Δ V P , and IPI. Moreover, the CMA deicer with 3Ca/7Mg mole ratio was recommended for deicing airport pavements to reduce the concrete frost deterioration caused by acetate-based deicers. In this study, we investigated the physical properties of acetate-based deicers, quantitatively analysed the destructive power of deicers in a freeze–thaw test, and studied the cause of freeze–thaw deterioration of concrete. Overall, this study provides a theoretical method for exploring new deicers. [ABSTRACT FROM AUTHOR]

• The effects of the different pH chemical additives on the hydration and hardening rules of BMSC were studied. • The mechanism of the effect of different pH chemical additives on the hydration process of BMSC was discussed. • To a certain extent, the performance of BMSC can be controlled through the selection of chemical additives. In this paper, based on the basic mechanical properties test and microstructure characterization of basic magnesium sulfate cement (BMSC), the effects of different pH chemical additives on the hydration and hardening rules of BMSC was studied, and its mechanism was analyzed. The results show that the chemical additives with acid pH value can inhibit the hydrolysing-bridging reaction of Mg2+ ions in liquid phase, and delay the hydration reaction of BMSC, which is beneficial to the growth and development of 5·1·7 phase and make it good crystallinity; the chemical additives with neutral and alkaline pH value can accelerate the hydrolysing-bridging reaction of Mg2+ ions in liquid phase, and promote the hydration reaction of BMSC, which is conducive to the crystallization and nucleation of 5·1·7 phase and increase the amount of its formation. Therefore, the performance of BMSC can be controlled to a certain extent through the selection of chemical additives. [ABSTRACT FROM AUTHOR]

The dynamic mechanical behavior of alkali-activated seawater and sea sand concrete (ASSC) in marine corrosive environments were investigated by splitting Hopkinson pressure bar (SHPB) impact experiment. This study investigated how corrosion age, strain rate, and various corrosion environments influence the mechanical characteristics and macroscopic failure behavior of ASSC. Furthermore, a three-dimensional (3D) mesoscopic model was employed to investigate mesoscopic damage mechanism of ASSC. The outcomes indicate that increasing corrosion age intensifies the corrosion effect on concrete, with the dry-wet cycling having a more pronounced erosive impact compared to solution immersion. The dynamic mechanical properties and failure morphology exhibit a pronounced strain-rate effect. Microscopic analysis showed that cracks first developed in the interfacial transition zone (ITZ) before propagating into mortar. As smaller cracks coalesced into larger ones, their progression ultimately led to the fragmentation of the aggregates. • Examines dynamic properties and mesoscopic damage of ASSC under marine corrosion using experiments and simulations. • Reveals strain rate effects on mechanical properties and failure patterns, influenced by corrosion environment and age. • Employs a 3D random aggregate model to simulate crack propagation and failure mechanisms under dynamic impact loading. [ABSTRACT FROM AUTHOR]

Magnesium phosphate cement (MPC) is widely recognized for its quick hardening, early strength, excellent durability, and strong adhesion. These traits make MPC ideal for rapid repairs, waste management, and stabilizing low-level radioactive waste. Despite its advantages, the high cost of magnesia is a significant barrier to its broader application. This study seeks to use lithium magnesium slag (LMS) from salt lakes, calcined at different temperatures, to produce boron-doped magnesia. This could serve as an alternative to conventional calcined magnesia in MPC production. The study explores the impact of boron-doped magnesia (B-MgO), treated at various temperatures, on MPC's properties: setting time, strength, hydration heat, heat release, and porosity.Techniques like Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD) were used to analyze the hydration byproducts and microstructure of the samples. Results indicate that raising B-MgO calcination from 800 °C to 1000 °C prolongs setting time to 20 minutes and boosts compressive strength by 29.4 %. Additionally, higher calcination temperatures reduce porosity after MPC hydration but do not alter the nature of the calcined byproducts. This research demonstrates the potential of using boron-doped magnesia as a cost-effective alternative in MPC, which could lead to its wider adoption in various applications. • MPC was prepared by extracting lithium magnesium slag from salt lake. • The change of calcination temperature affects the properties of MPC. • Elevated calcination temperatures significantly reduce porosity in MPC. • Underscores the potential for sustainable use of lithium extraction by-products. [ABSTRACT FROM AUTHOR]

Coral aggregate concrete (CAC) has been identified as a promising building material for reef engineering construction, yet its practical application remains challenging due to incomplete research on the mechanical behaviors of the CAC beam and column components. In this study, a 3D mesoscale modeling approach considering the heterogeneity of CAC was utilized to investigate the mechanical performance of coral aggregate reinforced concrete columns (CARCCs) under axial and eccentric compression. Through numerical delineation of failure processes and damage mechanisms at the mesoscale, comprehensive parametric analysis integrating both numerical simulations and experimental validations were executed to assess the load-bearing capacity of CARCC. Results indicate that the mesoscale model has significant reliability in simulating the deformation and cracking failure behaviors and analyzing the load-displacement relationships of CARCC under axial and eccentric compression loads. When under axial loads, macroscopic cracks in CARCC primarily develop in an oblique manner, precipitating significant concrete spalling and extensive cracking failure along the longitudinal axis. Under eccentric loads, failure advances from crack initiation at load points to abrupt vertical fractures in stressed zones, manifesting a decrement in the load-bearing capacity with escalating eccentricity. Furthermore, the strength grade and reinforcement ratio exert a profound influence on the load-bearing capacity, with disparate impacts under varied loading conditions. It is concluded that reinforcement strategies tailored to specific loading conditions have significant implications for the design and safety of reinforced concrete structures in reef engineering projects. • A 3D four-phase mesoscale modelling approach for CARCC. • Studies on the mechanical properties of CARCC under uniaxial compression. • Studies on the mechanical properties of CARCC under eccentric compression. • Parametric studies on the uniaxial and eccentric compressive properties of CARCC. • Mesoscopic cracking process and failure mechanism of CARCC components. [ABSTRACT FROM AUTHOR]

• Industrial waste silica fume (SF) is used to improve the performance of basic magnesium sulfate cement (BMSC). • Incorporation of SF can enhance the mechanical properties of BMSC significantly. • Hydration behavior and process of BMSC influenced by SF are studied. • Microstructure and phase evolution of BMSC are characterized and analyzed. Silica fume (SF) as an important supplementary cementitious material has been widely used in Portland cement, but few published articles have reported on the effect of SF on the performance and hydration mechanism of basic magnesium sulfate cement (BMSC). In the present work, the properties, microstructure and hydration mechanism of BMSC influenced by SF was studied systematically. The results show that the setting time and compressive strength of BMSC may increase with the increase of SF content, while the hydration heat will decrease with the increase of SF content. Mercury intrusion porosimetry (MIP), X-ray computed tomography (X-CT), scanning electron microscope- Energy dispersive spectrometer (SEM-EDS) results show that SF exhibits filling effect in the BMSC matrix, which makes the microstructure of BMSC matrix with SF more compact. In addition, solid-state magnetic resonance (NMR) and SEM-EDS analysis indicate that the activity of SF was excited in the BMSC matrix, resulting in the formation of M−S−H gel. [ABSTRACT FROM AUTHOR]

• Service life of coral aggregate concrete (CAC) under island reef environment were studied. • Influence of concrete cover thickness and concrete strength on service life of CAC were discussed. • Durability design parameters of CAC structural were proposed. Based on the reliability theory and modified chloride diffusion theory, the effects of concrete cover thickness and concrete strength on the service life of coral aggregate concrete (CAC) under island reef environment were analyzed, and the durability design parameters of CAC structural were proposed. As the result, the (1- m) power function boundary condition is recommended as the calculation model of service life. The service life of CAC structures are as follows: air zone (T) > underwater zone (P) > tidal zone (PG) > splash zone (PT). For the silane-coated CAC structures, the concrete cover thickness of C65T, C70P, C70PG and C80PT is 11.5–12.2, 5.8–7.2, 7.2–8.2 and 10.4–12.2 cm, respectively, which can meet the service life requirement of 50–100 a. It is suggested that 304 stainless steel and silane-coated should be used in CAC structures to effectively improve its service life. [ABSTRACT FROM AUTHOR]

• Using the linear polarization resistance (LPR) method to test the steel bars embedded in the CAC prismatic specimen and obtain the corrosion rate of them. • The anti-corrosion performances of different reinforcements in CAC were compared and ranked via corrosion rate measured by LPR method. • The surface free chloride content of steel bar in CAC was obtained through the two-dimensional chloride diffusion model. • The critical chloride content of different steel bars in CAC were determined based on the statistical relationship between surface free chloride content and corrosion rate of them. Chloride threshold is a significant index to judge the initial corrosion of rebar in concrete structures, and a key parameter for durability analysis and service life design of reinforced concrete structures. In the present wok, the corrosion rate of rebar embedded in coral aggregate concrete and the chloride ion diffusion law of coral aggregate concrete was studied by the linear polarization resistance method and two-dimensional chloride diffusion model, respectively. Depending on the relationship between the surface free chloride content of rebar and its corrosion rate, the chloride threshold values of different rebar in coral aggregate concrete were finally determined. The results indicate that the corrosion rate of rebar in coral aggregate concrete gradually decreases with the increase of concrete strength grade and cover thickness, and the order of anti-corrosion performance of reinforcements from high to low is: epoxy resin coated steel, 2205 duplex stainless steel, 316 L stainless steel, organic coated steel and ordinary steel. The chloride threshold values (% by weight of concrete) of these reinforcements in coral aggregate concrete were determined and ranked from small to large as follows: ordinary steel (＜0.15%), organic coated steel (＜ 0.21%), 316 L stainless steel (0.33% ∼0.41%), 2205 duplex stainless steel (＞0.46%) and epoxy resin coated steel (＞0.49%). Additionally, there is a remarkable regression relation of corrosion rate between surface free chloride contents and cover thickness of reinforcement, which has a great significance of the research of steel corrosion in coral aggregate concrete. [ABSTRACT FROM AUTHOR]

• The uniaxial compressive stress-strain curves of the MOSC capable of forming the 5 Mg(OH) 2 ·MgSO 4 ·7H 2 O phase were obtained. • The uniaxial compressive stress-strain curves of the MOSC are well fitted with the Guo's model. • The cracking load of MOSC is higher than that of OPC. • MOSC is more brittle than OPC. To investigate the uniaxial compressive stress-strain curves of magnesium oxysulfate cement concrete (MOSC), a new type of magnesium oxysulfate (MOS) cement capable of forming the 5 Mg(OH) 2 ·MgSO 4 ·7H 2 O phase (5·1·7 phase) was used to replace ordinary Portland cement (OPC) in specimens with C20-C65 strength grades, and the stress-strain curves of groups of three specimens with dimensions of 100 mm × 100 mm × 300 mm were obtained through uniaxial compression testing. The peak stress (σ p), peak strain (ε p), initial modulus of elasticity (E c), bending point and convergence point of the uniaxial compressive stress-strain curves of the MOSC are in good agreement with the model curves. Furthermore, the descending segment of the curve of MOSC is markedly steeper than that of OPC, indicating that MOSC is more brittle. According to the test results, the compressive stress-strain curves of the MOSC with C20-C65 strength grades are fitted. [ABSTRACT FROM AUTHOR]

• The properties of MOSC with L-FA or H-FA during water immersion are studied. • The compressive strength of MOSC decreases first and then increases after immersion. • Restricting the expansion contributes to the densification of the microstructures of MOSC. • A small number of 5·1·7 phases can form after immersion in water. To expand the application range of magnesium oxysulfate cement (MOSC) and the utilization of fly ash, the water resistance of MOSC mixed with low-calcium fly ash (L-FA) or high-calcium fly ash (H-FA) was studied. The softening coefficients and volume stability were tested to evaluate the water resistance of MOSC. The hydration products and microstructures of MOSC prepared without L-FA or H-FA (denoted as C) and MOSC prepared with L-FA (L-FM) or H-FA (H-FM) were characterized by quantitative X-ray diffraction (QXRD), mercury intrusion porosimetry (MIP), thermogravimetry (TG), and scanning electron microscopy (SEM). The results show that the expansion stress and the volume deformation caused by the transformation of residual MgO into Mg(OH) 2 are the main reasons for the decrease in the compressive strength of MOSC after its immersion in water. The formation of few 5·1·7 whiskers and the restraints on the expansion increase the compactness of the MOSC, which makes the compressive strengths and softening coefficients of C, L-FM and H-FM increase gradually with an increase in the immersion time. The softening coefficients of C, L-FM and H-FM immersed in water for 90 days are 0.97, 1.00 and 0.84, respectively. Although the softening coefficient of H-FM is not as good as that of C and L-FM due to the formation of CaSO 4 ·2H 2 O, the compressive strength and the compactness of H-FM immersed in water for 90 days are still higher than those of L-FM, which indicates that H-FA has good application prospects in MOSC. [ABSTRACT FROM AUTHOR]

• Basic magnesium sulfate cement coral aggregate concrete (BMSC-CAC) is a new type of concrete with excellent high-temperature resistance. • Mechanical properties of different strength grades of BMSC-CAC under impact loading after exposure to elevated temperature. • A dynamic constitutive model is proposed with the consideration of the elevated temperature weakening effect and strain-rate effect. • A 3D mesoscopic model which considered the random shape, size and distribution of the aggregate is established. This study investigates the effects of high temperature and strain rate on the dynamic compression behaviour of basic magnesium sulfate cement–coral aggregate concrete (BMSC–CAC). Specimens are exposed to temperatures ranging from 100 °C to 900 °C and then systematically tested using a split Hopkinson pressure bar (SHPB) apparatus. A three-dimensional mesoscale modelling approach is adopted in this study to account for the random shape and position of the aggregate. Results indicate that the dynamic impact damage is due to the coupled effect of high exposure temperature and high strain rate. At lower exposure temperatures, the dynamic compressive strength is similar to that at ambient temperature. However, at exposure temperatures of 500 °C and 700 ℃, the compressive strength is reduces to 50% and 30% of that at ambient temperature, respectively. Additionally, an increase in the exposure temperature increases the strain rate and continuously decreases the elastic modulus of concrete. A dynamic constitutive model is proposed considering the temperature and strain rate as influencing factor. This mesoscale modelling method is highly reliable and provides the best calculation parameters for simulating and predicting the dynamic compressive effect of BMSC–CAC after exposure to high temperatures. [ABSTRACT FROM AUTHOR]

This paper presents an experimental and numerical study on the compressive behaviors of a new carbon fiber-reinforced cement-based composite (CFRCC) that is composed of magnesium oxysulfate cement, carbon fiber and some chemical admixtures. Parametric studies on the effects of carbon fiber content and strain rate on the compressive behaviors of CFRCC, are conducted using a series of quasi-static and dynamic compressive tests. The research item includes the failure morphology, stress-strain curve, dynamic increase factor (DIF), etc. Besides, a three-dimensional (3D) mesoscale modelling approach is proposed to simulate and analyze the mechanical responses and the failure mechanisms of CFRCC subjected to dynamic compressive loading, where the random dispersion of chopped fibers are realistically considered. The results indicate that the cracking and failure modes of CFRCC are significantly affected by the bridging effect of carbon fibers randomly dispersed in cement matrix. With the increase of fiber content, the dynamic peak stress and peak strain of CFRCC at similar strain rates show an increasing and decreasing trend, respectively. An evident strain-rate hardening effect could be found on the initial elastic modulus and the peak stress of CFRCC as well. While the relationship between the DIF of CFRCC and the logarithm of strain rate could be described using a quadratic function. Moreover, the mesoscale modelling approach has been validated by a good agreement between the numerical and tested results, demonstrating the feasibility of the developed mesoscale model in simulating and investigating the dynamic behaviors of CFRCC. All in all, the CFRCC has proven to be of great significance in resisting to impact loads, and more insights should be provided into its dynamic properties. [Display omitted] • A new carbon fiber-reinforced composite (CFRCC). • Quasi-static and dynamic compressive tests of CFRCC. • A new 3D mesoscale model for fiber-reinforced composites. • Effects of strain rate and fiber content on the CFRCC. • Multi-scale analysis on the compressive behaviors of CFRCC. [ABSTRACT FROM AUTHOR]

[Display omitted] • Irregular polyhedron model with controllable shape and size characteristics. • Vector-growth algorithm for controlling convexity and concavity of particle. • An iterative growth algorithm for generating particles with different surface numbers. • Quantitative parametric analysis on shape and size configurations of irregular particles. • Random distribution algorithm for the irregular particles in spatial domain. • Compaction algorithm for controlling volume fraction of particulate system. Particles around us are generally in the form of irregular characteristics in shape and size. Establishing rational mesoscale models that are suitable for different types of particles is of great significance to comprehensively evaluate the mechanical properties of particulate systems assembled from irregular particles. The preponderances of previous works are mainly focused on particle simulations using regular-shaped geometries or simple polyhedrons, and little is known about quantitative characterization for the particles with complex shape characteristics. In this paper, a series of novel and efficient algorithms are presented to generate three-dimensional particulate systems assembled from particles with irregular characteristics in shape and size. According to the developed particle generation algorithm and vector-growth algorithm, the convex- and concave particles with different shape- and size configurations are obtained. Parametric analysis of corresponding algorithm parameters on the shape- and size configurations of particles are quantitatively investigated in terms of different indexes, such as sphericity, angularity and size, which provides an important guidance for the shape- and size control of irregular particles. Based on the generated irregular particles, a series of algorithms are proposed to generate the particulate systems with random spatial characteristics as well. Employing the developed compaction algorithm, mesoscopic particulate systems with different particle gradations and compactness are generated precisely. To sum up, the present particle model provides insights into capturing and studying the meso -structure characteristics and macroscopic mechanical properties of particulate systems. [ABSTRACT FROM AUTHOR]

The constitutive relationship of basic magnesium sulfate cement-coral aggregate concrete (BMSC-CAC) under uniaxial compression was investigated using cylindrical specimens. The equal strain monotonic loading method was applied to obtain the stress-strain curves. Mechanical parameters such as elastic modulus (E c) and Poisson's ratio(μ) were calculated. A 3D random aggregate mesoscopic model was used to simulate the failure process and crack propagation. The results show that when the strain approximated to maximum value, BMSC-CAC was rapidly destroyed in the shape of splitting. Compared with ordinary cement, the brittleness of concrete can be effectively improved due to the high toughness of basic magnesium sulfate cement (BMSC). The segmented constitutive equation of BMSC-CAC was given, which could express all of the compression test characteristics. The relative error of the peak stress and strain between the experiments and simulations were small. Analysis of the failure process reveals that damage first appears in the mortar and ITZ of the concrete, with micro-cracks converging into huge oblique cracks through the aggregate. [ABSTRACT FROM AUTHOR]

Ethylene glycol (EG) deicer and propylene glycol (PG) deicer were frequently used as the aircraft de-icing fluids in cold region. In this paper, the icing pressure (I p), icing volume expansion rate (Δ V P ), freezing point (T 0) and under-cooling (△T) of aircraft deicers with different concentrations were investigated, and the above indicators of aircraft deicers were also compared with that of water and NaCl deicers. The results showed that the icing volume expansion of deicers was the direct cause of icing pressure. During the cooling process, values of I p and Δ V P firstly increased sharply, then slowly, and finally became stable. The stabilized values of I p and Δ V P decreased with concentration. The stabilized I p value of EG deicer was slightly higher than that of 3.5% NaCl deicer and far greater than 3.5% PG deicer. Of all the tested deicers, the stabilized Δ V P value of NaCl deicer was the highest and that value of EG deicer was close to that of PG deicer. The results further confirmed the feasibility of using impulse of icing pressure as the indicator to evaluate the freeze-thaw deterioration to concrete. • The icing pressure and icing volume expansion rate of organic aircraft deicers was measured. • The heat of liquid–solid phase transformation was discussed. • The cooling process and the ice crystal growing process of organic aircraft deicers were divided into five stages. • The impulse of icing pressure was proposed to evaluate the frost damage to concrete induced by deicer. [ABSTRACT FROM AUTHOR]

For the island-reef engineering construction background in the South China Sea, the research studied the preparation technology, mechanical properties and chloride ion diffusion behavior of coral aggregate seawater concrete and discussed the steel bars corrosion behavior of coral aggregate seawater concrete (CASC) and reliability analysis of service life, which provide a low cost, high durability CASC and its application technology for island-reef engineering. The results showed that: according to the theory of grout concrete and the design principle of high-performance concrete, the cube compressive strength (f cu) of CASC can reach C80. Uniaxial compression whole stress-strain curve of CASC with different strength grades and constitutive relation were obtained. The "three high" Cl− diffusion characteristics, high initial chloride ion content (C 0), high surface chloride ion content (C s) and high apparent chloride ion diffusion coefficient (D a) in CASC, were established, which easily result in steel bars corrosion. It suggests to choose organic newly coated steel bars, which the thickness of the concrete protective cover is greater than 5.5 cm, concrete strength grade is greater than C50 and use "slow-release effect of micropump" of coral aggregate to pre-absorb and incorporate the amino alcohols inhibitor, which can significantly reduce steel bars corrosion of CASC structures. Finally, the study determined the durability design methods of CASC structures that meet the 20–100 years of service life requirement. [ABSTRACT FROM AUTHOR]

For better understanding the compressive behavior of coral aggregate concrete (CAC) with different volume fractions and maximum aggregate sizes (MASs) for coarse aggregate, an all-sided mesoscopic investigation was performed using a novel 3D mesoscale model assuming concrete to be a three-phase composite material composed of coarse aggregate, mortar matrix and interfacial transition zone. The random characteristics of coarse aggregate in shape and spatial distribution were considered adequately in the mesoscale model. Fourteen types of cubic mesoscale models (sized by 100 × 100 × 100 mm3) with various aggregate volume fractions (0–67.4%) and MASs (10–30 mm), were established to study the quasi-static compressive behaviors of CAC, such as the deformation processes, failure patterns and stress–strain relations. By comparing the simulation results and the available test results, it has proven to be that the present 3D mesoscale modelling approach was capable of simulating and investigating the compressive behavior of CAC. Besides, it was indicated from the simulation results that the compressive strength of CAC generally decreases with the increase of MAS, but when the MAS exceeds the critical value, the varying trend of compressive strength tends to slow down. Additionally, the compressive strength tends to decrease before reaching a certain aggregate volume, but when it exceeds a certain volume, that is, the optimum aggregate volume, the compressive strength exhibits a rebounding trend. And the optimum aggregate volume of CAC with the MAS of 20 mm was determined from 58.0% to 61.1%. Furthermore, a regressive relation between concrete strength and aggregate volume was preliminarily derived from simulation and available test results, which can provide some enlightenments for the mix design and performance prediction of CAC. [ABSTRACT FROM AUTHOR]

• Three-dimensional random mesoscale modelling for coral aggregate concrete (CAC). • Splitting-tensile failure patterns and failure processes of CAC. • Splitting-tensile failure mechanism of CAC. • Strain rate effect and size effect on the splitting-tensile behavior of CAC. • Strain effect law and size effect law of CAC under splitting-tensile loading. The paper presents a mesoscopic investigation into the effects of specimen size and strain rate upon the strength and failure modes of coral aggregate concrete (CAC) subjected to splitting-tensile load. A three-dimensional (3D) mesoscale model that assumes concrete as a three-phase composite, i.e., aggregate, mortar and interfacial transitional zone (ITZ), was established for simulating the splitting-tensile behaviors of CAC at different strain rates (10−5 ∼ 200 s−1). The mesoscopic deformation and failure patterns of CAC cylinders with different sizes were exhibited through the 3D mesoscale modelling. Emphasis was set on the specimen size- and strain-rate effect on the tensile strength of CAC and corresponding effect laws were finally expressed by the mathematical statistical method. The numerical results suggest that there is a rate-hardening effect on the tensile strength of CAC, and the strain-rate effect is more noticeable when the critical strain-rate (1 s−1) is reached. Taking into the varying strengths at low- and high strain rate, the piecewise formulas were determined by the numerical and available test results to represent the strain-rate effect on CAC. In accordance with the relationship between the splitting-tensile strength and fracture area of specimen, the size-effect law of CAC at various strain rates was mathematically expressed using a series of power functions, which is of great significance for future research and application of CAC. [ABSTRACT FROM AUTHOR]

A new magnesium-calcium oxysulfate cement (MCOSC) was proposed by replacing MgSO4 in magnesium oxysulfate cement (MOSC) with untreated flue gas desulfurization gypsum (FGDG) at levels of 25%, 50%, 75% and 100%. The synergistic effect of FGDG and chemical additives, including citric acid, ammonium citrate tribasic and ammonium dihydrogen phosphate on the mechanical performance and hydration mechanism of MCOSC was investigated. The results showed that three additives all promoted the formation of 5 Mg(OH)2·MgSO4·7H2O (5·1·7) phase and hindered Mg(OH)2 precipitation, improving mechanical properties of MCOSC. FGDG incorporation induced no new crystalline phase, but gypsum crystal provided the space for nucleation of 5·1·7 phase. FGDG also reacted with MOSC to form amorphous phase, which was identified as a magnesium-sulfide-calcium-hydrate gel. Moreover, it affected the morphology of 5·1·7 phase and lowered the temperature of endothermic peaks in the paste during the heating process. The additive-incorporated specimens with 25% FGDG replacement presented a superior compressive strength, water resistance and volume stability. This confirmed the effective application of FGDG in preparing MCOSC, and it also provided an approach for recycling waste gypsum. • Magnesium-calcium oxysulfate cement is prepared by replacing MgSO 4 with untreated FGDG. • The synergistic effect of FGDG and additives on mechanical performance and hydration mechanism of MCOSC is studied. • CA, ACT and ADP all induce the formation of needle-like 5·1·7 phase. • FGDG provides the space for nucleation of 5·1·7 phase and reacts to form a magnesium-sulfide-calcium-hydrate gel. [ABSTRACT FROM AUTHOR]

The dynamic compressive behavior of concrete is closely associated with the specimen size effect and the coupling effect of strain rate and specimen size on the compressive properties is something there worthy studying for the development and application of coral aggregate concrete (CAC). A novel 3D mesoscale modelling approach taking the random characteristics of aggregates is developed for the numerical study in the present work. The specimen diameter of numerical cylinder with the slenderness of 1/2 varies from 50 mm to 200 mm, and the strain rate is within the range of 10−3–200s−1. Employing the 3D mesoscale model, the dynamic compressive responses of CAC, i.e., stress-strain relation, failure pattern and process, energy evolution process, etc. have been simulated and analyzed systematically. Both the effect of strain rate and specimen size have been qualitatively and quantitatively discussed. Through the comparison of numerical and existing test data, it is verified that the compressive behaviors of CAC under different conditions could be characterized and predicted using the 3D mesoscale model. In addition, all above-listed properties are related to the strain rate and specimen size at different degrees. The strain rate effect law on the CAC's strength and absorbed energy can be well expressed using a quadratic function and an exponential function, respectively. And the relationship between strength and specimen size can be mathematically formulated by a power function. Furthermore, a coupling effect law of strain rate and specimen size on the strength of CAC has been proposed and validated based on numerical and test data, which is meaningful for the properties prediction of CAC. Image 1 [ABSTRACT FROM AUTHOR]

To explore the specimen aspect-ratio (height-to-diameter ratio) effect on the compressive behavior of coral aggregate concrete (CAC), a three-dimensional (3D) mesoscale modelling approach with considerations of random shapes and distributions for aggregates was developed in the present study. Through the comparison of numerical and available test results, the reliability of the numerical approach for CAC cylinder specimen with various aspect ratios (2–6) was validated. The stress-strain relations, failure patterns and failure processes of CAC in compression were numerically simulated and discussed in succession. Finally, the aspect-ratio effect law on the uniaxial compressive strength of CAC was obtained and compared with that of ordinary concrete. The results indicate that the post-peak branch of the compressive stress-strain curve of CAC tends to steeper with the increase of specimen length, which corresponds to the localization behavior of concrete. In addition, when the height-to-diameter ratio exceeds 2–3, the length of failure zone is close to three times of the specimen diameter (100 mm). Distinctive failure characteristics of each components (i.e., coral aggregate, mortar and ITZ) in CAC with different lengths were presented, and it was found that there is a faster failure process in the shorter specimen and the cracks in the longer specimen would obliquely propagate in the lateral surface. Furthermore, it has been demonstrated that the uniaxial compressive strength of CAC is related to the specimen aspect-ratio, while it becomes insignificant when the aspect ratio exceeds 2.5–3. Image 1 • The specimen aspect-ratio effect on the compressive behavior of coral aggregate concrete. • A three-dimensional (3D) random mesoscale modelling approach considering random shapes and distributions of aggregates. • The compressive stress and strain behaviors, failure patterns and failure processes of CAC. • The aspect-ratio effect law of uniaxial compressive strength. [ABSTRACT FROM AUTHOR]

[Display omitted] • The effects of two chemical additives on the performance of MOSC was compared. • The mechanism of the different effects of two chemical additives on the properties of MOSc was analyzed. • The decrease of hydration rate of MOSc is beneficial to the crystallization development of 5·1·7 phase. • The 5·1·7 phase with low crystallinity or 5·1·n phase may be another factor for the reduction of compressive strength of MOSc after immersion. Chemical additive is an indispensable component to improve the properties of magnesium oxysulfate cement (MOSc). In this paper, the differences of the effects of chemical additives phosphoric acid (PA) and calcium citrate (CC) on the performance of MOSc were investigated by analyzing the composition and content of MOSc hydration products and characterizing the microstructure. The results manifest that the hydration rate of MOSc with PA is relatively slow, but the crystalline grain of the hydration product 5 Mg(OH) 2 ·MgSO 4 ·7H 2 O (5·1·7 phase) is coarse, and the composition of the hydration product is excellent, so it has good mechanical strength and water resistance. The hydration rate of MOSc with CC is relatively fast, and the amount of 5·1·7 phase is relatively high, but its crystalline grain is not as coarse as that of MOSc with PA. Therefore, the MOSc with CC has high early strength, but its later mechanical strength and water resistance are not as excellent as that of MOSc with PA. Meanwhiles, it is found that 5·1·7 phase with low crystallinity and 5·1·n phase with high content of crystal water may exist in MOSc, which is another factor affecting the reduction of its mechanical strength after immersion. The research results are of great significance for promoting the progress of chemical additive modification technology and improving the performance of MOSc. [ABSTRACT FROM AUTHOR]

To solve the environmental problems caused by heavy metal pollution, a new cementitious material (basic magnesium sulfate cement, BMSC) was developed for the solidification of Cu2+/Zn2+. First, the effects of different amounts of Cu2+/Zn2+ on the properties (compressive strength, setting time, pH, and leaching toxicity) of the BMSC were investigated. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) were used to investigate the effects of different amounts of Cu2+/Zn2+ on the phase and microstructure of BMSC. The results showed that Cu2+/Zn2+ inhibited the hydration of BMSC, reduced compressive strength, and prolonged the setting time. The results of the leaching tests showed that the BMSC system exhibited high immobilization efficiency (up to 99%) for Cu2+/Zn2+. Further, the BMSC solidification matrix exhibited excellent acid resistance (compressive strength >40 MPa after 28 days of immersion). The physical phase analysis showed that the main phases of BMSC were the 5Mg(OH) 2 -MgSO 4 -7 H 2 O (5−1−7) phase and Mg(OH) 2 , and the crystal structure refinement analysis suggested that Cu2+/Zn2+ ions were substituted with Mg2+ in the 5–1–7 phase. It was confirmed that the solidification mechanism of BMSC on Cu2+/Zn2+ is mainly performed by chemical complexation and ionic substitution. [Display omitted] • An innovative cement materials (BMSC) was developed to solidify Cu2+/Zn2+ • Cu2+/Zn2+ were stabilized by the 5Mg(OH) 2 -MgSO 4 -7 H 2 O phase via ionic substitution. • BMSC matrix has good compatibility with Cu2+/Zn2+ (low leaching toxicity and good properties. [ABSTRACT FROM AUTHOR]

Herein, to reduce CO 2 emissions and energy consumption and to promote the recycling of waste resources, two types of boron-containing MgO by-products, which were obtained by lithium extraction from Qarhan Salt Lake, China, were used as substitutes for dead-burned MgO to prepare magnesium phosphate potassium cement (MKPC) as a rapid repair material. First, the phase composition and particle-size distribution of the MgO by-product were investigated. The effects of different MgO sources, molar ratio of MgO to KH 2 PO 4 (M/P), and curing age on the setting time and mechanical properties of MKPC were then studied. Based on the results, the mix proportion of MKPC was optimized. Finally, X-ray diffractometry, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), differential thermogravimetric (DTG) analysis, and mercury intrusion porosimetry were used to characterize the phase and microstructure evolution of MKPC prepared with different MgO contents. The results demonstrated that the by-product MgO prolonged the setting time of MKPC to more than 40 min. In addition, in the initial stage of hydration, the compressive strength of the MgO by-product was slightly lower than that of the dead-burned MgO; however, with increasing age, the mechanical properties of MKPC prepared by by-product MgO were excellent (up to 60 MPa). The phase and microstructure results revealed that the main hydration product of MKPC prepared using the three types of MgO was MgKPO 4 ·6H 2 O. Combined with the physical and chemical properties of the raw materials, it was confirmed that the larger particle size and the coexisting impurities from the salt lake were the main reasons for the longer setting time of the MKPC prepared by the by-product MgO. We believe that this research will be of great significance for the preparation of low-carbon, low-cost, and high-performance MKPC materials. [Display omitted] [ABSTRACT FROM AUTHOR]

• Three dimensional mesoscale model of SFRC. • Mesoscopic fracture responses of notched SFRC beam. • Fracture patterns of SFRC under bending loading. • Mesoscopic characteristics of steel fibers in concrete matrix. • Size effect laws of nominal fractural strength of SFRC beam. Steel fiber reinforced concrete (SFRC) can exhibit superior performances of high strength and toughness due to the enhancing effect of discontinuous steel fibers randomly distributed in concrete matrix. For better understanding the size effect on the fracture properties of SFRC structural elements, we developed a three-dimensional (3D) two-phase mesoscale model considering the random distribution and orientation of steel fibers in concrete, to investigate the quasi-static flexural responses of the notched SFRC beams with different beam depths, i.e., 30 mm, 60 mm, 90 mm, 120 mm and 150 mm. A new coupling method was employed to describe the interfacial relationship between steel fibers and concrete matrix in the present simulations. The fracture behaviors of SFRC beam were numerically studied in terms of the load versus cracking mouth opening displacement curve, the evolution of fracture process zone, the nominal flexural strength and the fracture failure patterns. Through the analysis and comparison between the present numerical results and the available test data, it was found that there is negligible size effect on the fracture patterns of SFRC beam. The size effect on the nominal flexural strength of SFRC beam can be well characterized and analyzed using the Bažant's size effect law. Further, the numerical results of SFRC beams showed a favorable agreement with the corresponding test results, demonstrating the great feasibility and potential of the developed mesoscale modelling approach in simulating and investigating the mechanical behaviors of SFRC. [ABSTRACT FROM AUTHOR]

• Coral aggregates sampled from different positions of coral cluster. • Morphology and pore characteristics of coral aggregates. • Quasi-static compressive behaviors of coral aggregates. • X-CT tests for mesoscopic characteristics of coral aggregates. • 3D mesoscale modelling of coral aggregate samples based on X-CT image data. Coral aggregate is a type of porous and light-weight construction material with intense prospects for the engineering applications in remote ocean islands. However, its macroscopic properties is directly determined by the mesoscopic characteristics, such as texture, porosity, etc. Thus, the mesoscopic characteristics and macroscopic properties of coral aggregates are investigated in the present work. Three types of coral aggregates sampled at different positions of coral clusters, such as the tip-, stem- and root segments, are selected for investigation. The zoom-stereo microscope and X-ray computed tomography (XCT) facility are used to conduct the morphology observations for coral aggregates. Afterwards, the pore information (i.e., size, number and connectivity) of different coral aggregates are tested and analyzed quantitatively according to the XCT scanning data. Besides, the uniaxial compressive properties of coral aggregates are studied using cylinder aggregate samples with dimension of φ10mm × 20 mm. The effect of porosity characteristic on the compressive strengths of coral aggregates is discussed comprehensively. Furthermore, based on the XCT scanning data, a series of algorithms are developed to establish the mesoscale models for different coral aggregate samples. Employing the established mesoscale models and the Mohr-Coulomb criterion, the uniaxial compressive tests of coral aggregates are simulated. Results indicate that there are high reliabilities for the developed mesoscale model and the determined material model parameters of different coral aggregates, providing further insights into the mechanical properties of coral aggregates. [ABSTRACT FROM AUTHOR]

[Display omitted] • 3D mesoscale modelling approach with random shape and distribution of aggregate. • Uniaxial direct-tensile properties of CAC under various strain rates. • Failure process, final failure pattern and stress-strain curves. • Tensile strength, elastic modulus, critical strain and energy absorption capability. • Tensile constitutive relations of CAC under different strain rates. The pre-existing studies of the dynamic tensile properties of coral aggregate concrete (CAC) are mainly performed through the splitting-tensile tests or simulations, which have certain discrepancies with the direct tensile properties. This paper employs a 3D mesoscale modelling approach considering the random characteristics (shape and distribution) of coarse aggregates to simulate the uniaxial direct-tensile behaviors of CAC. The dumbbell-shaped specimen model consisting of tri-phase components (aggregate, mortar and ITZ) was established for the numerical simulation of CAC under direct tension at different strain rates (10−5 to 100 s−1). Then the failure pattern and micro-cracking process at different strain rates were simulated, and corresponding stress-strain curves were subsequently obtained and analyzed. Besides, the effect of strain rate on the tensile properties, such as strength, elastic modulus, critical strain, and energy absorption capability, were quantitatively analyzed and discussed. And corresponding strain-rate effect laws were determined and validated by the available test results. Moreover, based on the tensile stress-strain relations of CAC, the tensile constitutive relations of CAC at different strain rates were finally obtained, which is of great significance for further investigation of the dynamic tensile properties of CAC. Most of all, the simulation results remain a good agreement with the existing test results, which demonstrates that the proposed 3D mesoscale modelling approach could be regarded as a powerful tool in simulating the direct-tensile behavior and understanding corresponding damage mechanism of CAC. [ABSTRACT FROM AUTHOR]

• State-of-the-art mesoscale modelling approaches and material models for concrete. • Generation methods for different mesoscopic models for aggregate and concrete. • General finite element meshing techniques for mesoscale concrete model. • Material models for mesoscopic modelling of concrete material. • Mesoscopic modelling of concrete materials under static and dynamic loadings. Further understanding of the mechanical responses and the failure mechanism of concrete under different loadings, the mesoscale modelling approach considering the heterogeneity of concrete has drawn extensive attention in investigating its mechanical behaviors. State-of-the-art mesoscale modelling approaches and material models for concrete are presented and compared with our developed 3D random mesoscale modelling approach, in which the randomness of particles in shape and spatial distribution in concrete is considered. This work mainly concentrates on the development and application of the 3D random mesoscale modelling approach in investigating the mechanical behaviors of concrete under static and dynamic loadings. Employing the 3D mesoscale modelling method, the mechanical properties of the ordinary concrete under dynamic compression, split- and direct tension are numerically studied. According to the example applications performed in this work, it is demonstrated that the 3D random mesoscale modelling approach has excellent validity in investigating the mechanical responses of concrete, which provides significant insights into the further development and application of concrete materials and structures under different conditions. [ABSTRACT FROM AUTHOR]

• Uniaxial compressive properties of ecological concrete (EC). • Three-dimensional (3D) random mesoscale modelling approach for EC. • Peak stress and stress-strain curves of EC in compression. • Effects of aggregate-to-cementitious material, water-to-cementitious material, void content, and aggregate size. • Failure pattern, deformation process and failure mechanism. In this study, the uniaxial compressive behaviors of ecological concrete were investigated using experimental and numerical methods. The uniaxial compressive test was performed on the cubic specimens with a side length of 150 mm. The three-dimensional (3D) mesoscopic model with the consideration of random shapes and sizes of aggregates was developed. The parameters considered were the aggregate-to-cementitious material ratio (A/C), the water-to-cementitious material ratio (W/C), the aggregate size, and the void content. The results indicate that the collapse of aggregate caused by the sliding failure of bonding points among aggregates is the main failure mode of ecological concrete under uniaxial compression. The void content and compressive strength of ecological concrete are associated with the A/C and W/C, and the A/C of 6 is suggested as the optimal value. The compressive stress-strain curve consists of three parts, in which the platform period represents the yield period that is relative to the compacting process. Besides, the peak stress and the peak strains in the axial- and lateral directions are related to the void content and aggregate size, and the lateral strain is obviously larger than the axial strain. Through the comparison of numerical and test results, the developed 3D mesoscale modelling approach together with the model parameters has been approved in investigating the compressive behaviors of ecological concrete. [ABSTRACT FROM AUTHOR]

The usage of manufactured sand concrete is widespread in modern engineering, and it is important to study its performance to improve the overall engineering quality. This paper presents an experimental study on the working performance and durability of 12 groups of manufactured sand high-performance concrete (MSHPC) with varying mix ratios, in the context of the construction of the Dalian Bay undersea immersed tube tunnel. The study reveals that the stone powder content significantly affects the physical and mechanical properties, as well as the durability, of manufactured sand concrete. At an approximately 9% stone powder content, the concrete achieves the highest slump and best workability. However, excessive stone powder reduces early crack resistance. Furthermore, an optimal stone powder content (ranging from 5% to 13%) enhances the compressive strength, with the 28-day compressive strength reaching 60 MPa at a 13% stone powder content, while the effect on the splitting tensile strength is negligible. The stone powder content does not significantly impact impermeability and frost resistance, but at 7-9%, the RCM method shows the lowest chloride ion diffusion coefficient. Additionally, a lower water-binder ratio enhances resistance to chloride ion diffusion. High-performance RCM concrete with a 9% stone powder content was used in the construction of the Dalian Bay Cross-Harbor Tunnel, achieving 28-day and 56-day compressive strengths of C45 and C50, respectively, an impermeability grade of P14, a chloride ion diffusion coefficient of 1.9 × 10 -12 m 2 /s, and a frost durability index of 92%, meeting the project's 100-year lifespan design requirements.