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Saturated concrete is significantly different from dry concrete in fracture mechanical properties. Using the wedge-splitting tensile method to research the rule of change in moisture content, double-K fracture toughness and fracture energy of three strength grades (C20, C30, and C40) of concrete immersed in a free water environment for 0 h, 2 h, 5 h, 24 h, and 120 h were studied in order to provide support for the safety evaluation of concrete structures in a water environment. The initial cracking fracture toughness of C20, C30, and C40 concrete in saturated state were, respectively, 29.6%, 23.2%, and 33.4% lower than that in dry state. The unstable fracture toughness of C20, C30, and C40 concrete in saturated state were, respectively, 22.7%, 23.9% and 33.8% lower than that in dry state. The fracture energy of C20, C30, and C40 concrete in saturated state are only 71.99%, 70.29%, and 66.11% of that in dry state, respectively. The initial cracking fracture toughness and unstable fracture toughness of concrete all show a linear, decreasing trend with an increase in moisture content. Before the crack initiation, the measured P–CMOD curve had an obvious linear elastic stage, stable expansion stage, and unstable expansion stage. The critical crack opening displacement gradually decreases with an increase in moisture content; the deformation capacity and toughness of concrete are shown to decrease. The humidity state should be fully considered when evaluating the fracture mechanical properties of concrete. [ABSTRACT FROM AUTHOR]

Using wedge-splitting method, the changes of moisture content and double K fracture mechanical properties of three strength grades (C20, C30 and C40) concrete immersed in free water environment for 0 h, 2 h, 5 h, 24 h and 120 h were studied, so as to provide support for the safety evaluation of concrete structure in water environment. The results show that the higher the concrete strength grade is, the slower the water absorption rate is, the lower the moisture content is, and the longer soaking time it takes to reach saturation state. The initial cracking fracture toughness and unstable fracture toughness of concrete decrease linearly with the increase of moisture content. The initial cracking fracture toughness of C20, C30 and C40 concrete in saturated state is 29. 6%, 23. 2% and 33. 4% lower than that in dry state, and the unstable fracture toughness of C20, C30 and C40 concrete in saturated state is 22. 7%, 23. 9% and 33. 8% lower than that in dry state, respectively. Under the same moisture content, the initial cracking fracture toughness and unstable fracture toughness of C20, C30 and C40 concrete decrease similar basically. The P-CMOD curves obviously have a linear elastic non-crack initial stage, a stable expansion stage and an unstable expansion stage. With the increase of moisture content, the slope of linear elastic stage and stable expansion stage gradually decreases, and the curve of unstable expansion stage drops more and more gently. The critical crack opening displacement decreases gradually with the increase of moisture content, the deformation capacity and toughness of concrete decrease gradually. [ABSTRACT FROM AUTHOR]

The extent of the impact of mega constellations on the low-orbiting geospace environment, which has not yet been assessed in more concrete quantitative terms, is an extremely important issue to consider as mega constellations are built. Satellite safety and lifetime can clearly represent the situation of space targets, and thus can reflect the impact of mega constellations on geospace security. Three target satellites with different characteristics were selected and the Accepted Collision Probability Level (ACPL) was calculated to obtain the impact of Starlink on satellite mission lifetime. Upon considering Starlink without early avoidance control, the lifetimes of the three target satellites were shortened by 56.21%, 99.09%, and 99.82%, respectively. After 10 revolutions of early avoidance control, two were shortened to 92.166% and 91.99%, while the lifetime of JILIN-01 was extended by 155.44%. After joining Starlink, the total risk became larger; even if the target satellite avoided control far more frequently than before joining Starlink, it will face a worse geospace environment. Adopting the most aggressive orbit avoidance control cannot avoid the deterioration of the geospace environment from the perspective of satellite lifetime, which is an irreversible and deteriorating process. [ABSTRACT FROM AUTHOR]

Moisture has a significant effect on the properties of concrete, and drying concrete in an oven is a common method to obtain specimens with different moisture content. In this paper, C30 concrete specimens with different sizes were oven-dried at constant temperatures of 60, 80, 105, 120, and 150 °C, respectively. The change of specimen mass, ultrasonic testing, and compressive and splitting tensile strength were tested before and after drying. The size effect and the size effect law (SEL) of concrete were analysed. The results indicated that the maximum drying rate and water loss ratio of the specimens with different sizes under the same drying temperature are basically the same. The larger the size, the longer the drying time. After drying, the concrete compressive and splitting tensile strength have increased to varying degrees, and the smaller the size, the greater the increase. For C30 concrete with different sizes, the drying temperature that has the minimal influence is 105 °C. The SEL was modified by introducing drying temperature, and the formulas were obtained after drying. [ABSTRACT FROM AUTHOR]

In order to comprehensively evaluate the properties of rock-filled concrete (RFC) with the strength of C15, lab experimental test and in-situ test are applied to explore the mechanical, hydraulic, ultrasonic characteristics of RFC in Hantang reservoir dam. Four types of defects within RFC are shown from the appearance of borehole cores specimens: (1) large sized voids existing in interfacial transition zone (ITZ) between self-compacting concrete (SCC) and rock block (RB); (2) bad cohesion in ITZ; (3) joints within rock block; (4) voids within SCC. For hydraulic aspects, the average porosity of RFC is 14.10%; the permeability rate of RFC ranges from 2.41 Lu to 10.41 Lu, with the average of 11.32 Lu, occasionally more than 25.52 Lu due to interconnected defects. For ultrasonic aspects, the ultrasonic velocity of RFC conforms to lognormal distribution, with the average of 2993.3 m/s and standard deviation of 650.5 m/s. For mechanical aspects, the average cubic compressive strength of RFC is 22.55 Mpa, with the standard deviation of 4.09 Mp. Thus the data shows a relatively great dispersion due to uneven distribution of some defects in RFC, which deteriorate the quality of RFC. Through the experimental investigation, it is shown that the quality of massive RFC in the Hantang dam is obviously non-homogeneous, mainly influenced by construction technology. [ABSTRACT FROM AUTHOR]

In order to obtain the drying control condition with no or minimal effect on concrete strength, various drying temperatures conditions are used as 150 °C, 120 °C, 105 °C, 85 °C and 60 °C, and the drying procedure of continuous and intermittent cyclic are presented independently in this study. The change of specimen mass is recorded in the drying process, and the compressive strength and splitting tensile strength are measured after the natural cooling. The results show that the concrete compressive strength initially decreases and then increases with the increase in drying temperature, while the splitting tensile strength always decreases. The relative compressive and splitting tensile strength of concrete in dry state are 1 and 0.99 respectively after 115.5 h of continuous drying at 105 °C, which denotes the optimal drying control condition has minimal effect on the strength, and efficiency is suitable. Finally, the change in components and defects structure of the concrete after minimal damage drying is analyzed through the thermo-gravimetric analysis, electron microscope scanning and industrial CT scanning. [ABSTRACT FROM AUTHOR]

In this research, we developed a four-phase model, which takes the aggregate gradation and porosity into account in the prediction of the elastic modulus of concrete, based on the micromechanical theories. The model has been verified with experimental results. First, using the Mori Tanaka and the differential self-consistent (DSC) methods, the pores in both the mortar and interfacial transition zone (ITZ) were homogenized. Then, the continuously graded aggregates were divided into finite aggregate size intervals. Further, using the generalized self-consistent model and multiphase composite model derived from the Mori Tanaka method, an aggregate gradation model for the prediction of the elastic modulus of concrete was developed. By simulating the pores in concrete with expanded polystyrene sphere (EPS) grains, the effect of overall porosity on the elastic modulus of concrete was investigated. The research results show that aggregate gradation and porosity have remarkable influence on the elastic modulus of concrete, and the proposed model is effective to estimate the elastic modulus of concrete, the deviation between the predicted elastic modulus and experimental elastic modulus is less than 8%. The elastic modulus decreases with increasing ITZ porosity. However, for ITZ porosity exceeding 40%, the decrease in the elastic modulus is large with increasing ITZ porosity. For a fixed overall porosity, the ITZ porosity owned more influences than the mortar porosity on the elastic modulus of concrete. Enhancing the ITZ elastic modulus and decreasing the ITZ thickness are efficient in increasing the elastic modulus of concrete. [ABSTRACT FROM AUTHOR]

Concrete structures are often in different humidity conditions that have a significant impact on the elastic modulus of concrete, therefore, systematic research on the evolution of the law of concrete elastic modulus under different humidity conditions is needed. In this study, the variation laws of the water saturation of concrete specimens with strength grades C15, C20, and C30 were obtained, and then the influence laws of the water saturation on the concrete axial compressive strength were carried out, and the prediction model of elastic modulus of concrete with respect to water saturation was constructed. The results showed that the water saturation of concrete with strength grades C15, C20, and C30 increased with an extension of immersion time, and the water saturation showed an approximately linear rapid growth within three soaking hours, reaching 47.56%, 71.63%, and 47.29%, respectively. Note, the concrete reached saturation state when the soaking time was 240 h. The axial compressive strength with strength grades C15, C20, and C30 decreased with increased water saturation, and the axial compressive strength of saturated concrete decreased by 27.25%, 21.14%, and 20.76%, respectively, as compared with the dry state concrete. The elastic modulus of concrete with strength grades C15, C20, and C30 increased with increased water saturation, and the elastic modulus of saturated concrete was 1.18, 1.19, and 1.24 times higher than those of dry concrete, respectively. [ABSTRACT FROM AUTHOR]

This paper presents an experiment to investigate the influence of moisture on the static mechanical properties of concrete, and prediction equations for strength and fracture toughness of concrete at different strength grades, relative to water saturation, were established respectively. The research results show that all of the compressive strength, splitting tensile strength, and fracture toughness of concrete exhibit an approximately linearly decreasing trend with the increase in water saturation. For saturated concrete specimens with w/c 0.65, 0.55, 0.42 compared with dry ones, compressive strength decreases by 40.08%, 36.08%, and 33.73%, respectively, splitting tensile strength decreases by 45.39%, 42.61%, and 35.18%, respectively, and fracture toughness decreases by 57.31%, 49.92%, and 46.76%, respectively. The higher the water saturation of concrete, the larger the slope of the ascending part of the uniaxial compressive stress-strain curve, and the smaller the peak strain corresponding to the peak compressive stress, then in this case, both crack mouth opening displacement and loading point deflection corresponding to the critical load on three-point bending beam, decrease. Ingress of water causes the deformation capacity to decrease, and the toughness to weaken, which are unfavorable to the mechanical properties of concrete. [ABSTRACT FROM AUTHOR]

Highlights • A micromechanics predictive model of the concrete strength was established. • The Drucker-Prager criterion was adopted to decide the failure of the micro-phase. • Cracking, yield and ultimate strengths were solved based on failure characteristics. • Effectiveness and reliability of the model were verified by comparing with test data. • Ultimate strength can increase by improving the elastic modulus of ITZ. Abstract On a microscopic scale, concrete was regarded as a three-phase composite consisting of coarse aggregate, cement mortar and interface transition zone (ITZ). The coarse aggregates and their peripheral ITZ were equated with equivalent particles blended in cement mortar through two equivalent processes. Taking the effects of ITZ, aggregate gradation, maximum aggregate size and the interactions among equivalent particles into consideration, the average stress of microscopic phase of concrete was calculated based on a micromechanics model, and the stress concentration within ITZ phase was further solved. Adopting the Drucker-Prager criterion to decide the failure of the microscopic phases, the cracking strength, yield strength and ultimate strength of the concrete were calculated respectively when the ITZ cracking, the ITZ failure and the mortar failure occurred. The results were compared with test data to verify the effectiveness and reliability of the model. Based on the model, the influence of the elastic modulus of coarse aggregate, the coarse aggregate content and the elastic modulus of ITZ on the yield strength and ultimate strength are discussed. This paper presents an approach to explore the connection between macroscopic strength and microscopic phase strength, which provides a theoretical reference for studying the failure mechanism of concrete. [ABSTRACT FROM AUTHOR]

Highlights • A drying model considering temperature and strength grade was obtained. • The changes of water mass loss and ultrasonic were analysed before and after drying. • Compressive strength decreases and then increases with increase in temperature. • Splitting tensile strength decreases and then increases with increase in temperature. • Optimal drying temperature with minimal effect on concrete strength is 100–125 °C. Abstract Dry-based concrete specimens must be obtained to investigate a moisture effect, but the drying law and strength change vary for concrete of different strength grades under different drying temperatures. In this work, concrete specimens with different strength grades are dried in a thermostatic electric drier at constant temperatures of 60, 80, 105, 120 and 150 °C, and the changes in mass, ultrasonic data and strength of the specimens before and after drying are analysed. The test results demonstrate that the drying stages of concrete with different strength grades at different drying temperatures are generally the same even with variations in drying rate, time and the total water mass loss. Considering the drying temperature and strength grade, a model for concrete drying is obtained based on the modified Page model II. After drying, changes in the concrete compressive and splitting tensile strengths of different strength grade materials are highly similar and are slightly higher than those before drying. The relative strength initially decreases and subsequently increases with increasing drying temperature. The drying temperature 100–125 °C is recommended as the optimal drying temperature for different strength grades of concrete according to the minimal strength change. Further analysis shows that the change in concrete strength is the result of mutual competition between the damage caused by water evaporation and the density increase caused by drying. [ABSTRACT FROM AUTHOR]

Pores, voids correspond to inevitable products in the formation of concrete and significantly affect its macroscopic effective strength. A lot of researches show that both porosity and pore structure have significant influence on the concrete strength and differences exist with respect to those influences on the tensile and compressive strengths. In this paper, first, the relationships between the effective tensile and compressive strengths of concrete and porosity are expressed respectively from the proposed simplified center pore model. Second, in order to further consider the influence of the pore structure, the pore size is chosen as the representative index of pore structure and an influence function related to the pore size is proposed based on the correlation analysis. The total influence coefficient of pore size is obtained by combining the pore size distribution function and influence function. Taking the total influence coefficient into account in the relation between the effective strength and porosity, explicit formulae of porous concrete effective tensile and compressive strengths are finally deduced. Through comparing the present approach with a few classical analytical solutions and experimental observations, the results indicate the reliability and accuracy of the proposed approach and the derived equations are simple and convenient to use. Analysis and discussion suggest that the effect of porosity on the compressive strength of concrete exceeds that of the tensile strength and that the effective concrete strength with the same porosity can be improved by decreasing the pore size. [ABSTRACT FROM AUTHOR]