Search

High-performance concrete (HPC) has been used extensively due to its good workability, lasting durability and high strength. However, the low water/cement ratio in HPC generally induces rather high autogenous shrinkage, resulting in high early-age cracking potential. Fly ash (FA) is an effective mineral admixture used to strengthen early-age concrete performance and lower the cracking potential. Although the early mechanical properties of concrete containing FA have been studied, literature on the cracking potential of FA HPC under adiabatic conditions and constant uniaxial restraint is limited. The influence of FA as a partial replacement of cement on the temperature, autogenous shrinkage, restrained stress, tensile creep and cracking potential of HPC was simultaneously investigated in this study. Three groups of large prismatic specimens with different FA replacement ratios (0, 20 and 50%) were tested using a temperature–stress test machine under adiabatic conditions at full restraint. The test results indicated that: autogenous shrinkage, cracking stress and restrained tensile stress rate of the HPC decreased with an increase in FA content; the use of FA as partial replacement of cement increased the ratio of cracking stress to axial tensile strength and the cracking age of the HPC; the specific tensile creep of mixtures with 20% (FA20) and 50% (FA50) replacement with FA was 0·96 and 1·16 times that of the mixture with 0% replacement (FA00), respectively; the integrated criterion used to evaluate the cracking potential decreased with an increase in FA content.

The decrease of internal relative humidity (IRH) in early-age concrete induces autogenous shrinkage. Autogenous shrinkage usually increases the risk of cracking if the concrete is restrained from shrinking freely at an early age. Predicting autogenous shrinkage of early-age concrete requires knowledge of IRH. Therefore, investigation of IRH is fundamental to evaluate cracking resistance. Although investigations of IRH in hardened concrete have been conducted, a prediction model for IRH in early-age concrete that considers the water/cement ratio (w/c) is still lacking. Therefore, experimental studies and analysis of IRH in early-age concrete with different values of w/c were conducted. Results showed that: (i) the IRH in concrete at 28 d after casting increased with increasing w/c under sealed and unsealed conditions; (ii) the average rate of IRH decrease in early-age concrete decreased with increasing w/c; (iii) the critical time of IRH (the length of the water-vapour saturated stage with 100% IRH) in early-age concrete increased with increasing w/c. Models for early-age IRH are proposed in consideration of w/c, critical time and age of concrete under sealed and unsealed conditions.

High-performance concrete (HPC) generally experiences rather high autogenous shrinkage. If restrained, residual tensile stress will be induced which may be sufficient to cause cracking. Ground granulated blast furnace slag (GGBFS) has been utilized as a mineral admixture to reduce shrinkage cracking in HPC. The restrained shrinkage cracking of concrete containing GGBFS appears to be less than that of concrete without slag. However, the data is limited, and the effect of the GGBFS on the restrained cracking resistance needs to be further investigated. Restrained ring tests on cracking resistance of HPC using GGBFS as 0%, 20%, 35%, and 50% by weight replacements of cement were performed. Four concrete mixtures with the same w/b ratio of 0.32 were placed in a 23 ± 1 °C, 60 ± 5% RH environment, and covered with vinyl and aluminum adhesive tape. The outer rings were removed at 24 h after casting to allow water evaporation through the height of the specimen. Test results showed that: (1) the residual tensile stress at the age of 14.25 days after casting decreased by 18.6%, 33.2%, and 40.9% when the amount of GGBFS increased from 0% to 20%, 35%, and 50%, respectively; (2) the stress rate decreased by 58.9%, 62.5%, and 66.8% when the amount of GGBFS increased from 0% to 20%, 35%, and 50%, respectively, indicating that the cracking resistance based on stress rate increased with increasing GGBFS replacement.

High performance concrete (HPC) has been applied in practical engineering for a wide range owing to its superior performances, including low permeability, high strength, high modulus, and other superior performance. However, with the high internal temperature and self-desiccation that induced by the low water-to-binder (w/b) ratio, HPC suffers high autogenous shrinkage, which leads to the premature cracking of HPC after peak stress at early age. For the purpose of increasing the cracking resistance of concrete effectively, Barchip fibers and the internal curing materials are used to improve the early-age properties of concrete. Although researches on the cracking resistance of HPC reinforced with different contents of Barchip fiber or internally cured with different kinds of internal curing materials have been conducted, studies on the effect of Barchip fiber length on the cracking resistance of internally cured HPC (ICHPC) with super absorbent polymers (SAPs) at early age are rather lacking. In present research, the effect of length of Barchip fibers (0, 42, 54, and 60 mm) on the early-age cracking resistance of ICHPC under the adiabatic condition was studied by Temperature Stress Test Machine. Results of the experimental research and related analysis indicated that (1) the splitting tensile strength and compressive strength of HPC increased as the Barchip fibers were applied; (2) the use of SAPs reduced autogenous shrinkage, restrained tensile stress rate, and tensile creep behavior, and increased temperature drop and the cracking resistance of HPC at early age; (3) the autogenous shrinkage decreased and the temperature drop, cracking age, cracking stress of ICHPC increased as the Barchip fiber length increased; (4) the tensile creep behavior of ICHPC increased as the Barchip fibers were applied, and decreased as the Barchip fiber length increased; (5) the cracking resistance of ICHPC increased first and then decreased as the Barchip fiber length increased, as obtained from the integrated criterion; (6) the poor dispersion of Barchip fibers may result in the decrease of the early age behavior and cracking resistance of ICHPC when the fiber length exceeded 54 mm.

In underwater radiated noise measurement using a vertical linear array, a diagonalization-decorrelation-based diagonal denoising method is proposed to improve the denoising effect for spatially correlated noise. Firstly, the ambient noise cross-spectral matrix is measured without the radiated noise source. Subsequently, the eigenvector matrix of the ambient noise cross-spectral matrix is utilized to implement a unitary transformation for the received data, which eliminates the correlation of the received noise and transforms the received noise cross-spectral matrix into a diagonal matrix, then the noise components are removed by diagonal denoising. Finally, the denoised cross-spectral matrix is used to estimate the power of the radiated noise by beamforming. Consequently, the influence of spatially correlated noise on radiated noise measurement is reduced. The effectiveness of the proposed method is validated and compared with the diagonal denoising method and the whitening-decorrelation-based diagonal denoising method via numerical simulations and experimental data. Under the ideal condition, the noise reduction performances of the proposed method and the whitening-decorrelation-based diagonal denoising method are equal and better than that of the diagonal denoising method. In practice, the number of snapshots is limited, so there is an inevitable mismatch between the ambient noise cross-spectral matrix and the received noise cross-spectral matrix due to the randomness of noise. The mismatch results in imperfect whitening and diagonalization, which reduces the denoising effect. However, the simulation results indicate that the proposed method still reduces more correlated noise and has a better performance on underwater radiated noise measurement compared with the diagonal denoising method and the whitening-decorrelation-based diagonal denoising method even if the number of snapshots is finite.

High performance concrete (HPC), which is a new-tech concrete, has been widely applied for its good workability, lasting durability, low permeability, and high strength. However, the high temperature rise and high autogenous shrinkage induced by the low water-to-cement (w/c) ratio would increase the early-age cracking potential in HPC. In order to reduce the cracking potential of HPC, Barchip fibers are applied to strengthen the early-age properties. Although the influence of Barchip fibers on the mechanical properties of concrete has been studied, investigations on the influence of Barchip fibers on the early-age cracking potential of HPC under adiabatic condition at uniaxial constant restraint degree are still lacking. The early-age cracking potential of HPC reinforced with different amounts of Barchip fibers (0, 4, 8, and 12 kg/m3) was tested by Temperature Stress Test Machine under adiabatic condition at full restraint degree in present study. Test results and corresponding analysis indicated that (1) the compressive strength and splitting tensile strength were enhanced by the addition of Barchip fibers in HPC; (2) Barchip fibers reduced autogenous shrinkage, restrained tensile stress rate and increased cracking age, specific tensile creep of early-age HPC; (3) the integrated criterion utilized to evaluate the cracking potential decreased first and then increased with increasing amount of Barchip fibers; (4) the optimal Barchip fiber amount of 8 kg/m3 was recommended for that poor dispersion and fiber clumping may occur in HPC with excessive addition of Barchip fibers.

The aerothermoelastic properties of composite laminated trapezoidal panels considering the compressibility of supersonic airflow and shock wave are studied. Due to the irregular configuration of composite laminated trapezoidal panel and complex thermal fluid–structure interaction, it is impossible to solve this problem by analytical methods. An effective finite element method (FEM) is developed to establish temperature equation and aerothermoelastic equation of motion of the complex trapezoidal panel. The first order piston theory is employed to evaluate the aerodynamic pressure. Based on the developed FEM, the thermal diffusion equation and the Galerkin’s method are applied to establish the temperature equation of the panel, and the complex temperature distribution of the panel is obtained. Combining the thermal and aeroelastic models, the aerothermoelastic equation of motion of the composite trapezoidal panel is established using Hamilton’s principle. Through the numerical analyses, some useful and significant results are obtained, and the advantages and disadvantages of various parameters of the panels are summarized. Because of the aerodynamic heating under the supersonic airflow, the buckling of the panel caused by thermal effect and the flutter caused by supersonic airflow are discussed considering the influences of various parameters. The interaction of the thermal and aerodynamic forces is also analyzed.

High performance concrete (HPC) has been extensively applied in practice. However, the water-to-cement (w/c) ratio of this concrete is low, high temperature rise and high self-desiccation will occur, and both of which can increase the cracking potential of HPC at early age and then decrease the service life of concrete structures. Therefore, super absorbent polymers (SAPs) are applied in HPC as an internal curing (IC) agent. Although the autogenous shrinkage, relative humidity, and stress relaxation of concrete internally cured with SAPs have been studied, investigations on the influence of SAPs on the tensile creep and cracking potential of HPC are still lacking. In present study, the influence of SAPs as an IC agent on the temperature, autogenous shrinkage, restrained stress, basic tensile creep, and cracking potential of HPC were simultaneously studied by Temperature Stress Test Machine. Test results and corresponding analysis showed that: (1) the adiabatic temperature rise of HPC was 27.6, 29.3, 31.0, and 34.9 °C and increased with the increase of amount of SAPs; (2) the rate of restrained tensile stress of HPC was 1.7, 1.5, 1.4, and 1.2 MPa/day and decreased with the increase of amount of SAPs; (3) the specific basic tensile creep of HPC at the age when the restrained specimen of mixture SAP-0 cracked was 45, 23, 13, and 7 μe/MPa and decreased with the increase of amount of SAPs; (4) a model for predicting the basic tensile creep compliance function of HPC considering the influence of amount of SAPs was proposed; (5) the cracking potential of HPC which was based on the integrated criterion decreased with the increase of amount of SAPs.

With the widely use of high-performance concrete, the lower and lower water-to-cement (w/c) ratio is applied in practice. This low w/c ratio can offer high strength and low permeability, however, coming with other drawbacks, such as high self-desiccation and high temperature rise, both of which can increase the early-age cracking potential of concrete. Creep is important to evaluate the early-age cracking resistance of concrete. Although studies on the influence of w/c ratio on the creep of mature concrete have been conducted, investigations on the influence of w/c ratio on the early-age tensile creep remain lacking. Therefore, investigations on the influence of w/c ratio on the tensile creep and cracking resistance of early-age concrete must be conducted. Experimental studies on the influence of w/c ratio (0.33, 0.40, and 0.50) on the temperature change, autogenous shrinkage, restrained stress, tensile creep, and cracking resistance of early-age concrete under adiabatic condition and at full restraint degree were conducted using Temperature Stress Test Machine. The test results indicate that (1) the temperature rise under adiabatic condition increased by 32.6%, and 74.2% when w/c ratio decreased from 0.50 to 0.40, and 0.33, respectively; (2) the restrained tensile stress rate under adiabatic condition and at full restraint degree increased by 16.7%, and 33.3% when w/c ratio decreased from 0.50 to 0.40, and 0.33, respectively; (3) the early-age specific basic tensile creep and basic tensile creep/shrinkage under adiabatic condition and at full restraint degree increased with the decrease of w/c ratio; (4) the early-age cracking potential under adiabatic condition and at full restraint degree increased with the decrease of w/c ratio according to the integrated criterion.

High-performance concrete (HPC) is widely used in practice due to its potential long-term benefits, such as high strength and low permeability. However, high self-desiccation and high temperature rise occur due to the low water-to-cement ( w / c ) ratio of HPC, both of which would increase the cracking potential of concrete at early age. Although the creep and cracking potential of early-age HPC have been investigated, studies on the tensile creep and cracking potential of internally cured concrete with pre-wetted lightweight aggregates (LWAs) at early age under adiabatic condition at various w / c ratios remain lacking. In present study, the tensile creep and cracking potential of concrete at early age were experimentally investigated under adiabatic condition using the temperature stress test machine. Test results and corresponding analysis showed that: (1) a model for predicting the compressive strength of concrete was presented in consideration of the influence of pre-wetted LWAs; (2) the adding of pre-wetted LWAs reduced the autogenous shrinkage of concrete with different basic w / c ratios; (3) the basic tensile creep/shrinkage and absolute value of basic tensile creep of internally cured concrete were lower than that of normal concrete when the basic w / c ratios were the same; (4) the basic tensile creep/shrinkage and absolute value of specific basic tensile creep of normal and internally cured concrete at the age of cracking both increased with the decrease of basic w / c ratio; (5) the cracking potential of normal and internally cured concrete both increased with the decrease of basic w / c ratio and the adding of pre-wetted LWAs reduced the cracking potential.

The early age cracking of concrete is a persistent problem for structures. Internal curing was developed to mitigate the shrinkage of low w/c concrete in recent decades by introducing additional water into concrete. Although shrinkage development and the early age cracking resistance of internally cured concrete under restraint were investigated, result on stress relaxation of internally cured concrete with super absorbent polymers (SAPs) by ring test is still lacking. Tests on the cracking resistance of concrete with different amounts of SAPs (0%, 0.05%, 0.16%, and 0.26% by weight of cement) were conducted using restrained ring test in present study. Test results showed that: (1) the strain in the restrained steel ring decreased with the increase of amount of SAPs; (2) the residual stress of concrete ring decreased with the increase of amount of SAPs; (3) the stress rate decreased with the increase of amount of SAPs; (4) the relaxed stress increased with the increase of amount of SAPs after the initiation of drying; and (5) the time to cracking for concrete increased with the increase of amount of SAPs.

Nonlinear vibration characteristics of composite laminated trapezoidal plates are studied. The geometric nonlinearity of the plate based on the von Karman's large deformation theory is considered, and the finite element method (FEM) is proposed for the present nonlinear modeling. Hamilton's principle is used to establish the equation of motion of every element, and through assembling entire elements of the trapezoidal plate, the equation of motion of the composite laminated trapezoidal plate is established. The nonlinear static property and nonlinear vibration frequency ratios of the composite laminated rectangular plate are analyzed to verify the validity and correctness of the present methodology by comparing with the results published in the open literatures. Moreover, the effects of the ply angle and the length-high ratio on the nonlinear vibration frequency ratios of the composite laminated trapezoidal plates are discussed, and the frequency-response curves are analyzed for the different ply angles and harmonic excitation forces.

High strength concrete (HSC) is used extensively in practice. However, HSC is prone to cracking at early age, which can be the first step in a construction’s deterioration and its malfunction. Reinforcement is one possible way to prevent the negative effect of early-age cracks in HSC structures. The early-age bond behavior is necessary to determine the cracking width of structures. Although the bond behavior between steel bars and normal strength concrete has been studied, study on early-age bond behavior between steel bars and HSC is still lacking. This paper presents an experimental investigation on the bond behavior between steel bars and HSC of different ages using a pull-out test. Test results showed that: (1) the early-age bond strength between steel bars and HSC increased with the increase of age; (2) the bond strength between steel bars and HSC increased with the increase of concrete strength and a model for the early-age bond strength between steel bars and HSC was proposed; (3) the slip corresponding to bond strength decreased with the increase of concrete compressive strength and a model for the early-age slip corresponding to bond strength was proposed; (4) a prediction model for early-age bond stress–slip relationship between steel bars and HSC was proposed based on BPE model, which showed good agreement with test results.

High-performance concrete (HPC) is used extensively in practice. However, this type of concrete has a low water-to-cement ( w / c ) ratio, which causes self-desiccation and leads to a rather high autogenous shrinkage (AS). AS then increases the risk of cracking if the concrete is restrained from shrinking freely at early age. Internal curing (IC) has been extensively used in reducing AS and consequently mitigating the high risk of early-age cracking of HPC. Super absorbent polymers (SAPs) can supply additional IC water for hydration of concrete, thereby counteracting the effects of self-desiccation. Although experimental studies were conducted to mitigate the AS of internally cured concrete with SAP, research on the relationship between the AS of concrete and the amount of IC water provided by SAP is still lacking. Thus, an experimental study on the effect of IC on the early age expansion, AS development, AS rate, and IC efficiency of concrete with SAP was conducted. Test results showed that (1) the early-age expansion occurred obviously prior to the first day for the internally cured concrete, and the maximum expansion increased with the increase of IC water; (2) the ultimate AS at 28 days and AS rate of internally cured concrete decreased with the increase of the amount of IC water; (3) the proposed model to estimate the AS of internally cured concrete with SAP considering the early-age expansion and amount of IC water indicated good correspondence with the experimental results; and (4) the IC efficiency of SAP decreased with the increase of IC water in concrete.

With the continuous progress of concrete technology, concrete with the low water-to-cement (w/c) ratio is utilized extensively. Concrete suffers significant shrinkage due to the high self-desiccation at early age. When shrinkage of concrete is hindered by the restraint, the tensile stress will develop and cause the generation of cracks. For reducing the shrinkage, internal curing (IC) has been employed in concrete by introducing the additional water which can be preserved in pre-wetted lightweight aggregates (LWAs). Although a number of studies focus on the results from autogenous and drying shrinkage, the result of the effect of IC with LWAs on the performance of concrete with low w/c ratio is still limited. In present study, the investigations on the stress relaxation and time to cracking of concrete with the different proportions of pre-wetted LWAs (0%, 10%, 30%, and 50% by the volume replacement of normal weight coarse aggregates) were conducted by the restrained ring test. Results obtained from tests were shown as follows: (1) the strain of steel ring decreased with the increase of the proportion of pre-wetted LWAs; (2) the maximum residual stress of concrete mixture decreased with the increase of the proportion of pre-wetted LWAs; (3) the relaxed stress of concrete mixture decreased with the increase of the proportion of pre-wetted LWAs at the different age; (4) the stress rate of concrete mixture decreased with the increase of the proportion of pre-wetted LWAs; (5) although the concrete mixture with the 50% proportion of pre-wetted LWAs cracked at the earliest age, the time to cracking was postponed when the proportion of pre-wetted LWAs increased from 0% to 30%.

Fibers reinforcement which is an effective mean to improve the cracking resistance of high performance concrete (HPC) is widely applied in practice. The early-age cracking resistance is improved by the addition of polypropylene plastic fibers, which is also affected by the length of fibers. Although investigations on the effect of content of polypropylene plastic fibers on the properties of HPC were carried out, investigations on the effect of length of polypropylene plastic fibers on the cracking resistance of HPC remain lacking. In present study, the effect of length of polypropylene plastic fibers on the early-age properties of HPC was investigated by Temperature Stress Test Machine. The test results and corresponding analysis indicated that: (1) the increasing length of polypropylene plastic fibers reduced the temperature drop at cracking time of HPC; (2) the increasing length of polypropylene plastic fibers reduced the absolute value of autogenous shrinkage of HPC; (3) the increasing length of polypropylene plastic fibers reduced the ratio of cracking stress to axial tensile strength and stress reserve of HPC; (4) the increasing length of polypropylene plastic fibers reduced the specific tensile creep of HPC; (5) the increasing length of polypropylene plastic fibers reduced the cracking resistance of HPC.

A unique bio-inspired multi-joint leg-like or limb-like vibration isolation structure is studied by mimicking the skeleton and joint structures of animal legs, and its advantage in passive vibration isolation is systematically investigated. This bio-inspired structure uses several small quadrilateral structures composed by short adjustable rods and a transverse spring to simulate the multi-articulation of animal legs and adopts long rods to simulate the skeleton. The equivalent static stiffness property, loading capacity and dynamic vibration isolation performance of the structure are systematically studied. It is shown that the nonlinear stiffness characteristics of the structure can lead to very excellent vibration isolation capability in the low-frequency band. Different from many existing quasi-zero-stiffness (QZS) isolators in the literature, this novel anti-vibration structure can demonstrate superior high static but low dynamic stiffness (HSLDS) property and thus excellent vibration isolation performance, subject to large vibration amplitude. Through adjusting proper structural parameters including the rod-length, assembly angle, spring stiffness and layer number according to the influence of the structural parameters on the nonlinear stiffness property, the new structure can have the optimal vibration isolation performance without sacrificing its loading capacity to meet actual requirements. Especially, the length of long rods which is used to simulate the skeleton can be adjusted to obtain better vibration isolation performance than adjusting other parameters, followed by the adjustment of the parameters of small quadrilateral structures. The analysis of the influence of structural parameters on the vibration isolation performance shows that the symmetry between different layers has an important role in maintaining the characteristic of the HSLDS. The comparison with other existing vibration isolation structures of the QZS property also indicates that, the new vibration isolation structure can have a better vibration isolation performance with relatively a smaller size of the overall structure. Experiments are successfully conducted and validated the beneficial nonlinear properties of the structure. This novelbio-inspired anti-vibration structure would have great advantages in practical engineering applications.

In recent years, high performance concrete (HPC) has been fully developed and put into widespread use in the actual project. Ground granulated blast furnace slag (GGBFS) has been widely used in HPC as a mineral admixture to improve the comprehensive workability. Nowadays, Temperature Stress Test Machine (TSTM) has been used to investigate the cracking potential of concrete. The influence of GGBFS on the early-age cracking potential of HPC has been investigated. However, investigations on the influence of GGBFS on the early-age cracking potential of HPC under adiabatic condition at full uniaxial restraint degree by using TSTM remain lacking. Investigations on the early-age cracking potential of HPC with different GGBFS contents (0%, 20%, 35%, and 50%) under adiabatic condition at full uniaxial restraint degree by using TSTM were conducted in the present study. The experimental results and related analysis indicated that (1) the addition of GGBFS reduced early-age temperature rise, temperature drop, cracking stress, cracking temperature, and stress reserve of HPC; (2) the autogenous shrinkage and integrated criterion of HPC increased with the increasing GGBFS content at early age; (3) the early-age cracking potential of HPC increased with the increasing GGBFS content, and the optimal GGBFS content shall not exceed 20% in the present study.

High-performance concrete (HPC) is widely used in practice. The water-to-cement ratio of HPC is low, and self-desiccation occurs which will induce marked autogenous shrinkage. Autogenous shrinkage usually increases the risk of cracking if the concrete is restrained from shrinking freely at early age. The autogenous shrinkage and cracking resistance of early-age concrete is influenced by curing temperature. However, the effect of curing temperature on autogenous shrinkage of early-age concrete is not in consistency and how the curing temperature affects the cracking resistance of concrete remains lacking. Thus, investigation on the effect of curing temperature on cracking resistance of early age concrete must be further studied. In this study, experimental studies on early-age cracking of concrete under 100% restraint and different curing temperatures were carried out using Temperature Stress Test Machine (TSTM). The present study investigated autogenous shrinkage of early-age HPC cured at different curing temperatures. The experimental results indicate that (1) the ratios of cracking stress to tensile strength for HPC specimens were all lower than 1.0; (2) the autogenous shrinkage of HPC increased with the increase of curing temperature; (3) a prediction model for autogenous shrinkage of HPC was presented considering the effect of curing temperature; (4) cracking temperatures and stress reserves were selected as the main cracking evaluation indicators of TSTM, and the HPC specimen cured at isothermal 20 °C showed better cracking resistance than that at isothermal 45 °C and adiabatic condition.

High-performance concrete (HPC) is widely used in practice. The water-to-cement ratio of HPC is low, and self-desiccation and high temperature rise occur. Internal curing (IC) with prewetted lightweight aggregates (LWAs) is applied to enhance the early-age behavior of concrete structures in terms of temperature, shrinkage, creep deformation, and stress, thus resulting in low cracking potential. Although tests on the effect of prewetted LWAs on the cracking potential of concrete under semi-adiabatic or isothermal conditions have been conducted, studies on the influence of the amount of prewetted LWAs on the cracking potential of internally cured concrete under adiabatic conditions remain lacking. In this study, the cracking potential of internally cured concrete with prewetted LWAs was experimentally investigated with a temperature stress testing machine. The effect of prewetted LWAs in reducing the cracking potential in HPC was investigated with different amounts of LWAs (0%, 10%, 30%, and 50%) in four pairs of large prismatic HPC specimens tested simultaneously under free and fully restrained shrinkage. Test results showed that (1) both temperature rise and drop of concrete occurred as prewetted LWAs increased; (2) the autogenous shrinkage and cracking stress of concrete decreased with the increase in the amount of prewetted LWAs; (3) the tensile stress rate of concrete with prewetted LWAs decreased; (4) the ratio of cracking stress to the tensile strength of concrete increased with the increase in the amount of prewetted LWAs; (5) the cracking age of concrete with the LWAs increased; (6) the specific tensile creep of concrete with prewetted LWAs decreased; and (7) the cracking potential of concrete with prewetted LWAs was reduced, as obtained from the integrated criterion. Internally cured concrete with prewetted clay LWAs is more robust for construction at early ages.

High-performance concrete is used extensively in practice. However, the water-to-cement ratio of this concrete is low and causes self-desiccation. Due to moisture diffusion and this self-desiccation, internal relative humidity (IRH) decreases in concrete structures that are exposed to ambient air and induces autogenous shrinkage. Internal curing with super absorbent polymer (SAP) is used to mitigate this shrinkage in high-performance concrete; nonetheless, IRH variation influences changes in the autogenous shrinkage of concrete with SAPs. Although tests have been performed to mitigate the autogenous shrinkage of concrete that was internally cured through SAPs, experimental study on IRH variation in such concrete remains lacking. Thus, such a study was conducted to determine the effect of internal curing with SAPs on the IRH of early-age concrete under sealed and unsealed conditions, as detailed in this paper. Test results showed that: (1) the IRH of concrete that was internally cured with SAPs increased with an increase in the content of internal curing water 28 days after casting under sealed and unsealed conditions. (2) The degree of decrease in the IRH of internally cured concrete was lower under sealed conditions than under unsealed conditions 28 days after casting. (3) The critical time of the IRH of internally cured concrete increased with increases in the content of internal curing water under both sealed and unsealed conditions. (4) The decrease rate of the IRH in early-age concrete that was internally cured with SAPs dropped with an increase in the content of internal curing water under sealed and unsealed conditions. (5) A formula was proposed to calculate the IRH of early-age concrete that was internally cured with SAPs in consideration of the content of internal curing water under unsealed conditions. This formula indicates the good accuracy of the experimental results.

A comparative study is conducted on a class of typical X-shaped structures to explore critical design parameters in passive vibration isolation. Several benchmark X-shaped structures are therefore studied, considering the effects of several critical structural parameters on potential vibration isolation performance in different cases. Theoretical calculations, finite element simulations and experimental results show that this class of X-shaped structures in vibration isolation is critically affected by the ratio between the horizontal displacement in the installed spring and the vertical displacement of the structure, which is further nonlinearly dependent on the values of other structure parameters. This critical design parameter can act as a convenient optimization objective for selecting structural parameters in various practical applications. The results of this study thus present a new understanding of the nonlinear dynamics of the X-shaped structures which can facilitate the design or parametric selection of these structures in practices.