Rebar debonding is one of the most common failure modes in reinforced concrete (RC) structures. In this paper, an effective method for identifying rebar debonding with concrete surface-bonded smart piezoelectric module (SPM) networks is proposed to address the non-destructive testing needs for in-service RC structures. The nonlinear harmonic energy entropy, calculated by the wavelet packet decomposition (WPD), is introduced as the damage indicator. The effectiveness of the identification method is validated through the numerical finite element analysis and experimental trials. The research results indicate that using the nonlinear harmonic energy entropy as the damage indicator can effectively achieve the detection, localization, and quantification of rebar debonding, achieving all Dice similarity coefficient (DSC) values exceeding the reliability threshold of 0.7. The selection of decomposition layer number and the wavelet basis functions can affect the amplitude and distribution discreteness degree of the nonlinear harmonic energy entropy, causing up to a 2-fold variation. However, these factors do not induce alterations in the distribution laws. A reasonable selection of the component range can effectively distinguish between the healthy and debonding specimens, which shows a significant difference in distribution variance, representing a greater than 1.5-fold change. From the perspective of the overlap degree, the evaluation results of DSC >= 0.8 confirm the high spatial accuracy of the proposed method in rebar debonding identification. The key innovation of this study lies in the integration of a novel nonlinear harmonic energy entropy indicator with a practical concrete surface-bonded SPM network, realizing the identification of rebar debonding for in-service RC structures.

Insulated Concrete Sandwich Panels (ICSPs) have been widely applied in engineering due to their excellent thermal insulation, vibration isolation, and mechanical properties, where the upper and lower concrete wythes are connected by shear connectors to achieve composite action. The degree of composite action (DCA) of ICSPs is primarily governed by the shear stiffness of the connectors. Most existing studies enhance the composite action of ICSPs by increasing the usage of connectors, which in turn degrades the panels' thermal insulation and vibration isolation performance due to the bridging effects. To address this issue, this study investigates the effectiveness of gradient-distributed connectors along the span direction to enhance composite action without increasing connector usage. An analytical model capable of considering non-uniformly distributed connector stiffness is developed to predict interlayer slip and structural deflection under bending conditions. The model is validated through both finite element (FE) simulations and comparison with existing experimental results. The FE and analytical results demonstrate excellent agreement and confirm that a gradient distributed stiffness, where stiffness increases from midspan toward the ends, significantly improves DCA. Under the test conditions, a linear gradient in connector stiffness increases the DCA by 22.35% compared to a uniform connector distribution. Parametric studies based on the analytical model further examines the effect of different parameters, including average stiffness of connectors Kave, concrete strength, the ratio of the interlayer spacing to the thickness of a single concrete wythe C'/t, and the length-to-width aspect ratio of the ICSPs L/b. Comparisons among linear, quadratic, and constant stiffness indicate that gradient distributions of connector stiffness consistently increases the DCA of the ICSPs. The most significant improvements are observed with low Kave, C'/t, and L/b, while quadratic distribution in connector stiffness gives higher DCA than linear ones. The enhancement effect diminishes as the structure approaches full composite action. The proposed analytical model offers a practical tool for optimizing connector layouts in ICSP design, enabling mechanical efficiency while maintaining insulation performance.

Effective identification of rebar debonding is crucial for health monitoring of reinforced concrete (RC) structures. In this article, a method for detection, localization, and quantification of interfacial debonding based on nonlinear harmonic energy entropy and using a combined surface-bonded and rebar end-mounted smart piezoelectric module system is proposed. The distribution of complex stress wave scattering field and the contact acoustic nonlinear phenomenon in the rebar debonding region are investigated by combining the numerical finite element analysis and experimental test, which reveal the energy distribution characteristics of nonlinear harmonics. The rebar debonding in RC structures is for the first time identified using the energy entropy as the damage indicator. The changing law of nonlinear harmonic energy entropy at different debonding locations is utilized to perform the identification of interfacial debonding, and at the same time, it eliminates the interference in the traditional wavelet packet analysis caused by the location of sensors. The numerical and experimental verifications of four RC beam specimens with different interfacial debonding lengths demonstrate the effectiveness of the present rebar debonding identification method. The findings from the combined numerical and experimental study manifest the potential of proposed rebar debonding identification method for practical applications.

Based on the key problem of insufficient durability of sensors in the health monitoring of concrete structures under freeze-thaw (F-T) cycle environments, a kind of encapsulated smart piezoelectric module (ESPM) suitable for concrete surface bonding and sensing is proposed, and the performance analysis, durability evaluation, and interface debonding monitoring of ESPM-concrete substrate assembly are systematically studied. Through multiple groups of comparative experiments combined with the electromechanical impedance (EMI) technology, the effects of the matching layer and the bonding layer on the performance of ESPMs are clarified, and the evolution mechanism of the impedance characteristics of ESPMs under F-T actions is revealed. The effects of three key factors (i.e., the self-aging of the sensor, the deterioration of the concrete substrate, and the degradation of the interface bond) on the EMI signals are experimentally isolated and independently quantified using the variable separation method. A novel application-oriented non-destructive identification method for ESPMconcrete interface debonding based on the changes of the impedance curves is proposed, and the long-term monitoring of interface degradation degree between ESPM-concrete bond is realized by combining the statistical parameters. The present study verifies the key role of a reasonable encapsulation structure in improving the service reliability of PZT sensors in harsh environments and expands the application range of the EMI technology in the field of monitoring the sensor-substrate interface bonding state. The theoretical support and technical assurance for reliability design and performance evaluation of long-term health monitoring systems for concrete infrastructure in cold regions are provided.

Insulated concrete sandwich panels (ICSPs) are widely utilized in modern building structures due to their excellent combination of energy efficiency and structural load-bearing capacity. However, compared to their mechanical and thermal properties, the sound insulation characteristics of ICSPs remain insufficiently studied, presenting a scientific deficit. In practical engineering, insufficient consideration of these acoustic properties—particularly the “acoustic bridging” induced by connectors—often leads to unpredictable noise transmission, making it difficult for building envelopes to meet stringent modern acoustic codes. To further investigate their acoustic characteristics, this paper extends existing theories on infinite periodic ICSPs to study the airborne sound insulation performance of finite-sized ICSPs. First, analytical models for ICSPs under simply supported on all edges (SS) and clamped on all edges (CC) boundary conditions are derived, wherein the connectors are equivalently modeled as elastic media and discrete elastic springs, respectively. Subsequently, the accuracy and applicability of the analytical models are verified through finite element (FE) models and an airborne sound insulation experiment. Finally, based on the analytical models, a parametric study is conducted to explore the effects of the stiffness of connectors, boundary conditions, and the thickness of the core layer on the sound insulation performance of the ICSPs. The results indicate that connector stiffness has a non-monotonic influence on the sound insulation performance of ICSPs. As the connector stiffness increases, the Rw first decreases and then increases, and the sound insulation performance gradually stabilizes when the connector stiffness becomes sufficiently high. Boundary conditions have a significant effect on the acoustic response. For the reference ICSPs, changing the boundary condition from SS to CC increases the Rw from 49 dB to 62 dB, corresponding to an increment of 13 dB and an approximately 95.0% reduction in the equivalent sound transmission coefficient. When the total panel thickness is kept constant, reducing the core layer thickness from 80 mm to 40 mm increases the Rw from 49 dB to 55 dB under SS boundary conditions and from 62 dB to 66 dB under CC boundary conditions, corresponding to increments of 6 dB and 4 dB, respectively. These improvements are equivalent to reductions of approximately 74.9% and 60.2% in the sound transmission coefficient, though this must be weighed against the inevitable reduction in thermal insulation capacity. Although the sound insulation performance of ICSPs is inferior to that of solid concrete panels (SCPs) of equivalent thickness, with reasonable parameter optimization, their sound insulation indices can significantly exceed the latest requirements of current building codes. By fully accounting for boundary effects in practical engineering, this study provides an analytical basis for the acoustic performance prediction and engineering-oriented optimization of finite-sized ICSPs.

Extensive studies have been conducted on surface waves in saturated soils with periodic pile barriers, which are mainly focused on analytical or numerical analyses. This paper presents an experimental study on vibration isolation performance of periodic pile barriers in saturated soils, based on a comprehensive testing program consisting of four groups, with four configurations in each group. The effects of four key parameters, including Young’s modulus, periodic parameter, structural configuration, and embedment depth, were considered. Based on the experimental results, it can be concluded that the bandwidth and central frequency of surface wave attenuation zones (SWAZs) are highly sensitive to the pile configurations and materials, where a denser arrangement broadens the high-frequency bandwidth and a specific shallow embedment enhances low-frequency isolation via coupled Bragg scattering and rocking resonance. While hollow PVC piles exhibit narrower SWAZs than solid concrete piles, they offer a cost-effective alternative for targeted frequency bands. Therefore, by rationally designing piles to tailor the attenuation domain, surface wave propagation can be effectively suppressed. Furthermore, the measured attenuation domains show good correlations with analytical predictions based on Biot’s theory, validating the accuracy of existing analytical models for saturated soils. This work can advance the engineering applications of periodic structures in vibration isolation.

Multi-ribbed wall structures (MRWS) represent an innovative prefabricated building technology that has seen increasing applications in recent years. However, the fire resistance of its primary component, the multi-ribbed composite wall (MRCW), remains insufficiently studied. This study first develops an analytical model for predicting the axial compressive capacity of MRCW under single-sided fire conditions, based on thermodynamic principles and limit equilibrium theory. The model incorporates the degradation of material strength at elevated temperatures and accounts for fire-induced load eccentricity. Next, a single-sided fire test was conducted on an MRCW specimen under a load ratio of 0.5, revealing a fire resistance limit of 181 min and a failure mode characterized by combined compression and bending. Additionally, a finite element analysis (FEA) model is established using a sequentially coupled thermal-stress method to simulate the fire performance of MRCW. Both the analytical and FEA models are validated against experimental results, demonstrating good agreement in predicting temperature distribution and load-bearing capacity. A series of FEA are then performed to investigate the fire resistance performance of MRCW under single-sided fire conditions. The FEA results obtained further validate the accuracy and reliability of the established analytical model. Finally, the analytical model is employed to analyze the key parameters influencing the residual bearing capacity coefficient of MRCW under fire conditions. It is found that increasing the compressive strength of autoclaved aerated concrete blocks, the concrete cover thickness and the thickness of the wall can effectively delay the degradation of the residual bearing capacity of MRCW during fire exposure. Conversely, a higher reinforcement ratio and increased wall height tend to negatively affect the residual bearing capacity under fire conditions.

The transient temperature field of reinforced concrete members under fire is critical for evaluating their fire resistance. Current methods largely depend on empirical or simplified equations derived from fitting experimental and finite element analysis (FEA) data. However, such equations often lack a sound physical basis and fail to adequately represent the influence of key parameters. Furthermore, existing analytical models typically assume constant thermal properties of concrete and are limited only to single-sided fire exposure, without accounting for double-sided fire conditions, which constrains their practical applicability and predictive reliability. To overcome these shortcomings, this study aims to establish an analytical model grounded in physical principles, capable of accurately predicting the transient temperature field in reinforced concrete walls and slabs under both single-sided and double-sided fire conditions. The proposed approach is based on heat conduction theory and partitioning the wall thickness into a high-temperature region and a low-temperature region. It incorporates the reduction of thermal conductivity in the high-temperature region and accounts for heat transfer between the wall surfaces and the external environment, thereby circumventing the need to directly solve nonlinear heat transfer equations. The accuracy of the model is verified through comparisons with existing experimental data and FEA results. Additionally, a parametric analysis using the model is carried out, which suggests that a reinforced concrete wall should have a minimum thickness of 150 mm to avoid failure due to thermal insulation loss in a fire. The analytical model presented here offers theoretical support and practical reference for the thermal response design and fire resistance assessment of reinforced concrete walls and slabs.

Concrete sandwich panels have been extensively used due to their cost-effective and multi-functional capacities. Building on this concept, concrete sandwich shells were developed, which extends their applications to curved structures. The objective of this paper is to study the mechanical performance of prototype concrete sandwich shells. First, full-size concrete sandwich and solid shells with the same dimensions were manufactured and tested using four-point bending test, considering both simple support and axially restrained support boundary conditions. Next, Finite Element (FE) models are constructed, and good correlations can be observed between experimental and FE results. It can be concluded that, before concrete cracks, the degree of composite action between the sandwich shell and the solid shell is similar. Under the simply-supported boundary condition, the existence of the connectors in the sandwich shell leads to an increased reinforcement ratio, resulting in a higher ultimate load (936 kN) compared with that of the solid shell (580 kN). Under axially restrained support boundary condition, the ultimate load of the sandwich shell (1847 kN) is lower than that of the solid shell (3505 kN), due to the significant axial force. The solid shell, which has a thicker concrete wythe, exhibits higher bending capacity. Finally, the FE model is used to conduct a parametric study on the sandwich shell. It is found that, as the number of the rows of connectors increases, the ultimate load of the shells increases. The load-bearing capacity of the sandwich shell with the thickness from 130 mm to 140 mm is similar, which tends to be stable when the thickness of the insulation is less than 120 mm. Based on this study, it can be shown that, when properly designed, the sandwich shells have enough capacities to be used as structure members, but with lower weight compared with that of solid shells.

Concrete sandwich shell (CSS) is a new type of freeform single-curved sandwich structure, which can provide multi-functionality of loading-carrying, insulation, and vibration reduction. It consists of exterior layers enclosing a functional layer connected together using shear connectors. Truss shear connectors have received extensive attention due to their excellent properties, but can only be used in flat sandwich panels. This paper proposes a closed form solution to predict the stiffness of CSS with curved truss connectors. To this end, an analytical model is constructed to calculate the equivalent shear modulus (Geq) using the energy method, based on which the shear stiffness of the curved truss connectors is obtained. Next, the accuracy of the analytical model is validated using prior experimental test and Finite Element models, showing the feasibility of using curved truss connectors for CSSs. Finally, parametric study is conducted using the analytical model to investigate the effect of vertex angle on Geq, based on which the optimal vertex angle for different configurations of the CSSs are obtained. The analytical model and optimization method proposed in this study can assist in designing optimal truss connectors for CSSs with different dimensions, resulting in more cost-effective structures.

The interface bonding properties of bi-material structures are influenced by a variety of factors, including surface roughness, presence of interface agents, moisture content of surface, etc., and they are critical factors in determining the overall performance and durability of structures. Interface between dissimilar materials exhibits heterogeneity and discontinuity. In this study, four types of interface agents (i.e., epoxy resin, nano-epoxy resin, high-strength mortar, and fiber-reinforced mortar) are proposed to investigate the multiscale effect and bonding mechanism of interface heterogeneity and continuity on the bond performance between hybrid recycled tire steel fiber reinforced concrete or ultra high performance concrete (UHPC) and normal strength concrete (NC). The multiscale analyses include the macroscopic (i.e., surface morphology, tensile bond strength, and shear bond strength), mesoscopic (i.e., fiber distribution, size of interface transition zone, and pore structure), and microscopic (i.e., micro-morphology and hydration product characteristics) characterization. The findings indicate that improving the interface heterogeneity and continuity enhances the bond performance of UHPC-NC interface. Compared with the reference group without the interface agent, the samples reach the highest tensile and shear bond strength, which are 71.46 % and 150 % higher than those of the reference ones, respectively, when the chemical interface agent with similar components and properties between the UHPC-NC layers is deployed. The present study reveals that the improvement in interface bond performance is attributed to the formation of hydration products and the densification of interface transition zone, governed by the chemical bonding, fiber reinforcement, and physical adsorption effects. Furthermore, the relationship between the size of interface transition zone and interface bond strength is established. Based on the theory of stress wave propagation, the influencing mechanism of interface heterogeneity and continuity on the failure mode and bond performance of UHPC-NC composites is elucidated. It concludes that altering the interface continuity between the UHPC-NC layers is the key to control the interface bond strength and its failure modes.

Grid connector has been widely used for insulated concrete sandwich panel (ICSP). This paper proposes an innovative curved grid connector for concrete sandwich shell and derives an analytical model using a continuum approach. Firstly, the curved grid is homogenized into a fan-shaped continuum and an equivalent stiffness matrix CH is obtained, reflecting the constitutive relations of the curved grid in macroscale. Next, the expression of the circumference shear stiffness of the curved grid is derived and verified by Finite Element (FE) models. It can be found that the curved grid connectors have the characteristics of trigonometrically distributed shear stiffness k(phi) along the circumferential direction phi. According to the parametric study, the maximum shear stiffness is at the vertex angle of grid members beta = pi /2, while k(phi) becomes constant at beta = pi /4 and beta = 3 pi /4. Additionally, denser arrangement and smaller cross-section of the grid members result in higher stiffness for the same number of connectors. Finally, the bending test of the sandwich shell with a grid connector is presented. The good correlation between the test and analytical results demonstrates the feasibility of applying the analytical model to sandwich shells. The method presented in this paper can be employed for structural optimization and design.

There is a lack of reliable carbon emission factors for recycled steel fiber (RSF) and quantitative studies assessing the carbon emissions of ultra-high performance concrete (UHPC) made with RSF (RSF-UHPC). Combined with experimental research and life cycle evaluation methods, the effects of fiber key parameters, strength, process and material components on mechanical properties and carbon emission of UHPC are studied. The results indicate that RSF-UHPC exhibits better flowability and comparable compressive strength and tensile strength to UHPC made with industrial steel fiber (ISF-UHPC) with the same fiber dosage. This is attributed to the short fibers reducing the loss of flowability and the long fibers help halt or slow the propagation of microcracks in RSF-UHPC. UHPC with 2% RSF is more environmentally friendly, with carbon emissions of 929.64 kg CO 2 e/m 3 and a carbon emission intensity of 8.03 kg CO 2 e/(m 3 ·MPa), which are 23.91% and 18.1% lower, respectively, than those of ISF-UHPC (2%). Based on the life cycle assessment, the carbon emission factor of RSF is determined to be 0.587 kg CO 2 e/kg. Furthermore, the carbon emissions during the raw material acquisition stage are the primary contributors to the total carbon emissions of UHPC, with cement and fibers being the main factors.

Accurately determining the tensile strength of interface bond in cementitious bi-layer hybrid materials and establishing the relationship between splitting tensile strength and direct tensile strength under different interface treatments are crucial in interface characterization and engineering application. However, research on the relationship between interface splitting tensile strength and direct tensile strength is limited. In this study, based on an improved direct tension test apparatus, the influence and bond failure mechanism of different treatment methods (i.e., smooth, high-pressure water jet, sandblasting, and chiseling) for ultra-high performance concrete (UHPC) and normal strength concrete (NC) interface bond are investigated through the splitting tension and direct tension tests. The results show that the improved direct tension test apparatus avoids the initial damage to the interface which may be caused by core sampling and is easy to operate with low variation of test results. The roughness of substrate surface is directly proportional to the tensile bond strength. For both the splitting tensile and direct tensile bond strengths, the order from the highest to lowest is: chiseling > sandblasting > high-pressure water jet > smooth surface, primarily due to the increased roughness, thus enhancing the effective contact area at the interface and strengthening the chemical bonding and mechanical interlocking at the interface. The tensile bond strength alone cannot manifest the failure mode, and the failure mode is jointly determined by the specimen stress level, crack morphology, and microstructure. In addition, the prediction models of splitting tensile and direct tensile strength of UHPC-NC interfaces are proposed, from which their relationship is established by considering the Weibull distribution with good reliability and accuracy. The present study provides viable test methods and models for tensile strength characterization of hybrid material interface bond.

Multifunctional multi-ribbed composite wall (MRCW) structural system has been increasingly used due to its dual load bearing and energy saving capacities. Most of existing studies were focused on its mechanical properties. This paper aims to evaluate its energy saving effect. To this end, two models are constructed using EnergyPlus for the same residential building in Beijing, China, but with different systems of MRCW and shear wall. The dynamic simulation of hourly energy consumption throughout one year is carried out at the building operation stage. The results show that the heating energy consumption is higher than the cooling energy consumption. The total energy consumption of the MRCW system can be reduced by 11.4% compared with that of the shear wall system. Further, the annual energy consumption of the residential building models in five typical climate regions in China with different orientations is simulated. The results show that the energy saving of MRCW system range from 5% to 13% in the five regions when compared with shear wall system, and the absolute value of energy saving is the largest in severe cold and cold regions. The north-south orientation is the best energy-saving orientation. Based on this study, it can be concluded that, other than carrying load, MRCW system can also save energy compared with shear wall system, especially in severe cold and cold regions.

Ultra-high performance concrete (UHPC) or fiber-reinforced concrete (FRC), one of the representatives of cementitious composites, has ultra-high compressive strength and high tensile strength, as well as excellent durability. Fibers are crucial for the tensile strength, post-cracking toughness, and control of cracks in UHPC. However, there are still many questions about the reinforcing effects and mechanisms of fibers on the tensile behavior of UHPC. The parameters, such as fiber type, shape, length-to-diameter ratio, etc., affect the tensile behavior of UHPC to different degrees, and the determination of the optimal mixing ratio of hybrid fibers is a difficult task. Therefore, it is necessary to review, summarize, and compare the studies on the influence of fibers to the tensile behavior of UHPC. The physical and mechanical properties of various fibers used in UHPC are thoroughly reviewed, and the effects of fiber type, content, shape, length, orientation, and ratio of hybrid fibers on the tensile behavior of UHPC are analyzed. Furthermore, three main enhancement mechanisms of fibers, including crack-resistant toughening, fiber shape effect, and chemical bonding, are discussed to improve the tensile behavior of UHPC. The constitutive models of UHPC in tension is also summarized and discussed. Finally, some recommendations and suggestions for future research and development of UHPC in tension are provided.

This study develops an innovative multifunctional concrete sandwich shell based on a combined experimental and finite element (FE) study on its flexural behavior. The sandwich shell is made up of inner and outer concrete layers connected by connectors, with a middle functional layer that provides various functions such as insulation, acoustic, and vibration control. Bending tests were conducted on four groups of specimens, including three groups of sandwich shells with different types of connectors, that is, truss, grid, and plate connectors, and one reference group of solid shells. Loads, displacements, and strains were recorded during the tests. The FE analysis showed good correlation with the experimental results. Furthermore, a parametric study was conducted using the FE model to evaluate the influence of different parameters, such as middle layer thickness and number of connectors. The results show that the performance of the sandwich shell is comparable to, and in some cases better than, that of the solid shell, depending on the type of connectors used. This study provides a proof of concept for the sandwich shell and establishes a prototype structure for future research.

This paper is focused on a new structural type - sandwich shell, which includes inner and outer shells connected by hoop connectors to achieve composite action; and a functional layer in the middle. Considering that the hoop connectors are not completely rigid, there will be limited slip between the inner and outer shells, which will lead to partial Degree of Composite Action (DCA). Due to the existence of the hoop connectors and shear lag effect, the stress distribution along the width direction of the shell is uneven, which can be described by effective width. Considering all these factors, this paper first develops an analytical model for sandwich shells with different configurations of connectors; and uses finite element analysis results to verify the analytical model. Next, a parametric study is conducted using the analytical model to study the effects of material strength, the ratio of the thickness of the functional layer to the thickness of the single layer shell, and the ratio of the arc length to the width of the sandwich shell on the effective width. It is found that DCA can significantly affect the stress and strain distributions of the sandwich shell. The effective width for 100% DCA can be used as a conservative value to represent those of other DCAs. Finally, a simplified calculation formula, which can be used in practical design, is derived based on the effective width for 100% DCA.