Thermomechanical Behavior of Fully Recycled Concrete Prefabricated Components under Elevated Temperature Environments: An Irreversible Thermodynamics Perspective

2.1 Raw materials and mix proportion design based on thermodynamic optimization

The cementitious material system is optimized based on the principle of Gibbs free energy minimization, breaking through the limitations of traditional empirical mix proportions and realizing the thermodynamically precise design of fully recycled concrete mix proportions. The Gibbs free energy calculation formula is: $G=H-T S$, where G is Gibbs free energy, H is enthalpy, T is thermodynamic temperature, and S is entropy. By systematically regulating the water–cement ratio (0.45–0.55) and the gradation of recycled aggregates (5–25 mm continuous gradation), the Gibbs free energy of the cementitious system is minimized, thereby preparing fully recycled concrete specimens with significant differences in initial internal energy and pore structure. The design of specimens with different initial internal energy and pore structures provides differentiated experimental samples for subsequent investigation of the quantitative relationship between activation energy and mechanical damage, avoiding the limitation of experimental data caused by excessive uniformity of specimen performance under traditional empirical mix proportions.

An isothermal calorimeter is used to carry out early hydration heat release tests of the specimens. The test temperature is controlled at 20±2 ℃, and the test duration is 72 h. The hydration heat release rate and cumulative heat release curves of each group of specimens are recorded in real time. Based on the test data, a correlation expression between initial internal energy and cumulative hydration heat release is established: $U_0=k Q+U_c$, where $U_0$ is the initial internal energy, k is the fitting coefficient, Q is the cumulative hydration heat release, and $U_c$ is a constant. This correlation expression quantitatively characterizes the intrinsic relationship between initial internal energy and hydration heat release characteristics, providing accurate initial boundary conditions for the subsequent thermodynamic energy balance analysis of fully recycled concrete, effectively improving the correlation and rigor between experimental data and theoretical analysis, and laying a reliable experimental foundation for the study of thermodynamic behavior at the material level.

2.2 Multi-physical field coupling experimental system

A four-in-one synchronous test platform is designed and constructed, breaking through the technical bottleneck that traditional experimental systems cannot simultaneously obtain multi-field parameters, and realizing the real-time synchronous acquisition of thermo-mechanical-optical multi-physical field information, providing comprehensive and accurate data support for entropy production calculation. The platform integrates a high-temperature environmental chamber, a universal testing machine, an infrared thermal imager, and a digital image correlation (DIC) system. The temperature control range of the high-temperature environmental chamber is from room temperature to 800 ℃, with a control accuracy of ±1 ℃, which can realize various thermal conditions such as uniform heating and constant temperature holding; the maximum load of the universal testing machine is 5000 kN, and the loading rate can be continuously adjusted within the range of 0.1–10 mm/min, meeting the loading requirements of real-time high-temperature mechanical tests; the temperature measurement accuracy of the infrared thermal imager is ±0.1 ℃, and the image resolution is 320 × 240 pixels, which can accurately capture the subtle changes in the surface temperature field of the specimen; the strain accuracy of the DIC system is ±0.005%, and the sampling frequency is 10 Hz, which can obtain the evolution law of the surface strain field of the specimen in real time.

In order to realize the precise alignment and real-time monitoring of multi-field parameters in the thermo-mechanical coupling process, a thermo-mechanical-optical multi-physical field coupling real-time synchronous test system platform as shown in Figure 1 is independently constructed in this study. The platform highly integrates the high-temperature environmental chamber, universal testing machine, infrared thermal imager, and DIC system. Through a high-speed data acquisition card and a master clock synchronous trigger controller, the synchronous acquisition of multi-dimensional information such as temperature field, mechanical load, surface thermal image grayscale, and real-time strain field is ensured, and the data acquisition time delay between hardware subsystems is strictly controlled within 10 ms. This high-precision physical-level synchronization mechanism completely overcomes the disadvantage of time difference in multi-field data in traditional discrete testing, and lays a solid and reliable experimental data foundation for the subsequent accurate calculation of real-time heat flux density, plastic strain rate, and transient local entropy production rate.

Figure 1. Schematic diagram of the thermo-mechanical-optical multi-physical field coupling real-time synchronous test system platform

Through the data acquisition card, the synchronous triggering of four sets of equipment is realized, and the sampling time difference is controlled within 10 ms. The temperature data, mechanical load data, thermal image grayscale data, and strain data are fused in real time through dedicated software. Combined with the basic formula for entropy production rate calculation:

$\dot{S}_{g e n}=\frac{q \cdot \nabla T}{T^2}+\frac{\sigma: \dot{\varepsilon}^p}{T}$                   (1)

where, $\dot{S}_{g e n}$ is the entropy production rate, q is the heat flux density, $\nabla T$ is the temperature gradient, $\sigma$ is the stress tensor, and $\dot{\varepsilon}^p$ is the plastic strain rate. The multi-field synchronous data collected by the platform can be directly substituted into the formula to complete the entropy production rate calculation without additional data conversion, significantly improving the accuracy and efficiency of entropy production analysis, and solving the problem of large errors in entropy production calculation caused by asynchronous multi-field data in traditional experiments, providing reliable experimental technical support for subsequent thermodynamic research at the interface and component levels.

2.3 Independent determination of key thermodynamic parameters

In order to support the accurate establishment of the subsequent mesoscopic numerical model and to break through the limitations of the macroscopic homogeneous assumption in traditional studies, this study independently determines the key thermodynamic parameters of each component of fully recycled concrete, accurately capturing the influence of internal heterogeneity of the material on thermodynamic properties. The laser flash method is used to measure the thermal diffusivity and specific heat capacity of recycled aggregates, new mortar, and old mortar within the temperature range of 20–600 ℃. During the test, one test point is set every 50 ℃, and each point is tested three times, and the average value is taken to reduce experimental error. A quartz push-rod dilatometer is used to measure the linear thermal expansion coefficient of each component, and the heating rate is controlled at 5 ℃/min. The length change of the specimen is recorded in real time throughout the process to ensure the accuracy and reliability of parameter determination. Different from the traditional method of treating fully recycled concrete as a homogeneous material for unified parameter measurement, this study independently tests the characteristics of each component, which can accurately reflect the differences in thermodynamic parameters between recycled aggregates and new and old mortars, providing a direct basis for parameter assignment of each component in the mesoscopic model.

In order to accurately reflect the profound influence of the highly heterogeneous internal structure of the material on macroscopic thermodynamic behavior, Figure 2 shows in detail the evolution curves of key thermodynamic parameters of the three independent components, namely recycled aggregates, new mortar, and old mortar, within the temperature range of 20–600 ℃. The test results clearly show that the thermal diffusivity, specific heat capacity, thermal conductivity, and linear thermal expansion coefficient of each component exhibit significant differentiated degradation characteristics with increasing temperature. In particular, compared with new mortar, old mortar shows more significant characteristics of lower thermal conductivity and higher thermal expansion. This decoupling and quantitative characterization of thermodynamic parameters at the component level not only intuitively explains the physical root cause of severe thermal stress concentration and thermal mismatch easily occurring inside fully recycled concrete during heating, but also provides indispensable underlying material data for the subsequent construction of a high-precision mesoscopic numerical finite element model considering full bidirectional thermo-mechanical coupling.

Based on the measured thermal diffusivity and specific heat capacity, combined with the density parameters of each component, the thermal conductivity of each component at different temperatures is calculated through the formula: $\lambda= \alpha \rho c$, where $\lambda$ is thermal conductivity, $\alpha$ is thermal diffusivity, $\rho$ is material density, and $c$ is specific heat capacity. The linear thermal expansion coefficient is calculated by the formula: $\alpha_L=\frac{\Delta L}{L_0 \Delta T}$, where $\alpha_L$ is the linear thermal expansion coefficient, $\Delta L$ is the length increment of the specimen after temperature change, $L_0$ is the initial length of the specimen, and $\Delta T$ is the temperature change. All measured parameters form complete temperature-dependent curves, clarifying the evolution law of thermodynamic parameters of each component with temperature, effectively solving the problem of parameter distortion in mesoscopic models caused by the traditional macroscopic homogeneous assumption, significantly improving the accuracy of subsequent numerical simulations, and providing reliable parameter support for thermo-mechanical bidirectional coupling simulation.

Figure 2. Temperature-dependent curves of thermodynamic parameters of each component of fully recycled concrete

2.4 Microstructural characterization after high-temperature damage

In order to reveal the microscopic mechanism of high-temperature damage of fully recycled concrete and to establish the correlation between microscopic damage and macroscopic thermodynamic response, this study adopts multi-method microscopic characterization techniques to carry out systematic testing of specimens subjected to different temperature conditions, breaking through the limitation that traditional microscopic observation only focuses on two-dimensional sections. Industrial CT scanning is performed on specimens subjected to temperatures of 20 ℃, 200 ℃, 400 ℃, and 600 ℃. The scanning resolution is set to 10 μm. A digital volume correlation algorithm is used to process the CT scanning data to accurately extract the internal three-dimensional crack network of the specimens. The complexity and propagation characteristics of cracks are quantified through fractal dimension and tortuosity, realizing three-dimensional quantitative description of microscopic damage.

The fractal dimension is calculated by the formula:

$D=\lim _{\varepsilon \rightarrow 0} \frac{\ln N(\varepsilon)}{\ln (1 / \varepsilon)}$                     (2)

where, D is the crack fractal dimension, $\varepsilon$ is the measurement scale, and $N(\varepsilon)$ is the number of cracks at that scale. A larger fractal dimension indicates a more complex crack network. The tortuosity is calculated by the formula:

$\tau=\frac{L_{\text {actual }}}{L_{\text {straight }}}$                 (3)

where, $\tau$ is the crack tortuosity, $L_{\text {actual }}$ is the actual crack length, and $L_{\text {straight }}$ is the straight-line distance between the two endpoints of the crack. Tortuosity reflects the degree of tortuous crack propagation. At the same time, a scanning electron microscope is used to observe the microscopic morphology of the interfacial transition zone of the specimens, with a magnification of 500–5000 times. The microstructural changes of the new–old mortar interface and the recycled aggregate–new mortar interface are mainly observed, capturing features such as interfacial debonding, micropore evolution, and decomposition of hydration products. The microscopic damage parameters, including fractal dimension, tortuosity, and interfacial porosity, are correlated and verified with macroscopic thermodynamic responses such as entropy production rate and strength degradation, clarifying the influence mechanism of the evolution law of microscopic damage on macroscopic thermodynamic properties, and providing direct microscopic experimental evidence for the in-depth analysis of the thermal damage mechanism of fully recycled concrete.

Figure 3. Microstructural damage characterization of fully recycled concrete after exposure to different temperatures

In order to further investigate the microstructural root causes of material performance degradation under high temperature, Figure 3 combines industrial CT three-dimensional reconstruction and scanning electron microscopy (SEM) technology to visually present the entire process of internal mesoscopic damage evolution of fully recycled concrete subjected to different temperature gradients from room temperature to 600 ℃. The CT three-dimensional crack network reconstruction image quantifies the exponential growth of crack volume by color coding, revealing that the damage evolves from isolated micropores at room temperature, to microcrack initiation at 200 ℃, to local network connectivity at 400 ℃, and finally to a fully developed unstable three-dimensional complex crack network at 600 ℃. The corresponding interfacial SEM microscopic morphology obtained simultaneously further confirms, from the microscopic scale magnified thousands of times, the degradation process from initially dense structure at the new–old mortar interface and aggregate transition zone to microcrack propagation, and finally to severe deterioration and collapse of the mortar matrix, providing clear visual and physical evidence for the staged sharp degradation of macroscopic mechanical strength.

As the weak link of fully recycled concrete prefabricated components, the thermal resistance evolution and thermo-mechanical debonding mechanism of the prefabricated composite interface under variable temperature environments directly determine the overall thermodynamic performance and service safety of the components. In this paper, through multi-physical field coupling tests, infrared thermography monitoring, and theoretical analysis, the thermo-mechanical coupling damage law of the interface is systematically revealed, the entropy production criterion for interfacial debonding is established, and the accurate monitoring and failure prediction of interfacial damage are realized. The experimental data and analysis results are as follows.

Table 2. Thermo-mechanical properties and thermodynamic characteristic parameters of prefabricated composite interface at different temperatures

Under variable temperature environments, the bonding performance and infrared thermography characteristics of the prefabricated composite interface show significant temperature dependence, and the two jointly reflect the whole process of thermo-mechanical debonding of the interface. As shown in Table 2, as the test temperature increases from 20℃ to 400℃, the interfacial bond strength continuously decreases from 3.82 MPa to 0.79 MPa, with a strength reduction rate of 79.3%, and the reduction rate shows a staged characteristic of “slow–fast–slow”: in the temperature range of 20–100℃, the strength reduction rate is only 17.5%, and the interfacial microstructure has not undergone obvious deterioration, and the bonding performance remains basically stable; in the temperature range of 100–300℃, the strength reduction rate reaches 41.4%, accounting for 52.2% of the total reduction rate, and in this stage, micropore expansion occurs at the interface between old mortar and new mortar, and the interfacial bonding force rapidly decreases; in the temperature range of 300–400℃, the strength reduction rate slows down to 20.4%, and the interface has undergone obvious debonding, and the remaining bonding force is mainly provided by aggregate interlocking.

The infrared thermography monitoring results reveal the dynamic evolution characteristics of interfacial thermo-mechanical debonding, which are highly consistent with the strength variation law. In the initial stage of loading, when the load is lower than 60% of the peak load, there is no obvious thermal anomaly in the interfacial region, and the maximum temperature rise of the thermal hotspot remains at 0℃, indicating that the interface is in the elastic deformation stage and no microscopic damage is initiated. At this time, the thermal conductivity changes little, and the damage variable D is less than 0.2. When the load is in the range of 60%–75% of the peak load, local thermal hotspots appear at the interface, and the maximum temperature rise of the thermal hotspot increases from 0.3℃ to 1.0℃. This phenomenon is caused by the thermoelastic effect, that is, the interfacial micro-elements undergo elastic deformation under mechanical load, and energy dissipation is converted into heat, marking the initiation of interfacial micro-damage. At this time, the damage variable D increases to 0.39–0.62, and the thermal conductivity decreases significantly. When the load exceeds 85% of the peak load, the thermal hotspots penetrate along the interface to form a continuous low-temperature zone, and the maximum temperature rise of the thermal hotspot stabilizes at 1.2℃. The formation of the low-temperature zone is due to the opening of interfacial cracks, and air enters the crack gaps to form a thermal barrier, hindering heat conduction. At this time, the interfacial thermal conductivity decreases to 0.47 W·(m·K)⁻¹, the damage variable D reaches 0.85, and the interface is completely debonded, which is completely consistent with the phenomenon of interfacial separation in the macroscopic failure mode. This staged change of infrared thermography characteristics can be used as an early warning signal for the initiation of interfacial damage and debonding, solving the technical problem that traditional methods are difficult to monitor interfacial damage in real time.

The degradation law of thermodynamic parameters of the interface layer further verifies the high sensitivity of the interface. Based on the interfacial thermal conductivity and damage variable data at different temperatures in Table 2, the degradation equation of interfacial thermal conductivity is obtained by inverse analysis $k(D)=k_0(1-D)^n$, where $k_0=1.86$ W·(m·K)⁻¹ and the degradation index $n=2.8$. Compared with the degradation index $n=1.3$ of the thermal conductivity of the fully recycled concrete matrix material, the degradation index of the interface layer is much larger than that of the matrix material, indicating that under the synergistic action of temperature and mechanical load, the degradation rate of thermodynamic parameters of the interface layer is faster and more sensitive to thermo-mechanical coupling. This characteristic originates from the microstructural defects of the interfacial transition zone: there are a large number of micropores and microcracks at the interface between old mortar and new mortar, and between recycled aggregates and new mortar. With the increase of temperature, these defects rapidly expand, leading to a sharp decrease in interfacial heat conduction capacity, thereby accelerating the interfacial debonding process, which is consistent with the rapid degradation of interfacial bond strength described above.

Based on the Clausius–Duhem inequality $\dot{S}_{{gen }} \geq 0$, combined with the characteristics of interfacial thermo-mechanical coupling damage, the calculation formula of local cumulative entropy production of the interfacial element is derived as:

$S_{gen,total}=\int_0^t \dot{S}_{ {gen }} d t=\int_0^t\left(\frac{q \cdot \sqrt{T}}{T^2}+\frac{\sigma: \dot{\varepsilon}^p}{T}\right) d t$                 (4)

where, $S_{gen,total}$ is the local cumulative entropy production and t is the loading time. The essence of interfacial debonding is the dissipation process of interfacial energy. When the local cumulative entropy production of the interface exceeds the interfacial energy stored during interface formation, the intermolecular bonding force of the interface is destroyed, and debonding failure occurs. Based on this, the entropy production criterion for interfacial debonding is proposed $S_{ {gen,total }} \geq U_i$, where $U_i$ is the interfacial energy.

Figure 5. Infrared thermography evolution characteristics of thermo-mechanical debonding process of prefabricated composite interface under variable temperature environments

As shown in Table 2, the interfacial energy is determined as 86.7 J·m⁻² through independent tensile tests. The local cumulative entropy production of the interface at different temperatures increases significantly with temperature, from 42.3 J·(m³·K)⁻¹ at 20 ℃ to 658.2 J·(m³·K)⁻¹ at 400 ℃. When the local cumulative entropy production exceeds 86.7 J·m⁻² for the first time, obvious interfacial debonding begins to occur, which is completely consistent with the experimental observations. The prediction accuracy of the entropy production criterion for interfacial debonding reaches 96.8%, which is significantly higher than that of the traditional stress criterion. The traditional stress criterion only considers the mechanical load on the interface and ignores the energy dissipation process under temperature action, and cannot accurately reflect the interfacial damage evolution law under thermo-mechanical coupling. The entropy production criterion directly relates thermodynamic energy dissipation to interfacial debonding, can comprehensively capture the synergistic effect of temperature and mechanical load, and accurately predict the timing of interfacial debonding, providing a new theoretical tool for damage early warning and failure evaluation of prefabricated composite interfaces.

For the prefabricated composite interface, which is the weakest link where thermo-mechanical coupling failure is most likely to occur in fully recycled concrete components, Figure 5 shows the dynamic evolution sequence of interfacial debonding captured by high-precision infrared thermography technology. The infrared thermography images clearly record the spatial distribution evolution of interfacial energy dissipation with the increase of load level: in the stage of 60% to 75% of the peak load, local micro-damage initiates and accumulates at the interface, and due to elastic deformation and microcrack friction generating heat dissipation, it appears as discrete “thermal hotspots” with local temperature anomalies; when the load approaches 85% of the peak load, interfacial cracks fully open and external air is drawn in, forming a “thermal barrier” effect that blocks heat conduction, and the thermography images show a “continuous low-temperature zone” penetrating along the interface. This polarity reversal evolution law from “local thermal hotspots” to “continuous low-temperature zone” not only directly reveals the transient physical process of the interface from damage initiation to complete debonding, but also verifies the high effectiveness of using infrared thermography characteristics as an early warning signal of interfacial thermo-mechanical coupling damage under variable temperature environments.

In summary, the thermo-mechanical debonding of the prefabricated composite interface is the result of the synergistic effect of temperature-induced thermodynamic parameter degradation and mechanically induced damage accumulation: the increase of temperature leads to a decrease in interfacial thermal conductivity and an increase in thermal resistance, forming temperature gradients and thermal stress, which aggravate the expansion of interfacial micro-defects; under mechanical load, interfacial micro-damage initiates and propagates, and energy dissipation is converted into entropy production. When the cumulative entropy production exceeds the interfacial energy, complete interfacial debonding occurs. The evolution characteristics of thermal hotspots monitored by infrared thermography can realize early warning of interfacial damage, while the entropy production-based debonding criterion can accurately predict interfacial failure. The combination of the two provides reliable experimental basis and theoretical support for the thermal safety design of interfaces of fully recycled concrete prefabricated components.

This paper focuses on the thermodynamic properties and failure mechanisms of fully recycled concrete prefabricated components under different temperature environments. Through experimental testing, numerical simulation, and theoretical analysis, systematic research is carried out at three scales: material, interface, and component. The thermodynamic behavior, interfacial debonding mechanism, and component failure law are clarified. The main conclusions are as follows:

Under real-time high-temperature environments, the strength degradation of fully recycled concrete has a direct quantitative relationship with the activation energy of the dehydration reaction of C-S-H gel. Based on the Arrhenius equation, the apparent activation energy of fully recycled concrete in the temperature range of 200–400℃ is calculated as 38.6 kJ/mol, which is 22.3% lower than that of conventional concrete. The lower activation energy makes fully recycled concrete more prone to hydration product decomposition and microstructural deterioration at high temperature, which is the core intrinsic thermodynamic mechanism for its higher thermal sensitivity compared with conventional concrete.

The thermo-mechanical debonding process of the prefabricated composite interface can be monitored for early warning through infrared thermography. The thermal hotspots appearing at the interface during loading are direct signals of micro-damage initiation, and the maximum temperature rise of thermal hotspots can reach 1.2℃. Their evolution characteristics are highly consistent with the interfacial damage stages. Based on the Clausius–Duhem inequality, the local entropy production criterion is established, taking interfacial energy as the critical threshold, which can accurately describe the interfacial debonding failure process. The prediction accuracy for interfacial debonding reaches 96.8%, which is significantly better than the traditional stress criterion, providing a reliable method for interfacial damage early warning.

A mesoscopic finite element model of fully recycled concrete prefabricated components considering full bidirectional thermo-mechanical coupling is established. By writing user material subroutines, the thermo-damage constitutive model is embedded into the numerical simulation, realizing the interactive feedback between temperature field and stress field, that is, temperature change induces thermal stress, and mechanical damage updates the thermal conductivity and specific heat capacity of materials in real time. The model successfully reveals the temperature-induced stress locking phenomenon under unilateral fire exposure, which is caused by the low thermal conductivity of fully recycled concrete, that is, the high-temperature region at the surface is under compression, and the low-temperature region inside is under tension, thereby triggering the unique failure mode of internal cracking occurring prior to surface damage.

A thermodynamic failure criterion of fully recycled concrete prefabricated components based on critical cumulative entropy production is proposed. It is clarified that the critical cumulative entropy production at component failure is stable at 1.02 MJ/m³. This value has good path independence and is not affected by heating rate and load path. Verified by experimental data at material and interface scales, the prediction agreement of this criterion reaches more than 96.8%, and the relative error between numerical simulation and experimental measurement is controlled within 2.5%, breaking through the limitation that traditional failure criteria only consider mechanical response, and providing a new theoretical tool and scientific basis for the thermal safety evaluation and fire resistance design of fully recycled concrete prefabricated components.

This study improves the thermodynamic theoretical system of fully recycled concrete, fills the gap in the application of irreversible thermodynamics at the component scale of recycled concrete, and provides important experimental support and theoretical guidance for promoting the large-scale and safe engineering application of fully recycled concrete prefabricated components.