Materials and Structures最新文献

The construction industry faces major environmental challenges due to the high carbon footprint of Portland cement (PC). Incorporating high volumes of coal thermal ashes provides a more sustainable alternative; however, the effectiveness of microwave curing for such concrete remains insufficiently explored. This study investigates the effects of microwave curing power and duration on the mechanical performance, durability-related properties, and sustainability of concrete incorporating high-volume coal thermal ashes (HVCTA). Concrete mixtures were produced using a ternary binder system of PC, fly ash (FA), and hydrated lime (HL) and subjected to microwave curing at power levels of 300, 600, and 900 W for durations of 10, 15, and 20 min. The mechanical properties were evaluated through compressive, flexural, and splitting tensile strengths, while physical and durability-related indicators were assessed using ultrasonic pulse velocity (UPV), drying shrinkage, water absorption, and rapid chloride ion penetration at 3, 28, and 90 days. The results indicate that microwave curing significantly enhances the performance of concrete incorporating HVCTA. The specimen cured at 900 W for 10 min achieved a 28-day compressive strength of 25.65 MPa, representing a 50% increase compared with the control (17.06 MPa). UPV increased by up to 36% for specimens cured at 600 W for 10 min, while drying shrinkage was reduced by 20.7% at 90 days under the 900 W–10 min regime. Improved pore refinement was evidenced by a 7.3% reduction in water absorption at 600 W for 10 min. Eco-efficiency assessment showed that microwave curing at 900 W for 10 min reduced relative carbon emissions by 48% (38.65 vs. 74.14 kg/m³ MPa) and relative energy consumption by 54% (352.42 vs. 762.67 MJ/m³ MPa) at 3 days compared with the control. Overall, appropriately controlled microwave curing provides concurrent improvements in strength, durability-related properties, and sustainability, indicating its potential as an efficient curing approach for precast concrete with HVCTA.

Design codes commonly adjust the withdrawal capacity of nailed angle brackets with a fixed group-reduction factor, regardless of bracket stiffness or rib geometry. This study proposes a mechanics-based method for predicting the effective number of nails directly from bracket dimensions, yield strength, rib height, and a single-nail withdrawal strength. The closed-form model useful for reliability-based design is obtained assuming a two-hinge kinematic mechanism and requires only one supplemental parameter, the plastic-moment ratio, which is obtained once per bracket series through a simple elastic–plastic finite-element analysis. The formulation is validated against 49 monotonic tests on seven S280GD steel brackets whose leg size, thickness, and rib height were systematically varied. The model reproduces both the observed hinge position along the timber leg and the fraction of nails mobilised at peak load, keeping the prediction error for the effective nail count within ±10 % across all specimens. In comparison, the fixed reduction coefficients overestimate nail utilisation by 25–55% and lead to capacity errors of 60–160%. Since the new expression depends only on mechanical and geometric properties, it provides an elementary but still accurate alternative to fixed group-reduction factors for designing nailed angle-bracket connections governed by withdrawal. The authors have presented illustrative examples of this expression for the reliability-based design of angle brackets for timber structures.

Wood exhibits absorption characteristics that deviate from theories that have conventionally been applied to liquid transport in porous building materials, leading to difficulties in interpreting experimental results and determining the essential hygric properties of wood. This study investigated the irregular water absorption patterns of wood, as well as the possible errors and improvements in water absorption testing. The results of long-term water absorption tests using specimens obtained from different wood samples and sealing methods were compared to understand the deviation patterns systematically. In addition, the causes of two typical patterns were discussed based on the water distribution visualized by X-ray computational tomography, absorption tests using non-aqueous liquids, and strain measurements. The results indicated that the use of non-adhesive sealing methods, which were adequate for mineral building materials, caused unexpected local water accumulation near the side and top surfaces, resulting in overestimation of the absorption. In addition, non-negligible increases (by 1.3–3.6 times) in the absorption rates, as measured against the square root of time, were observed during the early stages of absorption. Gradual decreases in the absorption rates were also noted, caused by the heterogeneity of water absorption and unexpected water accumulation close to the top surface. This led to difficulties in determining the capillary absorption coefficient and capillary water content. Approximation methods combining quadratic and linear functions to quantify and compare the initial and later absorption rates were proposed, and adequate test durations were discussed. The findings of this study will help to establish effective test methods, including those for surface treatment and data processing.

The primary objective of this study is to recover the performance of the recycled binders/mixtures incorporating Reclaimed Asphalt Pavement (RAP)—under three distinct cases: 40%RAP, 60%RAP and a hybrid case with RAP from two sources—to the level of the virgin material, relying on the application of green softening oils (including two bio-oils and refined engine oil bottom (REOB)). Prior to blending with RAP binders, the softening oils were first introduced to the base binder to evaluate their effectiveness on the Performance Grade (PG) of the base binder, meanwhile an oil modification index (OMI) was derived to quantify the efficiency of each oil in modifying both high- and low- temperature PG. Single Edge Notched Beam (SENB) tests were further employed to assess the oils’ effects on cracking property of binders beyond the linear viscoelastic range. Then, the oil contents were determined for each RAP recycling case to achieve cracking performance restoration targets aligned with the base binder, based on combined PG and SENB outcomes. Finally, the recycled binder and mixture samples prepared with the oil at the determined content were subjected to PG, PG plus, and Indirect Tensile Asphalt Cracking (IDEAL-CT) tests to evaluate binder and mixture performance, respectively. The findings highlight that two bio-oils outperform REOB in terms of improving both fatigue and thermal cracking performance, from both perspectives of binder and mixture, indicating that bio-oils may be more suitable in high RAP recycling scenarios.

Repairing damaged reinforced concrete (RC) structures to restore their load-bearing capacity and extend their service life remains a critical challenge in civil engineering. Achieving efficient repair is essential for minimizing losses of life and property while maintaining structural stability. This study proposes a active confinement method for repairing damaged concrete columns using nickel-titanium shape memory alloy (Ni–Ti SMA) strips. A series of uniaxial compression tests were conducted to systematically investigate the effectiveness of SMA strip confinement in restoring the performance of damaged concrete columns. The influence of three key parameters-the number of SMA strips, pre-strain level, and degree of pre-damage on the failure behavior of the columns was analyzed in detail. Experimental results indicate that increasing the number of SMA strips and enhancing the prestress level can significantly improve both the compressive strength and deformation capacity of the repaired concrete columns. The crack closure rate achieved by the confinement provided by the SMA strips ranged from 5 to 35.7%, and the peak stress of the specimens increased by up to 45.44%. Based on the test results, a stress–strain constitutive model applicable to SMA-repaired damaged concrete columns was developed. The model’s calculated results show good agreement with the experimental data and demonstrate satisfactory generality and potential applicability. Overall, the findings of this study provide valuable theoretical and practical insights for rehabilitating damaged reinforced concrete structures.

To investigate the effect of steel corrosion on bond degradation in underwater reinforced concrete (RC) structures, 18 RC specimens were prepared and subjected to three kinds of tests. Electrochemical corrosion was applied with target corrosion ratios of 3%, 6%, 9%, 12%, and 15%. Subsequently, underwater ultrasonic tests were performed to obtain corresponding wave velocities, followed by pull-out tests to evaluate bond strength. On the basis of these results, a bond-slip model incorporating ultrasonic velocity was developed to characterize bond behavior between corroded steel bars and concrete. The proposed finite element model was validated against experimental data from this work and prior studies, and further compared with existing models. The results show that the ultrasonic wave velocity in underwater specimens shows a positive correlation with the corrosion ratio. Failure mode shifts from splitting to pull-out as corrosion intensifies. The peak bond stress, the slip at peak bond stress, and the bond toughness all exhibit a trend of initial increase under slight corrosion. Underwater pull-out specimens exhibit superior bond performance compared to the terrestrial ones. Moreover, most of the predicted values from the model in this paper are lower than the experimental results, with deviations not exceeding 15%. These findings demonstrate that the proposed bond-slip model considering the corrosion of embedded steel bars can effectively predict bond performance, which can be used to optimize the design of underwater RC structures by reducing construction costs and extending service life.

The electrical and thermal properties of cement-based composites are garnering increasing attention due to their vast potential for applications in self-sensing, self-heating, and efficient thermal energy management. Although finite element simulations and machine learning have advanced performance prediction, current studies fail to capture special distribution of fine aggregate and interphase connectivity between aggregate and cement paste in microscopic images, limiting high-precision predictions. This study generates 1,000 sets of representative volume element (RVE) microscopic structural images along with their corresponding physical parameters through finite element simulations. Seven machine learning models—Support Vector Regression (SVR), Gaussian Process Regression (GPR), Convolutional Neural Networks (CNN), Multilayer Perceptron (MLP), Recurrent Neural Networks (RNN), Long Short-Term Memory (LSTM), and Transformer—are utilized to predict the electrical and thermal properties, and a comprehensive evaluation is conducted on the performance of each model in terms of prediction accuracy and applicability. The results show that the GPR model performs best in predicting electrical conductivity and thermal conductivity, with R² values exceeding 0.9985, MAE values of 0.0002% and 0.62%, and RMSE values of 0.0003% and 0.79%, respectively. The CNN and MLP models follow, with R² values exceeding 0.9948, MAE values for electrical and thermal conductivity predictions below 1.29%, and RMSE values below 1.69%. In comparison, the RNN, LSTM, and Transformer models exhibit larger prediction errors, with RNN performing the worst. In this case, R² drops to 0.9610, MAE rises to 3.58%, and RMSE climbs to 4.49%. This study significantly enhances the prediction accuracy of the thermoelectric properties of cement-based composites while simultaneously reducing computational cost, thereby facilitating rapid material design and optimization.

One of the major difficulties for reliably assessing the durability of salt laden building materials is the measurement of the in-pore salt crystals content without damaging the materials, which are frequently of great historical value. In this contribution, it was attempted to estimate the content of sodium sulfate decahydrate (mirabilite) by the combination of two methods: differential scanning calorimetry and ultrasonic pulse velocity. The proposed approach allowed to develop the relation between the P-wave velocity (non-destructive) and in-pore salt crystal content. Fully saturated red clay brick and sandstone with 10%, 15% and 20% Na₂SO₄ solutions were investigated. The idea was to first evaluate the content of crystallized in-pore mirabilite in the noted materials via differential scanning calorimetry. Subsequently, the content of in-pore mirabilite was related to the P-wave velocity increase that it caused with respect to the initial empty (no salt) dry state. Regardless of the anisotropic microstructure of the red clay brick, linear relations were obtained between the content of crystallized mirabilite and the P-wave velocity increase in all directions. The slope of such linear relations was higher in the direction which had a lower initial dry P-wave velocity. Linear relation was also obtained in the case of the isotropic sandstone, however, with a lower slope than all the directions of the red clay brick. Accordingly, the proposed non-destructive approach showed promising potential for the monitoring and preservation of the porous building materials which are susceptible to salt crystallization damage.

Chloride induced corrosion is a primary durability concern for reinforced concrete (RC) structures in marine environments, where premature corrosion of embedded reinforcement can shorten the service life of structures and compromise structural integrity. Traditional service life models account for the time-dependent reduction in chloride diffusivity by using the aging exponent (m), which is a crucial yet highly variable parameter influenced by binder composition, degree of hydration and pozzolanic reactions. This study investigates the aging exponents of 28 customized concrete mixes incorporating varying proportions of fly-ash and slag, with water-to-cementitious (w/c) materials ratio ranging from 0.33 to 0.55. Three different measurement techniques—rapid chloride migration, electrical resistivity and rapid chloride permeability tests—were used to evaluate chloride transport properties up to 365 days. The results indicate significant variability in m-values across different binder compositions, with fly-ash based mixes exhibiting considerably superior behavior. Empirical correlations were established between the key physiochemical parameters and the aging exponents, and the proposed model was calibrated using the experimental data. The model performance was evaluated against a set of data from different studies. Most of the validation data falls within the 95% prediction interval for the model, with satisfactory statistical significance. The findings underscore the potential of chemical constituent-based models in rapidly and economically evaluating the aging exponents of concrete mixes, thereby enhancing service life predictions and streamlining the durability assessment of RC structures.

This work presents a multi-scale investigation into the thermal aging mechanism of styrene–butadiene–styrene (SBS) modified bitumen. By employing extended Rolling Thin Film Oven Test (RTFOT) aging and integrating techniques including gel permeation chromatography, Fourier transform infrared spectroscopy, dynamic shear rheometry, and fluorescence microscopy, a critical transition in the SBS degradation mechanism was identified at approximately 160 min of thermal aging. Before this threshold, SBS undergoes cross-linking between polymer chains, while beyond it, dominant chain scission prevails, as conclusively evidenced by the reversal in the polydispersity index trend and a marked decrease in the Z-average molecular weight. Chemo-mechanical correlations were established, decoupling the competing effects of bitumen matrix oxidation and SBS degradation: penetration loss was correlated with carbonyl formation (increase in I_C=O) and the consumption of polybutadiene bonds (decrease in I_–CH=CH–), while ductility failure was primarily correlated with sulfoxide growth (increase in I_S=O) and large molecular structuring (increase in large molecular size (LMS)). Rheologically, the increase in overall stiffness (rise in modulus coefficient “a”) was mainly correlated with SBS chain scission (decrease in I_–CH=CH–) and bitumen oxidation (increase in I_C=O), whereas the shift in viscoelastic balance (decrease in viscoelastic index “n”) was more sensitive to the oxidized bitumen matrix (LMS, I_S=O). Overall, this work provides valuable guidance not only for optimizing construction control to prevent excessive SBS degradation, but also for designing advanced modifiers and rejuvenators that target the critical thermal aging transition.