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• Seawater (SW), sea sand (SS) and recycled coarse aggregate (RA) were mixed in SWSSRAC. • Effects of SW, SS, and RA on the compressive properties of SWSSRAC were studied. • Stress–strain curves of SWSSRAC at three curing ages were investigated. • Constitutive models for SWSSRAC at different curing ages were discussed. Resources such as fresh water and river sand have become scarce in some areas around the world because of the considerable increase in infrastructural construction. To overcome this issue, the utilization of recycled aggregates (RAs) in concrete is considered a sustainable construction method. Due to the growing scarcity of river sand and fresh water, this study explores the mechanical properties of recycled aggregate concrete (RAC) incorporated with seawater (SW) and sea sand (SS), referred to as SWSSRAC. To this end, a total of 18 mix ratios were designed to analyze the effects of different water-to-cement ratios, curing ages, and RA replacement rates on the mechanical properties of the SWSSRAC. The results suggest that the fluidity of SWSSRAC is slightly worse than that of RAC; further, the effect of the RA replacement rate on the fluidity of concrete is greater than that of SW and SS. The stress–strain relationship reveals that the deformation capacity of SWSSRAC at curing ages of 28 and 180 days show a higher improvement attributed to SW and SS for RAC compared to that for natural aggregate concrete (NAC). In addition, using SW and SS improves the compressive strength and elastic modulus of RAC, particularly after curing for 28 days; the enhancement effect of SW and SS on the mechanical properties of RAC is higher than that of NAC. Although the unloading segments in the stress–strain relationship of SWSSRAC are different from those of RAC at the 28-day curing age, it is necessary to use constitutive models of RAC for SWSSRAC considering long-term use. According to the findings of this study, SW and SS are more suited for RAC than NAC. [ABSTRACT FROM AUTHOR]

In the exploration of the moon and outer space, our preliminary mission lies in the construction of lunar base. To make lunar concrete with local materials on the moon becomes the key technology to promote the construction of lunar bases. In order to further study the feasibility of using lunar in-situ resources to prepare concrete and build a lunar base, the environment and resources on the lunar surface are analyzed, and the severe challenges brought by extreme environment to concrete preparation and the prerequisites of in-situ resources are clarified. In this paper, the research progress of lunar concrete is summarized from the aspects of raw material acquisition, concrete preparation methods and performance, and the comparative analysis of cement concrete, sulfur concrete, geopolymer concrete and polymer concrete is carried out. Existing studies have shown that in the acquisition of raw materials, sulfur concrete has more advantages, cement concrete and geopolymer concrete are also feasible, but polymer, as a scarce resource on the moon, is difficult to obtain. In terms of preparation methods, cement concrete and geopolymer concrete are more suitable for the production of prefabricated components in artificial environment due to the limitation of external environment and the demand of water circulation, while the in-situ preparation methods represented by sulfur concrete and polymer concrete can be used for the connection and node reinforcement of prefabricated components on the moon. In terms of performance, the mechanical properties of the four kinds of concrete all meet the basic requirements, but the service performance in the harsh environment of the moon needs to be further studied. Meanwhile, the key location characteristics for lunar base construction in different regions are analyzed in terms of topography, environment and in-situ resources. Finally, the future exploration direction of the construction of the lunar bases is proposed. • Comparative study on lunar concretes: cement, sulfur, geopolymer, and polymer. • Sulfur concrete's raw material acquisition advantage; polymer's scarcity on moon. • Prefabrication versus in-situ production methods for lunar habitat construction. [ABSTRACT FROM AUTHOR]

With the development of low-carbon sustainability, the recycling of waste concrete and waste tires to produce rubber-modified recycled aggregate concrete (RRAC) has become imperative. The compressive properties of RRAC after exposure to elevated temperatures were investigated in this study. The stress–strain curve, compressive strength, deformation performance, and bursting resistance of RRAC samples with rubber contents of 0%, 4%, and 9% were studied after exposure to 20, 200, 400, and 600 °C for 60 min. Furthermore, a single equation for the stress–strain curve of RRAC was selected and compared with the experimental results. The results suggested that the addition of rubber particles could significantly reduce the mass loss, improve the relative compressive strength, and increase the failure strain of recycled concrete. When the rubber content was large, additional pores formed after rubber melting, which led to a higher porosity in the specimen. Meanwhile, the internal structure became excessively loose, which resulted in a decrease in strength. Based on the obtained results, a rubber content of 4% is recommended for use in RRAC. [ABSTRACT FROM AUTHOR]

• The fracture tests on CRC were conducted by three devices. • The loading rate improved the dynamic fracture toughness and fracture energy. • The relationship between DIF (DIF g and DIF k) and loading rate was analysed. • The mechanism of loading-rate sensitivity on fracture characteristics was obtained. The dynamic mechanical properties of concrete materials significantly influences the safety of construction under impact loads. In this study, a comprehensive experiment on the crack straight-through flattened Brazilian disc specimens of crumb rubber concrete (CRC) is undertaken. Three specific experiments were performed to obtain the wide loading-rate ranges of fracture characteristics, i.e., the quasi-static test, drop-hammer impact test, and split Hopkinson pressure bar test. The measured and calculated results are also discussed. The results reveal that the fracture toughness and energy are both positively correlated with loading rate. Moreover, a transition loading rate (around 1 GPa × m1/2/s) exists between the high and low loading rates, and the enhancing effect of high loading rate on fracture toughness cannot be neglected. The analysis of the dynamic increased factor (DIF) on the fracture toughness (DIF k) and fracture energy (DIF g) of CRC shows that DIF k and DIF g are related to the rubber content. Furthermore, the analysis demonstrates that crumb rubber particles can enhance the crack resistance of concrete under dynamic loads. Accordingly, empirical models for the DIF k and DIF g of CRC are proposed based on the experimental data. In addition, the mechanisms by which the loading-rate affects the fracture characteristics were preliminarily identified by observing the failure modes under various loading-rate loads. This study is intended to help spread the application of crumb rubber in concrete materials. [ABSTRACT FROM AUTHOR]

Electromagnetic wave (EMW) pollution poses a tremendous effect on information security and human health. However, ordinary concrete structure incorporated by ferrite fiber lacks EMW absorption flexibility to form electromagnetic superstructures. 3D printing technology paves an effective way to facilitate the anisotropy of electromagnetic absorption capacity by generating directional effects on ferrite fiber. This research evaluates the influence of 3D-printed fiber-oriented superstructure on EMW absorption performance with an equivalent waveguide attenuator model. The microwave-absorbing cementitious composite (10% magnetite and 25% copper slag) was prepared to incorporate 0.5 wt% copper fibers (CF) and steel fibers (SF), respectively. Absorption elements in each group are prepared by laminar parallel printing, cross-printing, and zigzag printing. In addition, the EMW absorption capability (ranging from 2 GHz to 18 GHz) was investigated by the Naval Research Laboratory (NRL) equipment. The overall EMW absorption performance of the SF samples is superior to CF samples. The optimized order of the EMW absorption performance of CF-reinforced samples is determined as zigzag, parallel, and cross printing, while SF is parallel, cross, and zigzag printing methodology. Overall, the laminar parallel printed steel fiber element gave the best shot with a peak reflectivity of − 16.34 dB and an absorption bandwidth of 13.15 GHz. Meanwhile, SF-reinforced specimens all demonstrated absorption peaks around 8 GHz, while CF-incorporated samples' absorption peaks appeared at both 8 GHz and 12 GHz, offering multiple design and application choices according to engineering requirements. Finally, an equivalent attenuator model is suggested for illustrating the superimposed reinforcement, spatial impedance matching, and multiple scattering of dielectric properties. • The prominent fiber orientation effect induced by 3D printing facilitates the enhancement of EMW anisotropic properties. • Copper fibers with slender dimensions form abundant and flexible multi-size equivalent waveguide attenuator structures that promote vector superposition and interference effects, exhibiting RL peaks at both 7 GHz and 12 GHz. • The steel fiber-reinforced samples are superior to the fine copper fiber samples in terms of overall electromagnetic absorption performance. The steel fibers and the surrounding concrete exhibit various gradients in their ability to adsorb EMW, resulting in a completely absorbing superstructure. • The electromagnetic wave absorption mechanism of the equivalent waveguide attenuator model for the fiber-reinforced composite is analyzed in detail. [ABSTRACT FROM AUTHOR]

High-performance cement-based composite materials are commonly employed in marine structural engineering. However, the deterioration of mechanical and durability properties of composite structures caused by chloride erosion in marine environments is a typical engineering problem. This study aims to assess the impact of NS and PVA fibers on the strength, chloride resistance, and impact resistance of NS-PVA fiber mortar. The impact frequency distribution of NS-PVA fiber mortar was analyzed under two conditions: natural environment and after chloride erosion, utilizing both log-normal and two-parameter Weibull distribution models. The findings indicate that the impact energy consumption of NS-PVA fiber mortar increases after 28 days of chloride erosion compared to the natural environment, primarily due to the formation of a compact pore structure attributed to the reaction product Friedel's salt. The optimal composite dosage is determined to be 1.8% for NS and 1 vol% for PVA fiber. At this dosage, the PS18 mortar exhibits a 44.8% and 53.7% increase in initial cracking and failure impact energy, respectively, compared to conventional mortar in the natural environment. Furthermore, these enhancements are further amplified to 53.4% and 64.6% after chloride erosion. Concurrently, a negative correlation is observed between accelerated impact energy consumption and chloride diffusion depth in NS-PVA fiber mortar. A predictive model for impact energy consumption of NS-PVA fiber mortar following chloride erosion, considering various failure probabilities, is established, offering valuable insights for the practical application of NS-PVA fiber mortar in marine engineering. [Display omitted] • Synergistic Effect of NS and PVA fibers for superior strength and durability. • NS-PVA mortar enhanced the impact resistance especially after chloride erosion. • The optimal dosage is determined to be 1.8% for NS and 1 vol% for PVA fiber. • Predictive modeling for impact energy of NS-PVA mortar after chloride erosion. • Insights for optimizing NS-PVA fiber mortar in challenging marine engineering. [ABSTRACT FROM AUTHOR]

• A new concept of bio-based prestress technology. • Experimental proof-of-concept on bamboo-timber composite beams. • Industrial level of trial production of prestressed bamboo-timber composite beams. • Thorough lab investigation on the proposed new bio-based composite beams. This paper presents a pioneering experimental proof-of-concept study to validate a novel concept of prestress technology that used only pure bio-based composite materials while achieved consistent prestressed stress distribution within the structure member, and provided in-situ flexibility, improved structural performance, and maximised the rate of utilisation of each material. Industrial level of facilities were used during this development. The prestress is achieved by pressurised/forced lamination of multiple components with different materials and geometrical properties. The prestressing process is activated during the pressure release stage during which the components are interacting with each other, creating different stress statuses that would favour the weaker and adverse the stronger components to maximise the strength exploitation of different materials. Using laminated bamboo and timber as an example pair, twenty-two glulam, non-prestressed and prestressed laminated bamboo-timber composite beams were manufactured, tested, and analysed to provide an in-depth understanding of the structural behaviours of these novel structural members. Failure modes, yielding, ultimate and serviceability limit loads, and corresponding deflections, as well as the histories of strain development at key positions of the specimens were examined. The experimental study confirmed the feasibility, effectiveness and industrial scalability of the proposed technology. The novel concept provides a new approach for developing the prestress technology for bio-based materials, and this experimental study laid the foundation for its future analytical development and numerical studies. [ABSTRACT FROM AUTHOR]

• Bond behaviors between UHPC and deformed rebar were studied by a pullout test. • The effects of cover, embedded length and steel fiber on bond strength were analyzed. • A model for predicting the bond strength between pre- and post-yield rebar and UHPC was proposed. • The distribution of bond stress along the embedded length was analyzed. The ductility of reinforced concrete structures is often determined by the ductility of post-yield deformed rebar. However, to date, few studies have been conducted on the bond behavior of post-yield deformed rebars embedded in ultra-high performance concrete (UHPC). Therefore, a pull-out test was conducted to investigate the effects of the cover, embedded length, and steel fiber on the bond behaviors between UHPC, and pre- and post-yield deformed rebars. The test results showed that a 50% increase in the bond strength could be obtained when the cover was increased from 1 d to 2 d, where d is the diameter of the deformed rebar. As the anchorage length was increased from 2.5 d to 5 d , the bond strength decreased by approximately 50%. Meanwhile, the bond strength increased by 126% with an increase in steel fiber content from 0 to 2%. To predict the bond strength of the pre- and post-yield deformed rebar embedded in UHPC, a modified thick-walled cylinder model was proposed. Subsequently, the distribution of the bond stress along the embedded length was revealed, and the position function was provided. Finally, the bond stress–slip model including the anchorage position function provides an insight into the bond stress between UHPC and deformed rebars. [ABSTRACT FROM AUTHOR]

• The rubber-modified recycled aggregate concrete (RRAC) can be fabricated for low-carbon sustainability. The incorporation of RA and RPs can reduce the strength loss when concrete is exposed to high temperature. • Enhanced peak strain can be obtained through increasing the content of RA and RPs. The properties of RRAC are sensitive to the exposed temperature. • The hyperparameters of machine learning (ML) models were successfully tuned by beetle antennae search (BAS) algorithm. The established ML models possessed high accuracy and good generalisation performance. • The BPNN model possessed highest R value and lowest RMSE value compared to RF, LR, and MLR models, thus it had best prediction performance on the database in this study. The recycled aggregate (RA) and waste rubber particles (RPs) can be combined to prepare rubber-modified recycled aggregate concrete (RRAC) effectively contributing to low-carbon sustainability. However, the mechanical characteristics of RRAC must be investigated before the practical application. To this end, this study focused on the uniaxial compressive strength (UCS) and corresponding peak strain of RRAC with versatile design mixtures (i.e. varying contents of RA and RPs) after exposure to different temperatures ranging from 25 °C (room temperature) to 600 °C. The test results exhibited the negative relationship between UCS and RA replacement ratio, RPs content, and temperature. However, RPs positively affected both the loss of UCS and peak strain when RRAC was exposed to high temperatures. Besides, four machine learning (ML) models were developed based on a relatively comprehensive dataset including 120 groups of experimental results. The beetle antennae search (BAS) algorithm was applied to tune the hyperparameter of ML models. The high correlation coefficients (0.9721 for UCS and 0.9441 for peak strain) were determined in modelling using back propagation neural network (BPNN), presenting its accuracy and reliability. Furthermore, BPNN possessed optimal prediction performance since the lower root mean square error (RMSE) and higher correlation coefficient were obtained compared to the other three ML models (random forest, logistic regression, and multiple linear regression). [ABSTRACT FROM AUTHOR]

[Display omitted] • Shrinkage behaviour of geopolymer pastes with MgO was investigated. • The mechanism of shrinkage of geopolymer pastes with MgO were further analyzed. • The volume expansion produced by MgO reactions in geopolymer pastes could be calculated by a fitted model. • Low Ca/Si ratios of C-A-S-H gel exhibit compact microstructure in geopolymer pastes with MgO. Geopolymers (GI) possess excellent seawater erosion resistance because of stable hydration products and compact microstructures, and therefore, they have broad application prospects in marine engineering if their large volume shrinkage behavior can be mitigated. In this study, active MgO was used to compensate for different GI shrinkage stages and a scheme was proposed for staged compensation. Autogenous shrinkage was controlled by adjusting the content, activity, and combinations of active MgO with particle sizes of 1–100 µm. Based on the effects of MgO on shrinkage, GI paste composition and microstructure, the GI reaction process and mechanism of MgO-compensated volume shrinkage were examined and summarized. The results showed that the GI paste shrinkage decreased with MgO addition, and the higher the MgO activity, the smaller was the paste shrinkage in the early stages of hardening. Late-stage shrinkage of hardened paste was improved with the addition of low-activity MgO. Combining low and high activity MgO addition effectively ameliorated the paste hardening shrinkage at different stages. In the high-alkalinity environment of the liquid-phase GI reaction, the Mg(OH) 2 product presented microcrystalline structures, and it was dispersed in the cement matrix, which resulted in uniform volume expansion. Meanwhile, other reaction products containing magnesium were produced, filling pores in the hardened paste and effectively compensating for volume shrinkage during the hardening process. Based on the effects of MgO on the composition structure, reaction process, and volume shrinkage performance of GI slurries, this study proposes a mechanism for MgO to compensate for the volume shrinkage of GI. [ABSTRACT FROM AUTHOR]