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In practical engineering construction, differences in time intervals during concrete pouring arise due to issues in concrete quality control and construction procedures, thereby affecting the mechanical and durability properties of concrete. This study conducted compressive strength tests, splitting tensile strength tests, and natural immersion tests to investigate the influence of time intervals in layered pouring on the mechanical strength and chloride ion concentration distribution of staged pouring concrete. Additionally, the study elucidated the mechanism by which pouring interval time affects the mechanical properties and resistance to chloride ion erosion of staged pouring concrete at the microstructure level. The results indicate that compared to ordinary concrete specimens, the splitting tensile strength of staged pouring concrete demonstrates a continuous decrease with increasing pouring interval time. The most significant splitting tensile strength decrease occurred at a 24 h interval. The compressive strength of staged pouring concrete initially decreases and then increases with increasing pouring interval time. At a pouring interval time of 12 h, the compressive strength of staged pouring concrete decreased the most. Results from the natural immersion tests demonstrate that chloride ion concentrations at the bonding interface and on both sides of staged pouring concrete increase continuously with the extension of pouring interval time. The chloride ion concentration at the bonding interface is consistently higher than that on both sides, and the difference between them decreases with increasing diffusion depth. The chloride ion concentration difference ΔC was proposed to evaluate the influence of bonding interface performance on chloride ion concentration, which decreases to varying degrees with increasing depth. The findings of this study can provide guidance for the research on the mechanical properties and durability of staged pouring concrete in practical engineering construction, as well as for engineering protective measures. [ABSTRACT FROM AUTHOR]

In recent years, there has been unprecedented development of marine engineering construction in China. One such development has been the introduction of geopolymer (GP) concrete, which differs from portland cement in terms of durability and corrosion resistance in marine environments, and is also a low-carbon construction material. This study developed a low-shrinkage modified GP (MGP) concrete and researched its potential engineering applications. The research prototype was the Xigang seawall in the Guangdong Province of China. The effects of working performance, compressive strength, temperature rise, and strain of the concrete on the construction were assessed. The results showed that the autogenous shrinkage, chemical shrinkage, and drying shrinkage of the MGP paste were significantly lower than those of conventional GP concrete. The mechanism of shrinkage compensation of the MGP paste was discussed from the viewpoint of mesostructure and thermodynamics. The MGP concrete demonstrated smaller temperature rise and less difference between the internal and external temperatures than did portland cement concrete. Therefore, it featured a lower risk of cracking and was found to be more suitable than portland cement concrete for the rapid construction of seawalls. [ABSTRACT FROM AUTHOR]

• 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]

To improve the durability of Basalt fiber reinforced polymer (BFRP) bars reinforced geopolymer concrete (GPC), it is important to study the time-dependent variation of the corrosion resistance ability of GPC and BFRP in a seawater environment. This paper presents an experimental investigation to study the time-dependent mechanical properties and durability of BFRP bars and geopolymer materials synthesized by granulated blast furnace slag (GGBFS), fly ash, and silica fume. The resulting GPC and Portland cement (PC) concrete were exposed to artificial seawater. The mechanical properties of GPC were evaluated by analyzing and comparing the volume expansion and strength loss rates of GPC and PC concrete in an artificial seawater environment. The corrosion resistance of geopolymer (GP) mortar and PC mortar was evaluated by studying the migration ability and pore structure in corrosive ions attack (Cl−, SO 4 2−, Mg2+) in artificial seawater. Moreover, the time-dependent tensile strength of BFRP was comparatively investigated by immersing in different solutions (tap water, artificial seawater, and alkaline simulated seawater). In addition, the dual interface transition zones (ITZs) characteristics of BFRP reinforced GPC under artificial seawater were also investigated by SEM and BSE tests. The results showed that the volume expansion rate and strength loss rate of GPC decreased by 77.6% and 8.7%, respectively, after 360 days of seawater corrosion compared with PC concrete. This enabled the development of a time-dependent strength model of GPC in marine environments. The coefficient of ions diffusion in GP mortar is much lower than that of PC mortar, and GP mortar shows excellent resistance to ion migration. In addition, the effect of seawater corrosion on the tensile strength of BFRP bars increases with the increase of bars' diameter, and the ultimate strengths of BFRP bars with diameters of 6 mm and 8 mm were 695 MPa and 663 MPa, respectively. The tensile strength degradation model of BFRP bars in geopolymer concrete under seawater corrosion was established. After 360 days of seawater immersion, the average porosity of the ITZ between geopolymer and aggregates, and the average porosity of the ITZ between geopolymer and BFRP bars increased insignificantly compared to that of PC concrete. This research can provide a theoretical basis for the service life prediction of BFRP reinforced geopolymer concrete within marine environments. [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]

• A very early study on crack resistance of PSRC members under eccentric tension. • Using CFRP tendons for prestressing SRC members under eccentric tension. • Test data-enhanced analytical method for cracking load estimation. • Design formula for estimating the crack resistance. In recent decades, steel-reinforced concrete (SRC) members under eccentrical tension have been adopted more frequently in modern buildings due to the constraints of more complex structural design/detailing that accompanying contemporary architectural creations. Crack resistance of this type of structures has become a critical design consideration but received insufficient research attention in the past. In this context, the anti-cracking performance of prestressed SRC columns reinforced with CFRP tendons under eccentric tensile loads is investigated experimentally and analytically with the following first-hand experimental validations: 1) prestressed CFRP tendons can well strengthen SRC columns under eccentric tensile load and restrain the crack propagation; 2) an increase in the load eccentricity progressively weakened the influence of prestressing tension level on the cracking load; 3) specimens with the higher prestressed tension level, larger longitudinal reinforcement diameter, and flange thickness exhibited greater crack resistance capacity. Furthermore, the plane-section assumption was also validated in this study. A test data-enhanced analytical method was proposed for determining the cracking load of PSRC columns. [ABSTRACT FROM AUTHOR]