In marine structural engineering, the impact resistance of concrete holds high significance. The determination of whether the combined use of expansion agent (EA) and glass fiber (GF) has a synergistic effect on the impact resistance of seawater–sea-sand concrete (SSC) and plays a role in its performance and application. In this study, the dynamic Brazilian disc test at various strain rates was carried out with an SHPB device to investigate the effect of mixing 0% and 6% EA with 0% and 1% GF on the dynamic splitting tensile properties of SSC. The results show that strain rate effect on EA and GF-reinforced SSC during dynamic splitting tensile tests at higher strain rates, indicating strong strain rate sensitivity. The synergistic reinforcement of EA and GF consumed more energy under impact loading, thus maintaining the morphological integrity of concrete. However, the dynamic splitting tensile strength obtained in the Brazilian disc test had a significant overload effect which cannot be ignored. EA doped at 6% and GF doped at 1% showed a synergistic enhancement of SSC’s dynamic splitting tensile properties.

The use of seawater to prepare geopolymers has attracted significant research attention; however, the ions in seawater considerably influence the properties of the resulting geopolymers. This study investigated the effects of magnesium salts and alkaline solutions on the microstructure and properties of ground-granulated-blast-furnace-slag-based geopolymers. The magnesium salt–free Na₂SiO₄-activatied geopolymer exhibited a much higher 28 d compressive strength (63.5 MPa) than the salt-free NaOH-activatied geopolymer (31.4 MPa), with the former mainly containing an amorphous phase (C-(A)-S-H gel) and the latter containing numerous crystals. MgCl₂·6H₂O addition prolonged the setting times and induced halite and Cl-hydrotalcite formation. Moreover, mercury intrusion porosimetry and scanning electron microscopy revealed that the Na₂SiO₄-activated geopolymer containing 8.5 wt% MgCl₂·6H₂O exhibited a higher critical pore size (1624 nm) and consequently, a lower 28 d compressive strength (30.1 MPa) and a more loosely bound geopolymer matrix than the salt-free geopolymer. In contrast, MgSO₄ addition had less pronounced effects on the setting time, mineral phase, and morphology. The Na₂SiO₄-activated geopolymer with 9.0 wt% MgSO₄ exhibited a compressive strength of 42.8 MPa, also lower than that of the salt-free geopolymer. The results indicate that Cl⁻ is more harmful to the GGBFS-based geopolymer properties and microstructure than SO₄²⁻ is.

This study aims to investigate the influence of three typical gradation schemes, including continuous gradation, discontinuous gradation, and single gradation, on the mechanical properties and water permeability of pervious concrete using recycled coarse aggregate (RCA) as aggregates. The changes in compressive strength, flexural strength, failure mode, connected porosity, and permeability coefficient of Recycled Pervious Concrete (RPC) using different gradation schemes and the correlations among these factors were investigated. The results indicate that the RPC prepared with discontinuous gradation RCA achieves optimal compressive and flexural strengths of 15.70 MPa and 4.22 MPa, respectively. Furthermore, RPC with discontinuous gradation and single gradation exhibits superior water permeability compared to that made with continuous gradation RCA, and its optimal range of the permeability coefficient can reach between 6.36 and 8.32 mm·s⁻¹. The RPC prepared using multi-gradation RCA is effective in enhancing the stability of changes in connected porosity and permeability coefficients. Furthermore, the relationship between connected porosity and permeability coefficients of RPC using different gradation schemes can be effectively characterized through nonlinear fitting, and the correlation coefficients R² of RPC using continuous gradation, discontinuous gradation, and single gradation are 0.94, 0.92, and 0.90, respectively. This study focuses on the influence of several typical gradation schemes on RPC performance and provides valuable data support for further optimization of its properties.

The durability of a concrete structure is affected by temperature cycles that occur during the structure’s service life. This paper presents an experimental and theoretical study of the mechanical properties of seawater sea-sand concrete when exposed to temperature variations. By using compressive tests on cylindrical concrete specimens, the effects of thermal cycling (e.g., the amplitude of temperature variations and cycling times) on the mechanical properties of seawater sea-sand concrete, such as failure modes, compressive strength, stress–strain relationship, Young’s modulus, ultimate strain, Poisson’s ratio and toughness are investigated. Microstructures of both unconditioned and conditioned concrete samples are examined by using scanning electron microscopy (SEM) to understand the mechanisms behind the strength changes. Finally, the stress–strain model is proposed for seawater sea-sand concrete subjected to daily temperature variations, and the proposed model is verified by the experimental data.

Precast concrete structure is a low-carbon building system that has been attracting extensive attention in recent decades. Beam–column joints are the weak links in precast concrete structures. Past studies showed that the five-spiral stirrups had excellent confinement effects and had the potential to enhance the seismic performance of concrete structures. This study proposed the reinforcement of precast concrete beam–column joints by using five-spiral stirrups and investigated their seismic performance. Considering the influences of the joint failure mode, joint type, construction method, and stirrup type, low-cycle loading tests were conducted on six full-scale precast concrete beam–column joint specimens. Various seismic behavior indicators, such as failure modes, hysteresis curves, skeleton curves, ductility, and energy dissipation, were obtained. The results indicated that the deformation capacity of the precast joints with five-spiral stirrups was comparable to that of cast-in-place joints. Under different failure design criteria, the seismic performance of the precast joints was superior to that of cast-in-place joints. Furthermore, the experimental capacities of the precast joints, using five-spiral stirrups, were higher than the calculated values according to the design code, demonstrating an adequate safety margin. This research contributes to the development of low-carbon and sustainable construction practices in the field of precast concrete structures.

With the development of the automotive industry, a large amount of waste rubber is produced every year. The application and development of recycled rubber concrete (RRC) can effectively reduce ‘black pollution’ caused by waste rubber. However, the addition of recycled rubber particles can lead to a decrease in the compressive behavior of concrete. Previous research has demonstrated that by preventing crack growth, fiber addition can increase the strength and ductility of concrete. In this work, a total of 28 RRC mixes are designed, and the compressive behavior of RRC reinforced by steel fibers (SFs) and glass fibers (GFs) is investigated. The workability of fresh RRC can be negatively impacted by an increase in both fiber contents, with the GF content having a more notable effect. With the addition of fibers, the maximum increase rates for the compressive strength, elastic modulus, strain at peak stress, and compressive toughness were 27%, 8%, 45%, and 152%, respectively. A constitutive model is concurrently put forward to forecast the stress–strain curves of RRC with various fiber contents. These findings indicate that the maximum improvement in compressive behavior is achieved when the GF content was 0.4% and the SF content was 1.2%. The proposed constitutive model can be used to predict the stress–strain curve of hybrid fiber-reinforced recycled rubber concrete (HFRRRC).