Xun Zhang

Geopolymer composites have the advantages of low carbon and solid waste consumption, but they are highly brittle. However, the effect and mechanism of polyvinyl alcohol fiber (PVAF) on the toughening of geopolymers remain unclear. Therefore, this study investigated the bending toughness (BT) and crack fractal characteristics of PVAF‐reinforced geopolymer composites (PVAFRGC). By adjusting the water–binder ratio (0.44 and 0.47), fiber length (6 and 12 mm), fiber volume fraction (0.0%–2.5%) and fiber factor, combined with the four‐point bending test and crack fractal dimension calculation, the influence of PVAF parameters on the mechanical properties of the material was systematically analyzed. The results showed that the addition of PVAFs significantly improved the toughness of the material. Especially when the PVAF volume fraction exceeded 1.5%, the composite material exhibited typical pseudo‐strain hardening behavior, with a maximum deflection of up to 3.5 mm. There was a significant exponential relationship between the crack fractal dimension and the bending strength, bending–compressive ratio, and BT, indicating that the crack complexity was closely related to the energy dissipation capacity. In addition, the BT evaluation method based on JSCE‐SF4 was more suitable for the toughness analysis in the large‐deformation stage than ASTM C1018. This study provides a theoretical basis for the toughness optimization and crack evolution mechanism of PVAFRGC.

In this investigation, a U-shaped connection (U-C) was introduced for a prefabricated steel plate shear wall with beam-only connections. This design replaces a portion of shear bolts with tension bolts to enhance bolt efficiency and reduce the overall bolt count, and it is suitable for a prefabricated high-rise steel structure. Four 1/3 scale specimens were designed, and an array of performance aspects including failure modes, hysteretic behavior, skeleton curves, energy dissipation capacity, stiffness degradation, and key performance indicators were systematically investigated through a combination of quasi-static tests and finite element analyses. The results showed that the combination arrangement of tensile and shear bolts in U-C effectively reduced the usage of bolts used and reduced installation costs; under low cycle reciprocating loads, the ultimate bearing capacity and energy dissipation capacity of a beam-only-connected prefabricated steel plate shear wall with U-shaped connection (BPSW-U) had slightly decreased compared with the prefabricated steel plate shear wall with discontinuous cover-plate connection (DCPC). Nevertheless, the BPSW-U excelled in preserving the integrity of the frame beams, channeling structural plastic deformations primarily into the infilled steel plate. This design feature ensures that post-earthquake functionality recovery can be achieved by simply replacing the infilled steel plate.

Geopolymer is a green substitute for Portland cement but has low tensile strength, high brittleness and easy cracking. Therefore, fibers and nanomaterials are used to strengthen and toughen geopolymer composites. The influence of nano-calcium carbonate and PVA fiber on the properties of fresh and hardened geopolymer mortar were studied herein. The hybrid of long and short fibers with small content (0.8 vol.%) is conducive to flowability, while the hybrid with large content (1.6 vol.%) and nano-calcium carbonate is conducive to flowability. The slump flow and flow rate of geopolymer mortars with low fiber factor (product of fiber volume fraction and length-diameter ratio) decrease with the growth in nano-calcium carbonate content. As the PVA fiber factor reaches 464.8%, the slump flow and flow rate values of mortars with 0, 1 wt.% and 2 wt.% nano-calcium carbonate are close to each other. About 450% is the density packing threshold of PVA fiber in geopolymer composites. The combination of 0.8 vol.% 12 mm + 0.4 vol.% 6 mm fiber + 1 wt.% nano-calcium carbonate presents the highest flexural strength and flexural to compressive strength ratio, with a compressive strength of about 36 MPa. The optimal fiber factor range of PVA fiber in cement and geopolymer mortar is about 400% and higher than 600%, respectively. PVA fibers show more effective enhancement of flexural strength and toughness in geopolymer than cement mortar. The ultrasonic wave velocity and apparent density of geopolymer mortar show a downward trend as a whole with the increase in fiber factor. The intensity rise of the hump between 17° and 38° (2 θ) in the XRD pattern is observed. The SEM indicates that the surface of PVA fiber in geopolymer mortar with nano-calcium carbonate is heavily scratched, and the fiber filaments are rolled up, demonstrating improved bonding between PVA fiber and geopolymer mortar.

Fibre can enhance the mechanical properties of cement-based composites, but fibre also degrades their workability. However, the quantitative effects of fiber content and length-diameter ratio on alkali-activated materials are still unclear. Various aspect ratio, volume fraction of polyvinyl alcohol fibre (PVAF), and various water-binder ratio were employed to prepare a total of 26 groups of fibre reinforced alkali-activated composite (FRAAC). The influence of PVAF fibre factor (product of fiber volume fraction and length-diameter ratio) on flowability, compactness, strength, and crack fractal dimension of FRAAC was researched. The influence of water-binder ratio on the plastic viscosity of FRAAC was more significant than that on the yield stress. When fibre factor was lower than critical value of 150, the influence of fibres could be overlooked. The reason was that the space between fibres was distant, so the flowability of FRAAC was not impacted by PVAF. At this time, fibres were challenging to restrict the cracks in matrix and increase their mechanical properties. When fibre factor was higher than critical value 150 and lower than density packing value 450, the flexural strength, compressive strength and crack fractal dimension of FRAAC were considerably enhanced, and the FRAAC could still flow easily under dead weight. When fibre factor were above 450, noteworthy fibre balling considerably decreased the flowability, leading to poor solidity and reduced compressive strength. Hence, the ideal content of PVAF in alkali activated composite is between 150/(l/d) and 450/(l/d).

In order to realize the self-centering, high energy consumption, and high ductility of the existing building structure through strengthening and retrofit of structure, a method of reinforced concrete (RC) beam strengthened by using Shape Memory Alloy (SMA) and Engineered Cementitious Composites (ECC) was proposed. Four kinds of specimens were designed, including one beam strengthened with enlarging section area of steel reinforced concrete, one beam strengthened with enlarging section area of SMA reinforced concrete, beam strengthened with enlarging section area of SMA reinforced ECC, and beam strengthened with enlarging section area of steel reinforced ECC; these specimens were manufactured for the monotonic cycle loading tests study on its bending behavior. The influence on the bearing capacity, energy dissipation performance, and self-recovery capacity for each test specimens with different strengthening materials were investigated, especially the bending behavior of the beams strengthened by SMA reinforced ECC. The results show that, compared with the ordinary reinforced concrete beams, strengthening existing RC beam with enlarging section area of SMA reinforced ECC can improve the self-recovery capacity, ductility, and deformability of the specimens. Finally, a revised design formula for the bending capacity of RC beams, strengthened with enlarging sections of ECC, was proposed by considering the tensile capacity provided by ECC, and the calculated values are in good agreement with the experimental value, indicating that the revised formula can be well applied to the beam strengthening with enlarging section of SMA-ECC Materials.