This paper presents the authors’ newly developed hybrid ultra-high performance (HUHP) engineered cementitious composite (ECC) with steel (ST) and polyethylene (PE) fibers. From this point on it will be referred to as HUHP-ECC. The volumes of steel and PE fibers were adjusted to obtain different mechanical properties, including compressive strength, tensile, and flexural properties. We found that tensile and flexural properties, including bending strength and ductility indexes, increased with higher PE fiber amounts but reduced with the increased ST fiber volume. Notably, the compressive strength had the opposite tendency and decreased with increases in the PE volume. The ST fiber had a significantly positive effect on the compressive strength. The fluidity of HUHP-ECC improved with the increasing amount of ST fiber. The pseudo strain-hardening (PSH) values for all the HUHP-ECC mixtures were used to create an index indicating the ability of strain capacity; thus, the PSH values were calculated to explain the ductility of HUHP-ECC with different fiber volumes. Finally, the morphology of PE and ST fibers at the fracture surface was observed by an environmental scanning electron microscope (ESEM).

This paper presents the results of an experimental study on the behavior of predamaged concrete prisms that were strengthened with fiber-reinforced polymer (FRP) jackets. Tests were conducted on 46 concrete columns involving variations of corner radius ratios, damage levels, and confinement pressure. This paper investigates the effect of corner radii on the main parameters of the stress–strain curve for FRP-strengthened predamaged specimens, such as compressive strength, strain capacity, and initial elastic modulus. The test results demonstrated that the efficiency of repairing damaged columns greatly depends on the corner radius ratio. The strength gains after FRP strengthening decrease as the corner radius ratio reduces. Furthermore, this paper extends a well-established stress–strain model originally proposed for FRP-confined predamaged circular concrete columns to square columns. More accurate expressions for the strength, deformation capacity, and stress–strain relationship were developed considering both the corner radius ratio and predamaged level. The proposed model was verified by test results and can predict the stress–strain behavior of a FRP-confined predamaged concrete column with good accuracy.

In recent years, the application of engineered cementitious composites (ECCs) in structures subjected to cyclic fatigue loading, such as highway bridges, has gained widespread attention. However, most existing ECCs do not have sufficient strength and ductility, which limits their applications, especially in highway bridge structures under high-stress. In this work, an ultra-high performance engineered cementitious composite (UHP-ECC) was configured, which had a compressive strength of approximately 120 MPa, a tensile strength of up to 12 MPa, and a tensile strain capacity of more than 8%. This paper presents a study of the fatigue performance of UHP-ECC at four different fatigue stress levels through the four-point bending test. The mid-span deflection of the specimen was monitored along with the crack opening displacement (COD) of the pure bending section at the bottom of the specimen, and the crack width. In addition, the dissipated energy was calculated at various stress levels. The progressive formation of cracks under static loading was monitored using the digital image correlation (DIC) technique. The fibers at the fractured surface of the specimens were observed and analyzed by environmental scanning electron microscopy, and the morphology of the fibers was obtained at different fatigue stress levels. Eventually, the fatigue life under different stress levels was obtained, and the relationship between the fatigue life and the stress level was established.

The in situ application of recycled aggregate concrete (RAC) is of great significance in environmental protection and construction resources sustainability. However, it has been limited to nonstructural purposes due to its poor mechanical performance. External confinement using steel tubes and fiber-reinforced polymer (FRP) can significantly improve the mechanical performance of RAC and thus the first-ever study on the axial compressive behavior of glass FRP (GFRP)-confined RAC was recently reported. To have a full understanding of FRP-confined RAC, this paper has extended the type of FRP and presents a systematic experimental study on the axial compressive performance of carbon FRP (CFRP)-confined RAC. The mechanical properties of CFRP-confined RAC from the perspective of the failure mode, ultimate strength and strain, and stress–strain relationship responses were analyzed. Integrated with existing experimental data of FRP-confined RAC, the paper compiles a database for the mechanical properties of FRP-confined RAC. Based on the database, the effects of FRP type (i.e., GFRP and CFRP) and the replacement ratio of recycled coarse aggregate were investigated. The results indicated that the stress–stain behavior of FRP-confined RAC depended heavily on the unconfined concrete strength and the FRP confining pressure instead of the replacement ratio. Therefore, this study adopted eleven high-performance ultimate strength and strain models developed for FRP-confined normal aggregate concrete (NAC) to predict the mechanical properties of FRP-confined RAC. All the predictions had good agreement with the test results, which further confirmed similar roles played by FRP confinement in improving the mechanical properties of RAC and improving those of NAC. On this basis, this paper finally recommended a stress–strain relationship model for FRP-confined RAC.

This paper presents the development of a novel, multifunctional, floatable, lightweight cement composite (FLCC) using three different types of glass microspheres for structural engineering applications. Eight different mixtures of FLCC were produced and their matrix-related parameters were examined experimentally by adopting different types of microsphere fillers, fiber content (polyethylene fibers (PE)), and water-to-binder ratios. Along with the mechanical properties such as compressive, flexural, tensile strengths, and modulus of elasticity, the water tightness of the material was evaluated by sorptivity measurements and the energy efficiency by thermal conductivity. The optimal FLCC has an oven-dry density of 750 kg/m³, compressive strength (f_cm) up to 41 MPa after 28-day moist curing, low thermal conductivity of 0.152 W/mK, and very low sorptivity. It is found that an optimized amount of PE fiber is beneficial for improving the tensile resistance and ductility of FLCC while a relatively large amount of microspheres can increase the entrapped air voids in the FLCC matrix and reduce its density and thermal conductivity. Microstructural analysis by scanning electron microscopy (SEM) reveals that the microspheres are distributed uniformly in the cement matrix and are subjected to triaxial compression confinement, which leads to high strength of FLCC. Segregation due to density difference of FLCC ingredients is not observed with up to 60% (by weight) of glass microspheres added. Compared to the other lightweight aggregate concretes, the proposed FLCC could be used to build floating concrete structures, insulating elements, or even load-bearing structural elements such as floor and wall panels in which self-weight is a main concern.

Limestone calcined clay cement (LC³) is successfully used to fabricate engineered cementitious composites (ECC) exhibiting tensile strength σ_tu of 9.55 ± 0.59 MPa or tensile strain capacity ε_tu of 8.53 ± 0.30%. The high tensile strength of the composites is closely related to the improvement of fiber/matrix interfacial bond strength, and the high ductility is attributed to the enhancement of fiber dispersion homogeneity. For the case of ECC incorporating 50% LC³, the reduction of initial cracking stress σ_tc that favors the growth of the crack in a controlled manner also contributes to the improvement of strain hardening behavior. The composition analysis indicates that carboaluminates and additional hydration products including C-(A)-S-H and ettringite are generated, which contributes to the densification of the microstructure of the ECC matrix. The pore structure is thus remarkably refined. Besides, when ordinary Portland cement (OPC) is partly replaced by LC³, the consumed energy and equivalent CO₂ emission decrease, especially the equivalent CO₂ emission with the reduction ratio attaining 40.31%. It is found that ECC using 35% LC³ exhibits the highest mechanical resistance and ECC incorporating 50% LC³ shows the highest ductility from the environmental point of view.