Polyvinyl Alcohol Fiber Length Optimization for High Ductility Cementitious Composites with Different Compressive Strength Grades

The fiber length has a significant impact on the fiber bridging capacity and the mechanical properties of high ductility cementitious composites (HDCCs), which is related to fiber/matrix interfacial bonding. However, this fundamental knowledge of HDCCs design has rarely been investigated systematically. To this end, this study deeply investigates the effect of the fiber length on the bridging stress and the complementary energy with various fiber/matrix interfacial bonds in theory. Then, the mechanical performances of HDCCs with various fiber lengths and compressive strengths were evaluated experimentally. In micromechanical design, longer fibers can achieve stronger bridging stress and more sufficient complementary energy regardless of the fiber/matrix interfacial bonding properties. However, it should be noted that the increase in bridging capacity was quite slow for the overlong fibers and excessive interfacial bonding. The experiments indicated that overlong fibers (18 mm and 24 mm) easily twined on the mixer blade and were hard to disperse evenly. The HDCCs with shorter fibers displayed better workability. The compressive strength was less affected by the fiber length, and most striking differences were less than 5.0%, while the flexural properties and the tensile properties first increased and then decreased when the fiber length ranged from 6 mm to 24 mm. Consequently, the fibers with lengths of 9 mm and the fibers with lengths of 12 mm were better candidates for the HDCCs with compressive strengths of 30 MPa to 80 MPa, and fibers with lengths of 9 mm caused the HDCCs to exhibit higher ductility properties in general.

Toughening and Healing of CFRPs by Electrospun Diels–Alder Based Polymers Modified with Carbon Nano-Fillers

In the present investigation, thermo-reversible bonds formed between maleimide and furan groups (Diels–Alder (DA)-based bis-maleimides (BMI)) have been generated to enable high-performance unidirectional (UD) carbon fiber-reinforced plastics (CFRPs) with self-healing (SH) functionality. The incorporation of the SH agent (SHA) was performed locally, only in areas of interest, with the solution electrospinning process (SEP) technique. More precisely, reference and modified CFRPs with (a) pure SHA, (b) SHA modified with multi-walled carbon nano-tubes (MWCNTs) and (c) SHA modified with graphene nano-platelets (GNPs) were fabricated and further tested under Mode I loading conditions. According to experimental results, it was shown that the interlaminar fracture toughness properties of modified CFRPs were considerably enhanced, with GNP-modified ones to exhibit the best toughening performance. After the first fracture and the activation of the healing process, C-scan inspections revealed, macroscopically, a healing efficiency (H.E.) of 100%; however, after repeating the tests, a low recovery of mechanical properties was achieved. Finally, optical microscopy (OM) examinations not only showed that the epoxy matrix at the interface was partly infiltrated by the DA resin, but it also revealed the presence of pulled-out fibers at the fractured surfaces, indicating extended fiber bridging between crack flanks due to the presence of the SHA.

Effect of Raw Sugarcane Bagasse Ash as Sand Replacement on the Fiber-Bridging Properties of Engineered Cementitious Composites

The use of raw sugarcane bagasse ash (SCBA) as sand replacement in the production of engineered cementitious composites (ECCs) can improve its cost-effectiveness and practicality. A recent study by the authors showed that the use of raw SCBA as a replacement to sand in ECC mixtures substantially enhances the tensile ductility and provides mild improvements in tensile strength; however, it also indicated a need to further elucidate the mechanisms producing such improvements. Therefore, the present study examined the effects of raw SCBA as a sand replacement in ECC’s fundamental fiber-bridging relationship, [Formula: see text], through single crack tensile test (SCTT) using 1% polyvinyl alcohol (PVA) fiber volume fraction. The PVA fiber volume fraction was reduced from 1.5% in the previous study to 1% in this study to ensure that a single crack was produced, which is a necessary condition to obtain the fundamental [Formula: see text] relationship. A total of five mixtures were evaluated at different replacement levels of sand with raw SCBA (i.e., 0%, 25%, 50%, 75%, and 100%). SCTT results revealed that raw SCBA produced minor effects on the fiber-bridging capacity but significantly increased the complementary energy ( [Formula: see text]). A positive correlation was observed between the pseudo strain-hardening (PSH) strength index and raw SCBA content. Since the PSH strength index was higher than the recommended value (i.e., 1.3) for robust PSH behavior, it was concluded that the main factor contributing to tensile ductility enhancements was the increase in the PSH energy index resulting from the notable increase of [Formula: see text] and potential decrease in matrix fracture toughness.

Shear and flexure behavior of hybrid composite beams with high performance concretes

Shear and flexure performances of composite beams with different engineered cementitious composites (ECC) to self-consolidating concrete (SCC) depth ratio were investigated. Shear reinforced composite ECC/SCC beams showed similar behavior compared to their non-shear reinforced counterparts until the formation of diagonal cracks but exhibited higher ultimate shear resistance and ductility. Compared to the full depth SCC and full depth ECC beams, non-shear reinforced composite ECC/SCC beams showed higher ductility and energy absorption capacity. Composite ECC/SCC beams showed higher number of cracks with lower crack width because of fiber bridging and micro-cracking characteristics of ECC. Code based equations and other design specifications were conservative in predicting shear strength of shear/non-shear reinforced composite ECC/SCC beams. Composite ECC/SCC flexure beams showed satisfactory flexural performance compared to their full depth ECC and SCC counterparts.

Shear and flexure behavior of hybrid composite beams with high performance concretes

Shear and flexure performances of composite beams with different engineered cementitious composites (ECC) to self-consolidating concrete (SCC) depth ratio were investigated. Shear reinforced composite ECC/SCC beams showed similar behavior compared to their non-shear reinforced counterparts until the formation of diagonal cracks but exhibited higher ultimate shear resistance and ductility. Compared to the full depth SCC and full depth ECC beams, non-shear reinforced composite ECC/SCC beams showed higher ductility and energy absorption capacity. Composite ECC/SCC beams showed higher number of cracks with lower crack width because of fiber bridging and micro-cracking characteristics of ECC. Code based equations and other design specifications were conservative in predicting shear strength of shear/non-shear reinforced composite ECC/SCC beams. Composite ECC/SCC flexure beams showed satisfactory flexural performance compared to their full depth ECC and SCC counterparts.