Auxetic textile structures are defined as structures that exhibit a negative Poisson's ratio, which means they expand transversally up on application tensile forces and vice versa. Auxetic structures are potentially an up-and-coming option to reinforce concrete structures to enhance energy absorption, fracture toughness and crack resistance. The thesis aims to reinforce concrete structures with the developed high-stiffness auxetic textile structures. A novel modified helical auxetic yarn (MHAY) was made based on the modified helical auxetic yarn (HAY). The yarn was composed of a thin steel wire as secondary core II. The core II was parallel to the low modulus thick Nylon filament primary core I. The multifilament steel wires strand wrapped this core I and II assembly at a nominal wrap angle of 7º. The elastic moduli of the final resultant yarn samples were in the range of 11 to 15 GPa. The samples exhibited a high negative Poisson's ratio of about -7 to -63. The insertion of the secondary core II increased the initial secant modulus of the auxetic yarn by approximately 50%. To produce auxetic yarns at a large scale, a purposely built machine was designed and made. The auxetic structure-cementitious matrix interfacial bond was eva‎luated using a single-sided pull-out test. Results confirmed the superior pull-out resistance of the auxetic reinforcement compared to the conventional reinforcement due to frictional resistance against pull-out from the cementitious matrix. The pull-out test also confirmed the 75% increase in the auxetic reinforced structure's toughness compared to the non-auxetic reinforced cementitious structures. Based on the standard four-point bending test, the flexural strength of the concrete prism reinforced with the developed auxetic and non-auxetic fiber-reinforced polymer (FRP) strings was eva‎luated. The results demonstrated the increased flexural toughness in the auxetic reinforced concrete prism compared to the non-auxetic reinforced concrete prism. Auxetic FRP strings have a specific strength almost twice that of steel bars. Therefore, auxetic reinforced concrete structures provide a lighter design compared to steel-reinforced structures. Compared to steel bar reinforced structures, auxetic reinforced concrete structures show better crack distribution and multiple cracks, leading to better serviceability. Furthermore, results indicate a 42% increase in the toughness of the auxetic reinforced concrete prism compared to the steel bar reinforced concrete prism. The results also show that if there is a perfect bond between the auxetic FRP strings and the concrete matrix, the auxetic structures could fully participate in the flexural strength of the concrete prism. Results show that in the concrete section designed with identical flexural capacity, the concrete section reinforced by auxetic FRP structure compared to the concrete section reinforced by non-auxetic FRP structure shows higher flexural strength. In other words, a better bond between auxetic structure and cementitious matrix due to the auxetic effect is auspicious to benefit the full flexural capacity of the designed section.

محمد ذره بيني اصفهاني , داود مستوفي نژاد , مهدي ابطحي فروشاني

محسن شنبه

مهدي حجازي , محمدرضا افتخار , محمدعلي رهگذر