اثر جهت گیری الیاف در رفتار سازه از FRP پیچیده شده سیلندرهای بتن

In this study, 27 concrete cylinders with a diameter of 152.4 and a height of 304.8 mm were prepared. Among them, 18 cylinders were wrapped using two layers of fiber reinforced polymer (FRP) with six fiber orientations; six cylinders were wrapped using four layers of FRP with fibers in axial or hoop direction only; the remaining three cylinders were used as control. The FRP used was E-glass fiber reinforced ultraviolet (UV) curing vinyl ester. Fifteen coupon specimens were prepared to experimentally determine the tensile strength of the FRP with fibers oriented at 0°, 45°, and 90° from the loading direction. Co-axial compression tests were conducted on the wrapped cylinders and control cylinders. The test results were compared with existing confinement models. It is found that the strength, ductility, and failure mode of FRP wrapped concrete cylinders depend on the fiber orientation and wall thickness. Fibers oriented at a certain angle in between the hoop direction and axial direction may result in strength lower than fibers along hoop or axial direction. A larger database is desired in order to refine the existing design-oriented confinement models.

Concrete columns are fundamental structural components in civil structures. In recent years, repair, retrofitting, and strengthening of damaged concrete columns using fiber reinforced polymer (FRP) composite wrapping or jacketing are increasingly becoming important in civil infrastructures due to the high strength/stiffness to weight ratio, corrosion resistance, and ease of installation of FRPs [1], [2], [3] and [4]. Additionally, the structural capacity of FRP can be tailored and maximized by aligning fibers along the optimal orientation. For an ideal column, i.e., a column subjected to a co-axial compressive load, it is well established that fibers should be aligned along the hoop direction to confine the dilation of the concrete core. In practice, however, almost all the columns are subjected to an eccentric axial load, which can be resolved into a co-axial compressive load and a bending moment. Because of this, almost all the columns should be treated as beam-columns. For beam-column repairing/retrofitting/strengthening, hoop direction is the optimal fiber orientation only for co-axial compressive load; for a bending moment, fibers in the axial direction are more favorable. Therefore, fiber orientation is an important variable in the structural design of FRP wrapped concrete columns. A number of studies have been conducted using fibers aligned along a direction other than the hoop direction. Mirmiran and Shahawy [5] used fibers oriented at ±15° from the hoop direction in their FRP tube encased concrete columns. In the study by Rochette and Labossiere [6], fibers oriented at [±15°/0°] were used to wrap square concrete cylinders. Pessiki et al. [7] employed [0°/±45°] fibers to warp both small-scale and large-scale square and circular concrete columns. Fibers in both hoop and axial directions were used by Dias da Silva and Santos [8] to repair concrete columns. Fam and Rizkalla [9] and Fam et al. [10] studied filament wound FRP tube-encased concrete columns. In their tubes, fibers in various directions and stacking sequences were utilized to provide both hoop confinement and axial reinforcement. Li et al. [11] used ±54° filament wound tubes in their FRP tube-encased concrete cylinders. Although angle fibers other than the hoop direction have been used in FRP repaired or FRP tube-encased concrete columns, less attention has been paid to systematically investigate the effect of fiber orientation on the structural behavior of FRP wrapped concrete columns. Recently, Li et al. [12] conducted a 3-D finite element analysis of FRP wrapped concrete columns. They found that the structural behavior of FRP wrapped concrete columns depends on the fiber orientation and interfacial bonding strength. They also found that the effect of fiber orientation is coupled with the effect of interfacial bonding strength between the FRP shell and the concrete core. When the interfacial bonding strength is very weak, fibers in hoop direction are optimal; when the interfacial bonding strength is very high, fibers in axial direction play an important role. The objective of this study was to experimentally investigate the effect of fiber orientation on the structural behavior of thin-walled FRP wrapped concrete columns. The test results were also compared with the existing confinement models to validate their effectiveness in predicting the compressive strength of FRP wrapped concrete cylinders.

Based on the test results and discussion, the following conclusions are obtained: • Fiber orientation and FRP wall thickness have a considerable effect on the stress–strain behavior, strength, ductility, and failure mode of wrapped concrete cylinders. The FRP confinement effect cannot be fully realized without proper fiber orientation and sufficient wall thickness. • In practice, it is recommended that FRP coupon tests be conducted to provide the hoop tensile strength used in the confinement model. The mechanical properties provided by the manufacturers can be referenced, but cannot be directly used without experimental validation. • When fibers are aligned along a certain angle between hoop and axial direction, it may be possible that both in-plane shear and transverse tension mechanisms control the failure of the wrapped cylinders, leading to smaller compressive strength and ductility. It is suggested this failure zone should be found for each particular wrapping system to be used and fibers should be avoided to align in this region. • The existing database on which the design-oriented models are based need continuous expansion. More types of FRP materials need to be included such as the UV curing FRP.