Abstract
Columns were considered to play an especially important role in bearing vertical loads and resistance to lateral action in the structures,which would directly determine the safety of the structures.However,in recent years,the existing columns according to old design standards could not satisfy usage requirements due to more strict seismic design requirements in the new seismic codes.Therefore,in such case building should be strengthened by the proper method.The application of externally bonding fiber reinforced polymer jackets have been evidenced as a convenient and feasible solution.Numerous experimental studies have shown that adopt the FRP as a confinement system could increase both compressive strength and deformation capacity of concrete member under vertical and lateral loads[1,2].However,a few drawbacks and limitations have been discovered result from the presence of organic resin[3].A new composite material named textile reinforced concrete(TRC)system consisted of fiber textile meshes combined with high performance mortar was developed to solute these issues[4].
In this paper,the experimental study was carried out on the concrete square columns strengthened with TRC subjected to axial compression,taking into account the influence of the number of textile layers,concrete strength and glass short fiber on confinement ef fect of TRC system,with the aim to evaluate and analyze the effectiveness of different confinement scheme in increase compression strength and deformation capacity of TRC confined columns.
A two-dimensional knitted fabric that was made of carbon fiber bundles and alkali-free glass fiber bundles was used in this experiment.A total of thirteen square specimens consist of three unconfined columns and ten columns confined with TRC were designed and casted in the present investigation.For the specimen series,PC denoted plain concrete,L and the following number denoted the number of textile layers.N and F denoted with and without short fibers respectively,and C and the following number denoted the concrete strength.The experimental results were also discussed and analyzed in the following sections,mainly including the failure modes and load-axial deformation curves of specimens.
The unconfined columns occurred failure immediately once reaching the peak compressive strength,showing an obvious brittle feature.The finally failure mode was characterized by vertically splitting failure along a main crack.However,the typical failure pattern of columns strengthened with TRC was that multiply hair cracks instead of a major crack initialed at the mid-height of the column.Once the peak strength was achieved,a significant lateral deformation together with a progressive cracking of the cement-based mortar could be observed.The failure was characterized by a combination of the interfacial zone debonding in the overlapping zone and the textile rupture;moreover,the textile rupture was rather dominant.It was noteworthy that the thinner and denser cracks at the surface of TRC strengthening layer with short glass fibers added into for confined columns were observed.Figure 1 depicted the failure patterns of confined and unconfined specimens.
Figure 1 Failure mode of specimens
Figure 2 presents the load-axial deformation relationship of confined columns and unconfined columns.The loadaxial deformation response for each confinement configuration is characterized by three different branches.Firstly,a parabolic response with a similar slope to that of unconfined specimens is observed in the initial loading.Then the curve of second stage is represented by a non-linear ascending trend up to the peak strength of the confined specimens.The curve in the last stage is represented by a soft descending branch that dropped when the confinement fracture occurred.In addition,it can be seen that a significant gain in compressive strength and deformation capacity and a higher curves for square columns confined with TRC is obtained in comparison with unconfined columns.The TRC system is proved to be very effective in increasing peak strength and axial stiffness of the unstrengthened specimens.The increment is related to the number of textile layers and short fibers.The ultimate loads of TRC confined columns exhibit an increased gradually trend with the increase of the number of textile layers.A most high compressive strength will be achieved when the specimens strengthened with four layers of TRC.According to Figure 2(b),a certain increase in both ultimate load and axial deformation is obtained when added short glass fibers into TRC strengthened layer compared to that of without short glass fibers,indicating the short fibers also is the parameter affecting the confinement effect of TRC system.
In conclusion,TRC is a very promise composite material as confinement system to improve compressive strength and deformation capacity of concrete members.
Figure 2 Load-axial deformation relationship of columns