Abstract
Hybrid fiber-reinforced polymer(FRP)-concrete-steel multi-tube concrete columns(MTCCs)are a new tape of hybrid columns recently formed at the University of Wollongong.An MTCC consists of a number of inner tubes made of steel and an outer tube made of FRP,with the space inside all the tubes filled with concrete.In MTCCs,the combination of three materials(i.e.FRP,concrete,and steel)possesses several important advantages not available with existing columns including its excellent corrosion resistance as well as excellent ductility and ease for construction.The use of small circular steel tubes in MTCCs eliminates the difficulties connected with the manufacture,transportation and installation of large steel tubes.Furthermore,these steel tubes filled with concrete form a rigid wall to confine the concrete surrounded by them.Although large-size square/rectangular hybrid MTCCs may be needed for practical applications,due to aesthetic and other reasons,the existing experimental studies have only focused on the behavior of smallsize circular and square concrete specimens confined with glass fiber-reinforced polymer(GFRP)composites[1-3],which are referred to as conventional FRP composites.The data available for largesize square MTCCs columns are very limited and the previous studies have never focused on the usage of an FRP jacket made of Large Rupture Strain(LRS)FRP composites.Aiming to fill such a knowledge gap,this paper presents the results of an experimental investigation on the performance of largesize square MTCCs wrapped with Large Rupture Strain(LRS)FRP composites,namely,Polyethylene Naphtholate(PEN)FRP composites[4,5],which possesses a large rupture strain(usually larger than 5%)and is much cheaper and more environmentally friendly than conventional FRP composites.
A total of 4 large-size square MTCCs were manufactured and tested under monotonic axial compression(Figure 1).Details of the test columns are shown in Table 1.All specimens had a 1 500 mm height and a 500 mm square cross-section(measured from the inner side of the FRP tube)with a corner radius of 50 mm(Figure 1).The key parameters which were examined included the number of layers of FRP wrap,and the type and configuration of internal steel tubes.The 4 large-size square MTCC specimens covered two types of steel tubes(Type A and Type B,Table 2)and two steel tube configurations(i.e.eight-tube square configuration and four-tube con square figuration,Figure 1).For ease of reference,each specimen is given a name,which starts with a number 8 or 4 to represent the number of steel tubes in the specimens,followed by a letter A or B to represent the type of steel tubes.Another number 2 or 4 is to represent the number of plies of fiber sheets.The last Roman numeral at the end of“8A-2”is to distinguish two nominally identical specimens.
Figure 1 Cross-sections of test specimens
Table 1 Details of specimens
Table 2 _Dimensions of steel tube
During the test,the load kept increasing for“8A-2-Ⅰ”specimen until hoop rupture of the PEN FRP jacket occurred and the specimen lost its structural integrity with noticeable noises(Figure 2).For the sake of laboratory safety,the other three specimens terminated loading before the rupture of FRP jacket.
Figure 2 Failure modes of typical specimen
The key results of all the test specimens are summarized in Table 3.In this table,Pu is the ultimate load of the specimen from the test;εcu is the ultimate axial strain of the specimen from the test;εcu is normalized by the axial strain at the peak stress of unconfined concrete εco(0.002 7).It is evident that theεcu value is up to around 9 times theεco value,which indicates the highly ductile behavior of the 8A-2-Ⅰspecimen.
Table 3 Key results of compression test
The axial load-strain curves of all specimens are shown in Figure 3.Due to the use of a thinner PEN FRP jacket,all specimens displayed a post-peak descending branch and the maximum axial load capacity was reached before FRP rupture.The eight-tube MTCC specimens with four plies of fiber sheets had a higher load capacity than the corresponding specimens with two plies of fiber sheets.
Figure 3 Axial load-axial strain curves