Behavior Of Ice-filled Steel/FRP Tubular Stub Columns Under Axial Compression

The use of concrete has great limitations in cold areas due to the difficulty in mixing,pouring and curing concrete.In order to solve this problem,ice is suggested to be used as a building material to combine with steel or fiber-reinforced polymer(FRP)instead of using concrete.Based on bearing capacity enhancement,environmental protection and multifunctional design,a series of composite columns termed ice-filled steel tubular columns,ice-filled flax FRP tubular columns and ice-filled large rupture strain(LRS)FRP tubular columns are innovatively proposed,respectively.They consist of outer steel or FRP tubes filled with ice.Both the strength and ductility of the ice core can be greatly improved due to the sufficient confinement from the outer steel or FRP tubes.The inner ice is well isolated from surrounding disturbance due to the existence of the outer permanent formwork.They are expected to be used as compression members in cold regions.The axial compressive behavior of the proposed composite columns is exploratory investigated in this study.The main contents are summarized as follows:(1)Three circular plain ice stub columns,nine circular ice-filled steel tubular stub columns and nine square ice-filled steel tubular stub columns were tested under axial compression.The test variables included the diameter/width-to-thickness ratio.The results confirmed that the ice-filled steel tubular columns have high bearing capacity and good deformability.The bearing capacity of ice-filled steel tubular column was larger than the summation of that of hollow steel tube and unconfined ice column.The bearing capacity and initial stiffness of the ice-filled steel tubular column increased with the decreasing diameter/width-to-thickness ratio.The axial bearing capacity formulas were proposed for circular and square ice-filled steel tubular stub columns,respectively.(2)Three circular plain ice stub columns,three circular sawdust-reinforced ice stub columns,nine circular plain ice-filled flax FRP tubular stub columns and nine circular sawdust-reinforced ice-filled flax FRP tubular stub columns were tested under axial compression.The test variables included the number of FRP layers in FRP tubes(two,four and six layers)and the inclusion of sawdust in ice.The results revealed that the compressive strength and ultimate axial strain of confined plain ice and confined sawdust-reinforced ice were increased with the number of FRP layers in flax FRP tubes.The modification of sawdust improved the mechanical behavior of unconfined and confined ice.The axial bearing capacity formulas were proposed for circular plain ice-filled flax FRP tubular stub columns and circular sawdust-reinforced ice-filled flax FRP tubular stub columns,respectively.(3)Three circular plain ice stub columns,three circular sawdust-reinforced ice stub columns,nine circular plain ice-filled LRS FRP tubular stub columns and nine circular sawdust-reinforced ice-filled LRS FRP tubular stub columns were tested under axial compression.The test variables included the number of FRP layers in FRP tubes(one,two and three layers)and the inclusion of sawdust in ice.The results confirmed that the bearing capacity and deformability of the composite columns were greatly enhanced due to the use of LRS FRP composites.The axial stress-strain curves of confined plain ice and confined sawdust-reinforced ice featured monotonically ascending bilinear shapes.The compressive strength and ultimate axial strain of ice core were increased approximately linearly with the number of FRP layers in LRS FRP tubes.Both the compressive strength and ultimate axial strain of confined sawdust-reinforced ice were larger than those of plain ice core confined by the same number of FRP layers.Based on the definition and discussion on a series of key parameters including transition strain,transition stress and secondary stiffness,the axial stress-strain models of confined ice were developed.(4)In material level,tensile tests,luminance measurements,and scanning electron microscopy(SEM)imaging were conducted to examine the tensile properties,luminance properties and microstructure of self-luminous FRP composites.The results confirmed that the introduction of self-luminous powders into resin matrix had little influence on the mechanical properties of FRP composites.The luminance of self-luminous FRP laminates decayed in a logarithmical manner with the time after activation.In column member level,three circular plain ice stub columns,nine circular ice-filled normal FRP tubular stub columns and nine circular ice-filled self-luminous FRP tubular stub columns were tested under axial compression.The test variables included the number of FRP layers in FRP tubes(one,two and three layers)and weight ratio of self-luminous powders to the epoxy resin(0 and 80%).The test results realvealed that the compressive strength of confined ice increased approximately linearly with the increasing FRP layers.The introduction of self-luminous powders into FRP tubes had little influence on the behavior of composite columns under axial compression.The axial bearing capacity formulas were proposed for circular ice-filled normal FRP tubular stub columns and circular ice-filled self-luminous FRP tubular stub columns,respectively.