Study on the Mechanical Properties of BFRP Tube Confined Concrete Short Columns under Axial Compression

Axial compression tests were carried out on 6 square steel tube confined concrete short columns and 6 BFRP square pipe confined concrete axial compression tests. The concrete strength grades were C30, C40, and C50. The test results show that the failure modes of steel pipe and BFRP pipe are obviously different, and the BFRP pipe undergoes brittle failure. Compared with the short columns of concrete confined by BFRP pipes, the ultimate bearing capacity of axial compression is increased by -76.46%, -76.01%, and -73.06%, and the ultimate displacements are -79.20%, -80.78%, -71.71%.

conducted an axial compression test on reinforced concrete columns confined with FRP cloth. He changed the type of fiber cloth material, the number of layers of fiber cloth winding, the spacing of fiber cloth, the strength grade of concrete, and the size of the column to study the columns. The axial pressure performance. The research results show that the effect of restraining the lateral deformation of reinforced concrete columns confined by FRP sheets is significant, and at the same time, the ultimate compressive bearing capacity of the column is greatly improved and the ductility of the column is significantly improved.
Pan Yi ,Cao Shuangyin etc. [2]   Tong Lin [9] conducted axial compression tests on CFRP-constrained rectangular steel concrete short columns and ordinary rectangular steel-concrete short columns. Gao Peng, Wang Jingtang etc. [11] conducted axial compression tests on rectangular steel concrete short columns confined by CFRP cloth. They changed the concrete strength grade to study the load-displacement curve of rectangular steel concrete short columns confined by CFRP cloth. The research results show that the cross-sectional constrained area of the rectangular steel concrete short column confined by CFRP cloth is divided into strong constrained area and weak constrained area; the relationship between the limit value and concrete strength is deduced, and the test data result is calculated by the revised formula. Coincide.
Note: X indicates that the constrained material is BFRP tube, Z indicates the axial compression stub, L indicates the calculated length of the axial compression stub, B indicates the cross-sectional width of the axial compression stub, and the angle marks 0, 1, 2, and 3 represent the components respectively Number.

Specimen production
When pouring, the square steel pipe and square BFRP pipe are placed on the electric vibrating table, and the top layer is poured and vibrated and compacted. After pouring is completed, the top of the axial compression column specimen is smoothed with cement slurry to make the plaster higher than the outer restra int material. After

Analysis of test results
Axial compression test is carried out on the concrete column confined by the  Gradually increase.
The measured values of the axial compressive ultimate load of the square steel tube confined concrete column and the square BFRP pipe confined concrete column are shown in Table 2-1.  Based on the above analysis, it is known that the maximum displacement of the square steel tube-confined concrete column gradually decreases with the increase of the concrete strength level, while the square BFRP tube-confined concrete column shows the characteristics of first decreasing and then increasing. (GB50010-2010) [12] , and its expression is shown in the following formula:     The comparison between the ABAQUS analysis result and the test result of the load-displacement curve of the concrete column confined by the square BFRP tube is shown in Figure 3

Conclusions
As a new type of composite material, BFRP pipes have been studied relatively little, and are currently used in engineering fields less. In order to study its basic mechanical properties, this paper selects Q235 steel square pipes and BFRP square pipes for comparative research.
(1)The maximum compressive bearing capacity of concrete columns confined by square BFRP tubes is less than that of concrete columns confined by square steel tubes. With the improvement of the concrete strength level of the square steel tube confined concrete column, the increase in compressive bearing capacity is smaller, while the improvement of the square BFRP pipe confined concrete column is larger； (2)The failure characteristics of square BFRP pipe-confined concrete columns are different from those of square steel-tube-confined concrete columns. When the concrete-filled steel tube is damaged under compression, the middle bulge damages, while the BFRP tube-constrained concrete column has cracks running through the entire column height and damage occurs at the column corners. When the concrete-filled steel tube beam fails in bending, the cracks will crack from the bottom upwards in the middle of the span, causing the members to lose the bearing capacity.
From the point of view of the failure characteristics of BFRP pipe confined concrete columns, this is caused by the insufficient provision of BFRP pipe hoop restraint.