This paper presented a test with 108 squares pultruded concrete filled GFRP tube (CFGT) columns which were used to investigate the behavior of the columns strengthened with CFRP sheet. Categories of failure modes were grouped according to the failure characteristics of these columns under axial compressive loading. Cracks of the four edges in length directions of GFRP tube were effectively confined by CFRP sheet. The ultimate bearing capacity of the columns with CFRP sheet strengthened was greatly higher than that of the columns without the CFRP sheet strengthening. And the ultimate bearing capacity was further increased by increasing the GFRP tube thickness. The ductility coefficient and initial stiffness of the columns were greatly increased because of the strengthening of CFRP sheet. Besides, the strain development of ten square pultruded CFGT columns under axial compressive loading was investigated. Based on the mechanical mode of square pultruded GFRP tube confined by the core concrete and the test result, this paper proposed a calculated formula which was used to predict the ultimate bearing capacity of square pultruded CFGT columns strengthened with CFRP sheet, and a good agreement with the test results was achieved.
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