Cracking patterns and strength of CFT beams under different moment gradients

Rectangular concrete-filled tubes (CFTs), fabricated using steel and pultruded glass–fiber reinforced polymer (GFRP) tubes were tested in three-point bending, at different shear span-to-depth (a/d) ratios of 1–5 to examine crack patterns, strength and failure modes. It was shown that the critical (a/d) ratio, at which moment capacity drops, is between 4 and 5 for CFTs with GFRP tubes and between 1 and 2 for CFTs with steel tubes. On the other hand, for steel tubes, ductility is drastically reduced at (a/d) ratios below 3. The crack pattern and size were highly dependent on the magnitude of slip between concrete and tube. A major full depth flexural crack was developed in all CFTs with GFRP tubes, even at a/d = 1, however, when internal steel rebar was added, major diagonal cracks were formed in addition to fine flexural cracks. In CFTs with steel tubes, fine flexural cracks were developed, except at a/d = 1 where fine diagonal cracks were predominant. A strut-and-tie model was developed to predict the strength of CFT beams and provided reasonable agreement with test results.