Size effect on axial stress-strain behavior of CFRP-confined square concrete columns

• Axial stress-strain behavior of FRP-confined square concrete columns.
• Influence of size effect on column behavior and FRP effective strain.
• Experimental investigation and model development for FRP-confined concrete columns.
• Calibration of effective strain model considering size in FRP-confined columns.

Research published on the axial compressive behavior of fibre-reinforced polymer (FRP)-confined concrete columns has been generally based on small-sized specimens. There have been limited studies published on large-sized columns and there have also been limited studies on the validity of upscaling results obtained from small-sized specimens. On account of such knowledge gap, this paper presents the test results of 23 carbon FRP-confined square concrete columns of varying sizes subjected to monotonic axial compression. The specimens were sorted into 10 groups based on (i) specimen size, (ii) theoretical lateral FRP confining pressure, (iii) number of layers of FRP wrap, and (iv) inclusion and exclusion of internal steel reinforcement. In each group, the specimens consisted of different cross-sectional sizes but the same theoretical lateral confining pressure. The experimental results showed that specimen size had no significant effect on the axial stress-strain behavior of FRP-confined medium- and small-sized columns (i.e. sections defined herein equal to and less than 300 mm in width). The axial stress-strain responses exhibited differences as the specimen size increased. This was especially the case for FRP-confined large-sized columns (i.e. sections defined herein equal to and larger than 350 mm in width). The rupture strain of the FRP wrap at the corner regions is proposed to be defined as the effective lateral rupture strain of FRP, and this strain was shown to decrease with an increase of specimen size. Based on the test results, a modified FRP effective strain factor model considering the influence of size effect is proposed.