An investigation of the basic development length of GFRP bars in unsaturated polyester resin concrete

To derive a calculation formula for the fundamental development length of glass fiber-reinforced polymer (GFRP) bars in unsaturated polyester resin concrete (UPC), a series of forty pull-out tests were conducted on GFRP bar-UPC specimens. Analyses were performed on failure modes, bond strength, and bond-slip curves, and the differential equation governing the GFRP bar-UPC bond-slip curve was then proposed and solved. Based on existing criteria and the differential equation solution of the bond-slip curve, two formulas for calculating the basic development length of GFRP bars in UPC were derived. The results indicate that the extraction of reinforcement bar is the predominant failure mode in the GFRP bar-UPC pull-out tests. Bond strength and bond length show a negative correlation relationship. When the bond length increases from 3 to 7 times the bar diameter (with a bar diameter of 10 mm), the bond strength decreases from 35.71 MPa to 26.61 MPa, representing a reduction of 25.48%. The bonding strength is positively correlated with the thickness of concrete cover. When the concrete cover thickness is 2.5, 3, and 3.5 times the bar diameter, bond strength is 29.14 MPa, 29.71 MPa, and 30.25 MPa, respectively. The bond-slip curve of GFRP bars in UPC consists of three sections: rising stage, decreasing stage, and residual stage. The rising section of the curve fits well with the MBPE model curve.