Expansive Grout-Based Gripping Systems for Tensile Testing of Large-Diameter Composite Bars

Abstract

Tensile and shear testing to final fracture of large-diameter, fiber-reinforced polymer (FRP) composite round bars is often challenging because local stress triaxiality near the gripping ends can precipitate premature failure at these locations, instead of in the desired test gauge section. A method using expansive grout materials has been used, though its design is impaired by the lack of understanding of the gripping pressure developed by the confined expansive grout material. In this paper, an analytical solution has been derived to correlate the hoop strain on the outer surface of the confining steel pipe (caused by grout expansion in the steel pipe) to the grout’s elastic modulus and coefficient of linear expansion. By experimentally measuring the exterior surface hoop strains of two different steel pipes, the elastic modulus and coefficient of linear expansion were determined. This solution has been generalized to include the composite bar and predict the gripping pressure at the bar-grout interface for any given pipe and composite bar combination. Based on the analytical results, expressions for key design parameters for improved expansive grout-based gripping systems, including the minimum grip length, optimum dimensions of the confinement pipes, and minimum volume of the grout material, have been provided. Based on the improved design, glass FRP bars of diameters 19.0 to 38.0 mm (0.75 to 1.5 in.) were tested without gripping problems and with failure loads up to 534 kN (126 kip), which significantly exceeds 400 kN (90 kip), the load level identified as a threshold of concern.