Tensile and shear testing of large fiber-reinforced polymer composite (FRPC) specimens with circular cross-section to final failure has always been challenging. A major difficulty is that the local stress triaxiality near the gripping ends usually leads to premature failure at these locations, instead of the desired test gauge sections. Recently an alternative method using expansive grout materials has been proposed, but there have been no well-established design guidelines due to lack of understanding of the gripping pressure developed by the expansive grout material. In this study, an analytical solution has been derived to correlate the hoop strain on the outer surface of the confining pipe (caused by grout expansion in the pipe) to the grout elastic modulus and linear expansion coefficient. By measuring the exterior surface hoop strains of two different steel pipes, the elastic modulus and linear expansion coefficient have been determined to be 4.52±0.07 msi and 0.158±0.008 % respectivly. The solution has also been expanded to include the composite specimens and predict the gripping pressure at specimen-grout material interface for any given pipes, which makes it possible to select improved expansive grout material based gripping system parameters including the minimum gripping length, optimum confinement pipe dimensions, and minimum grout material volume.