A study on the size effect of the concrete matrix specimen and the radial mechanical behavior in bolt pullout tests

Pullout tests are usually used to test the anchoring capacity of rockbolts and to study their anchoring mechanisms. In this study, the influence of the diameter of the concrete matrix specimen to the ultimate pull force of the bolt was investigated through pullout tests in the laboratory. The bolt was grouted in the center of the concrete specimen in the test. The radial mechanical behavior of the bolt and the splitting failure of the concrete specimen under the pull force of the bolt were numerically studied. The pullout tests showed that splitting of the concrete specimen would reduce the ultimate pull force of the bolt. The pull force quickly dropped to zero without noticeable residual force after the splitting failure. The ultimate pull force increased with an increase in the diameter of the concrete specimen until the diameter reached 300 mm in which the concrete specimen did not split. The theoretically derived minimum diameter of the concrete specimen to prevent it from splitting is consistent with the results obtained in the laboratory tests and also in the numerical modelling. The results of this study provide a scientific basis for standardization of bolt pullout tests and for optimization of rockbolting design parameters. They are of great significance for the development of rockbolt mechanics.