Pull-out behavior and damage assessment of core concrete of Full-scale prestressed High-strength hollow square piles

Prestressed high-strength (PHS) hollow square piles have been used recently to improve the ability of structural foundations to resist uplift loads, and the connection between PHS hollow square pile and upper cap is often in the form of core concrete. The main purpose of this paper is to study the effect of the internal thread of the pile wall on the pull-out behavior of core concrete. For this reason, pull-out tests of the core concrete of full-scale PHS hollow square piles were carried out. And the influence of core concrete type, core concrete length, thread length, and thread depth on the pull-out bearing capacity of core concrete was discussed. Simultaneously, acoustic emission technology was used for damage monitoring in the whole process to characterize the progressive failure behavior of piles. The results show that the gain of ultimate pull-out bearing capacity caused by an increase in the core concrete length is highly dependent on the form of the inner wall of the pile. In addition, under the same core concrete length, the ultimate pull-out bearing capacity of core concrete with appropriate thread pitch (P) and thread depth (D) are increased by 79%, which means P/D should be considered in the optimal design of internally threaded PHS hollow piles. The test results also prove the great potential of UHPC for improving the pull-out behavior and safety of core concrete.