Analytical Modelling of Fully Bonded and Debonded Pre-Tensioned Prestressed Concrete Members

The research presented in this document is an in-depth analytical study of the bond behavior of strand in the end region of pre-tensioned prestressed concrete members, especially with respect to the transfer and development length of fully bonded and debonded strands. A detailed analytical investigation of several significant test specimens from Project 1210 and U-Beams from Project 9-580 was performed using the finite element method and other numerical methods. Additionally, an axi-symmetric finite element analysis of concrete cylinders containing fully bonded and debonded strands was performed to study the state of stress in the concrete surrounding the strands just after transfer of prestress.

The current investigation involved developing a technique to model the transfer and flexural bond zone of a pre-tensioned member. The modeling approach developed for this research allowed for slip between the strand and the concrete in the transfer and flexural bond zone of the member. The FE analysis also allowed for modeling the cracking and crushing of the concrete, and modeling of the non-linear behavior of the strand.

A typical analysis of a specimen involved comparing the external parameters such as the load-deflection relationship and the overall cracking response from the test and the analysis. Assuming a favorable comparison of the external parameters, which was usually obtained, the internal parameters such as the bond stress, steel stress and concrete stress profiles determined analytically were examined for the entire loading history of the test. This information could not be determined experimentally, and hence knowledge of the internal parameters provided an in-depth understanding of the actual behavior of each test specimen during the course of the test.

The results from the analytical and experimental work were used to develop a simple behavioral model to determine the development length of 0.5-inch and 0.6-inch strand for any pre-tensioned member containing fully bonded and debonded strands. The model was tested against the different test series from Project 1210 and other relevant research studies. The research study concluded with a brief discussion on code implications and general recommendations for design.