Lateral stiffness of post-tensioning tendons with corrosion and broken wires

A large number of post-tensioned prestressed concrete structures exist in corrosive environments that cause corrosion of their prestressing. This corrosion can cause wire breaks leading to a decrease in the tension and the reduction of the effectiveness of the tendons in prestressing the concrete. Finding the exact effect of the corrosion on the prestressing tendons is of great importance to determining the remaining strength of the structure.; One method of testing strands in existing structures is to use a device called a deflectometer which consists of a hydraulic jack mounted on a frame. The lateral stiffness of a 7-wire post-tensioning strand can be related to its axial tension. In order to improve these testing devices, an accurate estimate of bending stiffness of 7-wire steel strands is required and more specifically how the bending stiffness is affected by corrosion and wire breaks.; A review of several models relating the bending stiffness to the applied tension in 7-wire helical tendons has been carried out and a comparison of these models is presented. These models express the stiffness of a strand as a function of the applied tension, the internal friction between individual wires in the strand and the geometric properties of the strand.; An apparatus has been designed to allow testing of 7 wire post-tensioning strands under combined axial tension and lateral loading. The objective of this testing is to better understand the correlation between the bending stiffness of a corroded or damaged tendon and its remaining prestress force. The apparatus allows for the varying of tendon length, tendon diameter, prestress force and end conditions of the tendon.; From testing on 9 mm and 13 mm post-tensioning tendons it appears that the lateral stiffness of the tendons increases linearly with increasing tension at a given gauge length. The lateral stiffness of the strand is inversely proportional to the gauge length of the strand at a given axial tension in the range of gauge, lengths tested. Corrosion of the tendons can have a variety of effects on the lateral stiffness as both an increase in lateral stiffness in the 13 mm tendons and a decrease in lateral stiffness in the 9 mm tendons were observed at the 300 mm gauge length. Broken wires generally cause a decrease in lateral stiffness that is more noticeable at the lower gauge length. The current models available for predicting the lateral stiffness of post-tensioning tendons were determined to be insufficient when corrosion and broken wires were introduced.; The remaining prestress in a 13 mm post-tensioning tendon predicted using a deflectometer with a calibration curve based on an unblemished tendon may be in error by +/-15% at a gauge length of 600 mm and as much as +/-50% when the gauge length is 300 mm.