Stochastic Condition Assessment of External Post-Tensioning System of Bridges Based on NDE Results

Although post-tensioning elements bring efficiency by increasing the capacity and allowing more elegant spans for bridges, recent findings and observation have raised the need for health monitoring of post-tensioned (PT) bridges with a focus on PT tendons. One of the methods for verifying the health and safety or detecting potential damages for bridges in general is the application of non-destructive evaluation (NDE) methods. Dealing with nondestructive inspection results and updating structural model is the first stage of activities in order to assess the current condition of the bridge. The accuracy of nondestructive tests is always a concern because of the need for interpretations and special skills. Updating structural model based on inspection results is necessary, however the accuracy of nondestructive inspection results should be accounted for when making decisions for further action. This paper recommends a procedure for investigation on stochastic parameters of condition rating diagnosis in post-tensioning elements prior to structural model updating. In another word, the probability distribution for the accuracy (stochastic parameters) of the NDE method is needed to be able to obtain a distribution (stochastic parameters) for the structural condition of the bridge to be used in its model updating within a risk-based reliability framework. To demonstrate the procedure, an actual segmental post-tensioned bridge is used for which the inspection results for PT tendons are available. For this case, after spot inspection, impact testing was identified as the NDE method of choice through the procedure of selection of inspection method. Hands-on and nondestructive tests were performed on several external tendons to verify the accuracy of the method and to determine the stochastic parameters for condition assessment. True condition rating, overrated false condition rating, and underrated false condition rating have been defined and introduced as stochastic parameters of the condition rating. It was shown that the results from impact testing as NDE method and dissection as hands-on inspection are correlated, as such, the level of accuracy for the method was determined to be acceptable to continue the inspection in a large scale.