Two prestressed concrete (P/C) I-girder bridges along the coal haul route system of Southeastern Kentucky were constructed with a 50 degree skew angle. One of the bridges has concrete intermediate diaphragms, while the other bridge has no intermediate diaphragms. Bridges of similar design along coal haul routes have experienced unusual concrete spalling at the interface of the diaphragms and the bottom flange of the girders. The purpose of this report is to identify the cause of the damage, and to evaluate the effectiveness of intermediate diaphragms.

Experimental static and dynamic field testing was conducted on both bridges. All field tests were completed prior to the opening of the bridges. Once the calibration of the finite element models was completed using the test data, analyses were conducted with actual coal haul truck traffic to investigate load distribution and the cause of the spalling at the diaphragm-girder interface. Based on the results obtained in this research study, a significant advantage in structural response is generally not noted due to the presence of intermediate diaphragms. Although large differences were noted percentage wise between the responses of the two bridges, analyses suggested the bridge without intermediate diaphragms will experience displacements and stresses well within AASHTO and ACI design requirements.

Finite element analyses also revealed the cause of concrete spalling witnessed in the diaphragm-girder interface region. The tendency of the girders to separate as the bridge was loaded played a large role in generating high stress concentrations in the interface region. Other mitigating factors were the presence of the diaphragm anchor bars and the fact the bridge is subjected to the overloads of coal trucks. Resolving this problem would in some measure require the removal of the concrete intermediate diaphragm. However, the total elimination of intermediate diaphragms is not recommended since they are required during construction and in the event the deck is to be replaced. The use of temporary steel diaphragms, therefore, is recommended as substitutes for the concrete intermediate diaphragms.

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The contents of this report reflect the views of the authors who are responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the University of Kentucky, the Kentucky Transportation Cabinet, nor the Federal Highway Administration. This report does not constitute a standard, specification or regulation. The inclusion of manufacturer names or trade names are for identification purposes and are not to be considered as endorsement.