Comparison of stiffness identification methods for reinforced concrete structures

Damages of reinforced concrete result or appear in general by cracks due to its non linear material behavior. One of the most significant effects of cracks on reinforced concrete structures is given by its loss of stiffness within the region of this defect. Even with detailed monitoring data of a structure a localization or quantification of such stiffness loss is still very limited and difficult to handle. Therefore BOKU University initiated together with industrial and public partners the research project "AIFIT" aiming at methods resulting in simple identification and localization of local bending stiffness reductions in reinforced concrete bridges as significant help for detailed inspections. New approaches for such identification made it inevitable to conduct special tests on concrete beams to validate these approaches and compare them to common ones. The paper presents a total of 8 two-span beams, progressively damaged, which were used as an object for detailed static and dynamic surveys at each single damage state. The used measurement concepts are outlined including some new approaches for the detailed and precise determination of the modal behavior of the test specimens. New approaches based on direct measurements of influence lines and dynamic identification procedures using sensitivity and modal bending lines based model updates are introduced. Their applicability is confirmed by the comparison with results obtained from common and enhanced analysis approaches, like the "Direct Stiffness Method" or the analysis of deflection lines. The identified bending stiffness reductions of these beams are compared to the as far as possible unaffected crack pattern and width, which is initiated by progressively increased load. Based on the analysis results and experiences gained from these laboratory tests outlooks of the following work packages of this project are given. These will include numerical non linear analysis of the laboratory tests in context with a real reinforced concrete bridge structure, analyzed with the before mentioned methods, and in a field test, and in consequence in the development of enhanced upload measuring devices integrated into bridge bearings for the direct measurement of influence lines