In situ DIC method to determine stress state in reinforced concrete structures

Nowadays these structures are at the end of their theoretical life according to Eurocode 2. It is entitled to ask questions about their state of health, and whether these structures are still able to ensure their activity in respect of constantly evolving regulations, while ensuring public safety. To tender to answer these questions, we have developed a new method for analyzing in situ stresses in reinforced concrete structural elements, less invasive in order not to damage metal frames and able to give a 2D state of stresses. At the end the objectives are to obtain principal stresses (sigma I and sigma II) at the visible surface at +/- 1 MPa. Moreover, the value of Young's modulus is locally identified with an accuracy of 5,000 MPa. To have an accuracy of 1 MPa, the challenge is to identify dis-placements less than 1 mu m on site. This method is composed of two steps: firstly, a local relaxation of stresses in order to determine strains in the studied element, and then a reloading of the studied area to identify Young's modulus. With hypothesis of Hooke's law of behavior, the stress state is determined by assuming that the ma-terial is homogeneous, isotropic and elastic. The method uses the Digital Image Correlation to determine dis-placements. To validate our method, tests were realized at the laboratory on concrete column under controlled load. The method is applied on a concrete column in laboratory with a given vertical stress of 2.4 MPa. The vertical stress analyzed in situ is around 2.3 MPa with thus a relative error of 4 %. After polyethylene tere-phthalate (PETP) validation of the reloading within the relaxed zone, the analysis on the concrete structure gave a Young modulous equal to 33,000 MPa +/- 2,000 MPa. Results of standardized compression tests in laboratory, on the same concrete, return an average value of 34,769 MPa for the Young's modulus which demonstrate the good fitting of the in situ method (about 5 % of error). The real scale of the structure, the use of classical concrete, and the order of magnitude of the load applied, make this experimental campaign very relevant. The results demonstrate the high potential of this method for the diagnostic of existing concrete structures.