Accuracy of Measuring Rebar Strain in Concrete Using a Diffractometer for Residual Stress Analysis

Neutron diffraction is a noncontact method that can measure the rebar strain inside concrete. In this method, rebar strain and stress are calculated using the diffraction profile of neutrons irradiated during a specific time period. In general, measurement accuracy improves with the length of the measurement time. However, in previous studies, the measurement time was determined empirically, which makes the accuracy and reliability of the measurement results unclear. In this study, the relationship between the measurement time and the measurement standard deviation was examined for reinforced concrete specimens under different conditions. The aim was to clarify the accuracy of the measurement of rebar stress using the neutron diffraction method. It was found that if the optical setup of the neutron diffractometer and the conditions of the specimen are the same, there is a unique relationship between the diffraction intensity and the rebar stress standard deviation. Furthermore, using this unique relationship, this paper proposes a method for determining the measurement time from the allowable accuracy of the rebar stress, which ensures the accuracy of the neutron diffraction method.