Calculation the opening of neighboring surface cracks in concrete structure based on OFDR technology

Under the influence of temperature change, construction quality, and external force, concrete structures are prone to generate cracks which poses a significant threaten to the safety of the structure. With the advantages of distributed monitoring over traditional methods, the distributed fiber optic sensing (DFOS) technology has become a widely accepted method in concrete cracks detection and monitoring. Both laboratory tests and field monitoring indicate that the crack location can be determined by the strain peak and the width of cracks can be derived by integrating the strain exerted on the optical cable. Nevertheless, when the neighboring crack spacing distance (CSD) is sufficiently small, the strain induced by cracks will overlap and two strain peaks will be partially weighted, forming a saddle-shaped double strain peak. Thus, how to calculate the crack opening displacement (COD) becomes a big challenge. To tackle this issue, an analytical method to calculate the opening of neighboring cracks based on the shear strain transfer mechanism is proposed. The effectiveness of this method was evaluated through a crack simulation experiment and further validated by a concrete beam loading test. The strain variations induced by neighboring cracks were recorded by optical frequency domain reflectometry (OFDR) technology and the width of neighboring cracks was calculated using the proposed method. By comparing the calculation results with measured values, the accuracy of the proposed method was verified, which provided a feasible way to calculate the COD of neighboring cracks with strain overlap.