Coda wave entropy and implantable sensing technology: A novel approach for early-age crack assessment in concrete structures

Entropy is a fundamental physical concept used to measure the changes in disorder, randomness, or uncertainty within a system. Concrete, transitioning from a healthy state to the formation of massive cracks, can be regarded as a stochastic process that evolves from simplicity to greater complexity. This study introduces the concept of coda wave entropy (CWE) as a qualitative metric to assess early-age crack development in concrete, with the combination of implantable sensing technology (IST). In the IST, a novel implantable sensor that integrates an array of piezoceramic units is designed for generating cylindrical ultrasonic waves within concrete for omni-directional detection. The recorded coda waves, which undergo multiple scattering and reflection within the concrete material, exhibit high complexity and incoherence. The CWE is developed to indicate these characteristics from both the signal time and frequency domains. To validate the feasibility of the CWE to qualitatively characterize random concrete early-age cracks, numerical studies are carried out. The relationship between the CWE index, crack density, and signal frequency is preliminarily investigated. Afterward, the combined CWE and IST are applied in a shear wall reciprocating loading experiment to demonstrate the method’s effectiveness in the assessment of early cracks. Both numerical and experimental results confirm a correlation between the CWE index and the progression of cracking in concrete.