Non-Destructive Evaluation (NDE) of concrete’s ITZ characteristics by linear ultrasonic parameters inversion based on a numerical model

Studies about ITZ (Interfacial Transition Zone) effects on concrete/ultrasound interaction are very recent. Yet, they all confirm the important role played by ITZ on waves propagation. Characterizing ITZ - its thickness and/or its elastic modulus - allows to deduce the concrete’s state of health. This NDE of ITZ characteristics is here performed through ultrasonic measurements. In this study, a numerical forward model of ultrasound propagation within concrete that integrates ITZ was developed. Rheological spring-mass approaches are used to model the jump conditions at aggregate/mortar interface instead of a full ITZ layer modelling. Thickness and elastic modulus were combined in one single parameter, named “Contact Level (CL)”, easier to interpret and to handle. Parametric study was needed at the beginning to refine the model and to assess its limits, then some abacuses were constructed to link numerical measured ultrasonic parameters to ITZ thickness, or modulus for example. Those numerical based abacuses were then tested on some ultrasonic experimental measurements. The results are encouraging.