In situ embedded PZT sensor for monitoring 3D concrete printing: application in alkali-activated fly ash-slag geopolymers

An embedded PZT sensor is developed for in situ monitoring of 3D-printed materials formed by extrusion-based layer deposition. The PZT sensor with a two-layer protection coating is embedded in the material during the layer deposition, and continuous measurements are obtained through the post-printing period. The PZT sensor is used to detect the physio-chemical changes in the alkali-activated fly ash-slag geopolymer with time. The effect of the added weight of each layer of print is sensitively reflected in the electrical impedance (EI) measurement obtained from the PZT sensor. The changes in EI measurements obtained from the embedded PZT sensor are compared with the measurements on the material related to the changes in the rheological behavior, reaction kinetics assessed using calorimetry, and setting behavior in the material. The build-up of the internal structure within the material, which allows buildability, is assessed from the conductance signature of the embedded PZT sensor. The changes produced by the chemical reactions within the binder, which bring about irreversible changes leading to the setting of the printed structure, are also sensitively detected in the EI measurements from the embedded PZT sensor. The amplitude of conductance is sensitive to the setting of the material. The frequency changes from the recorded EI signature reflect the continuous increase in the material stiffness with time.