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The article explores the material and structural size effects on the electrical impedance response of the embedded PZT sensors which are used as an sensors/actuator in the smart sensing concrete. An embedded sensor for use in concrete monitoring is developed using Lead Zirconate Titanate (PZT). The electrical impedance (EI) response of the sensor placed inside concrete cubes of varying sizes are recorded for an applied electrical input of varying frequency. The dynamic response of the embedded PZT sensor to the applied electrical input is determined by the mechanical impedance offered by the surrounding medium. In specimens of finite size, the mechanical impedance to the motion of the PZT is influenced by the combination of dynamic material resistance and structural stiffness (size of the structure). The current study examined the effects of material resistance and size of structure on the EI response of the PZT sensor embedded in concrete medium. The influence of the size of the medium from small-sized specimens is seen in the form of secondary peaks, identified as structural peaks, on the resonance spectrum of the embedded PZT sensor. The structural peaks on the EI response are very dominant in the concrete cube of small sizes. Magnitude and intensity of these structural peaks attenuate with an increase in the size of the concrete cube. Also, the EI response from the smaller-sized specimens registered a significant leftward shift in the spectrum. The observations from the numerical simulations also showed the mechanical impedance of the surrounding concrete medium to the motion of the PZT also increases and converges at 150 mm. At sizes below 150 mm, the structural effects that are influenced by the specimen boundary must be separated to access the material properties of the concrete medium from the measured EI response of an embedded PZT sensor. At sizes, larger than 150 mm, the dynamic motion of the PZT patch and the measured EI response of the PZT patch depend only on the dynamic material properties of the concrete medium around the sensor. The zone of influence and its importance in standardization of PZT sensor for SHM of concrete structures is discussed.
A piezoelectric-based ultrasonic transducer that can be embedded at the time of casting is developed for the purpose of continuously monitoring the early-age strength development of cement paste. The transducers that have been developed are equipped with various levels of protection in order to enhance their resilience in an alkaline environment and mitigate the risk of short-circuits. The transducers are operated in actuator-receiver mode. The obtained ultrasonic signal is analyzed in frequency domain. The initial and final setting time estimated from calorimetry measurements demonstrated a strong agreement with the proposed frequency domain approach. The observed behavior of concrete in wave transmission measurements exhibits similarities to that of an electronic filter. Initially, concrete behaves similar to a low-pass filter, but as time elapses, it transitions into a high-pass filter.
Concrete undergoes physical and chemical changes from the time of casting. These changes in early stage of hydration of concrete, highly influence the final properties and overall performance of concrete structures. Comprehensive health monitoring sensors can help to improve the quality and performance of concrete structures by providing detailed and accurate real-time information about the properties and condition of concrete at different stages of its life cycle. In this study, an embedded smart PZT (Lead Zirconate and Titanium) sensor is developed and its potential for comprehensive health monitoring of concrete structures is explored. To protect the PZT sensor from short-circuiting and alkaline environment, a robust and sensitive protection scheme is developed. The developed embedded smart PZT sensor was placed in the concrete during casting to infer the in-situ properties of concrete through active and passive sensing techniques. Through active sensing techniques Electrical Impedance (EI) and wave propagation (WP), the hydration and damage are monitored, respectively; through passive sensing techniques vibration, and acoustic emission (AE), the development of Young’s modulus and cracking in concrete are monitored. The methodology to infer the changes inworkability, complete set behavior, property development, and crack opening in concrete structures is developed. The reproducibility of all these techniques using the developed embedded smart PZT sensor is established. Since the developed sensor is capable of both active and passive sensing, it allows for a more comprehensive and flexible approach to health monitoring. This contributes to increased data accuracy, reliability, and a more holistic understanding of the concrete structure’s condition.
Additive manufacturing of concrete structures is a novel and emerging technology. Free contouring in civil engineering, which allows for entirely new designs, is a significant advantage. Although several techniques and approaches demonstrate these advantages, quality control during printing is highly challeng-ing and rarely applied. Due to the continuous mixing process used in 3D concrete printing, it is impossible to exclude variations in the dry mixture or water content, and a single test sample is insufficient to represent the whole structure. A defect in one layer during printing can affect the integrity of the entire structure.
The growing interest of 3D Concrete Printing (3DCP) arises new challenges for quality control. A major challenge lies in evaluation of the interlayer bond strength. An optical approach is developed to monitor the surface properties of the printed layer. Commercial laser profile scanners are utilised to record the reflection intensity of the mortar’s surface. To be applicable in the printing process, the reflection intensity needs to be characterised for each material within the printable region and further effects, like geometry and environmental light, need to be considered. The presented work aims to provide a framework for implementing the laser reflection intensity to 3DCP monitoring.
Accurate assessment of damage in concrete structures requires monitoring techniques that can capture both global stiffness degradation and local cracking processes. Existing structural health monitoring approaches typically rely on separate sensors for vibration measurements and acoustic emission (AE) monitoring, while conventional surface-mounted devices often suffer from poor and variable coupling. This study presents an embedded piezoelectric (PZT) sensor developed for dual mode vibroacoustic monitoring in concrete structures.
The sensor is cast within the concrete matrix to improve mechanical coupling and enable robust measurement of structural response during damage evolution. Dual-mode monitoring is achieved through sequential operation of
the same embedded sensor in two distinct modes passive acoustic emission (AE) monitoring during fracture loading and impulse-excited vibration testing conducted before and after fracture test. Benchmarking experiments include comparison with commercial accelerometers and AE sensors, confirming that the embedded configuration enhances high-frequency sensitivity and coupling performance. The fracture process is interpreted by correlating AE activity with Digital Image Correlation (DIC)-based crack kinematics, enabling zone-wise understanding of crack development. The vibration response is interpreted using a stiffness-reduction framework consistent with hinge-type crack formation, explaining the observed modal-frequency reduction and in crease in damping. Electromechanical impedance measurements quantify sensor–matrix interaction, highlighting the role of epoxy-mediated impedance matching. Overall, the results demonstrate that the proposed embedded sensor provides a unified platform for validated AE-vibration sensing, offering a promising approach for integrated structural health monitoring of concrete infrastructure
Interlayer bonding in 3D concrete printing is influenced by the hydration progress and surface moisture of the previously printed layer. For effective quality control, continuous in situ monitoring of interlayer surface properties is required. This study investigated reflection intensity as a method for in situ measurements during the hydration of CEM I mixtures with varying retarder contents. Additional factors influencing the reflection intensity are also examined. Two laser line scanners with different wavelengths were used to track hydration over 72 h. Vicat tests and isothermal calorimetry served as reference methods. Across all the mixtures, the reflection intensity exhibited a repeatable pattern with five different stages. A sharp increase in intensity during the third stage was consistent with the acceleration period of hydration. These findings suggest that reflection intensity measurements could serve as a promising tool for evaluating interlayer bonding in 3D concrete printing.
In extrusion-based 3D concrete printing (3DCP), addressing challenges related to safety, reliability, and quality control is crucial for widespread adoption. Yet current limitations in monitoring material properties during and after printing hinder the development of effective 3DCP guidelines. Therefore, the development of an inline sensing system capable of real-time monitoring and adjustment of process parameters is necessary to overcome these challenges.
Building upon an existing inline sensing system developed by BAM, which currently monitors material properties during printing and the geometry of the print post-extrusion, this study extends its capabilities to post-extrusion monitoring using embedded piezoelectric (PZT) sensors. These PZT sensors provide localized measurements of material changes through electrical impedance (EI) measurements without disrupting the printing process. By embedding these sensors in 3D printed structures, continuous monitoring is achieved from layer deposition through 1-day of hydration. To achieve this, initially, PZT sensors were developed with multiple layers of protective coatings. Two different 3D printed mixtures, each with different hydration behaviors, were utilized, and PZT sensors were strategically placed between printed layers to maintain their integrity. EI measurements were collected continuously from printing through 1 day of hydration. Analysis of amplitude and frequency changes in the EI response spectrum provided insights into material behavior post-printing. The study highlights how continuous monitoring of frequency and conductance can track structural builtup and property development of the material. Rapid changes in conductance measurements, immediately post-printing indicate swift structural built-up, while key hydration phases are reflected in frequency measurements.
Real-time monitoring of structural build-up is critical for robust 3D concrete printing (3DCP). We evaluate electromechanical impedance (EMI) sensing with embedded piezoelectric (PZT) sensors to track early-age structuration of cementitious binders, a key phenomenon for 3DCP. Cement pastes (CEM I 42.5 R) spanning water-to-cement ratios w/c = 0.32, 0.36, 0.40 and a phosphate-retarded paste were cast with embedded sensors. Electrical impedance (EI) measurements were recorded from casting to 4 h for the w/c series (0–7 h for the retarded mix) and were normalized to each sensor’s in-air baseline. The fundamental resonance was very sensitive to the surrounding medium: immediately after embedding, the conductance peak decreased and the resonance shifted to lower frequency, followed by continued attenuation during the nominal dormant period. Over the hydration window studied, peak-conductance amplitude was more sensitive to microstructural changes than resonance-frequency shift, and this amplitude-based metric correlated with reference measurements: small-amplitude oscillatory shear (SAOS) rheometry and Vicat penetration. The rate of change in peak-conductance amplitude ranked with composition (WC32 > WC36 > WC40). The retarded mix showed an unusual early build-up signature that preceded the hydration-driven percolation knee seen in rheology, which was reflected in the conductance trends. The mapping between shear modulus and conductance-derived features was nonlinear. EI spectrum responded from the time of casting, providing the earliest actionable indicator for process control. Overall, this preliminary study with standard cement pastes indicates that the EMI sensing offers a sensitive, embedded, and physically interpretable route to in-situ monitoring of fresh-state evolution, with potential to define practical setpoints (for example, interlayer wait times), while Vicat and rheology serve as benchmarks and calibration companions.
3DCP introduces monitoring demands absent in cast concrete: the material is simultaneously being deposited, self-loading, and hardening, with failure modes — collapse, inter-filament cracking, interfacial voids — that are invisible to any external sensor. This presentation reported preliminary results from an embedded PZT sensor system addressing this gap through two complementary modalities operating from the same array.
The electromechanical impedance (EI) modality tracks local elastic property development through resonance frequency shift and conductance decrease. Paste-level validation across four mix designs (w/c = 0.32, 0.36, 0.40, and 0.36 + 0.5% NaP retarder) demonstrated sub-10-minute sensitivity to early hydration and a clear quantitative correlation between normalised conductance decrease and independently measured shear modulus. In full printed beam specimens (68 × 30 × 17 cm), three embedded sensors tracked EI evolution through 700 hours, corroborated by thermal kinetics, Vicat set behaviour, and external wave velocity measurements.
The wave propagation modality used the same array in actuator-receiver configuration to measure time-of-flight and signal energy between sensor pairs. A key finding was directional asymmetry: longitudinal pairs stiffened faster than the transverse pair crossing the inter-layer interface, providing direct in-situ evidence of deposition-induced mechanical anisotropy. Energy inflection points at approximately 100–200 hours were interpreted as the transition from stiffening-dominated to damage-dominated regimes, consistent with post-hardening XCT observations of drying shrinkage cracking along inter-filament contacts.
The presentation closed with an honest challenge: even with a functioning sensor system, four embedded sensors cover less than 1% of the element's information field. The sensing density problem was posed as an open question to the committee, framing it not as an instrumentation failure but as a geometric inevitability of the sequential deposition process demanding new thinking on distributed sensing architectures.
