Strength monitoring of early age concrete improves the efficiency of construction as it provides information on the optimum time for shoring removal and pre-stress transferring. Electromechanical impedance technique has been proven to be a useful tool for strength monitoring of cementitious materials. One of the key limitations of this technique is the lack of physical models, which resulted in strong reliance on statistical analysis tools to quantify the strength of structure being monitored. In this proof-of-concept study, a novel electromechanical impedance–based model with the potential of monitoring the strength of cementitious materials using the concept of Smart Probe is proposed. Instead of directly bonding a lead zirconate titanate patch on the host structure, a lead zirconate titanate was first surface-bonded on a pre-fabricated aluminum beam, which is termed Smart Probe. The Smart Probe was then partially embedded into cementitious materials for strength monitoring. The structural resonant frequencies of the Smart Probe can be identified from the conductance signatures measured from the lead zirconate titanate patch throughout the curing process and serve as strength indicator. By modeling the cementitious material as an elastic foundation supporting the Smart Probe, an analytical model was developed to predict the dynamic modulus of elasticity of cementitious materials based on the resonance frequency of the Smart Probe. Experimental study was carried out on a mortar slab specimen to verify the model and to investigate the performance of the Smart Probe. It was found that the dynamic modulus of elasticity of the host structure could be predicted from the conductance signatures using the proposed model. Compressive strength assessment was achieved by establishing an empirical relation with the dynamic modulus. The proposed electromechanical impedance–based model with Smart Probe is physically parametric in nature and shows high repeatability, which renders its superiority over the conventional statistical method–based electromechanical impedance technique for strength monitoring of cementitious materials.

No abstract

Electromechanical impedance (EMI) technique provides an alternative means of characterizing strength development of early age concrete on a real-time basis. However, most existing studies employing the technique heavily rely on statistical tools for strength development characterization. This article proposes a new impedance-based approach to strength and dynamic modulus assessment of cementitious materials. In this approach, a lead zirconate titanate patch is surfaced-bonded on a customized cementitious material specimen, known as ‘Miniature Prism’, in which the conductance signatures throughout the curing process are acquired. A 3D coupled field finite element (FE) model is then developed to compute the conductance signatures and model updating is performed using the experimental results. The conductance signatures computed by the updated FE model are found to be in good agreement with experimental results. The key contribution of this approach is the use of ‘Miniature Prism’ which ensures consistency of the resonance peaks in the conductance spectrum between identical specimens. This has been very difficult, if not impossible, to achieve with the conventional EMI technique. This merit allows for modelling of the electromechanical system and hence parametrically predicting the dynamic modulus of elasticity of the cementitious material throughout the curing process. Comparative study is also conducted on various conventional and advanced techniques and results indicate that the proposed technique is effective in strength assessment of cementitious materials. In addition, the technique is suitable for autonomous online monitoring purpose, and thus exhibits promising potential to substitute the conventional non-destructive testing methods.

Lamb wave based Structural Health Monitoring (SHM) has received much attention during the past decades for its broad coverage and high sensitivity to damage. Lamb waves can be used to locate and quantify damage in static structures successfully. Nonetheless, structures are usually subjected to various external vibrations or oscillations. Not many studies are reported in the literature concerning the damage detecting ability of Lamb wave in oscillating structures which turns out to be a pivotal issue in the practical application of the SHM technique. For this reason in this study, the propagating capability of Lamb waves in a vibrating thin aluminum plate is examined experimentally. Two circular shaped piezoelectric wafer active transducers are surface-bonded on the aluminum plate where one acted as an actuator and another as a sensor. An arbitrary waveform generator is connected to the actuator for the generation of a windowed tone burst on the aluminum plate. An oscilloscope is connected to the sensor for receiving the traveled waves. An external shaker is used to generate out-of-plane external vibration on the plate structure. Time of flight (TOF) is a crucial parameter in most Lamb wave based SHM studies, which measures wave traveling time from the actuator to sensor. In the present study the influence of the external vibrations on the TOF is investigated. Experiments are performed under different boundary conditions of the plate, such as free-free and fixed by gluing. The effects of external vibrations in the frequency range between 10 Hz to 1000 Hz are analyzed. Comparisons are carried out between the resulting Lamb wave signals from the vibrating plate for different boundary conditions. Experimental results show that the external vibrations in relatively low frequency range do not change the TOF during the application of Lamb wave based SHM.

No abstract