Design and performance analysis of an embedded wireless sensor for monitoring concrete curing and structural health

The monitoring of the properties of concrete introduces many challenges to the design of any non-destructive sensing system. The advance in recent years of wireless technologies has allowed the development of miniaturised sensing systems, which have now reached a stage of development that allows them to be embedded into concrete. This paper describes the design and performance analysis, under replicated building site conditions, of an embeddable wireless sensing node for monitoring concrete curing and structural health. Wireless sensors with temperature and relative humidity measurement capabilities were designed with onboard communication using the 433 MHz ISM band and embedded into concrete. Testing, which replicated on-site conditions, was carried out on the sensors to determine what the effect of the concrete itself, the steel reinforcement, and steel backed formwork had on the transmission of data. The transmission distance and reliability of receiving data was quantified. Test results confirm that it is possible to deploy an embedded sensor system and transmit live data from the embedded sensor to a data acquisition system located outside the concrete. Preliminary results show that steel backed formwork reduces the transmission distance of the sensors to 3.5 m from 5 m without formwork. Analysis of the data from the sensor showed that, while temperature readings were reliable, the method of measuring relative humidity, using a shielded humidity sensor, may not be suitable for use over the full lifetime of the structure. Accordingly, the electromechanical impedance (EMI) method was enhanced to allow it be used in an embedded system incorporating the AD5933 impedance chip. The EMI method has been successfully applied to monitor the strength development and deterioration of concrete but limited investigation has been carried out into advancing the method for deployment in embedded sensing systems. Analysis of the strength development of freshly mixed concrete and the effects of loading on the response of the sensor show that it is possible to monitor both the strength development and subsequent deterioration of concrete by monitoring the reactance antiresonant frequency shift and peak resistance of a piezoelectric material embedded into concrete. Analysis of the reactance antiresonant frequency shift showed that the technique successfully monitored the compressive strength development of the concrete structure, while monitoring the reactance antiresonant frequency shift in conjunction with the resistance peak provided important information on the condition of the concrete under loading conditions. This is believed to be the first time that the EMI method for monitoring concrete curing and structural health has been demonstrated for use in an embedded wireless sensing system.