An effective way to extend the service life of concrete structures is to continuously monitor their structural integrity, a process known as structural health monitoring (SHM). Early detection of structural problems enables targeted maintenance measures and prevents serious damage or even collapse.

Currently, SHM systems mostly use sensors attached to the surfaces of bridges, tunnels, and buildings. These sensors record parameters such as temperature, wind, rain, vibrations, and deformations. However, these measurements alone are insufficient. To make more precise statements about a structure's condition, it would be ideal to measure mechanical stresses directly inside the concrete.

Yet this poses challenges: cables inside the concrete could create weak points that allow water to penetrate, leading to further damage in winter due to salt corrosion. In addition, the sensors must have a service life of several decades, meaning that integrated electronics are not practical in many cases.

This is precisely where the i-MON project comes in. The project aims to develop a passive, wireless solution for monitoring mechanical stresses in concrete structures. With this innovative technology, structural loads inside concrete can be reliably monitored over the long term without compromising the building's structural integrity.

i-MON can contribute to sustainable construction by increasing the longevity of concrete structures and reducing resource consumption. The optimization of maintenance made possible by i-MON sensors results in shorter repair times. When used in transportation infrastructure, this can minimize traffic disruptions, reduce congestion, and lower air pollution. Last but not least, the technology helps prevent catastrophic events such as the sudden collapse of bridges in Genoa in 2018 and Dresden in 2024, thereby significantly increasing the safety of infrastructure.

The technology

i-MON technology is based on SAW (Surface Acoustic Wave) sensors. These sensors are completely passive and wireless. They essentially consist of a piezoelectric crystal with thin metallic grid structures applied to its surface. These grids are connected to an antenna. When a high-frequency signal is sent to the sensor, the antenna picks up the signal and transmits it to the grid, which converts it into acoustic waves. These waves propagate across the surface of the piezoelectric substrate.

By cleverly arranging the grid structures, a resonance cavity can be created on the surface of the piezoelectric substrate, which captures the acoustic waves. This resonance is very precise, enabling accurate measurements. This phenomenon is comparable to the vibration behavior of string instruments, such as violins and guitars. Plucking or striking the string excites the resonance. If the string is left untouched, the sound decays exponentially over time. The same reader that stimulates the resonance can then be used to "listen" to the resonance decay curve. This allows the frequency to be determined wirelessly and without integrated electronics.

In the case of the i-MON sensor, the resonance cavity is excited and read using a special, RADAR-like reader operating in the ISM band at 2.45 GHz. The decisive advantage is that: The resonance frequency depends on both temperature and strain! This phenomenon is comparable to the vibration behavior of string instruments, such as violins and guitars. The higher the tension, the greater the speed of the sound propagating along the string and the higher the frequency. Similarly, when the temperature fluctuates, the speed of sound changes, consequently changing the frequency. Thus, these sensors can wirelessly measure temperature and/or mechanical tension within a material without integrated electronics.

The i-MON project

Development of the i-MON technology began at Carinthia University of Applied Sciences in June 2021. The project is led by CiSMAT (Carinthian Institute for Smart Materials) and FuCoSo (Future Concrete Technologies). Industry partners Sensideon (reader technology) and SAW Components Dresden (sensor technology) are also involved. Together, these partners have proven the feasibility of the i-MON concept. SAW sensors for temperature and strain measurement were mounted on the steel reinforcement and embedded directly in the concrete. These sensors could be accurately read from the outside, even over distances of several decimeters.

For example, it was possible to measure the temperature of the concrete immediately after pouring and over several days with an accuracy of approximately 0.1 °C. Strain could also be measured precisely – in the range from -400 ppm to +1000 ppm. This represents a significant technological advance worldwide.

The i-MON project is funded by the FFG (Austrian Research Promotion Agency) as part of the COIN program and will run until May 2026. With a budget of around 1.2 million euros, the first demonstrators of the i-MON sensors will be developed and the feasibility of the concept further substantiated.

Despite these successes, there are still some highly complex technical challenges to overcome before the sensors can be launched on the market. The sensors must function reliably over the long term, for more than 70 years, in the chemically aggressive environment of concrete. In addition, work is underway on an extended sensor design that will enable measurements in a larger range (up to ±3000 ppm).

Further possible applications of i-MON technology

The latest results suggest additional applications in the construction industry. For instance, it is possible to precisely measure the temperature over a period of 28 days after pouring. This enables detailed monitoring of the concrete curing process. Construction companies are already using such solutions to ensure concrete quality and dismantle equipment quickly to move on to the next project. This saves them considerable costs each year. The i-MON solution could compete with or even replace the wired sensor measurement solutions currently in use.

The i-MON sensors also have an RFID function. With a handheld reader and a single click, a specific concrete block can be identified on site. This feature is helpful when assembling prefabricated parts to avoid errors. This feature is especially useful for modern architectural projects where elements are often not identical.

CisMAT, Sensideon, and SCD are further intensifying their joint research in order to enable significant advances in the construction industry with this promising technology. This technology has the potential to drive innovation in the field and set new standards for quality, efficiency, and sustainability.