Cloud-based concrete monitoring

Concrete is the world's most commonly-used construction material. Its strengths: extremely robust and durable, whilst at the same time versatile and extremely malleable. That's why concrete is used in the construction of so many buildings and critical elements of our infrastructure such as tunnels, bridges and dams. But the ravages of time also leave their mark on concrete structures, meaning that regular inspection and maintenance work are a must.

Keeping a close eye on the condition of the concrete

In an ambitious IoT research project, funded in part by the EU, umlaut has developed a wireless sensor network that works autonomously for the continuous long-term remote monitoring of concrete structures. The impetus for the project was provided by Concrefy B.V., an engineering firm from the Netherlands who are a member of the doka Group and who specialise in construction materials technology and concrete monitoring. Concrefy contacted umlaut as they were keen to find a solution that would allow them to precisely and reliably record the condition of concrete structures at regular intervals and over extended periods of time of up to several decades. The goal: to pick up at an early stage any chemical changes in the material and any changes in the distribution of moisture, for example on account of damp or the acute formation of cracks. Because the sooner maintenance work can be undertaken, the lower the level of complexity and investment required. They also wanted a system that could firstly be used with new buildings to monitor the correct mixing ratios of the concrete during the concreting phase and, secondly, that would be capable of monitoring the hardening process in order to secure quality and reduce the necessary waiting times, thereby cutting costs.

Taking measurements without an external power supply

To achieve these objectives, umlaut liaised with Concrefy and with the Institute of Materials Research at RWTH Aachen University (ibac) to design a new kind of distributed sensor system, and also assumed responsibility for the development of the measuring technology as well as the required wireless sensors, the solar-powered radio gateways and the cloud services. In order to be able to count on scientifically based hardening and ageing models for concrete in the evaluation process, complex impedance measurements via a depth profile were used rather than conventional resistance measuring. The procedure, which was inspired by (spectral) induced polarisation measurements (IP/SIP), is sophisticated in technical terms. It delivers the desired high value pool of data, but also involves higher energy consumption.

The particular challenge for umlaut was not only to be able to use the measurement process in the laboratory with a wired connection, but also on building sites where there is often no power supply and where the use of cables to supply power and transmit data needs to be avoided as it may cause a disruption. Through a novel kind of energy management and the use of customer-specific components that umlaut developed in collaboration with a leading manufacturer of semiconductors, they succeeded in equipping the sensors with radio transmission technology in accordance with LoRa and LTE Cat NB1 standards and, despite the complicated measurement process, in reducing the power requirements of the wireless sensors to the point that long-term measurements over a period of decades become a possibility. In order for the system to be tested out in practice, the local authorities of Mönchengladbach and Venlo made available various buildings and structures as well as a test bridge.

Solar gateway rests like Sleeping Beauty

Once the wireless sensors have been put in place in the concrete during the construction phase – or at a later point in time – they can independently take measurements at intervals that are dynamically adjusted in line with the boundary conditions, according to a prescribed measurement plan. They send the data via radio transmission to a wireless solar gateway located in the surrounding area. In order to also save energy here, this device is maintained in a kind of "deep sleep" and only becomes active when it receives new data. After a specified period of time, the gateway then transmits the bundled data to the cloud via a mobile phone connection such as LTE. The data is then stored here and made available for evaluation or for notification in the case that an acute situation has arisen. Data security has also been taken into account: during the wireless transmission of the data, the information is encrypted and it can only be accessed in the cloud with the appropriate authorisations.

The advantages of the wireless sensor network:

• Long-term monitoring of the concrete over several decades, suitable for both new and existing structures.
• Monitoring of the concreting and hardening phase for new builds for the purpose of quality assurance and to reduce waiting times.
• Complete wireless end-to-end solution for both power supply and for data transmission.
• Extremely energy-efficient sensors that can carry out up to 70,000 measurement cycles without requiring any maintenance.
• Automatic calibration and digital-based measurements and radio technology mean that the sensors are highly immune to the effects of ageing and potential disruptions.
• Solar-powered radio gateways that do not require any connection to a mains power supply or a wired network connection (in contrast to standard gateways).
• Simple adjustment to future transmission technologies can be achieved through centralised upgrades of the gateways.
• No requirement for an on-site service for manual reading of measurements.
• Transmission of all actual measured values almost in real time, rather than only at intervals according to maintenance schedules or in the case of acute problems.
• Central provision of all measured values in the cloud for secure and convenient storage of data, as well as for flexible and future-proof evaluations and emergency alerts.
• Time and cost savings achieved through considerably reduced expenditure for installation and maintenance.