The United States has over 600,000 bridges carrying traffic. Nearly forty percent of those are older than 50 years, and nine percent of all bridges are considered to be structurally deficient. Monitoring them is a gigantic effort. For every highly-visible collapse like the 2018 collapse of the Morandi Bridge in Genoa, Italy, there are many smaller ones, bridges with lanes closed, temporary patches and supports, and traffic restrictions.
Right now, structures are checked in person at fixed time intervals, and often an external sign of deterioration must appear before anyone is aware of a problem. Since most structures will show only minor degradation from year to year, there is a temptation to underfund the effort.
A wide variety of sensors are being developed that, connected by IoT to analytics, will transform how structural health monitoring is done, and make monitoring and investing in concrete infrastructure more efficient.
Patience and Subtle Attention
This type of situation is perfect for the wide distribution and long-term data gathering capabilities of IoT. Even as sensors are being tested and installed, analytic capabilities are being developed. What are the early warning signs of corrosion and structural weakness that are most useful? Sensors can’t measure actual damage, only various signs associated with it. So, in addition to sensor data, weather information, knowledge of loads, possible impacts and damage, and possible deficiencies in the original construction will all be inputs to structural models.
The more data generated over time, the better corrosion and structural health models can become. The information provided by the sensors can be tied to actual structural soundness by drone-enhanced inspection teams, and the models refined.
Looking Inside Structures
There are five parameters essential for structural health monitoring of concrete infrastructure:
- Temperature
- Humidity
- Corrosion rate
- pH
- Strain/stress/crack
Embedded or attached sensors track various factors, including service loads, various ions indicating corrosion (particularly chloride), pH, moisture, and visible cracks. These sensors need to be powered, be sturdy enough to survive for a long time, and be able to consistently transmit their data, most likely wirelessly. The necessary data rates are extremely low, making various types of low power wide area network (LPWAN) solutions possible.
The data rate does not need to be high. This data then needs to be analyzed, and the results conveyed to decision-makers and workers on site.
There is a wide range of possible sensors, and experimentation and analysis will be necessary to determine which provide the most useful data over their lifespan while requiring minimal maintenance, replacement, and recalibration.
Structural Sensor Examples
Fiber optic sensors detect deformations in the structure. Fiber optic sensors are immune to electromagnetic interference, work under a wide range of temperatures, and can be extremely long without much signal attenuation. They can be easily incorporated into materials or structures as they are manufactured.
Piezoelectric sensors generate a charge in response to force, and can be used to detect pressure, strain, and cracking.
Electrochemical sensors measure corrosion through a variety of methods. Chloride ions are the most frequent cause of corrosion in the steel reinforcement of concrete structures, and so detecting where they have penetrated can provide an indication of corrosion risk, particularly in parts of the structure that are encased in concrete, and so can’t be externally inspected.
Temperature sensors are particularly useful in tracking the curing of newly poured concrete, a more complex process than most people outside construction realize. Extreme temperatures during the life of the structure can also weaken it—tracking temperature over time can allow for calibration of structure life span, shortening or lengthening inspection intervals.
Moisture sensors detect water that causes corrosion, carbonation, and other types of damage, and can crack structures through freeze-thaw cycles.
Concrete structures depend on an alkaline pH to maintain strength and prevent rebar corrosion, so pH sensors can give an early warning of excessive acidity, and the likely presence of carbonation.
These are just a few of the possibilities under investigation. There is an incredible array of possible technologies and implementations, and the sensor literature is increasingly large, with possible new applications emerging from universities and research labs and being tested for their commercial possibilities.
The Future of Smart Infrastructure
All of these sensors have to last for the life of the structure, and either be located where they can have their power provided or their batteries changed, or have some way of generating their own power. They are subject to degradation and damage, just like the structures they are monitoring.
Most infrastructure problems remain invisible until they have significant negative consequences. IoT makes the invisible visible. Its data and analytics can support the creation of platforms for managing the funding of maintenance and capital investments. Someday, everyone will take for granted the long lifespan of bridges and other infrastructure, without any knowledge of the underlying IoT technologies that make that possible.
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