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The potential degradation of asphalt pavements results in expensive construction work and can be a serious safety concern for those traveling on affected roads. Although regular maintenance is often performed on paved roads, an urgent need for a precise pavement monitoring system remains. As a result, several smart sensor networks have been developed to monitor asphalt pavements' structural health in real-time.
An Introduction to Pavements
Historically, unpaved roads consisting of a mixture of gravels and stones were used by people for movement, transportation, hunting, and other activities. Since these traditional roads are unable to provide the high speed and safe surface required by pedestrians and vehicles, paved roads were developed to overcome these concerns.
Modern road pavements for foot and vehicular traffic are typically a combination of gravel and aggregate, as well as more high-quality materials such as asphalt, binder, and concrete.
Challenges in Pavement Maintenance
Paved roads require regular costly maintenance to ensure their safety, usability, and accessibility to the general public.
In the European Union alone, which contains a road network of more than 5.5 million kilometers (km), the annual maintenance spending in road infrastructures is estimated to cost approximately 30 billion Euros.
For the past several decades, around 90% of these costs are dedicated to the maintenance and upgrades of existing roads, with the remaining 10% being used for constructing new roads.
Despite the amount of money dedicated to maintaining the EU road network, there remains a lack of information available to both the general public and the European politicians to devote more resources towards maintaining the road infrastructure's integrity.
Without the proper investments, chronic underfinancing and deterioration of these pavements will result in a decay of the road network and a loss of asset value, and even higher costs that will be needed to manage roadway issues after a problem has already occurred.
Furthermore, energy consumption often required to correct these issues has an inevitable impact on the environment due to the natural resources involved in these construction projects and their greenhouse gas (GHS) production.
External vs. In Situ Pavement Sensors
Over the past several years, a growing number of efforts have been made to develop advanced, precise, and effective monitoring systems capable of continuously monitoring pavements to identify maintenance needs quickly.
The two primary types of non-destructive monitoring methods developed to investigate pavement conditions include external evaluation methods and in situ pavement sensors.
External evaluation methods, which can be manual or automated, are primarily used to measure the distress present within pavements through several different techniques, including deflection, acoustic emission, ground-penetrating radar, ultrasound, infrared thermal imaging, as well as laser technology.
Unfortunately, in addition to being limited in their ability to detect micro-damage and its development over time, most of these external methods require significant human resources and expensive equipment.
Comparatively, in situ pavement sensing technologies include a wide variety of sensors ranging from moisture and pressure sensors to those capable of detecting changes in the temperature and strain on pavements.
In situ pavement sensors guarantee the efficient and reliable transmission of data through communication wires between the sensor and the repository.
Although wired communication systems have been incorporated into various pavement sensors, their installation can often be labor-intensive and costly. As a result of these limitations, researchers have turned to wireless sensor networks as a low-cost alternative.
A Wireless Sensor Network Embedded into Pavement
One of the most recent developments in pavement monitoring devices includes a wireless sensor network (WSN) that is directly embedded into asphalt pavements for continuous monitoring of pavements.
The embedded WSN comprises a set of sensors, otherwise referred to as nodes, that are integrated into the structure of interest and specified positions determined before the system’s installation. The position of each WSN is determined by its susceptibility to experience a specific event, such as concealed cracks, or to measure a particular parameter such as temperature or pressure.
WSNs can be installed onto existing surfaces by drilling a deep hole into the pavement or incorporated into the pavement during construction through less invasive operations.
WSNs must be able to survive the daily pressure and strain that pavements are exposed to each day and resist the damage that can occur to their fragile sensors during their integration into the pavement.
As a result of these requirements, the packaging of a WSN and its components is critical to the design of these systems. WSNs must be packaged with a material that is compatible with that of the pavement and can protect the internal circuits from thermal and mechanical damage without compromising on their performance capabilities. For example, the strain gauges of WSNs are often packaged with epoxy resin, as it is associated with a high-energy harvesting efficiency and can simultaneously protect the internal circuits of the WSN from high pressure and potential harm from aggregates within the pavement.
Challenges of WSNs
Despite the advantages of a WSN, the primary challenges related to the implementation of this system are its energy supply. For example, most commercially available sensors either use batteries, which require periodic replacement every 102 years, or solar power systems. Therefore, these energy supply systems' limited lifespan causes WSNs to be cost-prohibitive in terms of their energy requirements.
Alternative approaches to this issue include energy harvesting, which could involve converting ambient energy, such as mechanical energy released from vehicles in the form of vibration, stress, deformation, and strain, into electrical energy that could ultimately power the sensor present within the pavement.
Piezoelectric materials such as lead zirconate titanate (PZT) offer a promising future to WSNs, as they are capable of harvesting dissipated mechanical energy from the pavements into electric energy.
The Future of Wireless Sensors in Pavement Maintenance
While still in its infancy, the field of pavement health condition monitoring is rapidly evolving to produce more accurate and less sensitive techniques to allow for the timely and efficient maintenance of road pavement.
As sensor technology advances, the integration of low-cost, self-powered, and autonomous wireless sensors embedded into the pavement will become a standard method of pavement maintenance.
References and Further Reading
Shtayat, A., Moridpour, S., Best, B., et al. (2020). A review of monitoring systems of pavement condition in paved and unpaved roads. Journal of Traffic and Transportation Engineering 7(5); 629-638. doi:10.1016/j.jtte.2020.03.004.
Di Graziano, A., Marchetta, V., & Cafiso, S. (2020). Structural health monitoring of asphalt pavements using smart sensor networks: A comprehensive review. Journal of Traffic and Transportation Engineering 7(5); 639-651. doi:10.1016/j.jtte.2020.08.001.
Xiao, J., Zou, X., & Xu, W. (2017). ePave: A Self-Powered Wireless Sensor for Smart and Autonomous Pavement. Sensors 17(10); 2207. doi:10.3390/s17102207.