Catastrophic failure of large civil structures like bridges, dams, embankments and buildings can result in fatal, costly and environmentally detrimental consequences. However, failures can also occcur in the surrounding groundwork, for example landslides and subsidence (sinkholes). Structural collapse during construction also poses high risk to people working on construction sites. There is a strong need for a sensing technology that is able to measure the performance of a structure over its entire lifetime, as well as its associated foundations and the surrounding soil and rock supporting the structure. This will help to provide early warning of impending failure, inform repair operations and optimize building methods.
The current gold-standard for monitoring structural stress and failure are distributed fibre optic sensors, which use the change in the properties of a thin fibre-optic cable to measure aspects such as strain. However, fibre-optic sensors are essentially wired into the structure, require deployment effort and provide a point of ingress, weakening the integrity of the structure. More importantly though, fibre-optic sensors can only measure strain along the fibre axis, meaning that the three-dimensional shape deformation of the structure cannot be directly measured. Additionally, it is time-consuming and costly to install fibre-optic sensors within the foundations and surrounding soil/rock, limiting their use to high risk projects.
This ambitious project seeks to develop a low-cost, wireless, embeddable sensing technology that can measure structural deformations in 3-D from deep within a structure, its foundations and surrounding ground, that are small enough to add to the concrete mix or injected into rock. Not only can each sensor measure changes in its position, it can also measure changes in orientation, yielding a full six degree of freedom sensor. Key to this is the use of low frequency magnetic fields that are able to penetrate rock, soil, concrete and water with minimal loss of signal, a marked advantage over current wireless technology based on high frequency radio that cannot penetrate even a few cm of concrete.
These cm-scale, low cost sensors are mixed in with the concrete pour, instantly forming a self-organizing and healing communication network. These devices start monitoring from the moment the element (e.g. a pillar or a beam) is poured, providing information over the entire lifetime of a particular structural element, from the concrete curing process to loading to monitoring cracks and corrosion. When structural elements are placed next to each other, the network will automatically extend to form a larger, merged communication system.
The sensors can measure their precise position and orientation within the structure and how this changes over time. With a number of these sensors the actual shape of the structural element and how it is bending or twisting with loads can be sensed. This is currently impossible to achieve using any other distributed sensing technology, a key advantage of low frequency vector fields, having both magnitude and direction in 3-D.
One of the issues of embedding sensors within a structure is maintaining operation over the lifetime of the structure, which can be many decades. The sensors use the same low frequency magnetic fields to harvest energy, which is either collected from ambient magnetic fields, such as mains wiring, or directly injected into the metallic reinforcement of the structure. This allows for battery-free, indefinite operation.
This technology has the potential to make buildings and large structures truly smart, using low cost, easy to deploy sensors that can operate from within the structure and the surrounding groundwork. This will enable real-time monitoring of key indicators of potential failure over the lifetime of the structure, providing early warning of impending disaster, with potentially life-saving results.
|