Concrete Gets a 'Sensing Skin'!

Back in 2009, a grade "D" was assigned to the overall quality of infrastructure in the United states by the American Society of Civil Engineers(ASCE) stating that current evaluation and maintenance of structures was essential to ameliorate the grade. Subsequently, federal stimulus funds made it possible for communities to repair some infrastructure, but more sophisticated methods for continual monitoring remained immature.  Rather, most evaluation structures is still done by visual inspection, which is slow, expensive, clumsy and grave.

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Simon Laflamme(L) and Jerome Connor(R)

A method for continual monitoring of structures was recently proposed by the civil engineers at #-Link-Snipped-#, working with the physicists at the University of Potsdam, Germany. Structural Control Health Monitoring and Journal of Material Chemistry have published some papers where the researchers describe a flexible fabric with electrical properties that could hold fast to areas prone to cracking- such as the undersides of bridges - and detect cracks whenever they come about.

The installation of this 'sensing-skin' would be as easy as unrolling it and gluing it to the surface of a structure. The rectangular patches on the skin can sense the changes in the electrical charge and orient it in a geometrical design apt for the type of the crack developed in the structure : for example, diagonal square patches to detect cracks caused by shear, or horizontal patches to detect the cracks caused by a sagging horizontal beam.

The crack formation causes a tiny movement in the concrete under the patch, varying the capacitance, or stored energy, of the sensing skin. A computer system attached to the sensing skin would send a current once a day, in order yo detect a flaw and its exact location within 24 hours- a task termed difficult by the use of any other types of sensors already in use. It may be because these tend to rely on detecting global changes in the entire structure using a few strategically placed sensors.

Simon Laflamme PhD ’11, who did this research as a graduate student at MIT's Department of Civil and Environmental Engineering, said "The sensing skin has the remarkable advantage of being able to both sense a change in the general performance of the structure and also know the damage location at a pre-defined level of precision. Such automation in the health-monitoring process could result in great cost savings and more sustainable infrastructures." Laflamme operated with Jerome Connor, professor of civil and environmental engineering at MIT, and University of Potsdam researchers Guggi Kofod and Matthias Kollosche.

Originally, researchers tried this idea using a commercially available, cheap elastic silicon fabric with silver electrodes. While this worked out in some labs with tests performed on both small and large concrete beams under stress, ultimately the material turned out to be too thin and flexible for this kind of an operation. The researchers have now developed a prototype sensing skin made of soft flexible thermoplastic elastomer mixed with titanium dioxide that is highly sensitive to cracks; painted patches of black carbon measure changes in the electrical charge of the skin. A patent has been filed for the same.

Professor Tzu-yang Yu of the University of Massachusetts at Lowell says "The innovation of this proposed sensor design is in its use of a material that provides mechanical flexibility and serves as a capacitor. This design allows the sensor to overcome the difficulties associated with conventional piezoelectric sensors which have strict contact conditions between the sensor and the structure’s surface. The proposed sensor is also superior to conventional fiber-optic sensors in the way that two-dimensional readings can be collected from one sensor." He further added, " Like all innovations in the development stage, there are additional issues this sensor needs to address, such as instrumentation, packaging and environmental vulnerability. Naturally, the next step would be to perform a small field test in order to investigate the field performance of the sensor."

Connor said," “Many of the types of infrastructures graded by the ASCE are made of concrete and could benefit from a new monitoring system like the sensing skin — including bridges, which received a ‘C’ grade, and dams and schools, which earned ‘Ds’. he safety of civil infrastructures would be greatly improved by having more detailed real-time information on structural health."