As an academic, you work on NDTs as part of civil engineering structures. What about?
I am the director of the Geophysics and Non-Destructive Evaluation (GeoEND) laboratory at Gustave Eiffel University in Paris. My research focuses mainly on the development of non-destructive auscultation methods of concrete with ultrasonic waves. At Cofrend, I participate in the animation of working groups that are of expanded competence, which deal with methods that go far beyond the framework of ultrasound. I participate in the "END & Civil Engineering" working group and lead a civil engineering working group as part of the Cofrend group dedicated to the SHM (Structural Health Monitoring).
First, can you tell us about your research work related to civil engineering?
Personally, I use ultrasound waves rather high frequency. These are waves that will mainly give information on mechanical properties such as modules of elasticity or porosity or on damage (cracks, etc.).
Concrete is cement with pebbles, aggregates. The waves, which can be multiplely diffused on the aggregates, make it possible to detect early defects or defects that are not detected by traditional methods. For example, we develop nonlinear acoustic techniques that offer the possibility of finding closed cracks, numerous in concrete, some of which must be found and quantified. There are already methods that are effective, but we try to improve the techniques and find defects that the usual techniques do not see. For example, we aim to detect defects earlier or defects smaller than those that can be detected by usual techniques.
Many laboratories are working on these ultrasonic techniques, using the diffuse field or surface waves. I also work on surface waves useful to monitor the surface that protects what are called metal reinforcements, reinforcements that corrode. With surface waves we try to examine and track the properties of concrete which constitutes a barrier for aggressive agents. Other researchers are working on these issues with other techniques. For questions of scale and environment (wetland, salty, etc.), a single technique is not enough. We need complementary techniques to have a multiscale and multiphysics approach.
You lead the Civil Engineering working group within a working group set up on the more general theme of the SHM. Can you tell us a bit more about that?
Gustave Eiffel University and the CEA are leading the SHM branch of Cofrend, with the aim of structuring the SHM sector in France. Working groups by subject have been set up. Hervé Lançon (Sites) and I lead the civil engineering working group which includes many participants, researchers and industrialists representing plural fields of activity such as energy, road networks and rail networks.
A first step was to establish the scope of the working group and to propose a definition of the SHM in Civil Engineering. Here is the definition we have chosen: the SHM is a set of measurement provisions for the collection and reporting of physical quantities remotely operated on civil engineering structures, with the aim of providing timely information for their management and operation (indicators, decision support, forecasting).
Included in "civil engineering structures" are: engineering structures (bridges, viaducts, dams, dikes, retaining walls, trenches, wharves, reservoirs, nuclear enclosures, aero-refrigerants, chimneys), buildings, pavements and geotechnics of the immediate environments of the structures.
The particularity of our theme is the taking into account of a particular temporality in terms of monitoring structures. With three times highlighted: a long time (to complete the knowledge of a part of a work, manage the heritage and monitor it in the long term), a short time (reinforced monitoring when a problem is identified, follow-up of evolution of a trend, punctual diagnosis) and a very short time (for immediate monitoring). Depending on the long, short or very short times, the locks and the stakes are not the same.
Is it possible to share concrete examples with us?
Take the case of sensors, for example. Over a long period of time, one of the challenges of sensors is that they are durable, with notions of costs that should not be too high. However, the cost that no longer matters too much for a very short time when we are in an emergency.
What about measurements and data? It is necessary to have real-time information when you are in a very short time. On a book that is examined 15 years after having installed the sensors, the question arises, for example, whether the data is well recorded and the formats are still transferable.
What are your proposals for the sector?
Our proposals for general actions for the sector are to promote French know-how and success stories, promote dialogue between actors who currently have little dialogue (as in other sectors), take into account the long time of the provisions of the SHM in civil engineering (transmission, open data, standardization, etc.), identify needs and make recommendations in terms of training, and publish a white paper (definition of the scope of the SHM civil engineering, state of the art of practices, identification of locks, AFOM analysis, success stories, responses of the SHM sector).
It should be noted that major structures such as the Millau viaduct or nuclear power plants are already instrumented for monitoring. However, for the moment, they are rather exceptional works. With the aging of all the structures in place, the SHM will be deployed and more and more new structures will be instrumented. Because the objective is to better monitor the state of the structures. This has a cost but used wisely it can be economical. Researchers are also formalizing this within the overall framework of the SHM.
The issue of sustainability is important especially for concrete in which it is possible, for example, to drown sensors such as optical fibers, deformation sensors or accelerometers. Some research laboratories are working to design innovative sensors to be buried.
Civil engineering is, in my opinion, a sector of the future. There is more and more auscultation and monitoring to ensure the safety and proper functioning of the structures, and also optimize maintenance. The Cofrend working group led by Vincent Garnier "END and concrete" is very active. A sectoral committee has also been set up because there are currently no regulations. In SHM either, and much remains to be done.