As an academic, you work on NDT in the context of civil engineering structures. What's your experience?
I'm director of the Geophysics and Non-Destructive Evaluation Laboratory (GeoEND) at the Gustave Eiffel University in Paris. My research focuses on the development of non-destructive testing methods for concrete using ultrasonic waves. At Cofrend, I'm involved in leading working groups with a broader remit, dealing with methods that go far beyond ultrasound. I take part in the "NDT & Civil Engineering" working group, and lead a civil engineering working group as part of the Cofrend group dedicated to SHM (Structural Health Monitoring).
First of all, can you tell us about your research work in the field of civil engineering?
Personally, I use rather high-frequency ultrasonic waves. These waves mainly provide information on mechanical properties such as modulus of elasticity or porosity, or on damage (cracks, etc.).
Concrete is cement with pebbles and aggregates. Waves, which can be multiply scattered over the aggregates, can be used to detect early defects, or defects that are not detected by traditional methods. For example, we are developing non-linear acoustic techniques that offer the possibility of finding closed cracks, of which there are many in concrete, and some of which need to be found and quantified. There are already methods that are effective, but we're trying to improve techniques and find defects that conventional techniques don't see. For example, we're aiming to detect defects earlier, or defects smaller than those detectable by conventional techniques.
Many laboratories are working on these ultrasonic techniques, using diffuse fields or surface waves. I'm also working on surface waves, which are useful for monitoring the surface protecting what are known as metal reinforcements, which corrode. With surface waves, we try to monitor the properties of concrete, which acts as a barrier to aggressive agents. Other researchers are working on these issues using other techniques. For reasons of scale and environment (wet, salty environments, etc.), a single technique is not enough. We need complementary techniques for a multi-scale, multi-physics approach.
You lead the Civil Engineering working group within the more general SHM working group. Can you tell us a little more about it?
Gustave Eiffel University and CEA are leading the SHM branch of Cofrend, with the aim of structuring the SHM sector in France. Subject-specific working groups have been set up. Hervé Lançon (Sites) and I lead the civil engineering working group, which includes a large number of researchers and industrialists from a wide range of fields, including energy, road networks and rail networks.
The first step was to establish the scope of the working group and propose a definition of SHM in civil engineering. Here's the definition we adopted: SHM is a set of measurement arrangements for remotely-operated collection and reporting of physical quantities on civil engineering structures, with the aim of providing information over time useful for their management and operation (indicators, decision support, forecasting).
Civil engineering structures" include engineering structures (bridges, viaducts, dams, dikes, retaining walls, trenches, quays, reservoirs, nuclear enclosures, air coolers, chimneys), buildings, pavements and the geotechnical engineering of the immediate surroundings of these structures.
The particularity of our theme is that we take into account a particular temporality when it comes to monitoring structures. Three timeframes are highlighted: long term (to complete knowledge of part of a structure, manage assets and monitor them over the long term), short term (reinforced monitoring when a problem is identified, trend monitoring, one-off diagnosis) and very short term (for immediate monitoring). Depending on whether the timeframe is long, short or very short, the issues and challenges are not the same.
Can you give us some concrete examples?
Take sensors, for example. Over the long term, one of the challenges facing sensors is to ensure that they are durable, and that their cost is not too high. However, when you're in a hurry, cost doesn't really matter for a very short time.
What about measurements and data? When you're working in a very short space of time, you need real-time information. On a structure that is examined 15 years after the sensors were installed, the question arises as to whether the data are properly recorded and the formats still transferable.
What are your proposals for the industry?
Our general action proposals for the sector are to promote French know-how and success stories, promote dialogue between players who currently have little dialogue (as in other sectors), take into account the long timeframe of SHM provisions 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 civil engineering SHM, state-of-the-art practices, identification of bottlenecks, SWOT analysis, success stories, etc.).), identify needs and make recommendations in terms of training, and publish a white paper (definition of the scope of SHM in civil engineering, state-of-the-art practices, identification of bottlenecks, SWOT analysis, success stories, responses from the SHM sector).
It should be noted that major structures such as the Millau Viaduct or nuclear power plants are already instrumented for monitoring purposes. For the time being, however, these are rather exceptional structures. As all existing structures age, SHM will be deployed and more and more new structures will be instrumented. The aim is to better monitor the condition of structures. This comes at a cost, but if used wisely, it can be economical. Researchers are formalizing this within the overall SHM framework.
The durability issue is particularly important for concrete, in which it is possible, for example, to embed sensors such as optical fibers, deformation sensors or accelerometers. A number of research laboratories are working on innovative sensors designed to be buried in concrete.
In my opinion, civil engineering is a sector with a bright future. There is a growing need for auscultation and monitoring to ensure the safety and proper functioning of structures, and also to optimize maintenance. The Cofrend working group headed by Vincent Garnier on NDT and concrete is very active. A sectoral committee has also been set up, as there are no regulations for the moment. Nor are there any in SHM, and much remains to be done.