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Critical analysis of FRS durability study Mar 2015

Pierre Rossi, Research Director, fiber reinforced concrete consultant, Paris, France
In critical analysis of a paper about the durability of fiber-reinforced shotcrete (FRS) in an aggressive environment, consulting engineer and research director Pierre Rossi suggests that the study parameters are flawed, the criteria for the study are not applicable for the concrete reinforced structures in an ‘in-service’ state, and that providing evidence that metal corrodes more than synthetic products is not a real or new scientific advancement.

The durability of different fiber-reinforced concretes used in shotcrete-type applications is the topic of a paper entitled Durability performance of fiber-reinforced shotcrete in aggressive environments, written by J P Kaufmann and presented and published in the Proceedings of the WTC Congress of Iguassu Falls, Brazil, in 2014.

More precisely, the comparison targets the effects of cracking of these materials when they are thrust into different environments – ranging from slightly aggressive to very aggressive. Five fiber-reinforced concretes are studied; one consisting of metal fibers, the remaining four of different synthetic fibers; and five increasingly aggressive environments were studied.

It must be specified that the matrix of all fiber-reinforced concretes studied is the same and representative of the usual matrix in the intended type of industrial application.

Set-up of test samples used in the durability study for exposure to liquid solutions (top), coverage (center) and different square panels for mechanical testing and prisms for microscopy (bottom).
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As a general opinion, the author evidently wishes his article to be considered as a scientific one. The beginning of the article – including the description of chosen materials, the production of tested samples, and the tests carried out – is relatively precise and supports the idea that it could be considered as a scientific article on this type of subject. However the subsequent parts do not keep standards with the beginning.

In fact, when the author begins to deal with the very heart of the subject matter, details become less specific. Different types of aggressive environments are selected without substantiating the choice, either by explaining the situations in which these environments are found or by citing examples of tunnels in which such environments have led to serious problems.

Further, there is an assumption that all square slabs consisting of different fiber-reinforced concretes are pre-cracked before undergoing chemical aggression, and that the crack openings initially created are between 0.5 and 1mm at a certain place on the slabs (the same for all of them); and further that they reach “around” 2mm at the center of the slabs, without specifying which slab has a 0.5mm crack opening and which has a 1mm crack opening. Such imprecision is not permissible considering the importance these crack openings have with respect to corrosion of fibers, particularly metal ones.

There are also some basic scientific weaknesses in the study in that the author does not specify the number of slabs and prisms tested per type of fiber-reinforced concrete and per type of aggressive environment.

It would seem to be a matter of:

  • one cracked slab per type of fiber-reinforced concrete
  • one prismatic sample pre-cracked to 1.2mm, plus
  • one prismatic sample pre-cracked to 0.5mm per type of fiber-reinforced concrete and per type of environment.

How can one draw such strong conclusions when so few results are available, especially in a supposedly scientific article that concerns concrete and the problems of durability and within the framework of a problem where dispersion of reinforcement is important?

The principal weaknesses of this work concern the outline of the problem that the author has chosen to study. Focusing on the cracking-durability relationship of fiber-reinforced concretes only makes sense in a situation where the structure is in a service state. Generally, when in a service state situation crack openings should not exceed 100 for a very aggressive environment, to 200 or 300µm in a slightly aggressive environment. This is far from the 2mm crack openings studied by the author.

The author should have known that when a crack opening is less than 300µm in fiber-reinforced concrete, it presents a very tortuous and, at times, discontinuous path, which makes the circulation of aggressive agents more difficult.

When the crack opening is less than 300µm, self-healing mechanisms can occur and the corrosion products (in the case of metal fibers) can be deposited in the interior of the cracks.

These two physical mechanisms consequently obstruct the cracks and therefore prevent circulation of aggressive ions.

The author, in a laudable attempt at scientific objectivity, should have cited other researchers and previous studies which show that metal fiber-reinforced concretes do not present problems, except, in certain cases, where soiling poses an aesthetic concern because of the above-mentioned physical phenomena.

In conclusion, it must be emphasized that, in a situation where the fiber-reinforced shotcrete plays a mechanical role in the tunnel (recovery of forces exerted by the ground), as well as a sealing role (for example, preventing water infiltration), it is easier to realize this by using metal fibers than synthetic fibers – for two reasons.

First, the Young’s modulus of synthetic fiber is much lower than that of metal fiber, even when the synthetic fiber in question is macro-synthetic.

Secondly, the crack openings in synthetic fiber-reinforced concrete are significantly larger than those in metal fiber reinforced concrete, for the same service load.

A crack crossed by a synthetic fiber is significantly more likely to increase in size over the course of time than a crack crossed by a metal fiber when kept under constant load over time. This is due to the fact that synthetic fibers have more of a tendency to creep than metal fibers.

For these various considerations, the article in question cannot be considered objective, rigorous, or truly scientific. It presents results that do not represent real or pertinent situations for fiber-reinforced concrete usage. Providing evidence that metal corrodes more than synthetic products is neither a real nor a new scientific advancement.

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