Since we know the history of reinforced concrete, whose origins towards the end of the 19th century are not easily attributable, we can say that the marriage between concrete and steel was stainless.

The birth of this union can be traced back to a series of characters who experimented with the combination of cement mortar and iron on several occasions.

We can mention William Wilkinson, English, who in 1854 filed a patent for the construction of fireproof roofs and walls made of reinforced concrete, while in 1855, during the universal exposition in Paris the French lawyer J.L. Lambot presented a metal boat model covered with a layer of concrete.

To quote the Italian C. Gabellini who in 1890 the construction of naval hulls in reinforced concrete began but, if we look at the world of construction to which he is associated normally reinforced concrete, it appears that the first slab for a building was designed and built in 1879 by the French engineer Francois Hennebique.

Many others followed, bringing the combination of cement (concrete) and steel reinforcement to the center of the works and applications, up to a very wide diffusion in all the structural works of the present day.

With the advancement of research and knowledge on alternative structural materials, it was discovered that the use of some composite polymers could improve the performance and durability of load-bearing structures in reinforced concrete, precisely in light of recent events in which structures have been seen collapsing due to the wear of the materials that compose them.

Eng. Casadei Paolo, who illustrates the recent discoveries about the use of reinforcement in reinforced composite materials (GFRP) to replace common steel reinforcing bars.

They are dramatically under the eyes of all the problems of Italian infrastructures, the result of a design and construction that dates back to the first post-war period and a lack of knowledge about the phenomena of degradation and durability. Today, thanks to technological innovation and research, alternative scenarios can finally open up.

Sireg Geotech has been working for some time and with foresight, on an important innovation that will have a strategic impact on the construction and infrastructure sector, guaranteeing the necessary durability for infrastructures and finally allowing the concrete to be applied successfully even in particularly aggressive environments subject to constant degradation.

The state of the art of Italian infrastructures

The collapse of various infrastructures, including that of the bridge in Lunigiana up to the striking and catastrophic collapse of the Morandi bridge in Genoa, have shown that it is no longer possible neglect a careful analysis of our dated infrastructures both from the point of view of the degradation of the materials with which they were made, and also from the simple point of view of the initial loads for which they were designed, to end up with the issue of poor conditions of maintenance.

The massive inspection plan currently underway is certainly a first step that will allow us to carefully evaluate the safety of our infrastructure assets, then intervening on existing structures in a way precise and targeted, but still leaves a question mark about our future open:

We will continue to build as we have always done or, with a view to sustainability, durability and reduction of costs associated with maintenance, we will evaluate new materials that are more durable and with less impact environmental?

Answering this question today becomes crucial for an effective investment in our infrastructures, be they major works or works of minor importance, but still strategic for the economic development of the our country.

Future scenarios for sustainable infrastructure renovation with GFRP bars

The use of fiber-reinforced composite FRP (Fiber Reinforced Polymer) bars to replace the steel rod for the construction of structural elements in reinforced concrete.

This type of bars is made with fibers of various kinds, among which glass and carbon are certainly the most used materials, with glass that unwinds without the dominant role is a shadow of doubt thanks to a series of chemical-mechanical characteristics which, in relation to costs, make it the most adopted solution for this type of applications to date.

The diffusion of bars in GFRP is favored primarily by the fundamental property of these materials, namely their undisputed greater durability due to the fact that they are not susceptible in any way to corrosion phenomena.

This makes them particularly suitable in all applications where the work or the structural element is particularly subject to corrosion phenomena.

Just think for example of the bridge decks which during the winter are particularly exposed to the chlorides adopted to prevent the formation of frost on the road surface, to the canals for the water drainage or at the docks and piers by the sea or, again, to any reinforced concrete product in an industrial environment exposed to particularly aggressive environments.

Recent studies have shown that the useful life of a structure reinforced with this new technology can reach up to 100 years without any particular precautions regarding the nature of the concrete or other construction details, necessary instead in the case of reinforced concrete structures traditionally reinforced with steel rods.

However, there are several other properties of these materials that must certainly be mentioned in the comparison with steel in order to make appropriate design choices.

The GFRP rods are non-magnetic and are not heat conductors, therefore they find a congenial application in all artifacts exposed to stray currents, solving the problem of typical corrosion steel reinforcements which are in fact incompatible with this type of application.

Just think, for example, of all the infrastructures related to the railway sector or motorway gates with electronic recognition systems.

Another not negligible advantage in the use of GFRP reinforcements is the ease and speed of installation thanks to their reduced weight, about a quarter of that of steel.

This undisputed lightness makes the product particularly easy to move on the ground, so much so that several studies have shown time savings of up to 40-50% compared to laying an equivalent steel armor.

Which parameters to keep an eye on in the design and construction of these materials

Alongside all these aspects that have made the technology particularly attractive according to the different uses, a series of other aspects must certainly be highlighted that require attention to those who want to start designing.

First of all, it should be noted that the GFRP bars for structural uses are produced according to the pultrusion technique using E-CR glass fiber - known for its mechanical characteristics and improved durability compared to traditional E-glass - and a resin matrix of vinylester or thermosetting nature.

This means that once hardened it can no longer be modeled, that is, the process by which the bars are machined to make brackets and / or bent parts must be performed in the production phase of the bar itself and not in subsequent times, as is usually the case with construction steel.

Again, the bending radii of the bars are not the same commonly known for steel rods, but they have slightly larger dimensions to try to minimize the negative impact of bending on the mechanical characteristics of the bent part with respect to the straight part of the bar itself, as well as for industrial production reasons that see this process as one of the main obstacles.

The table below shows the mechanical characteristics of the Glasspree® bars by Sireg Geotech in fiberglass and vinylester resin.

By observing the table you can see how the mechanical characteristics of the bars vary as the diameter varies, with smaller diameters having higher mechanical characteristics than larger ones and , in general, with mechanical traction performance much higher than that of a traditional steel rod with improved adhesion.

If on the one hand the tensile strength can induce higher mechanical performance, on the other hand the elastic modulus is about a quarter compared to that of steel, equal to 46Gpa in this specific case.

This therefore means that if, on the one hand, in an ultimate limit state check one could expect to be able to make an equivalent section with smaller or smaller diameters of material, on the other hand in the verifications at the limit states of exercise it will often be necessary to adopt more material due to the lower elastic modulus.

As regards the shear checks, for the reasons set out above, the bent part of a bar does not resist like the straight part, to the point that the table shows it can be seen how a bar bent by 90 ° loses about 60% of the declared resistance of the straight part.

This last aspect is absolutely fundamental and to be kept in mind when dealing with the design of shear reinforcements or those requiring the presence of bent bars.

It is therefore essential, when approaching a design with these materials, to refer to technical data sheets in which these parameters are clearly highlighted, together with the standard against which these values were obtained.

In Europe, the reference standard is ISO 10406-1 and other commonly recognized international standards.

In the USA and Canada, employment and regulations are one step ahead

In the United States and Canada, the use of these materials today is seeing an ever-increasing increase, certainly thanks to the great impulse favored by a development of the regulatory framework and standards qualification that allowed a rapid implementation.

Until twenty years ago, university laboratories were studying the use of these materials only for pilot applications, while today we are spectators of a gradual, but always more widespread use, mainly in the infrastructural field with permanent works such as bridges, canals and others in various sectors.

The success of this technology on the American and Canadian markets has certainly been favored by the rapid but still careful and gradual development of documents such as the ACI 440.1R-15 "Guide for the Design and Construction of Structural Concrete Reinforced with Fiber-Reinforced Polymer (FRP) Bars" from the American Concrete Institute and the "AASHTO LRFD Bridge Design Guide Specifications for GFRP-Reinforced Concrete" from the American Association of State Highway and Transportation Officials representing today the most up-to-date standards for the design of reinforced concrete elements reinforced with fiberglass bars.

Regulatory situation in Italy and Europe

In the old continent and especially in Italy, the regulatory framework presents a situation that requires rapid modernization and alignment with current design standards or the Technical Standards for Construction (NTC) 2018. The reference document is the CNR-DT 203-2006 published more than 15 years ago and therefore the son of the Ministerial Decree of 9 January 1996 and of studies that are now extremely conservative and dated.

However, one of the aspects that has hindered and still holds back the development of this very promising technology is certainly the absence of a regulatory framework to meet the requirements of chapter 11 of the NTC 2018, for which all construction materials for structural use must be CE marked or with national certification that allows them to define their essential characteristics and can guarantee their constant performance over time.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.