Reinforced concrete: the best of concrete and steel together

The strength and durability of a work can be guaranteed by design and construction solutions that guarantee the work itself to cope with two actions: compression and traction.

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In an extreme simplification, the first is the ability to withstand vertical loads, such as the weight of the same infrastructure added to the weight of the vehicles that pass through it, discharging it onto the ground on which the work itself is anchored. The second is related to the flexion that is determined for example in the event of an earthquake or in a bridge in the central part of the span. Following the bending, the lower part of the span is subjected to traction, while the upper part to bending.

Burj-Khalifa a concrete skyscraper. The material universally considered most suitable to withstand compression is concrete. Instead, steel is the most suitable material to withstand traction. Combined, they give life to reinforced concrete, ie concrete structures incorporating steel reinforcement and therefore capable of withstanding both compression and traction.

This solution, adopted everywhere in the world for almost a century now, has to cope with the natural aging process of materials that can be accelerated by external aggressive factors such as carbon dioxide, saltwater or the freeze/thaw cycle. In particular, the role of protection that the concrete guarantees to the steel reinforcement from phenomena that can oxidize and therefore corrode the material is fundamental.

Agents present in nature, over time, attack the concrete, progressively getting closer and closer to the reinforcement and causing corrosion. This can happen mainly for two reasons: the carbonation process or the presence of chlorides in the surrounding environment (marine environment, de-icing salts, etc.). In the first case, the carbon dioxide present in the atmosphere, penetrating inside the concrete, reduces its pH and causing the destruction of the protective oxide film that forms naturally around the steel thanks to the alkalinity of the concrete. At this point, the steel is no longer protected from corrosion (which triggers in the presence of water and oxygen). In the second case, the chlorides themselves perforate the protective film.

Today, this penetration process is made up to 100 times slower also due to the fact that the workability of the concrete is guaranteed by additives rather than by water. Using the polymers developed thanks to the progress of the chemical knowledge of the building materials industry, we obtain a workable concrete but at the same time less porous and able to adhere perfectly to the metal reinforcement, thus avoiding the creation of conditions that may favor steel deterioration processes.

Today’s concrete, therefore, is up to 100 times more resistant to external agents than in the past decades.

Maintenance and consolidation
Concrete is inherently durable by virtue of its mechanical and physical characteristics that allow it to withstand the agents that could cause its degradation. And in fact, a careful examination of the environmental conditions in which the work will be in operation is sufficient to choose the right concrete to guarantee durability and limit maintenance needs. The internal concrete structures can even be considered eternal since, under normal use conditions, there are no mechanisms that could damage the concrete present in closed environments without humidity.

The mechanical resistance to compression is the characteristic that has always identified the concrete and that has made it the leading material in construction. In the light of the great strides forward made by the sector, it is now possible to achieve values โ€‹โ€‹of resistance such as to make it the ideal material even for particularly complex structures in terms of mechanical performance such as buildings of considerable height or bridges of great light. It also has fire resistance characteristics that make it advantageous compared to other materials: in case of fire, the extraordinary fire-resistance properties of concrete are safe for people, things and the environment.

However, all the structures, made in concrete as well as in any other material, have an “expected life span”, that is a period of time during which, if subjected to adequate maintenance, they maintain adequate levels of safety, or higher than that minimum standard.

At the end of the service life, the works planned and carried out in a workmanlike manner, with materials of the right quality and subject to adequate maintenance, are not necessarily to be disposed of, but must be subjected to safety checks in order to prolong their life, possibly with extraordinary interventions.

These assessments must take into account several elements:

the significant variation in performance requirements over the last 50 years, in particular, traffic loads, which today are enormously more demanding;
the seismic risk;
the natural degradation of materials as a result of repeated actions and environmental agents.
In recent decades, materials preparation technologies and construction techniques have evolved considerably. So much so as to allow, today, to satisfy in a more than adequate way the needs of present and future solidity and security of each type of structure.

The aim of maintaining reinforced concrete structures is to maintain or restore the characteristics of the material and structure to maintain or restore the safety levels to the original ones or, in some cases, to improve them and adapt them to new conditions and/or regulations. To this end, specific materials and systems are used (cement-based, organic, composite, etc.) with the function of reconstructing, protecting, sealing, consolidating the existing material and/or structure.

The most applied techniques consist in the reconstruction of the degraded element through the application of new material, in the reinforcement of the structure by means of sheets, bars, fabrics in composite materials, in the sealing of the cracks, in the surface protection of the elements aimed at interrupting or slowing down the phenomena of degradation. The choice of the material and the method to be applied are fundamental in this sense.

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