CONCRETE DURABILITY

It is common to say that things made in the past were better build; built in an attempt to provide them with a lifespan longer than the life of the owner, much longer.

Unfortunately, this philosophy seems to be out fashioned when it comes about buildings (let´s say, often). Everything must be done quickly and trying to save every single penny. Nothing wrong with that, until it doesn´t interfere with long term safety for the users. Let´s see how concrete is involved.


The structure´s durability is represented by its ability to withstand the test of time, keeping its performance and safety standards for which it was designed. Of course, concrete plays an important role (even if it is not the only element contributing to the structure´s durability) that´s why we will focus, along this post, on concrete durability.


The most important causes for concrete decay are:


Carbon dioxide

Humidity

Freeze

Chlorides

Sulphates

Chemicals

Cracks


In Europe, at the regulatory level, are expressed 5 exposition classes which defines how to efficiently fight the majority of these causes; each of them has subclasses which defines the seriousness of the risk.

We are not going in depth here, according to the blog´s philosophy, but just trying to explain as easier as possible what are they about. Anyway, the following is the list of the macro-classes of environmental exposition:


XC = Carbonatation

XD = De-freezing

XS = Sea water

XF = Freezing

XA = Attack (chemical)

XC CARBONATATION


Is the air dangerous to reinforced concrete­? Actually YES. And I am not talking about polluted air, I am talking about normal carbon dioxide. This is the cause of concrete´s carbonatation.


Carbon dioxide flows into concrete´s porosity and combines with lime generating limestone (and some water). Lime was a “reinforcement allied” because of its PH (around 13). Limestone, on the contrary, has an aggressive PH (around 8). This originates reinforcement oxidation (rusting).


Oxidation is an expansive reaction that triggers Inside-out pushes to the extent that pieces of concrete are actually popped out. It´s interesting to notice that carbonatation is not that dangerous to the concrete itself, rather it is very dangerous for the reinforcement which, rusting, destroys its container, namely, the concrete.


In order to minimize Concrete´s Carbonatation and structure´s decay, w/c ratio should be lowered and the thickness of the cover increased.


XD DE-FREEZING and XS SEA WATER


This is the aggression that comes from chlorides like, for example, the chlorides used in wintertime to avoid icing on the streets (that´s why “de-freezing”) or sodium chlorides which comes from the sea water and its mist. Chlorides attack strikes both the concrete and the reinforcement bars.

In the situation described by the XD class the cement paste of the concrete is literally disintegrated as time goes by, while the sodium chlorides (XS) if combined with reactive aggregates, can trigger pop out reactions.

In both cases the reinforcement bars are going to be corroded, which means that they will be slowly consumed leaving the bars so weak to become totally useless.

In order to minimize the devastating effect of chlorides, w/c ratio should be lowered, the thickness of the cover increased and the shape of the elements designed in a way that fosters the drainage of the aggressive agents.

Chlorides can also be present in concrete´s “ingredients” that´s why the manufacturer have to declare its percentage, which must be lower than the limits imposed by the regulation.


XF FREEZING


The low temperatures cause the moisture (water) inside the capillary porousness of the concrete to freeze. As well-known, freezing causes an increase in volume of the water (about 9%) which causes strong pressure from inside out. Under this thrust, the concrete undergoes a progressive crumbling, starting from the outside.

The main way to fight this phenomenon is adding air to the fresh concrete. This air “bubbles” must have a precise dimension and space factor so to create, inside the concrete, a grid of “bubbles”, voids not interconnected, able to intercept both the capillarity and the fissures breaking from outside in.

These voids will become a sort of tank with one way out (the fissures themselves) so that the water trapped inside could freely expand during the freezing phase, actually, expanding throughout the fissure.

Again w/c ratio should be lowered but this time the compression strength value will decrease, because of the added air. Therefore, although with equal w/c ratio two different concretes, whereas one of them is in XF class, that last one will be weaker than the other (compression wise).


XA CHEMICAL ATTACK


One main enemy of concrete are sulphates (normally present in sewage) which literally blow away all the cement´s chemical bonds. But concrete is actually attacked by several chemical components. Of course, it is not the case to have a long list here, but if you are interested and need more explanations, I am sure you will have the chance to go in depth by means of p-concrete forum. Anyhow, the regulation defines specific w/c ratio and specific requirements for the cement used.


WHAT ABOUT CRACKS?


It´s trivial: cracks are an easy access for every kind of aggression, straight to the concrete core. That´s why it is vital to prevent them or at least to minimize them. But which are the main causes for cracks?

Shrinkage (loss of water, volume reduction). Please read more in my two posts “Concrete shrinkage part 1” and “Concrete shrinkage part 2”.

Mechanical stress (traction, hits). Concrete has good strength to compression but very low strength to traction, read how “Fiber-reinforced concrete” can help.

Thermal stress (expansions and contractions). To minimize this phenomenon joints are the best solution. Some hints in my post “Concrete flooring” and in my free booklet “Concrete flooring tips and tricks” available after registering as a member to Practically Concrete forum.


I want to conclude suggesting why durability should be a “must”:

  • It´s economic, because, although the higher initial cost, the final cost, in terms of maintenance, will drop drastically.

  • It´s eco-friendly, because, with lesser maintenance it is possible to save non-renewable resources and disposals.

  • It´s lifesaving. Structures built to last the more possible will preserve their users.


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