What is the concrete and where can you use it?

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What is the concrete and where can you use it?

The concrete is a material that we can build a variety of structures of, but the cooperation between the concrete and steel is often misunderstood and this leads to errors in design and execution.

I wanted to limit my articles to 1000 words, but I should mention that these words do nothing but paint a very weak image from all you need to know about the reinforced concrete. There is much more to say, but we will try to fix some essential knowledge.

The concrete is a cheap material with a good resistance to compression, but with a doubtful resistance to tension, although some sources claim that the concrete has a tensile resistance as 10% of the pressure resistance, I do not want to confirm this because that 10% percentage will vary depending on many factors.

The resistance of concrete to compression is directly dependent on the type of the aggregates (the sea sand, from the beach, it can not be compared with the quarry sand resulted from the diorite and basalt), the amount of the cement used and the amount of water used (the lesser water, the better) the length of the aggregates and their nature.

When a block of concrete is subjected to mechanical stress cracks appear. The small cracks are normal and healthy as long as they do not become too large.

Two things are certain when we are working with the concrete, it will strengthen and it will crack, so it is inevitable that sooner or later cracks will appear in the concrete.

What is perfectly normal if they are below certain limits, and in the Concrete degradation article we talked about what is acceptable and what is not, and many other things. For now I will say that if the thickness of the concrete cracks is as a hair and  if they are short, 1-25 cm, depending on the size of the concrete element, these cracks are totally normal.

The steel, a symmetrically material

The steel, as the wood, is a symmetrical material, this means that approximately, the resistance to tension is equal to the compressive resistance, but there occurs a phenomenon called buckling, that is, because of the length of the steel rod, we can benefit only for the tension because at compression the bar will bend relatively easily.

With a careful design of the steel element it can be reached to a point where to give a good use also to the compression part by removing the part of material from the center of the profile, so the inefficient steel form on the compression side is the bar, and the most efficient is the pipe.

When the steel is a bar we call it reinforcement (striated, as shown below, or without striations), and when is intended to find shape which increases the compressive resistance we call it profile. On the other side, to the tension, the rod design is better because it allows us to concentrate all the tension power in a small point.

For now we can conclude that the steel bar can help us complete the weakness of the concrete. And so we obtain an object made of a material resistant to compression and one resistant to tension. The steel also helps to compression by the fact that together with the stirrups it forms a cage for the concrete and they do not let it be aside even after shattering and the building will stand during an earthquake or an explosion even if the concrete is compromised.

Now that we have defined a few things about the concrete and the steel, I wanted to tell you that the steel and the concrete have the expansion coefficient almost similar, this thing is good because otherwise, if the steel could move without the concrete or in an opposite direction, the concrete would crack and the steel would shake off the concrete, so the cooperation between the steel and the concrete would not work.

None of them can be without the other and if tomorrow, miraculously, the expansion coefficients of one of them would change, the degradation of the building will be immediately felt.

Note: A good thing to know is that sometimes the concrete is reinforced with polymers. There are many reasons to give up steel and to insert a plastic (highly resistant) to reinforce the concrete. I want to divide the explanation into two parts, and these parts come from two reasons. The first reason for the substitution of the steel is that when the steel rusts, it expands its volume for 3 times, practically, superficial said, some part of steel turns into 3 parts of rust and this volume increase is a force so great that the concrete has no chance to counter it and to not crack. The reinforcement with polymers is recommended in the case where the exposure contains salt, such as in a marine environment. The salt is the number one enemy of the steel in the concrete. Below you can see how the sea water “ate” some railways buried in concrete several years ago.

The second reason is that steel is magnetic in some hospital rooms or military bases or industrial environments, where is keptequipment with powerful electromagnets and where the magnetic fields can attract the reinforcement from the wall or if the reinforcement is extremely well anchored the equipment comes to the wall.

In both cases, we saw special situations, and I do not recommend using the plastic for reinforcing the concrete. There are several reasons against their use such as the ecological impact or the price.

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