The history of concrete would go back to the Egyptians (-2600 years BC). It was a mixture of lime, clay, sand and water. It is really not far from what is currently used. We simply replaced lime by clinker which is also called “burnt lime” … and rightly so.
There are now several types of concrete that differ, among others, by the use of clay, bitumen or cement. Here we will talk about cement concrete, which is the most commonly used.
It is also called reinforced concrete when it is poured into a steel structure. We will first talk about simple concrete and then present the importance of steel in our structures.
The word “concrete” speaks to everyone, but are you able to explain a little more precisely what it is?
It all starts a bit like a dance step! And 1, and 2 and 3!
It simply means that for 1 volume of cement you need 2 volumes of sand and 3 volumes of gravel. In practice, we buy directly the mixture of sand and gravel. So we go to 1 volume of cement for 5 volumes of mixture. The final touch is made by the mason who adds water in a quantity of about 1/2 volume.
But let’s come back to the main components of concrete:
It is sold in 3 different families: A, B or C, which correspond to a different clinker content. “A” corresponding to a high content and “C” corresponding to a low content.
Other letters, such as L, LL, S, V and D, are added to this, corresponding to the main elements other than the clinker constituting the cement.
It is then divided into 3 common strength classes: 32.5, 42.5 and 52.5. These numbers indicate the strength in MPa (Megapascal) that a standardized mortar core (composed of the cement in question) acquires after 28 days of drying.
The main thing to remember is that it is the cement which, when mixed with water, produces a chemical reaction that allows the aggregates (the mixture) to stick together.
The famous mixture :
We use what is called, in the middle, 0/20. This means that the mixture is composed of elements ranging from 0 mm to 20 mm in diameter. This mixture must be judiciously carried out in order to leave the minimum gap to offer good mechanical performance to the work.
Strengths and weaknesses of concrete :
Concrete is very resistant in compression. We speak of a compressive strength of 30 N/mm2 ( 30 MPa) on average on a concrete cube. That is to say that on a cube with an edge of 2 cm (one side is 4 cm2) we could put 1200 kg evenly distributed or a city car!
On the other hand, concrete is 10 times less resistant in traction than in compression. Moreover, in the building industry, we consider that the tensile strength of concrete is nil. We must therefore fill this gap …
Why put steel in concrete?
Contrary to concrete, steel is very resistant in traction. Concrete has a yield strength of 250 N/mm2 (250 MPa), i.e. beyond this limit, steel deforms and therefore loses strength. Being 8 times more resistant in tension than concrete in compression, 8 city dwellers could be suspended from a steel cube with an edge of 2 cm without exceeding its elasticity range. Is it still necessary to achieve this feat …
A few figures to summarize:
0/20: It is the mixture of sand and gravel which is made up of elements with a diameter ranging from 0 to 20mm in diameter.
28: This is the number of days the concrete needs to reach its “conventional” strength. Its strength may continue to increase a little later.
From 32.5 to 52.35: these are the strength classes of the cements sold (in MPa).
10: Concrete is 10 times stronger in compression than in tension.
250: is the tensile strength in MPa (Megapascal) of the steel used in reinforced concrete. The mixture of the two allows to have a structure resistant in tension and compression.