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Concrete composition

The finished concrete mixture is a fluid composition of four components blended in a certain proportion: cement, crushed stones, sand, and water. The weight ratio of the components for producing the concrete mixture is approximately as follows: Cement – 1 portion, Crushed stones – 4 portions, Sand – 2 portions, Water – ½ of the portion.

For instance: cement – 330 kg, crushed stones – 1250 kg, sand – 600 kg, water – 180 litres. Naturally, these figures are rather approximate, and in practice they depend on numerous factors, such as: required concrete grade, cement grade, characteristics of crushed stones and sand, use of plasticisers, other admixtures, etc.

For example: when using cement M-400, the concrete with such a composition will show the grade M-250. When the cement is M-500, the concrete grade will be M-350. But even these figures are rather relative! When manufacturing concrete at the concrete plant, not a dozen of parameters and characteristics are taken into consideration.

Cement and water are the main components of the concrete. In fact, they make the main function – binding all the components in a whole monolithic structure. Adherence of the right proportion of these two components (water-cement ratio) is the basic object in the manufacture of concrete. This refers not only to the amount of water and cement added to the concrete. It is simple with that. It is important to take into account all the nuances: moisture of the crushed stones and sand, its moisture absorption, etc.

When interacting with the water (cement hydration), cement can stiffen and harden producing the so-called cement bricks. It is as if a completely self-sufficient material. However, it is not of a kind. When hardened, the cement bricks deform. Degree of shrinkage reaches 2 mm/m. It seems not too much, but due to the variation of the shrinkage processes, inner tension and tiny cracks appear. These tiny cracks are practically invisible, but the resistibility and longevity of the cement bricks decrease. In order to minimise these deformations, the composition should include aggregates:

  • Large aggregates: crushed stones
  • Minor aggregates: sand

The function of these aggregates is to create the structural framework that accepts the shrinkage tensions, and as the result, the finished concrete gives the less shrinkage. The resistibility and module of the concrete resilience (minimising deformations of the constructions under the load) increase, the creeping (when the concrete deforms irreversibly under the durable load) decreases. Aggregates considerably cheapen the concrete. Cement is much more expensive than crushed stones and sand.

At the beginning of the article data was provided on the approximate proportions of the basic components of the concrete mixture. Let us now convert weight portions into the volume, and calculate:

  • Cement 0.25 cubic metres (330 kg. Bulk density of the cement on average is 1300 kg per cubic metre)
  • Water 0.18 cubic metre (180 litres)
  • Crushed stones 0.9 of a cube (1250 kg. Bulk density 1350 kg per cubic metre)
  • Sand 0.43 of a cube (600 kg. Bulk density 1400 kg per cubic metre)

In total, if we resolve and pour it into different vessels, we will receive the total volume of 1.76 cubic meters! How it all can fit into a cube of concrete. Here is how. We take a one-litre jar and fill it fully with the crushed stones. Between the separate grains there will be a lot of free space (intergranular void). This void must be filled with two glasses of sand, one glass of cement, and a glass of water, by shaking and stirring it. And it all fits in! Eventually, we have a totally dense substance. All the pores are fulfilled; all the aggregates are set against each other. If not stirred and touched, the concrete soon hardens (solidifies). When vibrated and stirred, the concrete again becomes flexible (thixotropy).

Solidity (grade) of the crushed stones should be 2 times more than the rated grade of the concrete. It is for the reason that the project (28 days) grade of the concrete is always considerably lower than its real solidity that comes in half a year or a year's time. The solidity of the crushed stones does not increase with the time. For this reason they are levelled. Anyway, it all is done in the way of non-rated resistibility resources of the project requirements. As the saying goes – just to be safe!