What is the need for a suitable coarse aggregate

In-world, our construction industry needs 200 million cubic meters (approx) of concrete every year , In which total aggregate is 70 per cent, meaning 140 million cubic meter aggregate is required every year. And as the village of developing countries develops, the need will increase further. Therefore,It is very important to know about the coarse aggregate.

What does the quality of a suitable coarse aggregate depend on

The quality of a suitable coarse aggregate in the construction industry mainly depends on two things, parent rock of the aggregate and the gradation of the aggregate.

Sources of good coarse aggregate

the aggregate can be obtained from the rock, which is hard, strong and durable. But that aggregate shall not have the impurities such as coal, mica, clay, pyrites, lignite, clay, lime etc. Because it affects strength and durability of the concrete.

What is the need for gradation of aggregate

  • The purpose of a concrete mix design is that the Concrete can withstand more stress. And to withstand more stress, Concrete member shall have a minimum number of pores inside it. Therefore the total cross-section area of the member is equal to the effective cross-section area of the member, in other words, denser the Concrete is, there is more strength. Hence we have to select different size of aggregates in such a proportion so that the smaller size aggregates can enter into the hole of the larger size aggregates. Hence, the gradation of the aggregate is done in such a way that this thing can be achieved.
  • Workability means good finishing and good finishing cannot be achieved by just using the same size of aggregate. Further, when we measure the workability, then we desire that the Concrete should settle uniformly from all over, after the removal of cone. And this can be achieved only when we use different sizes of aggregate in the correct proportion. When we use the right amount of aggregates of various sizes, then we get an excellent finished surface during the slump test. Therefore, proper gradation of aggregate is very important.
  • When we use aggregates of the same size, in the concrete mix design, there is a problem of segregation of Concrete. Therefore, proper gradation of aggregate is very important.

the percentage of aggregate in concrete mix design

 quantity of aggregate to be used in the design mix, mainly depends on the  mix design. It can be understood from the lab report that the amount of aggregate, used in concrete mix design varies from 60 to 70 per cent of the volume of Concrete. It means, if we want to make one cum of Concrete, then we have to use aggregate upto 0.6 cum to 0.7 cum, and  we subdivide this aggregate quantity into coarse aggregate upto 66 per cent and fine aggregate upto 34 per cent. But for pumpable Concrete, there should be more finishing in the Concrete so that it can smoothly glide over the surface of the pipe, for which the quantity of fine aggregate can further increase upto 10 per cent. It means the use of coarse aggregate upto 56 per cent and fine aggregate upto 44 per cent of the total aggregate.

Grading zone of coarse aggregate

The size of the coarse aggregate varies from 4.75 mm to 80 mm. And we use the graded aggregate in our concrete mix design. Graded aggregate means aggregates of all sizes with different proportion  are available in that particular grading, so that maximum density can be achieved. When a graded aggregate has all the aggregates, then which aggregate size will lead the graded aggregates, of course, it is Maximum Nominal Size of aggregate. Where the maximum nominal size is the size of the sieve from which 90 to 100 per cent aggregate is passed.

As we know, the use of large size aggregates in the mix reduces the water requirement for the lubrication of the aggregate, if we compare it to the finer aggregate. Meaning if we look at the W/C, then we have to use less cement in the design mix. But by doing this, we face two issues. 

Firstly, If we have to cast a low width  wall or beam, then the distance between the two shuttering close to each other will be less. in such situation if we use coarser aggregate, such aggregate will work together to form as a bridge, create a blockage. as a result of this, there may be honeycomb concrete. So code restricts us to use the aggregate of size upto one-fourth of the distance measured between the two shuttering close to each other. 

Secondly, In our structure, we have very densely packed reinforcement. Due to which a large size aggregate does not enter between the two bars. So the code restricts us to use the aggregate of size 5 mm less than the distance measured between two reinforcement close to each other. 

Therefore when we select the maximum size of the aggregate for design mix, then three things should be taken care, 

  1. workability,
  2. the distance between the two shuttering close to each other and
  3.  distance measured between two reinforcement close to each other.Description: coarse aggregate
coarse aggregate

Grading zone of fine aggregate

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