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Understanding the Role of Aggregates in Concrete

What are Concrete Aggregates and Why do We Use Them?

The term concrete aggregates covers a variety of products, but are usually summed up as stone and sand, in coarse and fine grade. Coarse aggregates are defined as any material greater than 4.75 mm. A coarse aggregate is also identified as any aggregate retained in a #4 sieve. Fine aggregates are any material less than 4.75 mm that can pass through a #4 sieve and is retained on a #200 sieve.

 

Why do we use aggregates in concrete? There are many reasons. Perhaps the biggest reason in cost. Using aggregate as a filler can help concrete producers save a lot of money. Cement usually costs seven or eight times what stone and sand cost. Cement is necessary, but the strength can still be retained when using well-graded aggregates that cost significantly less. Aggregates make up 60-80% of the volume of concrete and 70-85% of the mass of concrete.

Aggregate is also very important for strength, thermal and elastic properties of concrete, dimensional stability and volume stability. Cement is more likely to be affected by shrinkage. Including aggregate in the mix can control the shrinkage level and prevent cracking.

What is the Importance of Grading?

If less cement equals less cost of concrete per meter, why not just add very large sized aggregates to take up more space? This where grading comes into play. Grading is a way of using the different sizes of objects to fill in the space that the bigger components can’t fill.

As an example, suppose you have a 2’x2’x2’ box that you want to fill with balls. First you may use bowling balls. Instead of just saying I took up a lot of space with a bowling ball and leaving it there, you decide to fill the extra small space with golf balls and then you fill up the remainder with marbles. If you used all marbles, all golf balls or all bowling balls, all those solutions would leave more remaining space then rationing some of each.

Now just take that example and substitute the balls with aggregates of different sizes. It is the same concept. Look at this illustration and notice the amount of cement paste needed for each type:

Shape and Content Ratios

When choosing aggregates, one must also consider their shape. Rough-textured and sharp aggregates will require more cement paste than round aggregates. Sharp aggregate will also be tougher to pump so make sure the aggregate fits your needs. Ideally, you will like spherical shaped aggregates. If they are too elongated, they may jam and block the voids when pouring or pumping.

Sand cannot be an overlooked component of concrete. Although the stone will supply the strength, sand also has an important purpose – workability. Being able to form your concrete according to the customer’s needs is vital. Therefore, one might over sand. Over sanding also has use when pumping concrete to allow a better flow as well as when stamping your concrete to allow more fine detail.

It is important to understand all components of concrete. Make sure your mix designs consider the purpose of the concrete, rather than just the strength and cost.

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