Affect aggregate size has on a concrete mix. 1

[MatthewMansfield]

Hi community

This is similar to a previous question but different as well.

I am trying to understand the affect aggregate size has on a concrete mix.

I know that the larger the aggregate size results in stronger concrete but what else? how else does aggregate size affect the final concrete product?

I have been told that using larger aggregates results in less water but WHY?

My gut feeling is also to say that larger aggregate size also improves a concretes resistance to weathering i.e. improve durability if the larger aggregates that are used is strong enough to absorb weathering action – would this be a true statement?

Any ideas?

Thank you.

 

[BUGGAR]

You are at the heart of concrete mix design.
I recommend "Concrete Construction Handbook" by Waddell, available on the net. He discusses everything you have noted, with graphs and all!

 

[JedClampett]

Another great reference is "Design and Control of Concrete Mixtures" by PCA. I've still got my 11th Edition copy from my concrete technology course and still use it on occasion (although, I use a newer edition when I can find it). Even if you only use it every once in a while, it's $75 well spent.

 

[delagina]

ACI states maximum aggregate size depending on concrete thickness, rebar spacing, etc.. so you need to check that.

 

[epoxybot]

I have been told that using larger aggregates results in less water but WHY?

This speaks to the fact that many small particles have a greater surface area, in need of wetting & coating with cement paste vs one large particle used for creating volume, while using less cement & water.

Larger aggregate in mass placed concrete can also function as a heat sink to regulate the exothermic heat of hydration.

 

[Ron]

The aggregate size has a significant influence on a concrete mix design. In general, the smaller the coarse aggregate, the higher the necessary cement content. This is because as coarse aggregate size goes down, its specific surface area goes up. Think of it this way.....

Consider a solid rock cube that is 12"x12"x12". That cube has a surface area of 864 square inches. Suppose you have to coat each surface equally with a 1/8" thick coating of cement paste. It would take 108 cubic inches of cement paste to cover the cube.

Now cut the cube in half on each face. You now have 8 cubes, each having dimensions of 6"x6"x6". Each cube now has a surface area of 216 square inches, but you have 8 of them, so the total surface area is 1728 square inches. Coating each of these cubes with the same thickness of cement paste would require 216 cubic inches of paste, or twice as much.

This same relationship holds for coarse aggregate, even though we have multiple sizes of coarse aggregate in the mix gradation.

The strength of concrete, in general, is related to its water-cement ratio, not the aggregate size. For example you can produce a 3000 psi mix using only sand, cement and water; just as you can produce a 3000 psi mix using No. 57 stone, sand, cement and water. Provided the aggregates are of good quality overall, they do not dictate the strength of concrete.

Using larger coarse aggregate requires less water because it also requires less cement. Again, keeping the same water-cement ratio, if you need less cement, you need less water.

Increasing the coarse aggregate size reduces the amount of cement required, reduces the amount of water required and as a consequence reduces the amount of shrinkage that will occur in the overall mix. That's a good thing and one of the reasons we want to use the largest coarse aggregate practicable for most applications, particularly flat slab work.