Grading of aggregate is very important from the point of view of packing theory. As well graded aggregates pack well and reduces the porosity and weak zones in the concrete.

Weakest link theory says that probability of failure increases with more small size aggregates.

However, in concrete well graded aggregates and proper mortar combination result in to good concrete.

Dear Mahdi

A very interesting topic.

There has been some research carried on this.

You may find the following articles interesting and answering your question:

Article Gradation of Aggregates and its Effects on Properties of Concrete

file:///C:/Users/cbx086/AppData/Local/Temp/EFFECTS-OF-AGGREGATE-GRADATION-ON-THE-PROPERTIES-OF-CONCRETE-MADE-FROM-GRANITE-CHIPPINGS.pdf

Best wishes

Mr Om Prakash Chhangani makes the very important point of aggregate packing. I've done similar research with regards to aggregate packing principles for asphalt design.

It is an industry-accepted fact that course aggregates contribute to increased compressive strength.

Varying the fines portion of your grading with concrete mixes will effectively vary the surface contact area exposed to curing and hardening of cement. Finer grading means you will need more cement to achieve the same strength in bonds. (simply put: if there are more things that the cement must touch and bond with, you're going to need more cement). A clever way to illustrate this will be to assess at least three concrete mixes by correlating their cube strengths (crushing or compressive strength values) to the specific surface are of the aggregate grading (whilst keeping the % cement the same). Your results will highlight that larger surface areas go hand-in-hand with lower compressive strength.

Other factors to consider:

• Type of aggregate (soft shales vs durable granites)
• Reinforcement spacing
• Thickness of concrete layer
• Shape and distribution of particles (angular vs cubic)
• Chemistry of aggregates (pertaining to free lime in some steel slag sources)

Based of my experience in concrete manufacturing, strenght of your concrete has straight relationship with greater aggregates and you sould use enough fine aggregates and cement to in fill the porosity of the greater aggregates and whatever you have different size from greater to fine of the aggregate you need less cement, and your concrete will stronger.

If you want reduce greater aggregates you should use more fine aggregates and cement to reach the same strenght!

The denseness of concrete increases there by voids gets reduced.Increased denseness will result better flow of forces resulting in higher load carrying capacity.

Dear Mahdi

Gradation of aggregates is one of the essential points in the control of strength, it controls the density and work-ability of fresh concrete while it controls the creep of hardened concrete. both dense and gap gradation can exhibit high compressive strength based on the mix design and other factors such as W/C ratio and cement content, additives ...etc

The well graded aggregate imparts better strength to the concrete. The well graded aggregate reduces the voids between the aggregate, which improve the strength and durability of concrete.

Gradation of aggregate is very important for the selection of appropriate size of aggregate , size of aggregates should be within limit. Concept of more Finess modules less compressive strength is totally wrong.

Compressive strength depends on following factors of aggregate properties.

1.Shape of aggregate - if flaky, elongated resulting less compressive strength

2.Sp.Gravity of aggregate - higher sp.gr can consider more dence and strong aggregate

3.Water Absorption- if WA is more - chances of less compressive strength .

4.Selection for proportion of aggregate..qty of each material should be calculated and fixed for better packing and minimise the voids in concrete.

Packing theory of concrete can be described as below.

Voids of coarse aggregate can be filled by fine aggregate, voids of fine aggregate fill the cement and again small voids can be filled using Flyash and for more packing microsilica or ultra high performance cementetious materials can be used

Gradation of materials and partical size analysis helps for fixing proportions of concrete ingredients..

Of course gradation of aggregates plays a vital role in increasing compressive strength. You will get the proper 'S curve' if and only if the sample is well-graded. It might be poorly-graded, gap-graded or even uniformly-graded. When you use well-graded aggregates, you will find different sizes of aggregates which will ultimately improve the interlocking between aggregates during bonding. So I think it is wise to check the gradation of your desired sample.

Gradation of aggregates play a very important role in increasing or decreasing the compressive strength in concrete.If aggregates aren't properly graded it effects the compressive strength of concrete.The theory of gradation is basically based on the concept of to decrease the void ratio in concrete mix.Even Fineness Modulus of fine aggregate also effects the compressive strength of concrete.

Traditionally strive for the most dense packing of aggregates.This is achieved in two ways: by providing continuous granulometry of aggregates; and granulometry with a rupture of the screening line.

This is reflected in the German norm DIN 1045

Aggregate occupying about 70% of concrete volume, so it's properties (type, shape, gradation,... etc.) will largely affect the concrete properties. Grading of aggregate is very important to strength and durability of concrete, it will affect the workability and packing of concret

Higher fineness modules means more coarse aggregate, this will affect the packing of particles and also increase the interfacial transition zone (ITZ), ITZ is the weakest plan at which failure starts. And as you mentioned, varying in the fine particles sizes, has greater effect because different sizes of fine particles can largely affect the packing of aggregate and voids ratio. Very fine particles will reduce the ITZ.