Strengths than larger sized coarse aggregates. Cook observed that the difference in compressive strengths due to aggregate size is increasingly larger with a decreasing water-to-cement ratio and increasing test age. The smaller sized coarse aggregate also increases the flexural strength of the concrete.

The effect of coarse aggregate size in compressive strength of concrete are as follows :-

(1) Coarse aggregate size is directly proportional to the slump (workability) of a fresh concrete with constant water cement ratio.

(2) Compressive strength of a concrete increases with increase in coarse aggregate size. Coarse aggregate size 13.2 mm, 19 mm, 25 mm, and 37.5 mm gave average compressive strength of 21.26 N/mm2, 23.41 N/mm2, 23.66 N/mm2 and 24.31N/mm2 respectively.

(3) Flexural strength of concrete beam is inversely affected by the increase in coarse aggregate size.

(4) Compressive strength of concrete is inversely proportional to flexural strength as coarse aggregate size increases when subjected to the same condition.

The optimum ratio of the maximum size of the fine aggregate to the maximum size of coarse aggregate for the highest compressive strength is 0.18 for the water/cement ratio selected. This shows that the greater the heterogeneity the lower compressive strength of concrete.

The coarse agg size is not as much of a factor as the gradation of sizes of agg, the shape of the agg, and the strength of the agg.

I worked on a bridge several years ago where there was a lot of river rock in it. The concrete on the bridge was very strong, but not lasting all that long. Where it was not delaminated, it was tough to demolish. This actually goes against some of today’s research.

Research today shows that a combined gradation where there is not a lot of voids seems to provide a stronger stone matrix. The picture below shows a well graded, poorly graded and gap graded mix. The combined gradation is closer to the well graded mix, but it is still a little different.

This, in combination with having a good percentage of fractured faces on the rock, tend to mechanically strengthen the concrete.

The slide below is discussing fractured faces in asphalt concrete, but the characteristics are similar in concrete.

Asphalt conc agg’s Superpave consensus agg properties. — Coarse agg ∠larity * ensures a high degree of internal friction and rutting resistance * percent by weight of agg’s larger than 4.75 mm with one or more fractured faces — Fine aggregate angularity * ensures a high degree of fine aggregate internal friction and rutting resistance * percent air voids in loosely compacted aggregates smaller than 2.36 mm

You need to have some voids though, because the chemical stabilization comes from the cement mortar.

The size of the aggregate is very important when you have reinforcement. If the aggregate cannot get around the reinforcement, you will have segregation with just mortar and no aggregate in spots where the aggregate can’t get through. This is not beneficial at all. The goal is for the concrete mixture to be homogeneous throughout the structure.

Effect of coarse aggregate sizes on the compressive strength of concrete :

Here I will be discussing how the sizes of coarse aggregate effect on various concrete properties like its compressive strength, workability, etc.

Effect of coarse aggregate sizes in concrete :

As the sizes of coarse aggregate increases then the compressive strength will also increase with constant water to cement ratio(w:c -0.5 and Mix proportion 1:2:4)

The workability of concrete get improved after increasing larger sizes of aggregates in concrete

The various result indicates that the sizes of coarse aggregate are directly proportional to the slump value of fresh concrete

Coarse aggregate sizes adversely affect on cement aggregate Bond

( Note: In the above figure various sizes of aggregates are listed)

As the course aggregate size increases, the compressive stress decreases.

It is due to the Interfacial transition zone.

The interfacial transition zone exists between the coarse aggregates and the mortar. When concrete is prepared, the water present in the mix forms a film around the aggregate. The film of water causes a relatively high-water cement ratio in the vicinity of the aggregates. This high water-cement ratio affects the hydration reaction by producing a porous network of Calcium Hydroxide and Ettringite. However, as the hydration reaction proceeds, the second generation of ettringite and calcium hydroxide along with the C-S-H gel starts filling this porous network.

As the size of the aggregate increases, the surface area of the aggregate increases, as a result, the size of the Interfacial Transition Zone also increases. Interfacial Transition Zone reduces the strength of the concrete by producing weaker sections and micro cracks. Therefore, as the size of the aggregate increases, the strength of the concrete reduces.