The size of the aggregate plays a crucial role in the strength of the concrete. Normally, small-sized aggregates are preferred. Larger sized aggregate reduces the strength of the concrete. This is because of the Interfacial Transition Zone (ITZ).
The interfacial transition zone is the zone that is formed on the surface of the aggregates. When concrete is mixed, the water present in the mix coats itself on the surface of the aggregates. This thin film results in a high water-cement ratio in the vicinity of the aggregates.
Owing to the high w/c ratio, during hydration reaction, a rather porous network is formed, resulting in a weaker section surrounding the aggregates. However, as the hydration reaction proceeds, the second generation hydration gels are produced, making the zone denser but still comparatively weaker than the rest of the concrete.
This happens in all sized aggregates. But as the size of the aggregate increases, the surface area of the aggregate also increases. As a result, the ITZ increases, making the concrete weak.