Earlier it was thought that the use of larger size aggregate leads to higher strength.This was due to the fact that the larger the aggregate the lower is the total surface area and, therefore, the lower is the requirement of water for the given workability. So, a lower water/cement ratio can be used which will result in higher strength of Concrete.
However, later it was found that the use of larger size aggregate did not contribute to higher strength as expected from the theoretical considerations due to the following reasons..
1.The larger maximum size aggregate gives lower surface area for developments of gel bonds which is responsible for the lower strength of the concrete.
2.Secondly bigger aggregate size causes a more heterogeneity in the concrete which will prevent the uniform distribution of load when stressed.
When large size aggregate is used, due to internal bleeding, the transition zone will become much weaker due to the development of micro-cracks which result in lower compressive strength.
Generally, high strength concrete or rich concrete is adversely affected by the use of large size aggregate. But in lean mixes or weaker concrete the influence of size of the aggregate gets reduced. It is interesting to note that in lean mixes larger aggregate gives highest strength while in rich mixes it is the smaller aggregate which yields higher strength. The Fig. below shows the influence of maximum size of aggregate on compressive strength of concrete..
Following Figure depicts the influence of size of aggregate on compressive strength of concrete for different w/c ratio.
