The strength of a concrete mix made of rough and angular aggregates is higher than that of smooth and rounded aggregates.

This is because a rough surface offers better bonding with cement particles than a smooth surface.

Additionally, compared to round aggregates, angular aggregates offer a larger surface area to adhere to for cement paste, and better interlocking exists between the aggregates. 

A strong bond in concrete is indicated by some aggregates breaking through and others detaching cleanly from the paste matrix during crushing.

When a concrete section is loaded, the stress at the contact points of aggregates is higher than the overall stress on the concrete, necessitating aggregates with strength greater than the concrete mix itself.

At the same time, when concrete undergoes volume changes and aggregates within it are too rigid, the stresses developed in the cement paste around the aggregate can lead to cracks in the cement paste.

The strength of aggregates can be determined indirectly by testing the parent rock sample or a bulk aggregate sample.