Why is it advised to avoid adding water to concrete after the initial mix?

Adding water to concrete after the initial mix can significantly compromise the quality of the final product, and this is primarily due to the impact it has on the concrete's strength characteristics. When additional water is introduced, it alters the water-to-cement ratio, which is critical in determining the concrete's strength and durability.

Concrete achieves its strength through a chemical reaction known as hydration, where water interacts with cement to form strong bonds. If extra water is added beyond the planned quantity, it dilutes the cement paste, resulting in a mix that has reduced overall density and increased porosity. This means air voids can form more easily within the concrete, which weakens the structure because the bond created by the hydration is not as strong. The concrete may demonstrate lower compressive strength and may have other performance issues like reduced wear resistance and increased permeability.

The other options don't accurately represent the consequences of adding water after mixing. While some might suggest that it leads to a 'too wet' mix, the more critical issue at hand is the impact on strength. Also, additional water does not improve setting time or bonding capabilities with aggregates, as it fundamentally compromises the integrity of the concrete itself.