How does an increase in slump affect concrete characteristics?

An increase in slump indicates a higher workability of concrete, which generally corresponds to an increase in the water content relative to the cement content. As the water-cement ratio increases, the overall strength of the concrete mix tends to decrease. This results in a weaker mix because the excess water creates more capillary pores within the hardened concrete, leading to a decrease in density and an increase in permeability.

In a structural context, a more permeable concrete can allow for the ingress of moisture, chlorides, and other deleterious substances, which can adversely affect the longevity and durability of the structure. Therefore, an increase in slump, while beneficial for workability, ultimately leads to drawbacks related to strength and durability of the concrete. Thus, the statement connecting an increase in slump to making the mix weaker and more permeable is accurate and reflects the underlying principles of concrete composition and performance.