If the water-to-cement (W/C) ratio is decreased, what happens to the watertightness of the concrete?

Decreasing the water-to-cement (W/C) ratio in concrete generally leads to improved watertightness. This is because a lower W/C ratio results in a denser cement paste, which effectively reduces the number and size of pores within the concrete. Fewer and smaller pores reduce the pathways for water to seep through the concrete, thereby enhancing its overall impermeability.

Moreover, a lower W/C ratio increases the strength of the concrete due to a higher cement content, which in turn contributes to its durability and resistance to water ingress. This improvement in watertightness is critical in various applications such as in foundations, water-retaining structures, and other scenarios where water penetration could compromise the structural integrity or lead to other issues like corrosion or degradation over time.

While it is important to manage the W/C ratio carefully, as excessively low ratios can lead to workability issues, a decrease within practical limits will typically yield better performance in terms of watertightness. Users should interpret this relationship when designing concrete mixtures for specific structural applications.