When should concrete masonry units be wet down?

The correct option is about wetting down concrete masonry units before laying the blocks because this process helps to ensure better adhesion between the mortar and the blocks. When masonry units are dry, they can absorb too much moisture from the mortar, leading to a weaker bond and potentially compromising the overall strength of the wall. Wetting the blocks not only helps in achieving a stronger bond but also minimizes the risk of the mortar setting too quickly.

This practice is essential for creating optimal conditions for the mortar to cure and adhere properly to the masonry units, making it a crucial step in masonry work.