Should CMU blocks be wetted prior to laying?

• A. Yes
• B. Only in hot weather
• C. No
• D. Only for dense blocks

Answer: (C)

Moisture control between the block units and the mortar is what this question is testing. In masonry, blocks can absorb water from the fresh mortar, which can cause suction that leads to a mortar mix that stiffens too quickly and doesn’t bond properly. While some situations call for dampening surfaces to reduce suction, the general rule reflected here is not to pre-wet CMU blocks before laying them. So, the blocks should go in with the intended mortar moisture balance and only light, case-specific dampening if required by conditions or specs. Excessive wetting can flood the joint, dilute the cement, weaken the bond, and promote efflorescence, which is why choosing not to pre-wet is considered the best default.