A mason is building a CMU wall and notices the blocks are absorbing water quickly from the mortar. What should be done before continuing?
Correct Answer
C) Wet the blocks before laying
When CMU blocks are very dry and absorbing water rapidly from mortar, they should be dampened before laying. This prevents the blocks from drawing too much water from the mortar, which would weaken the bond and mortar strength.
Why This Is the Correct Answer
When CMU blocks are excessively dry, they act like sponges and rapidly absorb water from the fresh mortar. This water absorption weakens the mortar's hydration process and reduces bond strength between the block and mortar. Pre-wetting the blocks to a surface-dry condition allows the mortar to maintain its proper water content for optimal curing and bonding. The blocks should be damp but not dripping wet to achieve the best mortar-to-block adhesion.
Why the Other Options Are Wrong
Option A: Use a drier mortar mix
While bonding agents can improve adhesion in some situations, they don't address the root problem of rapid water absorption. The dry blocks would still draw water from the mortar, and bonding agents are typically used for different applications like bonding new concrete to old.
Option D: Apply a bonding agent to the blocks
Using a drier mortar mix would actually worsen the problem by providing even less water for proper hydration. The dry blocks would still absorb what little water remains in the mortar, leading to poor workability and weaker bonds.
Memory Technique
Think 'Thirsty blocks steal water' - just like you'd give a thirsty person a drink, give thirsty blocks some water before laying them so they don't steal from the mortar.
Reference Hint
Florida Building Code, Chapter 21 - Masonry, or ACI 530 Building Code Requirements for Masonry Structures regarding mortar and unit preparation
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