Spalled bricks are bricks that have suffered from spalling, which is a type of damage where the surface of the brick begins to flake or peel away in small pieces. This can occur due to exposure to moisture, extreme temperature changes, or freeze-thaw cycles. Spalled bricks can be a serious issue for chimneys, as they can weaken the structural integrity of the chimney and lead to further damage. It is important to have spalled bricks replaced or repaired as soon as possible to prevent further damage to the chimney.