A hollow CMU is defined by having the cross-sectional area of the cells more than what percentage of the overall cross-sectional area?

A hollow concrete masonry unit (CMU) is classified based on the proportion of its cross-sectional area that is comprised of empty cells. In the case of hollow CMUs, the specification dictates that the cross-sectional area of the cells must exceed 25% of the overall cross-sectional area. This means that at least a quarter of the CMU's total area should be voids, which contributes to the lighter weight of the units and their thermal insulation properties.

The design of hollow CMUs allows for reduced material usage while still maintaining structural integrity and providing necessary load-bearing capabilities. This percentage threshold effectively differentiates hollow units from solid units, which do not have such a significant proportion of voids. Understanding this classification is essential for masonry professionals, as it influences choices in materials and construction methods suited for specific applications, ensuring compliance with building codes and achieving desired performance characteristics in masonry construction.