These days, builders are more likely to use poured concrete foundations than concrete block foundations. Poured concrete has become more popular for several reasons: wall forms that can be installed and dismantled quickly, concrete trucks that can go almost anywhere, and modern additives and pump equipment that make pouring “impossible” almost a routine.

Despite all the advantages mentioned above, concrete block still has its place as a viable base material. When the foundation is small, for example for a house addition, a builder can save money by laying a block foundation rather than outsourcing the work to a foundation contractor. Also, crawl space foundations can be built quickly and inexpensively with concrete blocks. And for a DIYer with masonry skills, this material provides the opportunity to complete a foundation one block at a time.

Before delving into this “heavy” topic, it is worth clearing up some terminology. In the construction industry, concrete blocks are referred to as “CMU”, short for concrete masonry units. Beyond the standard 16 x 8 x 8 in. concrete block stored by most home centers, CMU’s vast universe of sizes, shapes, and surface finishes is truly amazing. Most of this selection is intended for commercial construction.

Concrete blocks are made from the same ingredients that are used in poured concrete walls: gravel (called aggregate), sand, Portland cement, and water. Lightweight blocks are sometimes called cement blocks because fly ash (ash), a waste product of industrial combustion, is used as a partial substitute for heavier ingredients like sand and aggregates. The hollow cores in most CMUs make the block cheaper and easier to handle. The cores can be filled with concrete, sand or foam insulation to improve strength, thermal mass or insulation value.

Contrary to popular opinion, a concrete block foundation is not inherently inferior to a poured concrete foundation, provided it has been properly constructed. Both types of masonry require steel reinforcement and must be laid on a solid, stable foundation. Good drainage and waterproofing details are essential. When expansive soils or other external forces damage the base of a concrete block, cracks generally appear along the mortar joints that separate the individual blocks.

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