These days, builders are more likely to use poured concrete foundations than foundations built with concrete blocks. Poured concrete has become more popular for a number of reasons: wall forms that can be quickly set up and taken down, concrete trucks that can go almost anywhere, and modern admixtures and pumping equipment that make “impossible’ pours almost routine.
Despite all the advantages mentioned above, concrete block still has its place as a viable foundation material. When the foundation is small -for a home addition, for example-a builder can save money by laying up a block foundation instead of subcontracting the work to a foundation contractor. Likewise, crawl space foundations can be quickly and economically built with concrete block. And for a DIYer with masonry skills, this material provides an opportunity to complete a foundation one block at a time.
Before getting any deeper into this “heavy” topic, it’s worth clearing up some terminology. In the building industry, concrete blocks are referred to as “CMUs,” short for concrete masonry units. Beyond the standard 16 x 8 x 8 in. concrete block stocked by most home centers, the vast universe of CMU 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 used in poured concrete walls -gravel (called aggregate), sand, Portland cement and water. Lightweight blocks are sometimes called cinder blocks because fly ash (cinders), a waste product from industrial combustion, is used as a partial substitute for heavier ingredients like sand and aggregate. The hollow cores in most CMUs make the block more economical 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 isn’t inherently inferior to a poured concrete foundation, providing that it’s been properly built. Both types of masonry require steel reinforcement and must be laid on a strong, stable footing. Good drainage and damp proofing details are essential. When expansive soils or other outside forces damage a concrete block foundation, cracks typically appear along the mortar joints that separate individual blocks.