Nowadays the builders are more likely to build concrete foundations that are poured rather than foundations constructed using concrete blocks. Poured concrete is becoming more popular because of a variety of reasons, including wall forms that can be quickly built and then removed and concrete trucks that can go almost anywhere as well as modern admixtures and pumping equipment that create "impossible' pours almost routine.
In spite of the benefits mentioned above, concrete block can be used as a viable foundation material. If the foundation is small -for a home addition, for instance- a builder could save money by laying the foundation using block instead of outsourcing the work to an expert foundation contractor. In the same way, crawl space foundations can be easily and cost-effectively constructed with concrete block. For those who are DIYers who has masonry expertise it gives you an opportunity to complete a foundation one block at one time.
Before diving in this "heavy" subject it's helpful to know some terms. In the world of construction, 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 surfaces is truly astounding. The majority of the selection is designed for commercial use.
Concrete blocks are constructed of the same materials used in poured concrete walls -gravel Phoenix (called aggregate), sand, Portland cement, and water. The blocks that weigh less than a pound are often referred to"cinder blocks" due to the fact that the fly-ash (cinders) is a waste product of industrial combustion, is used as a substitute for larger quantities of sand or aggregate. The hollow cores of most CMUs make the block much more affordable and easy to work with. Cores may be filled using concrete foam insulation or sand to increase strength, thermal mass as well as insulation.
Contrary to what many believe, a concrete block foundation isn't necessarily superior to a poured concrete foundation, if it's constructed correctly. Both types of masonry require steel reinforcement, and they must be laid on a strong firm, solid footing. A good drainage system and damp proofing are essential. When expansive soils and other external forces cause damage to the concrete block foundation cracks are typically seen in the mortar joints which separate individual blocks.