These days construction companies are more likely to use poured concrete foundations than foundations constructed using concrete blocks. Poured concrete is becoming more popular for a number of reasons, including wall forms that are quick to built and then removed as well as concrete trucks that are able to travel almost everywhere as well as modern pumps and admixtures which make "impossible pours' almost common.
Despite the many advantages discussed above, concrete block still is an appropriate foundation material. If the foundation isn't too big such as a house addition for instance- a builder could save money by building the foundation using block instead of subcontracting the work to an expert foundation contractor. Additionally foundations for crawl spaces can be quickly and economically built with concrete block. And for a DIYer who has masonry expertise the block material offers the chance to build the foundation block by block one time.
Before getting any deeper into this "heavy" subject it's helpful to know the basics of terminology. In the construction industry concrete blocks are referred to as "CMUs," short for concrete masonry units. In addition to the standard 16 x 8 x 8 in. concrete block stocked by most home centers, the vast variety of CMU sizes, shapes and surface finishes is truly amazing. A majority of the CMUs are specifically designed for commercial construction.
Concrete blocks are made using the same components that are used to make concrete walls, which are gravel (called aggregate), sand, Portland cement, and water. Lightweight blocks are sometimes called"cinder blocks" because the fly-ash (cinders), a waste byproduct of industrial combustion is used to substitute larger quantities of sand or aggregate. The hollow cores found in many CMUs make the block more economical and easier to handle. Cores may be filled using concrete, foam insulation, or sand to increase durability, thermal mass and insulation.
Contrary to what many believe it concrete block wall is true that a block foundation made of concrete isn't necessarily inferior to a poured concrete foundation, providing that it's been properly built. Both types of masonry need steel reinforcement, and they must be placed on a sturdy and stable foundation. Proper drainage and damp proofing features are essential. When expansive soils and other external forces harm the foundation of a concrete block, cracks typically appear in the mortar joints which divide the blocks.