Today the builders tend to prefer poured concrete foundations than foundations constructed using concrete blocks. Poured concrete is becoming more popular for a number of reasons. Wall forms that can be quickly built and then removed as well as concrete trucks that are able to go almost anywhere as well as modern pumps and admixtures that create "impossible' pours almost routine.
In spite of the benefits mentioned above, concrete block still 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 a block foundation instead of outsourcing the work to foundation contractors. In the same way, crawl space foundations can be easily and cost-effectively constructed with blocks of concrete. And for a DIYer who has masonry expertise the block material offers the opportunity to construct the foundation one block at a time.
Before getting any deeper into this "heavy" topic it's important to understand the basics of terminology. In the construction industry concrete blocks are often known as "CMUs," short for concrete masonry units. In addition masonry contractor to the standard 16x8x8 in. concrete block stocking most home centers, the array 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 found in concrete walls, which are gravel (called aggregate) and sand Portland cement and water. Blocks that are light weight can be referred to as"cinder blocks" because fly ash (cinders) which is a byproduct of industrial combustion is utilized to substitute heavier ingredients like sand and aggregate. The hollow cores in most CMUs allow the block to be cheaper and easier to handle. They can also be filled with cement sand or foam insulation to increase the durability, thermal mass or insulation value.
Contrary to popular belief the concrete block foundation isn't inherently inferior to a concrete foundation, providing that 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 outside forces damage the concrete block foundation, cracks typically appear in the mortar joints which separate individual blocks.