Today, builders are more likely to build poured concrete foundations than foundations made of concrete blocks. Concrete poured has become more popular for a number of reasons. Wall forms that are quick to set up and taken down as well as concrete trucks that are able to travel almost everywhere, and modern pumps and admixtures that make "impossible pours" almost routine.
Despite the many advantages discussed in the previous paragraphs, concrete block has its place as an acceptable foundation material. When the foundation is small such as a house addition for example-a builder can save money by laying up the foundation using block instead of subcontracting the work to an expert foundation contractor. Additionally foundations for crawl spaces can be quickly and economically constructed using blocks of concrete. If you are a DIYer with masonry skills the block material offers the chance to build the foundation one block at each step.
Before we dive into this "heavy" topic, it's worth clearing up the basics of terminology. In the construction industry concrete blocks are called "CMUs," short for concrete masonry units. In addition 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) as well as sand, Portland cement and water. Blocks that are light weight can be referred retaining wall to as"cinder blocks" because fly ash (cinders), a waste byproduct of industrial combustion can be used to substitute more heavy ingredients such as sand and aggregate. The hollow cores in most CMUs allow the block to be cheaper and easier to handle. The cores can be filled with concrete sand or foam insulation to increase the durability, thermal mass or insulation value.
Contrary to popular belief the concrete block foundation isn't necessarily inferior to a poured concrete foundation, if it's been properly built. Both types of masonry need steel reinforcement and should be laid on a strong and stable foundation. Good drainage and damp proofing features are essential. If expansive soils or other outside forces damage the foundation of a concrete block, cracks typically appear on the mortar joints that separate individual blocks.