Hollow and solid concrete blocks, which are collectively known as "aggregate concrete masonry units," are very similar. Corresponding units are made of the same concrete mix, are dimensionally alike and interchangeable in many building situations. However, they have one essential difference engineered into their design. This affects the mechanical properties of each block type.
According to The Concrete Block Association, a cellular block is designed with one or more hollows, or voids. However, these do not go all the way through the block. In most cases, where the plane of the voids is vertical, this means that one of the surfaces to which mortar is applied is voided while the other is whole. Containing proportionately less concrete, a cellular block is lighter than a corresponding solid concrete block.
Hollow blocks also have hollows, or voids, designed into them. In this case, they go all the way through the block. In most cases, where the plane of the voids is vertical, this means that both of the surfaces to which mortar is applied are voided. Containing proportionately less concrete, a hollow block is lighter than a corresponding cellular block or solid concrete block.
Solid concrete blocks
A solid concrete block is a more straightforward concrete masonry unit, in terms of design, being essentially a concrete cuboid. Naturally, the additional concrete in a solid block means greater weight than a corresponding cellular or hollow block. However, this may be associated with greater strength, depending on the concrete mix used. Further, solid blocks can be less expensive than their hollow counterparts, as they are easier to manufacture.
Concrete blocks are classified as group 1, 2, 3 or 4 depending on the percentage of volume occupied by the voids and the plane of the voids. Most blocks belong in groups 1 and 2. For group 1 blocks, the volume occupied by the formed voids is less than a quarter of the overall volume. For group 2 blocks, this can be from 25 to 60 percent.