Concrete blocks are made from cast concrete (e.g. Portland cement and aggregate, usually sand and fine gravel, for high-density blocks). Lower density blocks may use industrial wastes, such as fly ash or bottom ash, as an aggregate. Lightweight blocks can also be produced using autoclaved aerated concrete.
Sizes and Structure
Concrete blocks may be produced with hollow centers (cores) to reduce weight or improve insulation. The use of blockwork allows structures to be built in the traditional masonry style with layers (or courses) of staggered blocks. Blocks come in many sizes. In the US, the most common nominal size is 16 in × 8 in × 8 in (410 mm × 200 mm × 200 mm); the block measures a 3/8 in shorter, allowing for mortar joints. In Ireland and the UK, blocks are usually 440 mm × 215 mm × 100 mm (17.3 in × 8.5 in × 3.9 in) excluding mortar joints. In New Zealand and Canada, blocks are usually 390 mm × 190 mm × 190 mm (15.4 in × 7.5 in × 7.5 in) excluding mortar joints.
Block cores are typically tapered so that the top surface of the block (as laid) has a greater surface on which to spread a mortar bed and for easier handling. Most CMU’s have two cores, but three- and four-core units are also produced. A core also allows for the insertion of steel reinforcement, tying individual blocks together in the assembly, with the goal of greatly increased strength. To hold the reinforcement in proper position and to bond the block to the reinforcement, the cores must be filled with grout (concrete). Reinforcement is primarily used to impart greater tensile strength to the assembly, improving its ability to resist lateral forces such as wind load and seismic forces.
A variety of specialized shapes exist to allow special construction features. U-shaped blocks or knockout blocks with notches to allow the construction of bond beams or lintel assemblies, using horizontal reinforcing grouted into place in the cavity. Blocks with a channel on the end, known as “jamb blocks”, allow doors to be secured to wall assemblies. Blocks with grooved ends permit the construction of control joints, allowing a filler material to be anchored between the un-mortared block ends. Other features, such as radiused corners known as “bullnoses” may be incorporated. A wide variety of decorative profiles also exist.
Concrete masonry units may be formulated with special aggregates to produce specific colors or textures for finish use. Special textures may be produced by splitting a ribbed or solid two-block unit; such factory-produced units are called “split-rib” or “split-face” blocks. Blocks may be scored by grooves the width of a mortar joint to simulate different block modules. For example, an 8-by-16-inch (200 mm × 410 mm) block may be scored in the middle to simulate 8-by-8-inch (200 mm × 200 mm) masonry, with the grooves filled with mortar and struck to match the true joints.
Concrete block, when built in tandem with concrete columns and tie beams and reinforced with rebar, is a very common building material for the load-bearing walls of buildings, in what is termed concrete block structure (CBS) construction. American suburban houses typically employ a concrete foundation vibrant and slab with a concrete block wall on the perimeter. Large buildings typically use copious amounts of concrete block; for even larger buildings, concrete blocks supplement steel I-beams.”
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