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When referring to a concrete speed block, a mason usually means a mortar-less head joint, or H-block. A speed block is rectangular concrete masonry unit (CMU), or cement block, that is constructed in an open-ended “H” configuration. These blocks differ from the standard CMU, which is usually closed-ended, and will have two or three core holes.
CMU’s are manufactured in a wide variety of sizes and configurations to accommodate the various convolutions of building construction. These vagaries include window and door jambs, corner braces, columns, etc. As a rule, speed blocks are used at the top joints of a structure because of their lighter weight and more versatile configuration.
The common speed block is nominally 8 inches wide (20.32 cm) by 8 inches (20.32 cm) in height by 16 inches in length (40.64 cm). Standard concrete blocks have the same dimensions, and all are actually 3/8-inch (1.02 cm) less all around to allow for a joint of mortar between blocks. The common speed block, however is manufactured with a tongue-and-groove specification, thus needing little or no mortaring to secure these particular blocks.
Although the speed block is recognized as a head joint, for smaller projects, it is very often used in construction of an entire wall facing. Because of their versatile, open-ended construction, and lighter weight, speed blocks are quickly placed. This saves time and labor costs when constructing a concrete masonry building. Additionally, because of its tongue-and-groove configuration, less mortar is needed for joining speed blocks, resulting in significant savings in material costs. Use of speed blocks is encouraged by most architects today for just this reason.
All concrete blocks, including speed blocks, are usually manufactured out of Portland cement and an aggregate of sand or gravel. A cinder speed block uses fly ash or boiler plate, each the result of a burning or sintering process, as a major aggregate. CMUs, including speed blocks, are also constructed using plastic and rubber aggregates to reduce weight, and render the blocks more eco-friendly.
In addition, concrete speed blocks, as well as standard CMU's, may be manufactured of aerated concrete. This manufacturing process uses proportionally less concrete than that of the standard CMU, replacing much of the concrete with an ammonium powder aggregate. This type of cement block is a stronger, lighter unit, but is considerably more expensive in comparison to the typically manufactured cement block. Since a speed block is lighter in weight simply by the nature of its construction, it is occasionally used as a less expensive alternative to aerated concrete block.