Gravel is often used to make a speed block.
When referring to a quick concrete block, a mason typically means a mortarless head joint, or H block. A speed block is a rectangular concrete masonry unit (CMU), or cement block, that is constructed of an open “H” configuration. These blocks differ from the standard CMU, which is usually closed and will have two or three center holes.
CMUs are manufactured in a wide variety of sizes and configurations to accommodate the various convolutions of construction. These quirks include window and door jambs, corner brackets, columns, etc. As a rule, speed blocks are used in the upper 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) high by 16 inches long (40.64 cm). The standard cinder blocks have the same dimensions and are all actually 1.02 cm shorter to allow for a mortar joint between the blocks. The common speed block, however, is manufactured to a tongue-and-groove specification, therefore needing little or no mortar to hold these particular blocks.
Although the speed block is recognized as a front joint, for smaller projects it is very often used in the construction of an entire wallcovering. Due to their versatile, open construction and lighter weight, speed blocks are placed quickly. This saves time and labor costs when constructing a concrete masonry building. In addition, because of their tongue and groove configuration, less mortar is required to join velocity blocks, resulting in significant savings in material costs. The use of velocity blocks is encouraged by most architects today for this reason alone.
All concrete blocks, including quick blocks, are usually made from Portland cement and an aggregate of sand or gravel. An ash velocity block uses fly ash or boiler plate, each the result of a burning or sintering process, as a main aggregate. CMUs, including speed blocks, are also constructed with plastic and rubber aggregates to reduce weight and make the blocks more environmentally friendly.
In addition, fast concrete blocks as well as standard CMUs can be manufactured from aerated concrete. This manufacturing process uses proportionately less concrete than standard CMU, replacing much of the concrete with an ammonium powder aggregate. This type of cinder block is a stronger and lighter unit, but it is considerably more expensive compared to the normally manufactured cinder block. Since a speed block is lighter simply by the nature of its construction, it is occasionally used as a less expensive alternative to aerated concrete block.