Some construction projects call for a larger masonry block than a standard brick, but solid concrete blocks can be very expensive and very heavy. One common compromise are largely hollow masonry blocks known as cinder blocks. These are also sometimes referred to as concrete blocks, breeze blocks, or concrete masonry units (CMUs), though these terms have nuanced differences among them. Cinder blocks are generally lighter than solid concrete blocks, which makes them easier for brick masons to place in position. The hollow spaces in the blocks also provide some natural insulation or allow grout to be poured inside the rows of masonry.
Cinder blocks differ from concrete blocks in other ways besides their hollow design. Concrete blocks are made from a slurry of Portland cement and small aggregate, such as small stones or gravel. This makes them heavier and smoother than cinder blocks, which are made from a combination of Portland cement and cinders, the dusty remnants of burned coal.
When bricklayers work with cinder blocks, they generally use techniques similar to standard brick laying. The alternative rows of blocks are carefully offset so that the second layer stabilizes the first. A line of mortar is put down between each layer, so the actual dimensions of a standard cinder block may be adjusted slightly to accommodate the mortar. Corners may be finished out with half blocks, or interlaced to create a four-cornered structure.
Because cinder blocks do not have a significant amount of tensile strength, concrete is often poured vertically into the hollow chambers to provide more stability and strength. An metal rod called rebar is often placed vertically in the hollow chambers as well to reinforce the poured grout and the wall in general. It is not unusual to see rows of blocks with lengths of exposed rebar on construction sites.
The problem with using grout with standard cinder blocks is block placement. The blocks must be carefully threaded over the rebar before they can be put into place. This may not be a problem for shorter projects such as home foundations, but it would be difficult and time consuming to thread individual blocks over 20 feet sections of rebar. There is a solution to this problem, however. Some cinder blocks, called speed blocks in the construction industry, are open-ended, generally shaped like the letter H. These speed blocks can be maneuvered around existing rebar and tilted into place by a skilled bricklayer.
By Michael Pollick