Cinder blocks are lighter and cheaper than solid concrete blocks, but lack tensile strength. They are made from Portland cement and cinders, while concrete blocks contain small aggregates. Speed blocks, which are open-ended, can be used to maneuver around existing rebar.
Some building projects require a masonry block larger than a standard brick, but solid concrete blocks can be very expensive and very heavy. A common compromise is the largely hollow masonry blocks known as cinder blocks. These are also sometimes referred to as concrete blocks, breeze blocks, or concrete masonry units (CMUs), although these terms have nuanced differences from each other. Cinder blocks are generally lighter than solid concrete blocks, making them easier for masons to place in place. Block cavities also provide natural insulation or allow for mortar to be poured within 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 aggregates, 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 remains of burnt coal.
When masons work with concrete blocks, they generally use techniques similar to standard brick laying. The alternate rows of blocks are carefully staggered so that the second layer stabilizes the first. A line of mortar is laid between each layer, so the actual size of a standard cinder block can be adjusted slightly to accommodate the mortar. Corners can be finished with half-blocks or interlaced to create a four-edge structure.
Because cinder blocks don’t have a significant amount of tensile strength, concrete is often poured vertically into hollow chambers to provide extra stability and strength. Often a metal rod called a rebar is also placed vertically in the hollow chambers to reinforce the poured mortar and the wall in general. It’s not unusual to see rows of blocks with exposed rebar lengths in construction sites.
The problem with using standard cinder block mortar is the placement of the blocks. The blocks must be carefully tucked over the scaffolding before they can be placed. This may not be a problem for shorter projects like home foundations, but it would be difficult and time-consuming to string individual blocks over 20-foot sections of rebar. There is a solution to this problem, however. Some concrete blocks, called speed blocks in the construction industry, are open-ended, usually in the shape of the letter H. These speed blocks can be maneuvered around existing rebar and tilted into place by a skilled mason.
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