Since 1882, when the first concrete block was molded, concrete masonry has become a standard building material. Concrete blocks create structures that are economical, energy efficient, fire-resistant, and involve minimal maintenance. In addition, concrete masonry allows architectural freedom and versatility.

The standard concrete block is a rectangular 8X8X16-inch unit (200X200X400 mm) made mainly of portland cement, gravel, sand, and water. The concrete mixture may also contain ingredients such as air-entraining agents, coloring pigment, and water repellent. During the manufacturing process, a machine molds moist, low-slump concrete into the desired shapes. These blocks then undergo an accelerated curing process at elevated temperatures inside a special chamber. This is generally followed by a storage or drying phase.

Concrete masonry is widely used to construct small and large structures. The most common application of concrete masonry is walls for buildings. However, other uses for concrete masonry units include retaining walls, chimneys, fireplaces, and firesafe enclosures of stairwells, elevator shafts, and storage vaults.

Concrete masonry units can be manufactured for virtually any architectural or structural function. Split-face block units have been fractured lengthwise or crosswise by machine to produce a rough stone-like texture. The split face exposes the aggregates in the various planes of fracture. A patented slotted concrete block provides high sound absorption, making it ideal for use in gymnasiums, factories, bowling alleys, or other places where noise generation is high. Glazed concrete masonry units are used in swimming pools where sanitation and a durable, attractive finish are needed.

For more information, contact the National Concrete Masonry Association

Architect Magazine Profiles Block Plant

A modern block plant in operation. Blocks move along conveyor throughout plant.

The March 2008 issue of Hanley-Wood's Architect magazine takes a closer look at how concrete masonry units (CMU) are manufactured. They note how prevalent these units are (nearly 8 billion produced in 2007 in North America), and how they are nearly taken for granted, too. They say how the “block’s value lies in its versatility—certainly not in portability” and that the plant they visited ships its nearly 3 million units within a 50-mile radius.

They hit it right on the head: block, or CMU, is versatile. They also hit on a sustainability topic that deserves emphasizing. The fact that most units are manufactured and used locally makes them sustainable in all locations. Whether the cement is shipped in from a distance or manufactured locally as well, it represents a small portion of each unit (8.5% to 12% by weight), or only about 3 lbs per block (each block weighs from 25 to 35 pounds each).

In the U.S., CMU are manufactured to conform to ASTM C140, Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units. C140 and it annexes cover the standard CMU and various other concrete masonry products such as concrete brick, segmental retaining wall units (SRWs), interlocking pavers, grid pavers, and roof pavers. This standard ensures consistent properties like size, density (weight), absorption, and strength.

These units are made from very dry concrete mixes that are placed into steel molds, vibrated and compacted, then demolded and cured. Architect says that molding takes a mere 6.5 seconds at the plant they visited, but the curing goes on for 24 hours. Some other plants may be on cycles of 18 hours or 36 hours. Read the article from Architect.

Additional information about block manufacturing can be found in PCA’s Concrete Masonry Handbook for Architects, Engineers, Builders, (EB008) and in the NCMA TEK sheets at www.ncma.org.