| Concrete Masonry Units
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
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.
ASTM C140 Standard on CMU Reorganized
The Standard Test Methods for Sampling and Testing Concrete Masonry
Units and Related Units, ASTM
C140, was reorganized late in 2007 and published in January 2008.
Designers and specifiers should be familiar with this document,
as it is used for evaluating characteristics of concrete masonry
units and related concreate units. The big change in this version
from previous ones is that annexes were added to address each type
of masonry unit, outlining the appropriate test procedures for that
type of unit.
Not all units are tested the same way, and not all methods are
applicable to all unit types. Units serve different purposes and
need different properties in service. They should be tested for
properties pertinent to each type of unit.
For instance, dimensions of CMU and concrete brick should both
be verified. While it is necessary to check the width, height, and
length of the overall unit, for CMU, it is also necessary to check
the thickness of face shells and webs.
C140 provides testing procedures that are commonly used. Methods
describe sampling, measurement of dimensions, compressive strength,
absorption, unit weight (density), moisture content, flexural load,
and ballast weight. Specifically, the annexes are:
- Annex 1, Concrete Masonry Units
- Annex 2, Concrete Brick
- Annex 3, Segmental Retaining Walls
- Annex 4, Concrete Interlocking Paving Units
- Annex 5, Concrete Grid Paving Units
- Annex 6, Concrete Roof Pavers
- Annex 7, Determining Plate Thickness Requirements for Compression
Some common units covered under ASTM C140: CMU, concrete brick,
|Concrete masonry unit (CMU)