Page 30 - Masonry and Concrete

Understanding Concrete
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UNDERSTANDING CONCRETE

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Reinforcing Bars: Reinforcing bars range in size from ⁄4-in. to 1-in. in
diameter and have surface ridges to provide better bond with the con-
crete paste. Reinforcing bars are numbered according to their diameter
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in eighths of an inch. A #3 bar, for example, is /8 in. in diameter, a #4
5
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bar is /8 in. or 1 /2 in., a #5 is /8 in., and so on. Reinforcing bars are used
for concrete carrying heavy loads such as footings and foundation
walls, slabs, and columns. There are several different types of steel
used to make reinforcing bars, and there are two common grades,
Grade 60 and Grade 40. Grade 60 has a higher yield strength and is
required by building codes for some applications.

Reinforcing Mesh: Reinforcing mesh is made from steel wires woven
or welded into a grid of squares or rectangles. The wires are usually 6,
8, or 10 gauge and may have smooth or deformed surfaces. Reinforcing
mesh comes in rolls and mats and is used primarily in flatwork
such as sidewalks, patios, and driveways. For most residential work,
6 in. 6 in.–10 gauge mesh provides adequate strength and distributes
shrinkage stresses to minimize cracking.

2.5 Control, Construction and Isolation Joints

Concrete shrinks irreversibly as it cures and dries out. After this initial
shrinkage has occurred, concrete expands and contracts reversibly with
changes in temperature and moisture content. This movement can cause
concrete to crack uncontrollably unless it is reinforced with steel and
built with special joints that are designed to control cracking locations.
The amount of expansion and contraction that concrete will experi-
ence is influenced by several things, including the water content of the
mix, and the weather conditions during the curing period. Mixes made
with a high water content are more prone to cracking from initial shrink-
age than drier, stiffer mixes. Reinforcing steel increases the strength of
concrete and absorbs the stress of expansion and contraction, but it can-
not prevent cracking altogether—it can only distribute the stresses so
that there will be many minute cracks instead of a few big ones. While
reinforcement limits the amount of expansion and contraction to some
extent, cracking and movement can also be regulated by subdividing the
concrete into smaller sections with control joints and construction
joints, and separating it from adjacent construction with isolation joints.

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