Page 25 - Masonry and Concrete

Understanding Concrete
24 CHAPTER TWO

Within the range of normal concrete strengths, compressive
strength is inversely related to water content. That is, the more water
you use, the lower the concrete strength. But increasing water content
increases fluidity and workability. Since water is required for worka-
bility, and since workability is required for high-quality concrete, the
low water requirements for strength and high water requirements for
workability must be balanced. The ratio of water to cement is the
weight of water divided by the weight of cement. Water-cement ratio
affects the consistency of a concrete mix. The consistency, in turn,
affects how easily the concrete can be poured, moved around in the
forms, compacted, and finished. Up to a point, a mix with more water
is easier to work with than one that has less water and is therefore
stiffer. Too much water, though, will cause the ingredients to separate
during the pouring, placing, and handling and will destroy the
integrity of the concrete. Too much water also lowers strength,
increases the porosity and water permeability of the cured concrete,
and makes it more prone to shrinkage cracking. The trick is to use
enough water to make the fresh concrete workable, but not so much
that it creates weak or porous structures.
1
Air content for ready-mix concrete should generally be 3 to 6- /2%,
depending on the maximum aggregate size (Figure 2-1). Concrete that
is batched on site can be made with either an air-entrained cement or
an air-entraining admixture. Using an air-entrained cement will yield
an air content within the proper range. When using a separate air-
entraining agent, carefully follow the manufacturer’s instructions to
determine the correct amount to add to the mix. For job-site mixing,
air-entrained cement is usually easier to work with.
It is easier to measure concrete consistency or slump than to calcu-
late water-cement ratio. The concrete mix consistency produced by
adding various amounts of water is measured by slump tests in which
fresh concrete is poured into a special mold called a slump cone. You
can buy one from a building supply yard. Place the concrete into the
cone in three layers. Tamp each layer with a metal rod to assure that it
is completely consolidated and does not contain air pockets. When the
cone is full, scrape off any excess concrete, leaving a level top. Then
remove the cone and measure the amount of slump or settlement with
a rod and ruler (Figure 2-11). The wetter the mix, the higher the slump
measurement, and the drier the mix, the lower the slump measure-

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