Concrete in Sewage and Industrial
Waste Plants
E. P. SELLNER, Manager, Conservation Bureau
Portland Cement Association
Chicago, Illinois
By mixing portland cement, water and aggregates together in the proper
quantities, we are able to produce a fundamental building material -- con-
rete. In general, concrete may be of a high quality or of a poor quality for
the required project, depending upon the specifications prepared by the design
engineer.
No doubt the term "quality concrete" is a familiar one, but does the
term mean the same thing to everyone? Quality in this case means more than
sufficient strength to carry the structural loads. It also embodies such factors
as durability, watertightness, wear resistance, workability, economy and,
last but not least, appearance. When all of these desirable features have
been obtained,   the results will be "quality concrete. "
Concrete for sewage and industrial waste plants must be able to withstand
exposure to freezing and thawing, wetting and drying, mild chemical corrosion, and resistance to any erosion that might be caused by abrasive material
in the flowing water. In some industrial waste plants, it might be necessary
to protect the concrete from the effects of liquids having exceedingly low pH
values. These conditions make it mandatory that a concrete of high quality
be used in construction in order to obtain a long service life.
Concrete is formed by mixing a portland cement with water and such inert materials as sand and gravel or crushed stone. Portland cement is a ce-
menticious material that requires water to hydrate and harden. Hydration is
nothing more than the chemical reaction that takes place when the cement
powder combines with water. The cement and water combine together to
form a cement paste, and the function of the aggregate is primarily that of a
filler or extender.
The cementing properties of the paste are due to hydration of the cement.
Although only a small amount of water is necessary for hydration (approximately three gal per sack), more water is added to facilitate the placement
of the concrete. With the addition of the extra water, more aggregate can be
used with resulting economy. For quality concrete, a proper proportion of
water to cement is essential.
FACTORS AFFECTING CONCRETE QUALITY
To produce a good quality concrete, there are five important items to be
considered. These are; structurally sound aggregates of low porosity; a port-
land cement paste of low water-cement ratio; a properly designed air-
entrained mix; proper placement; and adequate curing.
Aggregates constitute from two-thirds to three-fourths of the volume of
concrete. Consequently, their characteristics influence to a large degree the
workability, strength, and durability of concrete.
Hardness of an aggregate means its resistance to abrasion. Any concrete
subjected to heavy moving loads, as in loading platforms, should be made
with hard aggregates.
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