Page 23 - Masonry and Concrete

Understanding Concrete
22 CHAPTER TWO

Admixtures marketed as “dampproofing” or “waterproofing” com-
pounds are of little practical use and may, in fact, be detrimental to the
concrete. Most water leakage problems can be traced to poor construc-
tion practices, cracks, or lean, high-slump mixes. No admixture or sur-
face-applied treatment is a substitute for high-quality ingredients and
good workmanship.

Air-Entraining Agents: Both natural and chemical admixtures can
be used to improve workability. Lean or harsh concrete mixes can be
improved by the addition of finely ground material such as fly ash
or natural or manufactured pozzolans. Some set-retarding or water-
reducing admixtures also improve workability, but they are not used
primarily for this purpose. Air-entraining agents improve workabil-
ity and are particularly effective in lean mixes and in mixes con-
taining poorly graded or sharp, angular aggregate. Air entrainment
reduces segregation, slows the rate of bleeding, and shortens finish-
ing time. Either a separate air-entraining agent or an air-entrained
cement may be used, but total air content is generally recommended
not to exceed 4 to 7% of the total concrete volume. Better control of
air content is achieved using a separate air-entraining admixture
batched at a ready-mix plant. For job-site mixing, air-entrained
cements are easier to use but require mechanical rather than hand
mixing.

Coloring Pigments: One of the ways to introduce color to concrete is
the addition of natural or synthetic mineral coloring pigments to the
mix. The pigments must be insoluble in water, free from soluble salts
and acids, colorfast in sunlight, chemically stable in the alkaline
cement paste, and have no adverse effect on the setting time, strength
development, or durability of the concrete. Synthetic oxide pigments
are stronger than natural oxide pigments so less is required, but the
cost is higher. Many manufacturers package their pigments in amounts
appropriate to color one cubic yard of concrete containing six bags of
cement. Both liquid and powder pigments are available. Using white
portland cement instead of grey produces cleaner, brighter, more vivid
colors. Figure 2-10 lists various colors that can be achieved using dif-
ferent pigments.

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.