Civil Engineering Reference
In-Depth Information
concrete. One of the best methods of decreasing perme-
ability in concrete is to increase the moist-curing period
and reduce the water-cement ratio to less than 0.5. Most
admixtures that reduce water-cement ratio consequently
reduce permeability.
Some supplementary cementing materials, especially
silica fume, reduce permeability through the hydration and
pozzolanic-reaction process. Other admixtures that act to
block the capillaries in concrete have been shown to be ef-
fective in reducing concrete corrosion in chemically ag-
gressive environments. Such admixtures, designed for use
in high-cement content/low-water-cement ratio concretes,
contain aliphatic fatty acid and an aqueous emulsion of
polymeric and aromatic globules (
Aldred 1988
).
Fig. 6-19. Red and blue pigments were used to color this
terrazzo floor. (69873)
PUMPING AIDS
Pumping aids are added to concrete mixtures to improve
pumpability. Pumping aids cannot cure all unpumpable con-
crete problems; they are best used to make marginally
pumpable concrete more pumpable. These admixtures
increase viscosity or cohesion in concrete to reduce dewater-
ing of the paste while under pressure from the pump.
Some pumping aids may increase water demand,
reduce compressive strength, cause air entrainment, or
retard setting time. These side effects can be corrected by
adjusting the mix proportions or adding another admix-
ture to offset the side effect.
A partial list of materials used in pumping aids is
given in Table 6-1. Some admixtures that serve other
primary purposes but also improve pumpability are air-
entraining agents, and some water-reducing and retarding
admixtures.
Unmodified carbon black substantially reduces air
content. Most carbon black for coloring concrete contains
an admixture to offset this effect on air. Before a coloring
admixture is used on a project, it should be tested for color
fastness in sunlight and autoclaving, chemical stability in
cement, and effects on concrete properties. Calcium chlo-
ride should not be used with pigments to avoid color
distortions. Pigments should conform to ASTM C 979.
DAMPPROOFING ADMIXTURES
The passage of water through concrete can usually be
traced to the existence of cracks or areas of incomplete
consolidation. Sound, dense concrete made with a water-
cement ratio of less than 0.50 by mass will be watertight if
it is properly placed and cured.
Admixtures known as dampproofing agents include
certain soaps, stearates, and petroleum products. They
may, but generally do not, reduce the permeability of
concretes that have low cement contents, high water-
cement ratios, or a deficiency of fines in the aggregate.
Their use in well-proportioned mixes, may increase the
mixing water required and actually result in increased
rather than reduced permeability.
Dampproofing admixtures are sometimes used to
reduce the transmission of moisture through concrete that
is in contact with water or damp earth. Many so-called
dampproofers are not effective, especially when used in
concretes that are in contact with water under pressure.
BONDING ADMIXTURES AND
BONDING AGENTS
Bonding admixtures are usually water emulsions of organic
materials including rubber, polyvinyl chloride, polyvinyl
acetate, acrylics, styrene butadiene copolymers, and other
polymers. They are added to portland cement mixtures to
increase the bond strength between old and new concrete.
Flexural strength and resistance to chloride-ion ingress are
also improved. They are added in proportions equivalent to
5% to 20% by mass of the cementing materials; the actual
quantity depending on job conditions and type of admix-
ture used. Some bonding admixtures may increase the air
content of mixtures. Nonreemulsifiable types are resistant
to water, better suited to exterior application, and used in
places where moisture is present.
The ultimate result obtained with a bonding admix-
ture will be only as good as the surface to which the
concrete is applied. The surface must be dry, clean, sound,
free of dirt, dust, paint, and grease, and at the proper
temperature. Organic or polymer modified concretes are
PERMEABILITY-REDUCING ADMIXTURES
Permeability-reducing admixtures reduce the rate at
which water under pressure is transmitted through
