Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.13
greatly facilitates placing concrete around heavy, closely spaced reinforcing steel,
or in complicated forms, or both, and reduces the need for vibrating the concrete.
It is important that the slump of the concrete be verified at the jobsite prior to the
addition of the superplasticizer. This ensures that the specified water-cementitious
materials ratio required for watertight impermeable concrete is in fact being
achieved. The superplasticizer is then added to increase the slump to the approved
level.
Waterproofing. A number of substances, such as stearates and oils, have been
used as masonry-mortar and concrete admixtures for ‘‘waterproofing.’’ Indiscriminate
use of such materials in concrete without extremely good quality control usually
results in disappointment. The various water-repellent admixtures are intended
to prevent capillarity, but most severe leakage in concrete occurs at honeycombs,
cold joints, cracks, and other noncapillary defects. Concrete containing waterrepellent
admixtures also requires extremely careful continuous curing, since it will
be difficult to rewet after initial drying.
Waterproof concrete can be achieved by use of high-strength concrete with a
low water-cementitious materials ratio to reduce segregation and an air-entraining
agent to minimize crack width. Also, good quality control and inspection is essential
during the mixing, placing, and curing operations. Surface coatings can be used to
improve resistance to water penetration of vertical or horizontal surfaces. For detailed
information on surface treatments, see ‘‘Guide to Durable Concrete,’’ ACI
201.2R.
Cement Replacement. The term ‘‘cement replacement’’ is frequently misused in
reference to chemical admixtures intended as accelerators or water reducers.
Strictly, a cement replacement is a finely ground material, usually weakly cementitious
(Art. 9.1), which combines into a cementlike paste replacing some of the
cement paste to fill voids between the aggregates. The most common applications
of these admixtures are for low-heat, low-strength mass concrete or for concrete
masonry. In the former, they fill voids and reduce the heat of hydration; in the latter,
they fill voids and help to develop the proper consistency to be self-standing as the
machine head is lifted in the forming process. Materials commonly used are fly
ash, silica fume, ground granulated blast-furnace slag, hydraulic lime, natural cement,
and pozzolans.
Special-Purpose Admixtures. The list of materials used from earliest times as
admixtures for various purposes includes almost everything from human blood to
synthetic coloring agents.
Admixtures for coloring concrete are available in all colors. The oldest and
cheapest is perhaps carbon black.
Admixtures causing expansion for use in sealing cracks or under machine bases,
etc., include powdered aluminum and finely ground iron.
Special admixtures are available for use where the natural aggregate is alkali
reactive, to neutralize this reaction.
Proprietary admixtures are available that increase the tensile strength or bond
strength of concrete. They are useful for making repairs to concrete surfaces.
For special problems requiring concrete with unusual properties, detailed recommendations
of ‘‘Chemical Admixtures for Concrete,’’ ACI 212.3R, and references
it contains, may be helpful.
For all these special purposes, a thorough investigation of admixtures proposed
is recommended. Tests should be made on samples containing various proportions
for colored concrete. Strength and durability tests should be made on concrete to