Civil Engineering Reference
In-Depth Information
acceptable for patching and thin-bonded overlayment,
particularly where feather-edged patches are desired.
Bonding agents should not be confused with bonding
admixtures. Admixtures are an ingredient in the concrete;
bonding agents are applied to existing concrete surfaces
immediately before the new concrete is placed. Bonding
agents help “glue” the existing and the new materials
together. Bonding agents are often used in restoration and
repair work; they consist of portland cement or latex-
modified portland cement grout or polymers such as
epoxy resins (ASTM C 881 or AASHTO M 235) or latex
(ASTM C 1059).
FUNGICIDAL, GERMICIDAL, AND
INSECTICIDAL ADMIXTURES
Bacteria and fungal growth on or in hardened concrete
may be partially controlled through the use of fungicidal,
germicidal, and insecticidal admixtures. The most effec-
tive materials are polyhalogenated phenols, dieldrin
emulsions, and copper compounds. The effectiveness of
these materials is generally temporary, and in high
dosages they may reduce the compressive strength of
concrete.
ANTIWASHOUT ADMIXTURES
GROUTING ADMIXTURES
Antiwashout admixtures increase the cohesiveness of con-
crete to a level that allows limited exposure to water with
little loss of cement. This allows placement of concrete in
water and under water without the use of tremies. The ad-
mixtures increase the viscosity of water in the mixture
resulting in a mix with increased thixotropy and resistance
to segregation. They usually consist of water soluble cellu-
lose ether or acrylic polymers.
Portland cement grouts are used for a variety of purposes:
to stabilize foundations, set machine bases, fill cracks and
joints in concrete work, cement oil wells, fill cores of
masonry walls, grout prestressing tendons and anchor
bolts, and fill the voids in preplaced aggregate concrete. To
alter the properties of grout for specific applications, vari-
ous air-entraining admixtures, accelerators, retarders, and
nonshrink admixtures are often used.
COMPATIBILITY OF ADMIXTURES AND
CEMENTITIOUS MATERIALS
GAS-FORMING ADMIXTURES
Fresh concrete problems of varying severity are encoun-
tered due to cement-admixture incompatibility and
incompatibility between admixtures. Incompatibility
between supplementary cementing materials and admix-
tures or cements can also occur. Slump loss, air loss, early
stiffening, and other factors affecting fresh concrete prop-
erties can result from incompatibilities. While these prob-
lems primarily affect the plastic-state performance of
concrete, long-term hardened concrete performance may
also be adversely affected. For example, early stiffening
can cause difficulties with consolidation of concrete, there-
fore compromising strength.
Reliable test methods are not available to adequately
address incompatibility issues due to variations in materi-
als, mixing equipment, mixing time, and environmental
factors. Tests run in a laboratory do not reflect the condi-
tions experienced by concrete in the field. When incom-
patibility is discovered in the field, a common solution is
to simply change admixtures or cementing materials
(
Helmuth, Hills, Whiting, and Bhattacharja 1995
,
Tagni-
Hamou and Aítcin 1993
, and
Tang and Bhattacharja 1997
).
Aluminum powder and other gas-forming materials are
sometimes added to concrete and grout in very small
quantities to cause a slight expansion of the mixture prior
to hardening. This may be of benefit where the complete
grouting of a confined space is essential, such as under
machine bases or in post-tensioning ducts of prestressed
concrete. These materials are also used in larger quantities
to produce autoclaved cellular concretes. The amount of
expansion that occurs is dependent upon the amount of
gas-forming material used, the temperature of the fresh
mixture, the alkali content of the cement, and other vari-
ables. Where the amount of expansion is critical, careful
control of mixtures and temperatures must be exercised.
Gas-forming agents will not overcome shrinkage after
hardening caused by drying or carbonation.
AIR DETRAINERS
Air-detraining admixtures reduce the air content in
concrete. They are used when the air content cannot be
reduced by adjusting the mix proportions or by changing
the dosage of the air-entraining agent and other admix-
tures. However, air-detrainers are rarely used and their
effectiveness and dosage rate should be established on
trial mixes prior to use on actual job mixes. Materials used
in air-detraining agents are listed in Table 6-1.
STORING AND DISPENSING
CHEMICAL ADMIXTURES
Liquid admixtures can be stored in barrels or bulk tankers.
Powdered admixtures can be placed in special storage
bins and some are available in premeasured plastic bags.
Admixtures added to a truck mixer at the jobsite are often
