Civil Engineering Reference
In-Depth Information
the color and amount of the material used in concrete.
Many supplementary cementing materials resemble the
color of portland cement and therefore have little effect on
color of the hardened concrete. Some silica fumes may give
concrete a slightly bluish or dark gray tint and tan fly ash
may impart a tan color to concrete when used in large
quantities. Ground slag and metakaolin can make concrete
whiter. Ground slag can initially impart a bluish or green-
ish undertone.
STORAGE
In most cases, moisture will not affect the physical
performance of supplementary cementing materials. These
materials should, however, be kept dry to avoid difficulties
in handling and discharge. Class C fly ash and calcined
shale must be kept dry as they will set and harden when
exposed to moisture. Equipment for handling and storing
these materials is similar to that required for cement.
Additional modifications may be required where using
silica fume, which does not have the same flowing charac-
teristics as other supplementary cementing materials (and
may be supplied as a liquid).
These materials are usually kept in bulk storage facili-
ties or silos, although some products are available in bags.
Because the materials may resemble portland cement in
color and fineness, the storage facilities should be clearly
marked to avoid the possibility of misuse and contamina-
tion with other materials at the batch plant. All valves and
piping should also be clearly marked and properly sealed
to avoid leakage and contamination. Fly ash, slag, and
natural pozzolans should be weighed after the portland or
blended cement in the batching sequence to avoid over-
dosing in case valves stick.
CONCRETE MIX PROPORTIONS
The optimum amounts of supplementary cementing mate-
rials used with portland cement or blended cement are
determined by testing, by the relative cost and availability of
the materials, and by the specified properties of the concrete.
Several test mixtures are required to determine the
optimum amount of pozzolan or slag. These mixtures
should cover a range of blends to establish the relationship
between strength and water to cementing materials ratio.
These mixtures should be established according to
ACI
Standard 211.1
or
211.2
, taking into account the relative
densities of the supplementary cementing materials. These
are usually different from the relative density of portland
cement. The results of these tests will be a family of
strength curves for each age at which the concrete is
required to meet certain specified requirements. The
dosage of a cementitious material is usually stated as a
mass percentage of all the cementitious materials in a
concrete mixture.
Typical practice in the United States uses fly ash, slag,
silica fume, calcined clay, or calcined shale as an addition
to portland cement or as a partial replacement for some of
the portland cement. Blended cements, which already
contain pozzolans or slag, are designed to be used with or
without additional supplementary cementitious materials.
Concrete mixtures with more than one supplementary
cementitious material are also used. For example, a concrete
mixture may contain portland cement, fly ash, and silica
fume. Such mixes are called ternary concretes. When fly
ash, slag, silica fume, or natural pozzolans are used in
combination with portland or blended cement, the propor-
tioned concrete mixture should be tested to demonstrate
that it meets the required concrete properties for the project.
REFERENCES
AASHTO,
Guide Specification
on
Alkali-Silica Reaction,
Section 56,
http://leadstates.tamu.edu/ASR/library/
gspec.stm
, 2000.
Abrams, Duff A., “Effect of Hydrated Lime and Other
Powdered Admixtures in Concrete,”
Proceedings of the
American Society for Testing Materials,
Vol. 20, Part 2, 1920.
Reprinted with revisions as Bulletin 8, Structural Materials
Research Laboratory, Lewis Institute, June 1925, 78 pages.
Available through PCA as LS08.
ACAA,
http://www.acaa-usa.org
, American Coal Ash
Association, Alexandria, Virginia, 2001.
ACI,
Concrete Durability, Katherine and Bryant Mather
International Conference,
SP100, American Concrete Insti-
tute, Farmington Hills, Michigan, 1987.
ACI Committee 211,
Standard Practice for Selecting
Proportions for Normal, Heavyweight, and Mass Concrete,
ACI
211.1-91, American Concrete Institute, Farmington Hills,
Michigan, 1991, 38 pages.
AVAILABILITY
ACI Committee 211,
Standard Practice for Selecting Pro-
portions for Structural Lightweight Concrete,
ACI 211.2-98,
American Concrete Institute, Farmington Hills, Michigan,
1998, 14 pages.
All supplementary cementitious materials may not be
available in all areas. Consult local material suppliers on
available materials. Class F fly ashes are usually available
in the Eastern United States, while Class C ashes are avail-
able in the Midwest and West. Silica fume is available in
most locations because only small dosages are used.
Calcined clays and shales are available in select areas.
Ground granulated slags are available in most regions.
ACI Committee 211,
Guide for Selecting Proportions for High
Strength Concrete with Portland Cement and Fly Ash,
ACI
211.4R-93, American Concrete Institute, Farmington Hills,
Michigan, 1993, 13 pages.
