Civil Engineering Reference
In-Depth Information
Table 9-8 shows limits on the amount of supplemen-
tary cementing materials in concrete to be exposed to
deicers. Local practices should be consulted as dosages
smaller or larger than those shown in Table 9-8 can be
used without jeopardizing scale-resistance, depending on
the exposure severity.
ficient to affect the water-cementing materials ratio by 0.01
or more.
An excessive use of multiple admixtures should be
minimized to allow better control of the concrete mixture
in production and to reduce the risk of admixture incom-
patibility.
Admixtures
PROPORTIONING
Water-reducing admixtures are added to concrete to reduce
the water-cementing materials ratio, reduce cementing
materials content, reduce water content, reduce paste con-
tent, or to improve the workability of a concrete without
changing the water-cementing materials ratio. Water
reducers will usually decrease water contents by 5% to 10%
and some will also increase air contents by
1
⁄
2
to 1 per-
centage point. Retarders may also increase the air content.
High-range water reducers (plasticizers) reduce water
contents between 12% and 30% and some can simultane-
ously increase the air content up to 1 percentage point;
others can reduce or not affect the air content.
Calcium chloride-based admixtures reduce water
contents by about 3% and increase the air content by about
1
⁄
2
percentage point.
When using a chloride-based admixture, the risks of
reinforcing steel corrosion should be considered. Table 9-9
provides recommended limits on the water-soluble chlo-
ride-ion content in reinforced and prestressed concrete for
various conditions.
When using more than one admixture in concrete, the
compatibility of intermixing admixtures should be
assured by the admixture manufacturer or the combina-
tion of admixtures should be tested in trial batches. The
water contained in admixtures should be considered part
of the mixing water if the admixture's water content is suf-
The design of concrete mixtures involves the following:
(1) the establishment of specific concrete characteristics,
and (2) the selection of proportions of available materials
to produce concrete of required properties, with the
greatest economy. Proportioning methods have evolved
from the arbitrary volumetric method (1:2:3—
cement:sand: coarse aggregate) of the early 1900s
(
Abrams 1918
) to the present-day weight and absolute-
volume methods described in ACI's Committee 211
Standard Practice for Selecting Proportions for Normal,
Heavyweight and Mass Concrete
(
ACI 211.1
).
Weight proportioning methods are fairly simple and
quick for estimating mixture proportions using an assumed
or known weight of concrete per unit volume. A more accu-
rate method, absolute volume, involves use of relative den-
sity (specific gravity) values for all the ingredients to
calculate the absolute volume each will occupy in a unit
volume of concrete. The absolute volume method will be
illustrated. A concrete mixture also can be proportioned
from field experience (statistical data) or from trial mixtures.
Other valuable documents to help proportion con-
crete mixtures include the
Standard Practice for Selecting
Proportions for Structural Lightweight Concrete
(
ACI 211.2
);
Guide for Selecting Proportions for No-Slump Concrete
(
ACI
211.3
);
Guide for Selecting Proportions for High-Strength
Concrete with Portland Cement and Fly Ash
(
ACI 211.4R
);
and
Guide for Submittal of Concrete Proportions
(
ACI 211.5
).
Hover
(
1995
and
1998
) provides a graphical process for
designing concrete mixtures in accordance with
ACI 211.1
.
Proportioning from Field Data
Table 9-9. Maximum Chloride-Ion Content for
Corrosion Protection
A presently or previously used concrete mixture design
can be used for a new project if strength-test data and
standard deviations show that the mixture is acceptable.
Durability aspects previously presented must also be met.
Standard deviation computations are outlined in
ACI 318
.
The statistical data should essentially represent the same
materials, proportions, and concreting conditions to be
used in the new project. The data used for proportioning
should also be from a concrete with an ˘ that is within
7 MPa (1000 psi) of the strength required for the proposed
work. Also, the data should represent at least 30 consecu-
tive tests or two groups of consecutive tests totaling at
least 30 tests (one test is the average strength of two cylin-
ders from the same sample). If only 15 to 29 consecutive
tests are available, an adjusted standard deviation can be
Maximum water-soluble
chloride ion (Cl
-
) in
concrete, percent by
Type of member
mass of cement*
Prestressed concrete
0.06
Reinforced concrete exposed to
chloride in service
0.15
Reinforced concrete that will be
dry or protected from moisture
1.00
in service
Other reinforced concrete
construction
0.30
*ASTM C 1218.
Adapted from
ACI 318
.
