Civil Engineering Reference
In-Depth Information
It should be kept in mind that changing the amount of
any single ingredient in a concrete mixture normally
effects the proportions of other ingredients as well as alter
the properties of the mixture. For example, the addition of
2 kg of water per cubic meter will increase the slump by
approximately 10 mm (10 lb of water per cubic yard will
increase the slump by approximately 1 in.); it will also in-
crease the air content and paste volume, decrease the ag-
gregate volume, and lower the density of the concrete. In
mixture adjustments, for the same slump, a decrease in air
content by 1 percentage point will increase the water
demand by about 3 kg per cubic meter of concrete (5 lb per
cu yd of concrete).
tity of cementing materials to be used should be not less
than shown in Table 9-7.
To obtain economy, proportioning should minimize
the amount of cement required without sacrificing con-
crete quality. Since quality depends primarily on water-
cementing materials ratio, the water content should be
held to a minimum to reduce the cement requirement.
Steps to minimize water and cement requirements include
use of (1) the stiffest practical mixture, (2) the largest prac-
tical maximum size of aggregate, and (3) the optimum
ratio of fine-to-coarse aggregate.
Concrete that will be exposed to sulfate conditions
should be made with the type of cement shown in Table 9-2.
Seawater contains significant amounts of sulfates and
chlorides. Although sulfates in seawater are capable of at-
tacking concrete, the presence of chlorides in seawater
inhibits the expansive reaction that is characteristic of sulfate
attack. This is the major factor explaining observations from
a number of sources that the performance of concretes in
seawater have shown satisfactory durability; this is despite
the fact these concretes were made with portland cements
having tricalcium aluminate (C
3
A) contents as high as 10%,
and sometimes greater. However, the permeability of these
concretes was low, and the reinforcing steel had adequate
cover. Portland cements meeting a C
3
A requirement of not
more than 10% or less than 4% (to ensure durability of rein-
forcement) are acceptable (
ACI 357R
).
Supplementary cementitious materials have varied
effects on water demand and air contents. The addition of
fly ash will generally reduce water demand and decrease
the air content if no adjustment in the amount of air-
entraining admixture is made. Silica fume increases water
demand and decreases air content. Slag and metakaolin
have a minimal effect at normal dosages.
Cementing Materials Content and Type
The cementing materials content is usually determined from
the selected water-cementing materials ratio and water con-
tent, although a minimum cement content frequently is
included in specifications in addition to a maximum water-
cementing materials ratio. Minimum cement content re-
quirements serve to ensure satisfactory durability and
finishability, to improve wear resistance of slabs, and to
guarantee a suitable appearance of vertical surfaces. This
is important even though strength requirements may be
met at lower cementing materials contents. However,
excessively large amounts of cementing materials should
be avoided to maintain economy in the mixture and to not
adversely affect workability and other properties.
For severe freeze-thaw, deicer, and sulfate exposures,
it is desirable to specify: (1) a minimum cementing mate-
rials content of 335 kg per cubic meter (564 lb per cubic
yard) of concrete, and (2) only enough mixing water to
achieve the desired consistency without exceeding the
maximum water-cementing materials ratios shown in
Tables 9-1 and 9-2. For placing concrete underwater, usu-
ally not less than 390 kg of cementing materials per cubic
meter (650 lb of cementing materials per cubic yard) of
concrete should be used with a water to cementing mate-
rials ratio not exceeding 0.45. For workability, finishability,
abrasion resistance, and durability in flatwork, the quan-
Table 9-8. Cementitious Materials Requirements for
Concrete Exposed to Deicing Chemicals
Maximum percent of
Cementitious
total cementitious
materials*
materials by mass**
Fly ash and natural pozzolans
25
Table 9-7. Minimum Requirements of Cementing
Materials for Concrete Used in Flatwork
Slag
50
Silica fume
10
Total of fly ash, slag, silica fume
and natural pozzolans
Nominal maximum size
Cementing materials,
50
†
of aggregate, mm (in.)
kg/m
3
(lb/yd
3
)*
Total of natural pozzolans and
silica fume
37.5 (1
1
⁄
2
)
280 (470)
35
†
25 (1)
310 (520)
19 (
3
⁄
4
)
320 (540)
* Includes portion of supplementary cementing materials in blended
cements.
** Total cementitious materials include the summation of portland
cements, blended cements, fly ash, slag, silica fume and other poz-
zolans.
† Silica fume should not constitute more than 10% of total cementi-
tious materials and fly ash or other pozzolans shall not constitute
more than 25% of cementitious materials.
Adapted from
ACI 318
.
12.5 (
1
⁄
2
)
350 (590)
9.5 (
3
⁄
8
)
360 (610)
* Cementing materials quantities may need to be greater for severe
exposure. For example, for deicer exposures, concrete should con-
tain at least 335 kg/m
3
(564 lb/yd
3
) of cementing materials.
Adapted from
ACI 302
.
