Civil Engineering Reference
In-Depth Information
Air entrainment is accomplished by using an air-
entraining portland cement or by adding an air-entraining
admixture at the mixer. The amount of admixture should
be adjusted to meet variations in concrete ingredients and
job conditions. The amount recommended by the admix-
ture manufacturer will, in most cases, produce the desired
air content.
Recommended target air contents for air-entrained
concrete are shown in Fig. 9-4 and Table 9-5. Note that the
amount of air required to provide adequate freeze-thaw
resistance is dependent upon the nominal maximum size
of aggregate and the level of exposure. In properly pro-
portioned mixes, the mortar content decreases as maxi-
mum aggregate size increases, thus decreasing the re-
quired concrete air content. This is evident in Fig. 9-4. The
levels of exposure are defined by
ACI 211.1
as follows:
Mild Exposure.
This exposure includes indoor or outdoor
service in a climate where concrete will not be exposed to
freezing or deicing agents. When air entrainment is
desired for a beneficial effect other than durability, such as
to improve workability or cohesion or in low cement con-
tent concrete to improve strength, air contents lower than
those needed for durability can be used.
Moderate Exposure.
Service in a climate where freezing
is expected but where the concrete will not be continually
exposed to moisture or free water for long periods prior to
freezing and will not be exposed to deicing or other
aggressive chemicals. Examples include exterior beams,
columns, walls, girders, or slabs that are not in contact
with wet soil and are so located that they will not receive
direct applications of deicing chemicals.
Severe Exposure.
Concrete that is exposed to deicing or
other aggressive chemicals or where the concrete may
become highly saturated by continual contact with mois-
ture or free water prior to freezing. Examples include
pavements, bridge decks, curbs, gutters, sidewalks, canal
linings, or exterior water tanks or sumps.
When mixing water is held constant, the entrainment
of air will increase slump. When cement content and
slump are held constant, the entrainment of air results in
the need for less mixing water, particularly in leaner con-
crete mixtures. In batch adjustments, in order to maintain
a constant slump while changing the air content, the water
should be decreased by about 3 kg/m
3
(5 lb/yd
3
) for each
percentage point increase in air content or increased
3 kg/m
3
(5 lb/yd
3
) for each percentage point decrease.
A specific air content may not be readily or repeatedly
achieved because of the many variables affecting air con-
tent; therefore, a permissible range of air contents around
a target value must be provided. Although a range of ±1%
of the Fig. 9-4 or Table 9-5 values is often used in project
specifications, it is sometimes an impracticably tight limit.
The solution is to use a wider range, such as -1 to +2 per-
Nominal maximum aggregate size, in.
0
0.5
1
1.5
2
2.5
3
8
7
6
Severe exposure (deicers)
5
Moderate exposure
4
3
Mild exposure
2
Non-air-entrained concrete
1
0
0
10
20
30
40
50
60
70
Nominal maximum aggregate size, mm
Fig. 9-4. Target total air content requirements for concretes
using different sizes of aggregate. The air content in job
specifications should be specified to be delivered within -1
to +2 percentage points of the target value for moderate
and severe exposures. Adapted from Table 9-5,
ACI 211.1
and
Hover
(
1995
and
1998
).
centage points of the target values. For example, for a
target value of 6% air, the specified range for the concrete
delivered to the jobsite could be 5% to 8%.
Slump
Concrete must always be made with a workability, consis-
tency, and plasticity suitable for job conditions. Workabil-
ity is a measure of how easy or difficult it is to place,
consolidate, and finish concrete. Consistency is the ability
of freshly mixed concrete to flow. Plasticity determines
concrete's ease of molding. If more aggregate is used in a
concrete mixture, or if less water is added, the mixture
becomes stiff (less plastic and less workable) and difficult
to mold. Neither very dry, crumbly mixtures nor very
watery, fluid mixtures can be regarded as having plasticity.
The slump test is used to measure concrete consis-
tency. For a given proportion of cement and aggregate
without admixtures, the higher the slump, the wetter the
mixture. Slump is indicative of workability when
assessing similar mixtures. However, slump should not be
used to compare mixtures of totally different proportions.
When used with different batches of the same mix design,
a change in slump indicates a change in consistency and in
the characteristics of materials, mixture proportions,
water content, mixing, time of test, or the testing itself.
