Civil Engineering Reference
In-Depth Information
and is affected by the size of the aggregates. The exposure levels are defined as
follows:
Mild exposure
—Indoor or outdoor service in which concrete is not ex-
posed to freezing and deicing salts. Air entrainment may be used to
improve workability.
Moderate exposure
—Some freezing exposure occurs, but concrete is
not exposed to moisture or free water for long periods prior to
freezing. Concrete is not exposed to deicing salts. Examples include
exterior beams, columns, walls, etc., not exposed to wet soil.
Severe exposure
—Concrete is exposed to deicing salts, saturation, or
free water. Examples include pavements, bridge decks, curbs, gut-
ters, canal linings, etc.
Table 7.6 presents the recommended air contents for different combi-
nations of exposure conditions and maximum aggregate sizes. The values
shown in Table 7.6 are the entrapped air for non-air-entrained concrete
and the entrapped plus entrained air in case of air-entrained concrete. The
recommended air content decreases with increasing maximum aggregate
size.
5. Workability Requirements
The next step in the mix design is to determine
the workability requirements for the project. Workability is defined as the
ease of placing, consolidating, and finishing freshly mixed concrete. Concrete
should be workable but should not segregate or excessively bleed (migration
T ABLE 7.6
Approximate Target Percent Air Content Requirements for Different Nominal
Maximum Sizes of Aggregates*
Maximum Aggregate Size
9.5 mm 12.5 mm 19 mm 25 mm 37.5 mm 50 mm 75 mm 150 mm
(3/8 in.)
1
2
in.
3
4
in.
1
1
(
)
(
)
(1 in.)
(
2
in.
)
(2 in.)
(3 in.)
(6 in.)
Non-air-entrained
concrete
3
2.5
2
1.5
1
0.5
0.3
0.2
Air-entrained concrete**
Mild Exposure
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
Moderate Exposure
6.0
5.5
5.0
4.5
4.5
4.0
3.5
3.0
Severe Exposure
7.5
7.0
6.0
6.0
5.5
5.0
4.5
4.0
*American Concrete Institute (ACI 211.1 and ACI 318).
**The air content in job specifications should be specified to be delivered within
- 1
to
+ 2
percentage
points of the table target value for moderate and severe exposures.
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