Civil Engineering Reference
In-Depth Information
amount of air-entraining admixture must be adjusted to
field conditions to maintain the specified air content.
Water Reducers.
Water reducers are used to increase
workability without the addition of water or to reduce the
water-cement ratio of a concrete mixture to improve per-
meability or other properties.
Using the final mixture developed in the last example,
assume that the project engineer approves the use of a
water reducer to increase the slump to 5 in. to improve
workability for a difficult placement area. Assuming that
the water reducer has a manufacturer's recommended
dosage rate of 4 oz per 100 lb of cement to increase slump
2 in., the admixture amount becomes
624
100
Example 3. Laboratory Trial Mixture
Using the PCA Water-Cement Ratio
Method (Metric)
With the following method, the mix designer develops
the concrete proportions directly from the laboratory trial
batch rather than the absolute volume of the constituent
ingredients.
Conditions and Specifications.
Concrete is required for
a plain concrete pavement to be constructed in North
Dakota. The pavement specified compressive strength is
35 MPa at 28 days. The standard deviation of the concrete
producer is 2.0 MPa. Type IP cement and 19-mm nominal
maximum-size coarse aggregate is locally available. Pro-
portion a concrete mixture for these conditions and check
it by trial batch. Enter all data in the blank spaces on a trial
mixture data sheet (Fig. 9-6).
Durability Requirements.
The pavement will be exposed
to freezing, thawing, and deicers and therefore should
have a maximum water to cementitious material ratio of
0.45 (Table 9-1) and at least 335 kg of cement per cubic
meter of concrete.
Strength Requirements.
For a standard deviation of
2.0 MPa, the  (required compressive strength for pro-
portioning) must be the larger of
 = ˘ + 1.34
S
= 35 + 1.34(2.0) = 37.7 MPa
or
 = ˘ + 2.33
S
- 3.45 = 35 + 2.33(2.0) - 3.45 = 36.2 MPa
Therefore the required average compressive strength
= 37.7 MPa.
Aggregate Size.
The 19-mm maximum-size coarse aggre-
gate and the fine aggregate are in saturated-surface dry
condition for the trial mixtures.
Air Content.
The target air content should be 6% (Table
9-5) and the range is set at 5% to 8%.
Slump.
The specified target slump for this project is
40 (±20) mm.
Batch Quantities.
For convenience, a batch containing 10
kg of cement is to be made. The quantity of mixing water
required is 10 x 0.45 = 4.5 kg. Representative samples of
fine and coarse aggregates are measured in suitable con-
tainers. The values are entered as initial mass in Column 2
of the trial-batch data sheet (Fig. 9-6).
All of the measured quantities of cement, water, and
air-entraining admixture are used and added to the mixer.
Fine and coarse aggregates, previously brought to a satu-
rated, surface-dry condition, are added until a workable
concrete mixture with a slump deemed adequate for place-
ment is produced. The relative proportions of fine and
coarse aggregate for workability can readily be judged by
an experienced concrete technician or engineer.
Workability.
Results of tests for slump, air content, den-
sity, and a description of the appearance and workability
are noted in the data sheet and Table 9-13.
x 4
= 25.0 oz per cu yd
The amount of air-entraining agent may also need to be
reduced (up to 50%), as many water reducers also entrain
air. If a water reducer was used to reduce the water-
cement ratio, the water and sand content would also need
adjustment.
Pozzolans and Slag.
Pozzolans and slag are sometimes
added in addition to or as a partial replacement of cement
to aid in workability and resistance to sulfate attack and
alkali reactivity. If a pozzolan or slag were required for the
above example mixture, it would have been entered in the
first volume calculation used in determining fine aggre-
gate content. For example:
Assume that 75 lb of fly ash with a relative density (specific
gravity) of 2.5 were to be used in addition to the originally
derived cement content. The ash volume would be
75
2.5 x 62.4
= 0.48 cu ft
The water to cementing materials ratio would be
W
C + P
=
27
643
0
+75
= 0.38 by weight
The water to portland cement only ratio would still be
W
C
=
270
643
= 0.42 by weight
The fine aggregate volume would have to be reduced by
0.48 cu ft to allow for the volume of ash.
The pozzolan amount and volume computation could
also have been derived in conjunction with the first ce-
ment content calculation using a water to cementing mate-
rials ratio of 0.42 (or equivalent). For example, assume
15% of the cementitious material is specified to be a poz-
zolan and
W/ CM
or
W/ (C
+
P)
= 0.42.
Then with
W
= 270 lb and
C
+
P
= 643 lb,
P
= 643 x
15
100
= 96 lb
and
C
= 643 - 96 = 547 lb
Appropriate proportioning computations for these and
other mix ingredients would follow.
