Civil Engineering Reference
In-Depth Information
Coarse-Aggregate Content.
The quantity of 25-mm
nominal maximum-size coarse aggregate can be estimated
from Fig. 9-3 or Table 9-4. The bulk volume of coarse ag-
gregate recommended when using sand with a fineness
modulus of 2.80 is 0.67. Since it has a bulk density of 1600
kg/m
3
, the ovendry mass of coarse aggregate for a cubic
meter of concrete is
1600 x 0.67 = 1072 kg
Admixture Content.
For an 8% air content, the air-
entraining admixture manufacturer recommends a dosage
rate of 0.5 g per kg of cement. From this information, the
amount of air-entraining admixture per cubic meter of con-
crete is
Air content 8% (±0.5% for trial batch)
Estimated concrete
= 135 + 435 + (1072 x 1.005*)
density (using
+ (634 x 1.007*)
SSD aggregate)
= 2286 kg/m
3
The liquid admixture volume is generally too insig-
nificant to include in the water calculations. However, cer-
tain admixtures, such as shrinkage reducers, plasticizers,
and corrosion inhibitors are exceptions due to their rela-
tively large dosage rates; their volumes should be included.
Moisture.
Corrections are needed to compensate for mois-
ture in and on the aggregates. In practice, aggregates will
contain some measurable amount of moisture. The dry-
batch weights of aggregates, therefore, have to be in-
creased to compensate for the moisture that is absorbed in
and contained on the surface of each particle and between
particles. The mixing water added to the batch must be
reduced by the amount of free moisture contributed by the
aggregates. Tests indicate that for this example, coarse-
aggregate moisture content is 2% and fine-aggregate mois-
ture content is 6%.
With the aggregate moisture contents (MC) indicated, the
trial batch aggregate proportions become
Coarse aggregate (2% MC) = 1072 x 1.02 = 1093 kg
Fine aggregate (6% MC) = 634 x 1.06 = 672 kg
Water absorbed by the aggregates does not become part of
the mixing water and must be excluded from the water
adjustment. Surface moisture contributed by the coarse
aggregate amounts to 2% - 0.5% = 1.5%; that contributed
by the fine aggregate is, 6% - 0.7% = 5.3%. The estimated
requirement for added water becomes
135 - (1072 x 0.015) - (634 x 0.053) = 85 kg
The estimated batch weights for one cubic meter of con-
crete are revised to include aggregate moisture as follows:
Water (to be added) 85 kg
Cement 435 kg
Coarse aggregate (2% MC, wet) 1093 kg
Fine aggregate (6% MC, wet)
672 kg
Total 2285 kg
Air-entraining admixture 0.218 kg
Water reducer 1.305 kg
Trial Batch.
At this stage, the estimated batch weights
should be checked by means of trial batches or by full-size
field batches. Enough concrete must be mixed for appro-
priate air and slump tests and for casting the three cylin-
ders required for 28-day compressive-strength tests, plus
beams for flexural tests if necessary. For a laboratory trial
batch it is convenient, in this case, to scale down the
weights to produce 0.1 m
3
of concrete as follows:
0.5 x 435 = 218 g or 0.218 kg
The water reducer dosage rate of 3 g per kg of cement
results in
3 x 435 = 1305 g or 1.305 kg of water reducer
per cubic meter of concrete
Fine-Aggregate Content.
At this point, the amounts of
all ingredients except the fine aggregate are known. In the
absolute volume method, the volume of fine aggregate is
determined by subtracting the absolute volumes of the
known ingredients from 1 cubic meter. The absolute
volume of the water, cement, admixtures and coarse
aggregate is calculated by dividing the known mass of
each by the product of their relative density and the den-
sity of water. Volume computations are as follows:
135
1 x 1000
Water
=
=
0.135 m
3
435
3.0 x 1000
Cement
=
=
0.145 m
3
8.0
100
Air
=
= 0.080 m
3
1072
2.68 x 1000
Coarse aggregate
=
= 0.400 m
3
Total volume of known ingredients 0.760 m
3
The calculated absolute volume of fine aggregate is then
1 - 0.76 = 0.24 m
3
The mass of dry fine aggregate is
0.24 x 2.64 x 1000 = 634 kg
The mixture then has the following proportions before
trial mixing for one cubic meter of concrete:
Water 135 kg
Cement 435 kg
Coarse aggregate (dry) 1072 kg
Fine aggregate (dry)
634 kg
Total mass 2276 kg
Air-entraining admixture 0.218 kg
Water reducer 1.305 kg
Slump 75 mm (±20 mm for trial batch)
* (0.5% absorption ÷ 100) + 1 = 1.005
(0.7% absorption ÷ 100) + 1 = 1.007
