Civil Engineering Reference
In-Depth Information
both) would have been added to reduce slump. Had the
slump been less than required, water and cement in the
appropriate ratio (0.50) would have been added to in-
crease slump. It is important that any additional quantities
be measured accurately and recorded on the data sheet.
Mixture Proportions.
Mixture proportions for a cubic
yard of concrete are calculated in Column 5 of Fig. 9-8 by
using the batch yield (volume) and density (unit weight).
For example, the number of pounds of cement per cubic
yard is determined by dividing 27 cu ft (1 cu yd) by the
volume of concrete in the
batch and multiplying
the result by the number
of pounds of cement in
the batch. The per-
centage of fine aggregate
by weight of total aggre-
gate is also calculated. In
this trial batch, the
cement content was 539
lb per cubic yard and the
fine aggregate made up
33.5% of the total aggre-
gate by weight. The air
content and slump were
acceptable. The 28-day
strength was 4950 psi
(greater than Â). The
mixture in Column 5,
along with slump and air
content limits of 1 in. to 3
in. and 3.5% to 5.5%,
respectively, is now
ready for submission to
the project engineer.
Mixture Adjustments.
To determine the most
workable and econom-
ical proportions, addi-
tional trial batches could
be made varying the per-
centage of fine aggregate.
In each batch the water-
cement ratio, aggregate
gradation, air content,
and slump should re-
main about the same.
Results of four such trial
batches are summarized
in Table 9-14.
Fig. 9-8. Trial mixture data sheet (inch-pound units).
Table 9-14. Example of Results of Laboratory Trial Mixtures (Inch-Pound Units)*
Fine aggregate,
Air content,
Density,
Cement content,
percent of total
Batch no.
Slump, in.
percent
lb/cu ft
3
lb/cu yd
3
aggregate
Workability
1
3
5.4
144
539
33.5
Good
2
2
3
⁄
4
4.9
144
555
27.4
Harsh
3
2
1
⁄
2
5.1
144
549
35.5
Excellent
4
3
4.7
145
540
30.5
Excellent
*Water-cement ratio was 0.50.
