PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 250

of the fine aggregate required. The volume of the fine aggregate is then con-
verted to a weight using the unit weight. Generally, the bulk SSD specific
gravity of aggregates is used for the weight-volume conversions of both fine
and coarse aggregates.

10. Moisture Corrections Mix designs assume that water used to hydrate
the cement is the free water in excess of the moisture content of the aggre-
gates at the SSD condition (absorption), as discussed in Chapter 5. There-
fore, the final step in the mix design process is to adjust the weight of
water and aggregates to account for the existing moisture content of the ag-
gregates. If the moisture content of the aggregates is more than the SSD
moisture content, the weight of mixing water is reduced by an amount
equal to the free weight of the moisture on the aggregates. Similarly, if the
moisture content is below the SSD moisture content, the mixing water
must be increased.

11. Trial Mixes After computing the required amount of each ingredient, a
trial batch is mixed to check the mix design. Three
cylinders are made, cured for 28 days, and tested for com-
pressive strength. In addition, the air content and slump of fresh concrete
are measured. If the slump, air content, or compressive strength does not
meet the requirements, the mixture is adjusted and other trial mixes are
made until the design requirements are satisfied.
Additional trial batches could be made by slightly varying the material
quantities in order to determine the most workable and economical mix.

0.15 m * 0.30 m

(6 in. * 12 in.)

Sample Problem 7.3

You are working on a concrete mix design that requires each cubic yard of concrete to
have a 0.43 water-cementitious materials ratio, of dry gravel, of
water, and 4% air content. The available gravel has a specific gravity of a
moisture content of 2.3% and absorption of 4.5%. The available sand has a specific
gravity of a moisture content of 2.2% and absorption of 1.7%. Air entrainer
is to be included using the manufacturers specification of 0.1 fl. oz / 1% air / 100 lb ce-
ment.
For each cubic yard of concrete needed on the job, calculate the weight of cement,
moist gravel, moist sand, and water that should be added to the batch. Summarize and
total the mix design when finished.

2077 lb/yd 3 244 lb/yd 3

G gravel = 2.6,

G sand = 2.4,

Solution

567 lb/yd 3 Step 7: W cement = 244/0.43 =

Step 8: air entrainer = 1 0.1 21 4 21 567/100 2 = 2.27 fl. oz

62.4 lb/ft 3

3

1684.8 lb/yd 3

Step 9: g w = 1 3 ft/yd 2 =