Figure 17-2.—Wet sand.
Figure 17-3.—Very wet sand.
The procedure for adjusting the mixing water
caused by free surface moisture is as follows:
. Determine the approximate FSM of the fine
aggregate by the squeeze test.
. Estimate the FSM of the coarse aggregate by
observation. Usually, 2-percent FSM is the maximum
amount gravel will hold without actually dripping.
. Multiply the percentages of FSM on the
aggregates by their respective weights per cubic yard.
This will yield the weight of the FSM on the aggregates.
. Divide the total weight of the FSM by 8.33
pounds per gallon to determine the number of gallons
of water. Subtract those gallons from the mixing water
requirements in the original mix design.
. If you are batching your concrete mix by weight,
you need to account for the weight contributed by the
FSM by increasing the total weights of the aggregates
per cubic yard by the weights of the FSM.
Example Problem: Using the final mix proportions
as determined, adjust the design mix to account for
6-percent FSM on the fine aggregate (FM = 2.70) and
2-percent FSM on the coarse aggregate. Original mix
design for a 1-cubic-yard trial batch was
Cement:
6.49 sacks (Type IA)
Water:
36.6 gallons
Coarse aggregate:
1,735.0 pounds
Fine aggregate:
1,153.0 pounds
Air content:
5.0 percent
Step 1. Determine the amount of water (in gallons)
on the coarse and fine aggregate.
l
l
l
Coarse aggregate
= 1,735 x 0.02 = 34.70 pounds
Fine aggregate
= 1,153 x 0.06 = 69.18 pounds
Total weight of water
= 103.88 pounds
Converted to gallons
= 12.47 gallons
Step 2. Adjust the original amount of mixing water
by subtracting the amount of water contributed by the
aggregates. The adjusted water requirement then is
24.13 gallons (36.6 - 12.47).
Step 3. Adjust the weights of the aggregates by the
amount contributed by the water.
. Coarse aggregate
= 1,735 + 34.7
. Fine aggregate
= 1,153 + 69.18
= 1,770 pounds
= 1,222 pounds
Step 4. The adjusted mix design to account for the
actual field conditions is now
Cement:
6.49 sacks (Type IA)
Water:
24.13 gallons
Coarse aggregate:
1,770.0 pounds
Fine aggregate:
1,122.0 pounds
Air content:
5.0 percent
You should check the moisture content of the aggregates
and make appropriate adjustments as conditions change
(such as after rains, after periods of dryness, or when the
new material arrives). This quality control step assures
that the desired concrete is produced throughout the
construction phase.
Materials Estimation
After proportioning the mix, you must estimate the
total amount of material needed for the job. This is
simply done by computing the total volume of concrete
17-9
