Civil Engineering Reference
In-Depth Information
P
= 100
(M - D)/D
the sample is dry; (5) after burning, the sample is cooled
for a few minutes and weighed; and (6) the percentage of
moisture is then calculated.
When drying equipment is not available a field or
plant determination of surface (free) moisture in fine
aggregate can be made in accordance with ASTM C 70.
The same procedure can be used for coarse aggregate with
appropriate changes in the size of sample and dimensions
of the container. This test depends on displacement of
water by a known mass of moist aggregate; therefore, the
relative density (specific gravity) of the aggregate must be
known accurately.
Electric moisture meters are used in many concrete
batching plants primarily to monitor the moisture content
of fine aggregates, but some plants also use them to check
coarse aggregates. They operate on the principle that the
electrical resistance of damp aggregate decreases as mois-
ture content increases, within the range of dampness
normally encountered. The meters measure the electrical
resistance of the aggregate between electrodes protruding
into the batch hopper or bin. Moisture meters using the
microwave-absorption method are gaining popularity
because they are more accurate than the electric meters.
However, both methods measure moisture contents accu-
rately and rapidly, but only at the level of the probes.
These meters require frequent calibration and must be
maintained properly. The variable nature of moisture
contents in aggregates cause difficulty in obtaining repre-
sentative samples for comparison to electric moisture
meters. Several oven-dried moisture content tests should
be performed to verify the calibration of these meters
before trends in accuracy can be established.
Table 16-1 illustrates a method of adjusting batch
weights for moisture in aggregates.
where
P
= moisture content of sample, percent
M
= mass of original sample
D
= mass of dried sample
The surface (free) moisture can be calculated if the
percentage of absorbed moisture in the aggregate is
known. The surface moisture content is equal to the total
moisture content minus the absorbed moisture. Historic
information for an aggregate source can be used to obtain
absorbed moisture content data if the mineral composition
of the pit or quarry has not changed significantly. How-
ever, if recent data is not available, they can be determined
using methods outlined in ASTM C 127 (AASHTO T 85)
for coarse aggregate and ASTM C 128 (AASHTO T 84) for
fine aggregate.
Only the surface moisture, not the absorbed moisture,
becomes part of the mixing water in concrete. Surface
moisture percentages are used to calculate the amount of
water in the aggregates to reduce the amount of mix water
used for batching. In addition, the batch weight of aggre-
gates should be increased by the percentage of surface
moisture present in each type of aggregate. If adjustments
are not made during batching, surface water will replace a
portion of the aggregate mass and the mix will not yield
properly.
Another method to determine moisture content,
which is not as accurate, is to evaporate the moisture by
burning alcohol. In this method: (1) a measured sample of
damp fine aggregate is placed in a shallow pan; (2) about
310 ml of alcohol for each kilogram (5 oz for each pound)
is poured over the sample; (3) the mixture is stirred with a
rod and spread in a thin layer over the bottom of the pan;
(4) the alcohol is then ignited and allowed to burn until
Table 16-1. Example of Adjustment in Batch Weights for Moisture in Aggregates
Aggregate data
Absorption, %
Moisture content, %
Fine aggregate
1.2
5.8
Coarse aggregate
0.4
0.8
Aggregate mass
Aggregate mass
Mix water correction for
Mix design mass
(SSD condition),**
(in moist condition),
surface moisture in
(aggregates in dry
kg/m
3
(lb/yd
3
)
kg/m
3
(lb/yd
3
)
aggregates, kg/m
3
(lb/yd
3
)
Adjusted
Concrete
BOD condition),*
BOD
•
(Absorbed %)
BOD
•
(Moisture %)
BOD
•
(Moist%-Absorb%)
batch weight,
ingredients
kg/m
3
(lb/yd
3
)
kg/m
3
(lb/yd
3
)
100
100
100
Cement
355
(598)
355
(598)
Fine aggregate
695
(1171)
703
(1185)
735
(1239)
32
(54)
735
(1239)
Coarse aggregate
1060
(1787)
1064
(1793)
1068
(1800)
4
(7)
1068
(1800)
Water
200
(337)
164
(276)
Total
2310
(3893)
36
(61)
2322
(3913)
†
* An aggregate in a bulk-oven dry (BOD) condition is one with its permeable voids completely dry so that it is fully absorbent.
** An aggregate in a saturated, surface-dry (SSD) condition is one with its permeable voids filled with water and with no surface moisture on
it. Concrete suppliers often request mix design proportions on a SSD basis because of batching software requirements.
† Total adjusted batch weight is higher than total mix design weight by the amount of water absorbed in the aggregate.
