28-09-2012, 12:17 PM
Structural Concrete Design
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Properties of Concrete
Workability is the ease with which the ingredients can be mixed and the resulting mix handled, transported,
and placed with little loss in homogeneity. Unfortunately, workability cannot be measured
directly. Engineers therefore try to measure the consistency of the concrete by performing a slump
test.
The slump test is useful in detecting variations in the uniformity of a mix. In the slump test, a mold
shaped as the frustum of a cone, 12 in. (305 mm) highwith an 8 in. (203 mm) diameter base and 4 in.
(102 mm) diameter top, is filled with concrete (ASTM Specification C143). Immediately after filling,
the mold is removed and the change in height of the specimen is measured. The change in height of
the specimen is taken as the slump when the test is done according to the ASTM Specification.
Awell-proportionedworkable mix settles slowly, retaining its original shape. Apoor mix crumbles,
segregates, and falls apart. The slump may be increased by adding water, increasing the percentage of
fines (cement or aggregate), entraining air, or by using an admixture that reduces water requirements;
however, these changes may adversely affect other properties of the concrete. In general, the slump
specified should yield the desired consistency with the least amount of water and cement.
Concrete should withstand the weathering, chemical action, and wear to which it will be subjected
in service over a period of years; thus, durability is an important property of concrete. Concrete
resistance to freezing and thawing damage can be improved by increasing the watertightness, entraining
2 to 6% air, using an air-entraining agent, or applying a protective coating to the surface.
Chemical agents damage or disintegrate concrete; therefore, concrete should be protected with a
resistant coating. Resistance to wear can be obtained by use of a high-strength, dense concrete made
with hard aggregates.
Lightweight Concrete
Structural lightweight concrete is usually made from aggregates conforming to ASTM C330 that are
usually produced in a kiln, such as expanded clays and shales. Structural lightweight concrete has a
density between 90 and 120 lb/ft3 (1440 to 1920 kg/m3).
Production of lightweight concrete is more difficult than normal-weight concrete because the
aggregates vary in absorption of water, specific gravity, moisture content, and amount of grading of
undersize. Slump and unit weight tests should be performed often to ensure uniformity of the mix.
During placing and finishing of the concrete, the aggregates may float to the surface. Workability
can be improved by increasing the percentage of fines or by using an air-entraining admixture to
incorporate 4 to 6% air. Dry aggregate should not be put into the mix because it will continue to
absorb moisture and cause the concrete to harden before placement is completed. Continuous water
curing is important with lightweight concrete.
No-fines concrete is obtained by using pea gravel as the coarse aggregate and 20 to 30% entrained
air instead of sand. It is used for low dead weight and insulation when strength is not important.
This concrete weighs from 105 to 118 lb/ft3 (1680 to 1890 kg/m3) and has a compressive strength
from 200 to 1000 psi (1 to 7 MPa).
Heavyweight Concrete
Heavyweight concretes are used primarily for shielding purposes against gamma and x-radiation
in nuclear reactors and other structures. Barite, limonite and magnetite, steel punchings, and steel
shot are typically used as aggregates. Heavyweight concretes weigh from 200 to 350 lb/ft3 (3200 to
5600 kg/m3) with strengths from 3200 to 6000 psi (22 to 41 MPa). Gradings and mix proportions
are similar to those for normal weight concrete. Heavyweight concretes usually do not have good
resistance to weathering or abrasion.
Proportioning Concrete Mix
Aconcrete mix is specified by theweight of water, sand, coarse aggregate, and admixture to be used per
94-pound bag of cement. The type of cement (Table 4.4), modulus of the aggregates, and maximum
size of the aggregates (Table 4.5) should also be given. A mix can be specified by the weight ratio of
cement to sand to coarse aggregate with the minimum amount of cement per cubic yard of concrete.
In proportioning a concrete mix, it is advisable to make and test trial batches because of the many
variables involved. Several trial batches should be made with a constant water-cement ratio but
varying ratios of aggregates to obtain the desired workability with the least cement. To obtain results
similar to those in the field, the trial batches should be mixed by machine.
When time or other conditions do not allow proportioning by the trial batch method, Table 4.6
may be used. Startwith mix B corresponding to the appropriate maximumsize of aggregate. Add just
enough water for the desired workability. If the mix is undersanded, change to mix A; if oversanded,
change to mix C. Weights are given for dry sand. For damp sand, increase the weight of sand 10 lb,
and for very wet sand, 20 lb, per bag of cement.
Admixtures
Admixtures may be used to modify the properties of concrete. Some types of admixtures are set
accelerators, water reducers, air-entraining agents, and waterproofers. Admixtures are generally
helpful in improving quality of the concrete. However, if admixtures are not properly used, they
could have undesirable effects; it is therefore necessary to know the advantages and limitations of the
proposed admixture. The ASTM Specifications cover many of the admixtures.