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) high with
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.
A
well-proportioned workable mix settles slowly, retaining its original shape. A
poor 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 water tightness,
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.
Excess
water leaves voids and cavities after evaporation, and water can penetrate or
pass through
the
concrete if the voids are interconnected. Water tightness can be improved by
entraining air or
reducing
water in the mix, or it can be prolonged through curing.
Volume
change of concrete should be considered, since expansion of the concrete may
cause buckling and drying shrinkage may cause cracking. Expansion due to
alkali-aggregate reaction can be avoided by using nonreactive aggregates. If
reactive aggregates must be used, expansion may be reduced by adding pozzolanic
material (e.g., fly ash) to the mix. Expansion caused by heat of hydration of
the cement can be reduced by keeping cement content as low as possible; using
Type IV cement; and chilling the aggregates, water, and concrete in the forms.
Expansion from temperature increases can be reduced by using coarse aggregate
with a lower coefficient of thermal expansion.
Drying
shrinkage can be reduced by using less water in the mix, using less cement, or
allowing adequate moist curing. The addition of pozzolans, unless allowing a
reduction in water, will increase drying shrinkage. Whether volume change
causes damage usually depends on the restraint present; consideration should be
given to eliminating restraints or resisting the stresses they may cause [8].
Strength
of concrete is usually considered its most important property. The compressive
strength
at
28 d is often used as a measure of strength because the strength of concrete
usually increases with time. The compressive strength of concrete is determined
by testing specimens in the form of standard cylinders as
specified in ASTM Specification C192 for research testing or C31 for field
testing. The test procedure is given in ASTM C39. If drilled cores are used,
ASTM C42 should be followed.