Civil Engineering Reference
In-Depth Information
paste bond. Washing of coarse aggregates may be neces-
sary. Combining single sizes of aggregate to produce the
required grading is recommended for close control and
reduced variability in the concrete.
The quantity of coarse aggregate in high-strength
concrete should be the maximum consistent with
required workability. Because of the high percentage of
cementitious material in high-strength concrete, an in-
crease in coarse-aggregate content beyond values recom-
mended in standards for normal-strength mixtures is
necessary and allowable.
In high-rise buildings and in bridges, the stiffness of
the structure is of interest to structural designers. On cer-
tain projects a minimum static modulus of elasticity has
been specified as a means of increasing the stiffness of a
structure (Fig. 17-5). The modulus of elasticity is not neces-
sarily proportional to the compressive strength of a con-
crete. There are code formulas for normal-strength
concrete and suggested formulas for high-strength con-
crete. The modulus achievable is affected significantly by
the properties of the aggregate and also by the mixture
proportions (
Baalbaki and others 1991
). If an aggregate has
the ability to produce a high modulus, then the optimum
modulus in concrete can be obtained by using as much of
this aggregate as practical, while still meeting workability
and cohesiveness requirements. If the coarse aggregate
being used is a crushed rock, and manufactured fine aggre-
gate of good quality is available from the same source, then
a combination of the two can be used to obtain the highest
possible modulus.
Due to the high amount of cementitious material in
high-strength concrete, the role of the fine aggregate
(sand) in providing workability and good finishing char-
acteristics is not as crucial as in conventional strength
mixes. Sand with a fineness modulus (FM) of about 3.0—
considered a coarse sand—has been found to be satisfac-
tory for producing good workability and high
compressive strength. For specified strengths of 70 MPa
(10,000 psi) or greater, FM should be between 2.8 and 3.2
and not vary by more than 0.10 from the FM selected for
the duration of the project. Finer sand, say with a FM of
between 2.5 and 2.7, may produce lower-strength, sticky
mixtures.
these trial batches, it will be possible to determine the
workability, setting time, and amount of water reduction
for given admixture dosage rates and times of addition.
The use of air-entraining admixtures is not necessary
or desirable in high-strength concrete that is protected
from the weather, such as interior columns and shearwalls
of high-rise buildings. However, for bridges, concrete piles,
piers, or parking structures, where durability in a freeze-
thaw environment is required, entrained air is mandatory.
Because air entrainment decreases concrete strength of rich
mixtures, testing to establish optimum air contents and
spacing factors may be required. Certain high-strength
concretes may not need as much air as normal-strength
concrete to be frost resistant.
Pinto and Hover (2001)
found
that non-air-entrained, high-strength concretes had good
frost and deicer-scaling resistance at a water to portland
cement ratio of 0.25.
Burg and Ost (1994)
found good frost
resistance with non-air-entrained concrete containing silica
fume at a water to cementing materials ratio of 0.22 (Mix
No. 4 in Table 17-5); however, this was not the case with
other mixtures, including a portland-only mixture with a
water to cement ratio of 0.28.
Proportioning
The trial mixture approach is best for selecting propor-
tions for high-strength concrete. To obtain high strength, it
is necessary to use a low water to cementing materials
ratio and a high portland cement content. The unit
strength obtained for each unit of cement used in a cubic
meter (yard) of concrete can be plotted as strength effi-
ciency to assist with mix designs.
The water requirement of concrete increases as the
fine aggregate content is increased for any given size of
coarse aggregate. Because of the high cementing materials
content of these concretes, the fine aggregate content can
be kept low. However, even with well-graded aggregates,
a low water-cementing materials ratio may result in con-
crete that is not sufficiently workable for the job. If a
superplasticizer is not already being used, this may be
the time to consider one. A slump of around 200 mm
(8 in.) will provide adequate workability for most appli-
cations.
ACI Committee 211 (1998)
,
Farny and Panarese
(1994)
, and
Nawy (2001)
provide additional guidance on
proportioning.
Admixtures
The use of chemical admixtures such as water reducers,
retarders, high-range water reducers or superplasticizers
is necessary. They make more efficient use of the large
amount of cementitious material in high-strength concrete
and help to obtain the lowest practical water to cementing
materials ratio. Chemical admixture efficiency must be
evaluated by comparing strengths of trial batches. Also,
compatibility between cement and supplementary ce-
menting materials, as well as water-reducing and other
admixtures, must be investigated by trial batches. From
Mixing
High-strength concrete has been successfully mixed in
transit mixers and central mixers; however, many of these
concretes tend to be sticky and cause build-up in these
mixers. Where dry, uncompacted silica fume has been
batched into a mix, “balling” of the mix has occurred and
mixing has been less than complete. In such instances it
has been found necessary to experiment with the
sequence in which solids and liquids are added, and the
