Civil Engineering Reference
In-Depth Information
cement being considered for the project. Amounts will
vary depending on target strengths. Other than decreases
in sand content as cement content increases, the trial
mixtures should be as nearly identical as possible.
Supplementary Cementing Materials
Fly ash, silica fume, or slag are often mandatory in the pro-
duction of high-strength concrete; the strength gain
obtained with these supplementary cementing materials
cannot be attained by using additional cement alone. These
supplementary cementing materials are usually added at
dosage rates of 5% to 20% or higher by mass of cementing
material. Some specifications only permit use of up to 10%
silica fume, unless evidence is available indicating that
concrete produced with a larger dosage rate will have sat-
isfactory strength, durability, and volume stability. The
water-to-cementing materials ratio should be adjusted so
that equal workability becomes the basis of comparison
between trial mixtures. For each set of materials, there will
be an optimum cement-plus-supplementary cementing
materials content at which strength does not continue to
increase with greater amounts and the mixture becomes
too sticky to handle properly. Blended cements containing
fly ash, silica fume, slag, or calcined clay can be used to
make high-strength concrete with or without the addition
of supplementary cementing materials.
Fig. 17-5. The Two Union Square building in Seattle used
concrete with a designed compressive strength of 131 MPa
(19,000 psi) in its steel tube and concrete composite
columns. High-strength concrete was used to meet a design
criteria of 41 GPa (6 million psi) modulus of elasticity.
(59577)
tion and honeycomb. Superplasticizing admixtures are
invariably added to HPC mixtures to produce workable
and often flowable mixtures.
Production of high-strength concrete may or may not
require the purchase of special materials. The producer
must know the factors affecting compressive strength and
know how to vary those factors for best results. Each
variable should be analyzed separately in developing a
mix design. When an optimum or near optimum is estab-
lished for each variable, it should be incorporated as the
remaining variables are studied. An optimum mix design
is then developed keeping in mind the economic advan-
tages of using locally available materials. Many of the
items discussed below also apply to most high-perform-
ance concretes.
Aggregates
In high-strength concrete, careful attention must be given
to aggregate size, shape, surface texture, mineralogy, and
cleanness. For each source of aggregate and concrete
strength level there is an optimum-size aggregate that
will yield the most compressive strength per unit of
cement. To find the optimum size, trial batches should be
made with 19 mm (
3
⁄
4
in.) and smaller coarse aggregates
and varying cement contents. Many studies have found
that 9.5 mm to 12.5 mm (
3
⁄
8
in. to
1
⁄
2
in.) nominal
maximum-size aggregates give optimum strength.
In high-strength concretes, the strength of the aggre-
gate itself and the bond or adhesion between the paste and
aggregate become important factors. Tests have shown that
crushed-stone aggregates produce higher compressive
strength in concrete than gravel aggregate using the same
size aggregate and the same cementing materials content;
this is probably due to a superior aggregate-to-paste bond
when using rough, angular, crushed material. For specified
concrete strengths of 70 MPa (10,000 psi) or higher, the
potential of the aggregates to meet design requirements
must be established prior to use.
Coarse aggregates used in high-strength concrete
should be clean, that is, free from detrimental coatings of
dust and clay. Removing dust is important since it may
affect the quantity of fines and consequently the water
demand of a concrete mix. Clay may affect the aggregate-
Cement
Selection of cement for high-strength concrete should not
be based only on mortar-cube tests but should also
include tests of comparative strengths of concrete at 28, 56,
and 91 days. A cement that yields the highest concrete
compressive strength at extended ages (91 days) is prefer-
able. For high-strength concrete, a cement should produce
a minimum 7-day mortar-cube strength of approximately
30 MPa (4350 psi).
Trial mixtures with cement contents between 400 and
550 kg/m
3
(675 to 930 lb/yd
3
) should be made for each
