Document 6492451

How to produce
high strength concrete
It really takes the best of everything
BY PIERRE-CLAUDE AITCIN, PROFESSOR, DEPARTMENT OF CIVIL
ENGINEERING UNIVERSITY OF SHERBROOKE
T
hough a longtime major aim of the concrete industries has been to provide the least expensive material possible, these industries have also been concerned
with constantly improving their product. New vanguard
producers of ready mixed concrete are directing their
promotion efforts toward commercialization of highstrength concretes (9000 psi and more).
Traditionally concrete has been the predominant material for horizontal construction while steel has taken
most of the vertical construction market. As a matter of
fact, many designers still tend to think of concrete as a
heavy material with an effective top limit on compressive strength of about 4500 psi.2 Yet, if we combine the
use of structural lightweight concrete in horizontal elements with high-strength concrete in vertical elements,
the construction of skyscrapers of a respectable height
can be achieved at a lower cost than with steel design.
In a field that until now has been exclusively reserved for
steel stru c t u re s, the Water Tower Place building in
Chicago, with its 74 floors and 859-foot3 height is surely
one of the most spectacular breakthroughs among all
concrete buildings. Such construction has been made
economically possible only because the designer could
obtain ready mixed concrete having a guaranteed
strength on the jobsite of 9000 psi.1
High-strength concretes offer a technological breakthrough in the building of columns, beams and shear
walls because, with concrete two or three times as strong
as ordinary, the concrete cross-sectional area or the
amount of steel can be reduced, thus lessening the dead
weight of the building, relieving the strain on the lower
columns and foundations, and reducing costs. Reducing
the cross-sectional area of the columns and shear walls
also means slightly increasing the rentable space on
each floor.
Without daring to assert that high-strength concrete
will finally supplant ordinary concrete in most of its applications, one cannot deny that in the near future many
ready mix producers should be prepared to offer their
customers high-strength concretes. This evolution to-
wards the commercialization of high-strength concrete
should become more marked with the constantly growing and irreversible concern for energy conservation.
Even though concrete is a building material with a very
low energy content, the fact remains that in industrialized countries the production of cement consumes great
quantities of energy, so that each time we use a pound
of cement we ought to be certain that we make the most
of its structural potential.
The purpose of this article is to recall some basic principles that could aid the commercialization of highstrength concrete. These are important because highstrength concrete has not been achieved by chance; it is
a concrete in which all the factors that contribute toward
an increase in strength must be maximized whereas
those that can lessen strength must be reduced to a minimum. Developing a high-strength concrete requires intensive research work, the establishment of an efficient
system of quality control and a good knowledge of what
helps and what hinders achieving a concrete of good
quality.
Compressive strength of concrete
Before trying to improve the compressive strength of a
c o n c re t e, it is worth observing the different possible
forms of rupture of a specimen of hardened concrete
subjected to a compression test. The specimen will rupture when the shear tension or strains induced by the
uniaxial compression load reach a critical value in one of
the three following zones:
• in the hydrated cement-sand mortar (Photo 1)
• along the interface between coarse aggregate and hydrated cement-sand mortar (Photo 2)
• in the coarse aggregate (Photo 3)
Consequently, in order to make a high-strength concrete we must improve the concrete strength in these
three zones.
Improvement in strength of cement sand mortar
The hardened cement-sand mortar is composed of
h yd rated cement grains, sand grains and air. The
stronger this hydrated mortar, the stronger the concrete.
As a first step, we must therefore optimize the different
factors which can affect the strength of the mortar .
Photo 1 The hydrated cementsand mortar was the weakest
part of this air-entrained
concrete.
Photo 2. The bond of the aggregate to the
hydrated cement-sand mortar was weak in
this concrete (made with ceramic balls of
1/2 inch17 maximum size to clearly
illustrate the phenomenon.
Photo 3. The aggregate is the
weakest part of this concrete. This is
a close-up view of the concrete of the
broken cylinder shown in photo 6.
Cement
Water
The choice of cement is of utmost importance to a
high-strength concrete because the success of the operation depends mostly on the binding power of the cement used. Experience shows that not all cements are
alike from this point of view, because the standards of
quality which all cements must meet are only minimum
standards and they leave a rather large working latitude
to cement manufacturers.
The cement content will have to be rather high, ranging from 850 to 1000 pounds per cubic yard,4 in order to
increase the proportion of binder in the mortar. Yet the
cement should not exceed such amounts. Too much cement may cause problems by too-rapid liberation of
heat during hydration (which lessens the final strength
of concrete) and problems with too much drying shrinkage. Whenever economically feasible we should try to reduce the cement content and add a good quality fly ash
so that the negative effects of heat liberation and drying
shrinkage are reduced to a minimum. Yet it should be remembered that this change in ingredients always lowers the short-term strength of concrete .
Experience has shown that the only way to find the
best cement to make high-strength concrete is to test
mortar cubes of all the types and brands of cement economically available as a basis for comparing the comp re s s i ve strengths to be expected in the concretes. It
should be remembered, howe ve r, that the compressive
strengths of standard cubes of mortar do not always vary
in the same way as compressive strengths of cylinders
of concrete.
The amount of mixing water used should be minimized; that is, the water-cement ratio should range from
0.28 to 0.30. Still, for such a concrete to be placed easily
in the forms a superplasticizer must be used. It is desirable also to use the Coldest water possible in order to
limit the temperature of Concrete during setting.
Superplasticizers
The commercialization of superplasticizers promises
to greatly improve the production of high-strength concrete; it is now possible to make concrete with a watercement ratio of only 0.28 at a slump of 8 inches.5 It is
necessary to use a superplasticizer at a higher dosage
than recommended by the manufacturer, though not so
high as to adversely affect the final strength of the conc re t e. A field concrete of 14,000 psi6 has thus been delivered in Japan. The ratio of cement to superplasticizer
that will yield the strongest concrete has to be determined experimentally.
Air
Since air voids reduce the effective surface area of the
concrete section that resists compressive stresses, the
amount of air entrapped in the cement mortar during
the mixing and placing of the concrete should be reduced as much as possible. The use of a superplasticizer
makes this easier. (In this article we obviously are not
considering the production of air-entrained highstrength concrete, which has already been discussed in
the article by Weston Hester in the February 1977 issue
Photo 4. The bond of smooth grains of coarse sand,
A, was weak in this high-strength concrete.
Particle A is about the largest that would pass a
Number 418 screen.
Photo 5. Coarse aggregate, A, in this
high-strength concrete ruptured by
slipping along a cleavage plane while
other aggregate, B, was split. These two
particles are about 1/2 inch17 size.
of CONCRETE CONSTRUCTION, page 77.)
Sand
Since the mix is already rich in fines, it is advisable to
use a relatively coarse sand so that the concrete will not
be too stiff. Usually a sand having a fineness modulus of
about 3.0 is suitable. The proportion of sand should be
kept as low as possible, but the mix should not be made
too harsh, which could cause problems when the concrete is placed at the jobsite. Finally, it is worth remembering that the mineralogical composition of the sand
can be of some importance too.
Improvement in cement aggregate bond
The bond strength of cement to sand affects the
strength of the mortar fraction. The bond strength of cement to coarse aggregate affects the concrete. These
bond strengths can be of two kinds: mechanical and
chemical. The surface roughness creates fixing points for
the hydrated cement that help prevent any movement of
either material relative to the other. That is why it is not
advisable to use natural, rounded gravels as coarse aggregate in the production of high-strength concrete
(Photo 4).
In certain circumstances bonding can also be mineralogical when some kind of chemical adhesion forms
between the hydrated cement and the minerals included in the aggregates. It is plain that this adhesion can appreciably augment the mechanical bond strength. On
the other hand, crushed granite aggregates, even if they
are intrinsically quite strong, are not very desirable be-
cause the hydrated cement will not adhere to any extent
to the micaceous and quartz grains present on the surfaces of such aggregate.
It is not enough, howe ve r, simply to improve the bond
of hydrated cement to the aggregate. When the interface of the hydrated cement and aggregate is subjected
to stress, the two materials must yield the same amount
in order to avoid a decrease in the bond strength. Suppose that in the production of concrete we used an aggregate whose Young’s modulus is 11⁄2 times that of the
h yd rated cement. This would mean that, for a given increase in stress, the hydrated cement will be deformed
roughly ll⁄2 times as much as the aggregate, inevitably
leading to the movement of one surface over the other.
Ac c o rd i n g l y, to obtain a high-strength concrete we
should preferably use freshly crushed fine-grained
metamorphic limestones (for the surfaces to be more effective). Metamorphic limestones with grains that are
too coarse can yield too low a strength in the direction
of cleavage planes, very quickly causing rupture of the
aggregate in that direction as shown in Photo 5.
Optimization of coarse aggregate characteristics
The characteristics of coarse aggregates that are of
most importance to high-strength concrete are compressive strength, shape and maximum size.
Compressive strength of coarse aggregate
To make high-strength concrete we must obviously
use coarse aggregate that has a high compressive
strength to prevent rupture from occurring in the coarse
aggregate, as it has done in Photo 3. We must therefore
find coarse aggregates that come from quarries that produce rocks with compressive strengths above 16,500
psi7 and absolutely avoid rocks that are too soft or which
present cleavage planes. So before making laboratory
trial batches, we should determine the compressive
strengths of all the coarse aggregates economically available. Yet, as already noted, it is not necessarily the
strongest coarse aggregate which will produce the
strongest concrete, since the bond of the hydrated cement to that same aggregate must be taken into account.
Shape of coarse aggregate
Because the bond between the coarse aggregate and
the hydrated cement is more of a mechanical type at the
beginning, to make high-strength concrete we ought to
use a cubically shaped crushed stone rather than a natural gravel or a crushed gravel. The type of crusher used
by the aggregate producer is important in this respect.
Fu rt h e rm o re, the surfaces of the coarse aggregate must
be clean and free of any dust which would impair mechanical bonding. In certain cases, washing of the aggregate may prove necessary. Careful examination of aggregate samples from local quarries is sufficient to
choose the coarse aggregate that offers the most useful
characteristics from this point of view.
Photo 6. A champion cylinder (5- by 10-inch).19
Maximum size of coarse aggregate
Curing of concrete
We could show that for a given aggregate there is a relation between its maximum diameter and the maximum compressive strength possible from concrete
made with it. The absolute maximum strength seems to
be obtained with aggregates having a maximum size of
3
⁄8 or 1⁄2 inch.8 Standard coarse aggregates of Number 4
to- 3⁄8-inch 9 or Number 4-to-5⁄8-inch10 sizes are the most
suitable.
The ideal temperature for the curing of concrete
ranges from 50 to 60 degrees F.” The hydration reaction
seems to reach its fullest development in this temperature range. It is obviously very hard to limit the temperature of field concrete to such a low level, particularly
when we deal with cement contents of 850 to 1000
pounds per cubic yard.4 At higher temperatures the heat
of hydration raises the temperature of concrete excessively; this can lead to:
• cracks due to thermal shrinkage if the concrete does
not cool down uniformly
Quality control
It is obvious that after having so carefully chosen his
sources of raw materials, the concrete producer should
control no less closely the constancy of their quality. The
p ro p o rtioning and mixing of a high-strength concrete
obviously requires much more care than that of ordinary
concrete. Once a production procedure has been perfected, we should not depart from it lest we lose the last
psi of strength gained with so much difficulty. Delivery
and placement—particularly placement—should not be
left to partially qualified personnel. As a rule, in order to
control the fabrication, delivery and placing of a highstrength concrete, it should be sufficient to increase the
frequency of the routine tests normally performed (air
content, slump, temperature, unit weight) in the quality
control of ordinary concretes so as to immediately detect the least variation and apply the necessary correction.
• tension strains during cooling at the aggregate-hydrated cement interface if the aggregate and cement
have different coefficients of thermal expansion
• a too-rapid and incomplete hydration of cement
grains
Once concrete has hardened, its temperature should
be controlled as much as possible by adequately spraying or immersing it.
Measurement of compressive strength
To measure the compressive strength of high-strength
concrete it is best to use rigid steel cylinder molds. These
molds enable us to produce cylinders whose compressive strengths are 10 or 15 percent higher than those obtained with cardboard cylinder molds—and the results
are more reproducible.
The rigidity of the walls of steel cylinder molds ensures better reproducibility of the compaction of the
concrete and less deformation of the mold walls during
Casting. The Capping material should be strong enough
and the Caps as thin as possible so that it does not alter
the test results. The Compression test machine should
have a Capacity at least twice the maximum load at
which the Concrete cylinders will break.
Design strength
When high-strength concretes are to be used, it is desirable to make the designer aware that design strength
should be based on the compressive strength at 56 or
even 90 days rather than 28 days. Very few concrete
structures are subjected to loads equivalent to their full
design capacity before such dates.
This lengthening of the period of evaluation of concrete is in harmony with energy conservation since a
greater saving can be derived from using later-age
strength data, thus using most efficiently the amount of
energy necessary to make cement. The ratio, psi obtained/Btu spent,l2 can thus be increased.
Conclusion
These reflections do not offer any miraculous solution
for the production of concrete of 9000 psi1 and higher,
for only an intensive research program and an efficient
system of quality control can enable a concrete producer to obtain such concretes. Howe ve r, having specified
the main points on which research work and quality
control should be based, we hope to help concrete producers who are willing to launch into such an enterprise
and at the same time show the most skeptical of them
that after all it is not difficult to make a concrete of 9000
psi’ when you know how to manage it.
As proof, the writer, as part of an underg ra d u a t e
course at Sherbrooke University, organizes a contest for
his students each year for the purpose of obtaining the
strongest concrete possible with a minimum slump of 4
inches13 and a maximum cement content of 1000
pounds per cubic yard. 14 During the 5 years this competition has taken place, all the students have easily
gone beyond the desired 9000 psi, the absolute record
being 14,390 psi15 at 42 days, slightly higher than the
previous record of 14,350 psi16 illustrated in Photo 6.
The students, who are all new in the field, only apply studiously the few principles developed above.
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