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How to avoid problems with
lightweight mixes
Don’t despair . . .
lightweight concrete is pumped successfully by those who know the rules
I
t could easily be inferred from an
account like that in the article just
preceding that pumped lightweight
concrete is inferior or that successful pumping of lightweight concrete
cannot be assured ahead of time.
This is not so; the principles of successful pumping are well established and widely practiced.
There appear to have been a large
number of possible inadequacies in
the pumping operation described,
including incompletely saturated
aggregate, poor aggregate gradation, high air content, too small a
line and an inadequate pump. Unfortunately, these and other possibilities cannot all be positively identified with respect to that particular
job. When the troubles began to occur the measures taken to alleviate
them were not effective in delivering concrete of the design strength
because they did not get to the root
of the problem. It seems likely that
the only real assurance of success in
such an operation would have
called for attention to a whole group
of well established factors which
this and the final article in this series
will outline.
Pumping improves quality
When mixes are properly designed pumping does change the
properties of concrete, and for the
better, as shown by tests reported by
the Virginia Highway Research
Council.* These show that pump
transit alone produced strength increases in normal weight concrete
as high as 14 percent as measured
by careful tests on the same concrete before and after pumping hor-
izontally through 600 feet (180 metres) of line. These high increases
were obtained even though the mixes contained the arbitrary extra 10
percent of coarse aggregate permitted in ACI 211.1-70 for paving mixes.
More workable mixes usually show
smaller gains in strength but gains
are the rule. Furthermore, on large
jobs where the coefficient of variation has been calculated it has been
found that the variability between
batches has been decreased by
pumping. These gains are the logical results of the remixing, consolidation and homogenizing effects involved.
Mix design for pumping
Pumpable concrete starts with
the aggregate. It is the combined
gradation of the aggregates and
their residual moisture absorption
that are most important.
When an experienced engineer
designs a mix for pumping he will
take into consideration the surface
texture of the aggregate, its void
content, water content, water absorption and gradation. For a lightweight pumping mix he will usually
increase the quantity of fine aggregate, the cement and water contents
and will incorporate admixtures.
A pumpable lightweight mix may
differ from one that is not pumpable
in having a higher plastic unit
weight, especially if more natural
sand is used. It may have a higher
slump as it enters the pump and its
dry unit weight at 56 days is greater
by about two to five pounds per cubic foot (32 to 80 kilograms per cubic metre).
Gradation of fine aggregates
A mix with natural sand graded
within the limits of ASTM C 33 or
lightweight sand within the limits of
ASTM C 330 will not necessarily be
pumpable. It is most desirable for it
to have a fineness modulus between
2.2 and 2.7 and have a grading
preferably between the middle and
upper limits shown in Figure 1. Unfortunately, few aggregates fall entirely within the range, so the supplier must rely on experience and
good judgment.
Even more important than fineness modulus is that 15 to 30 percent of the fine aggregate should
pass the Number 50 screen and five
to 10 percent should pass the Number 100 screen.
Size of coarse aggregate
The maximum size of coarse aggregate is generally controlled by
job specifications and the pipe diameter must be large enough to
handle it. Larger percentages of
sand must be used when coarse aggregate size is reduced as shown in
Figures 2 to 4. These graphs show
the ranges of grading needed for
pumpability when using combined
aggregate of various maximum
sizes. The grading curve should be
as smooth as possible.
Volume of coarse aggregate
The next consideration is the volume of coarse aggregate that can be
pumped. A footnote to Table 5.3.6 of
ACI 211.1-74, “Recommended Practice for Selecting Proportions for
Normal and Heavyweight Concrete,” allows reduction of the
Figure 1. Recommended grading of fine aggregate for concrete pumping mixes
Figure 2. Recommended grading of coarse aggregate for
pumping mixes when maximum size is 3/8 inch
coarse aggregate content (of hard
rock concrete) by as much as 10 percent to provide more workable concrete, when required, for pumping.
Nevertheless, as noted later, if the
gradation is poor it should first be
improved. How much reduction
should be made can be determined
only by experience with the aggregates and with the particular pump
being used. Some pumps can accept more coarse aggregate than
others without blocking because of
differences in design of the machine, the valving system and the
amount of reduction in the pipe. A
mix that may be capable of being
pumped through straight pipe may
be impossible to move through a reducer.
Designing the mix
Figure 3. Recommended grading of coarse aggregate for pumping
mixes when maximum size is 3/4 inch
Figure 4. Recommended grading of coarse aggregate for
pumping mixes when maximum size is one inch
ACI 211.2, “Recommended Practice for Selecting Proportions for
Structural Lightweight Concrete,” is
not a recipe, but a “method for selecting and adjusting mix proportions” [italics ours]. Depending on
the competence and available time
of the mix designer and inspector,
the trial mix of 211.2 can be improved in strength-related properties and economy by making a series of adjustments invoking all
available experience with the particular materials. Adjustments that
improve pumpability may also improve strength and economy.
ACI 211.2 relates primarily to solid volumes and makes no provision
for the myriad other qualities of the
mix that affect pumping results materially, such as variations in gradation, particle shape, surface texture,
porosity, surface area and number
of voids, particularly in the fine aggregates. The needed mix improvements, described at some length in
the ACI 304 report titled “Placing
Concrete by Pumping Methods” go
beyond the brief basics of ACI 211.2
How not to modify the mix
The ACI 304 report states that the
addition of water does very little if
anything to improve pumpability
and may cause segregation. It is far
better to find and correct deficiencies in the ingredients than to succumb to the costly and useless addition of water.
Addition of sand is not likely to
help the mix if the sand is poorly
graded, too coarse, too fine or of
poor shape, such as 100-percentcrusher product. It is far better to
improve the sand by blending or
classifying or by making additions
to fill the gaps in grading. The less
sand used the less the area that
must be coated with paste to improve workability and pumpability.
Prewet the aggregate
To make lightweight aggregate
concrete pumpable it is essential to
prewet, presoak or presaturate the
lightweight particles so that they
will not take water out of the mix
during pumping and make it less
pumpable. The wide variation in
porosity of structural lightweight
aggregates can cause a range of 12
to 50 percent in their ability to absorb water.
A comparison of typical values of
water absorption for two particular
aggregates obtained by various wetting methods is given in Table I. The
differences may more than double
the absorption from one method to
another. This extremely important
fact demonstrates that an aggregate
may seem to have taken up a maximum of water by some kinds of
treatment yet be capable of absorbing more.
It was undoubtedly incomplete
saturation that caused the 11⁄2-inch
(3.8-centimetre) decrease in slump
between pump and deck early in the
job cited by Mr. Hersey. This decrease indicates that the aggregate
was probably absorbing about 11⁄2
gallons of water per cubic yard (7.42
kilograms per cubic metre) of mix.
Mr. Hersey also reported that the
concrete later lost as much as four
inches slump between the pump
and the end of the pipeline, even
though the lightweight aggregate
pile was kept wet. The presence of
surface moisture on lightweight aggregate does not assure that the in-
TABLE I Water absorptions of two commercial lightweight
aggregates under various conditions.
(Percentages are percent by weight of aggregate.)
Method
Absorption, percent
Aggregate
Aggregate
A
B
24-hour absorption
(ASTM C 127)
6-8
–
Spray absorption
for five days
12-16
–
Soaking,
inundating
or premixing
–
24
Hydrothermal
saturation
at 350°F (177°C)
16-20
–
Vacuum saturation
20-30
49
terior is completely wetted. Nor
does surface moisture have anything to do with the amount of water which can be absorbed under
the pressure of pumping. On the job
described either the designer of the
mix or the supplier of the concrete
appears to have been in error in not
requiring an internally saturated aggregate.
Common saturation procedures
Experience shows that from two
to three days of sprinkling may be
required when presoaking lightweight aggregate for pumping. During this time sprinkling should be
discontinued whenever free water
appears at the base of the material
pile but should be resumed and repeated periodically as long as the
aggregate will take on water. The total water that the presoaked aggregate will finally contain should be at
least as much or more than the 24hour absorption obtained by the
standard ASTM test methods C 127
or C 128.
Although a few lightweight aggregates can be readily pumped after
thorough sprinkling or soaking, and
a few without any special treatment
at all, these procedures for saturat-
ing lightweight aggregate are usually only marginally successful.
Treat aggregate at source
Pretreatment by the aggregate
producer, using hydrothermal or
vacuum methods at the manufacturing plant almost guarantees freedom from the trouble caused by water being absorbed from the
concrete mix. The pumping of lightweight aggregate concrete should
not be attempted, especially if line
pressures may exceed 150 psi (10.5
kilograms force per square centimetre), unless either the aggregates are
pretreated or alternative procedures
can be proven to be effective on the
specific job.
For many but not all lightweight
aggregates such plant treatment can
be successful only if done when the
material is thoroughly dry. Attempts
to treat such aggregates after storage, shipment, rain and air drying
are destined in most cases to produce erratic results. The vacuum
saturation method should be used
only on “bone-dry” coarse aggregate. The process is complete in 30
to 45 minutes.
Vacuum-saturated lightweight
aggregate concrete responds to
pumping much the same as hard
rock concrete. Hydrothermal saturation, which is also highly effective,
has the advantage of saturating the
lightweight fines. Concretes made
with such treated aggregates pump
superbly with no loss of slump, with
routinely predictable ease and with
strengths actually higher than those
with nonpumped concrete. At the
same time, a net savings in pumping costs is experienced.
In several parts of the United
States, both north and south, lightweight aggregate suppliers now offer aggregates treated by one of
these two methods. The economics
of pretreatment are such that it is
practical even for a medium-sized
project such as an apartment house.
For small volumes a portable vacuum processing rig has been successfully used.
Saturated aggregate makes
heavier plastic concrete
The more complete the saturation, the heavier the aggregate and
the heavier the unit weight of the
fresh plastic concrete. This internal
water will be available for curing but
will slowly evaporate until the concrete is air dry. Use of the heaviest
unit weight allowed by the specifications will permit the concrete to be
designed with the least amount of
lightweight aggregate and thus will
improve its pumpability.
Cement content, pumping aids
and air entraining agents
When pumping nonprocessed
lightweight aggregate concrete consideration should be given to the
use of additional cement in the mix
due to the higher sand content or
higher slumps that may be used. No
less than 517 pounds of cement per
cubic yard (307 kilograms per cubic
metre) should ever be used. When a
height above five floors is reached
the cement content should be at
least 564 pounds per cubic yard,
and if the concrete is to be pumped
higher than 10 stories another 25 to
50 pounds of cement per cubic yard
(15 to 30 kilograms per cubic metre)
should be added to limit the slump
loss in the line to two inches (five
centimeters). The apparent watercement ratio is maintained constant.
Use of a pumping aid should also
be considered when pumping nonprocessed lightweight aggregate.
This can be fly ash or natural pozzolan, if recommended by the con-
PUBLICATION #C750234
Copyright © 1975, The Aberdeen Group
All rights reserved
crete supplier, or it can be an admixture. Pumping aids are not intended to compensate for excessive aggregate
porosity
or
water
absorption, however. Use of an air
entraining agent sufficient to obtain
four to seven percent air also helps.
Too much air in the mix may give
problems similar to the air within
the aggregates; pressure rebounding
effects have been noted.
Slump
Slump affects the frictional resistance of the pumping mix. Mixes
can be pumped most successfully at
slumps between three and seven
inches. Low slumps cause high
pumping pressures and require a
pump that is capable of putting on
adequate pressure to move the concrete. High slumps may cause segregation and blockage.
*Newlon, Howard, Jr., and Ozol,
Michael, A., “Delayed Expansion of
C o n c rete Delivered by Pumping
Through Aluminum Pipeline,” Conc rete Case Study Number 20, Vi rginia Highway Research Council,
Charlottesville, Vi rginia, October
1969, 3 pages.