Pumped concrete can be used for most structural concrete construction
but is most useful where space for construction equipment is limited or
access is difficult. Concrete pumps can be either truck- or trailer
mounted and range from small units, exerting pressures from 250 to 300
psi and outputs of 15 to 30 yd3/hr, to large units, exerting pressures of 1,000 psi and outputs up to 150 yd3.
The effective capacity of a pump depends not only on the pump itself
but also on the complete system. Several factors including line length,
number of bends in the line, type of line, size of line, height to which
the concrete is being pumped, and the concrete mixture affect the
effective working capacity of a concrete pump. An excellent reference is
ACI 304.2R.
1. Pump lines
Pump lines are usually a combination of rigid pipe and heavy-duty
flexible hose. Acceptable rigid pipe can be made of steel or plastic and
is available in sizes from 3 to 8 in. in diameter. Aluminum alloy pipe
should not be used as pump line. Flexible hose is made of rubber, spiral
wound flexible metal, and plastics. It is useful in curves, difficult
placement areas, and as connections to moving cranes but exhibits
greater line resistance to the movement of concrete than rigid pipe and
may have a tendency to kink. To obtain the least line resistance, the
pipeline should be made up primarily of rigid pipe with flexible hose
only where necessary. If possible, the pipeline should be of one size
and laid out so as to contain a minimum number of bends.
2. Mixture proportions
Concrete mixture proportions of pumpable mixtures are essentially the
same as those to be placed by other methods, except that more emphasis
should be placed on the grading of the fine aggregates. Concretes which
are pumped must be cohesive. Harsh mixtures do not pump well. Pressure
exerted by the pump can force the mortar away from the coarse aggregate
causing a blockage in the line if the mixture is not proportioned
properly. The cement content will generally be somewhat higher for
pumped mixtures than those of mixtures placed by conventional methods.
The higher fine aggregate content will have a higher water demand, which
in turn will require higher cement content. However, extra cement
should not be used to correct pumping deficiencies resulting from poorly
graded aggregates. It is usually more preferable to correct
deficiencies in the fine aggregates by blending in additional fine
aggregates or pozzolana than by adding cement.
3. Coarse aggregates
The nominal maximum size of the coarse aggregate is limited to
one-third of the smallest inside diameter of the pump line for crushed
aggregates or 40 percent of the smallest inside diameter of the pump
line for well-rounded aggregates. Oversize particles should be
eliminated. Higher mortar content will be necessary to effectively pump a
concrete containing crushed aggregates than for a concrete containing
rounded aggregates. Depending upon the type and size of the coarse
aggregate, it may be necessary to reduce the coarse aggregate content
from 5 to 10 percent as compared to mixtures placed by conventional
methods.
4. Fine aggregate
The properties of fine aggregates are more critical in proportioning
pumpable mixtures than are the properties of the coarse aggregates.
Together with the cement and water, the fine aggregates constitute the
mortar which conveys the coarse aggregates in suspension through the
pump line. Fine aggregates should conform to the requirements given in ASTM C 33
for fine aggregates. In addition, for pump systems having lines 6-in.
in diameter and smaller, 15 to 30 percent of fine aggregate should pass
the 300-μm (No. 50) sieve and 5 to 10 percent should pass the 150-μm
(No. 100) sieve. Fine aggregates that are deficient in either of these
two sizes should be blended with selected finer aggregates to produce
the desired grading. Pumpability of concrete is generally improved with a
decrease in the fineness modulus. Fine aggregates having a fineness
modulus between 2.40 and 3.00 are generally satisfactory provided that
the percentages passing the 300- and 150-μm (No. 50 and No. 100) sieves
meet the previously stated guidelines. Fineness modulus values alone
without stipulations on the finer sizes may not produce satisfactory
results. Both manufactured fine aggregates and natural sands can be used
in pumped mixtures provided their gradings are appropriate; however,
natural sands are preferred due to their rounded shape.
5. Slump
The water requirements to establish the optimum slump and to maintain
control of that slump throughout the course of a pumping placement are
both extremely important factors. Concretes having slumps less than 2
in. when delivered to the pump are difficult to pump. Concretes having
slumps over 6 in. can segregate causing a blockage in the pump line and
may require a pump aid to increase the cohesiveness of the concrete to
prevent the aggregate from separating from the mortar during pumping.
It is much more important to obtain a cohesive concrete through
proper proportioning than to try to overcome deficiencies by adding
extra water. In fact, the use of excess water creates more problems than
it solves.
6. Admixtures
Materials which improve workability, such as water-reducing,
high-range water-reducing, and air-entraining admixtures, as well as
pozzolanas, usually improve pumpability. It is common to experience a
decrease in air content during pumping. The specified air contents
required for durability should be obtained at the point of placement in
the structure. Therefore, it may be necessary to entrain a higher air
content into the concrete mixture prior to pumping. Pumping aids are
admixtures which can reduce friction, reduce bleeding, and increase
cohesiveness, all of which make concretes pump easier.
7. Pump-ability tests
There is no standard laboratory test method available to accurately
test the pumpability of a concrete mixture. Testing a concrete mixture
for pumpability involves duplicating anticipated job conditions from
beginning to end. A full-scale field test for pumpability should be
considered to evaluate both the mixture proportions and pumping
equipment. Prior use of a mixture and pumping equipment on another job
may furnish evidence of pumpability if job conditions are duplicated.
8. Planning
Proper planning of the entire pumping operation including pump
location, line layout, placing sequence, and concrete supply will result
in savings of time and expense. The pump should be as near the
placement area as possible. Concrete delivery systems should have easy
access to the pump. Lines from the pump to the placement area should be
made up primarily of rigid pipe and contain a minimum number of bends.
For large placement areas, alternate lines should be laid for rapid
connection when required, and standby power and pumping equipment should
be readily available to replace an initial piece of equipment should a
breakdown occur.
Other requirements
When pumping downward 50 ft or more, an air release valve at the
middle of the top bend will prevent vacuum or air buildup. When pumping
upward, a shutoff valve near the pump will prevent the reverse flow of
concrete during the fitting of cleanup equipment or when working on the
pump. Direct communication should be maintained between the placing crew
and the pump operator. Good communication between the pump operator and
the concrete batch plant is also important. It is desirable to have the
concrete delivery such that the pumping can proceed continuously. When a
delay occurs, it may be difficult to start the concrete moving in the
line again, especially if the delay has been for a considerable length
of time. This critical delay time will depend upon such factors as the
concrete mixture, temperature, length of pipeline, and type of pump. It
may be necessary to clean the line and start again if the delay becomes
extended. A grout or mortar should be used to lubricate the pipeline
anytime pumping is started with clean lines, but it should not be pumped
into the forms.
Quality verification
A high level of quality control must be maintained to provide
assurance that the concrete is of the desired quality. Concrete should
be sampled at both ends of the pumpline to determine what, if any,
changes in the slump, air content, and other concrete properties occur
during pumping. However, the quality of the concrete being placed in the
structure can only be measured at the placement end of the pumpline.