Multiquip ST70 - Manual (Page 12)

PAGE 12 — ST-70 — PARTS & OPERATION MANUAL — REV. #4 (05/29/07)
ST-70 — GENERAL INFORMATION
CONCRETE MIX DESIGN
Mix design is most important to achieve maximum pumpability.
Pumpability is affected by, among other factors, the type and
gradation of aggregate used. Natural aggregates make a more
workable mix and pump more readily than crushed aggregates.
A blend of natural and crushed aggregates will produce a
workable mix. The type and gradation of aggregates is equally
important for workability as the size and percentage of coarse
aggregates in the mix.
The term “aggregates” describes all of the solid materials, from
the largest rock to the smallest grain of sand, contained in the
concrete mix.
Concrete mixes with a consistency as dry as one-inch slump
and as wet as ten-inch slump have been pumped; but for
maximum efficiency from the pump, a slump ranging from two
to six inches will produce a more workable mix than one that
contains more or less water.
A slump rating should be used with discretion; it is not always a
real indication of the pumpability of the mix. The concrete may
be workable in the sense that it will readily flow into place, but
the same mix may not respond to pressure. Overly wet mixes
tend to separate. In addition to affecting the strength and quality
of the concrete, the delivery system will not tolerate separation.
Overly dry mixes are similarly unsatisfactory if they lack plasticity
and tend to be crumbly. To be properly pumped, the mix must be
able to continuously coat the inside of the line with a lubricating
seal of mortar.
The principle of concrete pumping is based on self-lubrication.
As it moves through the transfer line, the concrete takes the
shape of a plastic cylinder. It is forced through the transfer line
on a film of mortar that is self-troweled to the service of the
transfer line around its full periphery by the slug of concrete
itself.
There are four ways in which this seal can be lost:
1. By pumping excessively wet mixes which do not have
enough cohesion to hold together.
2. By pumping harsh undersanded concrete with poorly graded
aggregates which can jam together when the pressure
becomes too great for the insufficient amount of sand to
hold the aggregates apart.
3. By getting a rock pocket, such as mixer tailings, into the
pump valve. This rock pocket will have an insufficient coating
of mortar and the mix will not be plastic enough to allow the
valve to operate or the mix to move in the line.
4. Through excessive bleeding. If the mix is short or fines, but
the sand is otherwise fairly well graded, bleeding will not
normally create any problems as long as the pump continues
operation. But, if the pump is shut down, bleeding can result
in a loss of lubrication and blocked erratic flow.
The above are bad concrete practices, regardless of how the
mix is to be placed. But, these points do show that special
mixes are not always needed, within limits, for pumping
concrete. Good aggregate gradation is most important to
pump concrete the maximum distance.
The use of admixtures can have a beneficial effect on
pumpability. Most of the dispersing agents will fatten, retard
bleeding, and increase workability. Thus, the average concrete
can be pumped for appreciably longer distances. Air entraining
agents will also improve workability, although they cannot be
used as a substitute for good gradation of the aggregate.
Pumping will not appreciably affect the final air content of the
mix. High-early cement tends to give a more readily pumpable
mix with superior water retaining qualities. However, if delays
are likely to occur, extra care must be exercised due to the
faster setting time over regular cement.
The Mayco Model ST-70 will pump a wide variety of concrete
pump mixes. But, there are guidelines that must be followed.
Use this information in conjunction with “The Delivery System”
on page 32, Sections 18 thru 18.10.