CONCRETE LIBRARY OF JSCE NO. 26, DECEMBER 1995
STUDY OF DISCHARGE EFFICIENCY FROM TRUCK AGITATOR
(Translation from Concrete Research and Technology, Japan, Vol.5, No.2, July 1994)
Yoshihiro HAYASHI
Chikanori HASI-IIMOTO
Yukikazu TSUJI
Truck agitators are generally used to transport fresh concrete after mixing at a concrete plant. In
placing the concrete at a construction site, the discharge efficiency from the truck agitator
influences construction efficiency and concrete quality. In this report, we observe the flow of
fresh concrete in the agitator, which has hitherto been impossible to observe, with the help of
visualization techniques. We also investigate the influence of blade spiral pitch angle and
rotational speed on discharge efficiency.
Results demonstrate that the agitator has an optimum angle for discharge efficiency, and that if
the sliding resistance between concrete and blade surface is higher, discharge efficiency is lower.
Keywords : truck agitator, similarity law, discharge efficiency, blade, visualization technique
Yoshihiro Hayashi Works for the R&D Div. of ShinMaywa Industries, Ltd. in Japan. He
received his D.Eng. Degree from the University of Gumna in 1995. His research and
development‘ interests include special—purpose trucks and environmental systems. He is a
member of the JSCE and JCI.
Chikanori Hashimoto is an Associate Professor in the Department of Civil Engineering at
Gunma University, Gunma, Japan. He received his D.Eng. Degree from the University of Tokyo
in 1989. His research interests include the pumpability of flesh concrete, the flowability of fresh
concrete in a concrete pump or truck agitator, the mixing efficiency of concrete in a mixer, and
visualization techniques for concrete flow. He is a member of the JSCE, JCI, AJI, and VSJ.
Yukikazu Tsuji is a Professor in the Department of Civil Engineering at Gunma University,
Gunma, Japan. He received his D.Eng. Degree from the University of Tokyo in 1974. His
research interests include the behavior of reinforced concrete structures, chemically prestressed
concrete, and the properties of fresh concrete. He is a member of the JSCE, JCI, ACI, and
LABSE.
1.
INTRODUCTION
After mixing at a concrete plant, &esh concrete is generally transported to the constructionsite
in a truck agitator. If the discharge efBciencyof the truck agitator is poor, a number of problems
arise, as follows.
1) Placingthe fresh concretetakes excessivetime.
2) ne quality of the &eshconcretechangesduring placing.
3) Air mixeswith the fresh concreteas it is pumped.
4) ne bladesand irmerdrumsurface of the agitatorsuffer excessivewear.
5) Undischargedfresh concretehardensinside the agitator,and removalis time-consuming.
This has led to recent demands for improvementsin dischargeefBciency'especially to prevent
the fh5Shconcretehardening.
At the user end, dischargeefficiency can be improvedby increasingthe rotation of the agitator,
while the maker can attempt to improve discharge characteristics.h this respect discharge
efBciencydepends on the form of agitator in the drum, and of the blades welded to the irmer
surface of the drum. rrhe drum shape cannot be changed &eely, since the Japan Automobile
Body hdustry Associationregulates of that. So to improve dischargeefBciency,the blades must
be made more suitable for discharge.Many blades have been designed to achievethis type of
improvement,but they have not been investigatedtheoretically.Designs have largely depended
on observationsand the experienceof the designer.Blade shape, in theory,be decided based on
the now conditionsof fresh concrete in the agitator, but this is impossibleto observe as things
stand.
h this paper, we describe experimentsin which we observedthe flow of fresh concrete in the
agitator and quantified discharge efficiency with the help of a newly developed visualization
teclmique [1]-[4]. We were able to clarify the factors that have negative effects on discharge
efBciencyand investigatethe followingcharacteristics:
2@
1) Relationbetweenblade spiralpitch angle and dischargeefficiency
2) Relationbetweenrotationalspeedof the agitatorand dischargeefBciency
2. SUMMARY OF EXPERIENCE
The interior of an agitator is illustrated in Fig. 1. Two spiral blades are welded to the inner
surfaceof the drum with a 180-degree phase angle. When the agitatorrotates, the blades force
the &esh concrete inward and agitateit. When the agitatorrotates in the opposite direction,the
blades push the &esh concrete toward the outlet and dischargeit. Figure 2 illustrateshow the
agitatordischargesthe fresh concrete.
Drum
21BggipgW
We designed equipmentthat would give a
visualization of the situation in order to
observe and quantify the Bows of &esh
concrete in the drum. This equipment
comprises a model agitator, a model of
&eshconcrete,an imageanalyzer,and other
Systems.
i/
(Interi(or)
scale is 1/5, and the
-20-
& Io{tlet
0
/
((
Photo 1 showsthe model agitator.This is a
model of the type of agitator fitted to a
10-ton chassis. ne
Blades
Fig.1
drum
nlustrationof agitaorinterior
-y
A-
blade
->
A
Photo 1 Modelagitaotr
Halfrotation
model consistsof the drum, a drive motor, and
->
A-
Ban:STreelt
Theac,yd.TfSsin?adewhOi:h
COlarll.eLs;
observations of the flows of model concrete
inside. ne drive motor has a rating of 100
watts, and its rotation is reversible. Rotational
speed can be control &eely between 0 and 24
n
rpm.
Fig.2 Discharge motion of &esh conaete
We assumed that the fresh concrete consists of
two phases : coarse aggregatesand mortar. On this basis, we made the model fresh concrete. It
is a mixture of artificiallight aggregates (of particle size 5-10 mm and specific gravity 1.45) as
coarse aggregatesand a high-polymer resin solutionas the mortar.The consistencyof the model
concreteis controlledby adjustingthe consistencyof the solution and the ratio of light aggerates
to model mortar. (Thisis known as the volumeratio here.)The consistencyof the resin solution
can be controlledby adding water to the high-polymer resin.
ne consistency of the model mortar is defhed by its flow time (accordingto the criterionof
Japan Society of Civil Engineering JSCE-1986 : ne Bowing test of the injected mortar for
prepacked concrete.)The flow time is the time taken for all model mortar to flow out hm a
Blled P-type funnel. The relationshipbetween model concrete, consisting of resin solution and
light aggregates,and real fresh concretewill be mentionedlater.
23*
ne image analyzer comprises CCD cameras for B1mingthe flow of model concrete in the
model agitator, a video tape recorder for recording and playing back the images, and ancillary
equipment.
Two main factors are considered in the discharge efficiency experimentson model concrete
using this visualizationequipment : blade spiral pitch angle and rotationalspeed. An outlineof
how dischargeefficiencyis affectedby each factor is givenbelow.
(1) hfluence of spiral pitch angle
h this experiment,we used model agitatorswith spiral pitch angles of ll.3, 12.8, 14.9, 16.0,
18.0, and 20.0 degrees.ne spiral pitch angle is the angle betweenthe blade welding line and a
perpendicularhm the rotationalaxis of the agitator(see Fig. 3). When the spiral pitch angle is
large, the spiral pitch (representingthe blade spacing) is large and the angle of attack of the
blade surface is easy. ne experiment is illustrated in Fig. 4. The model agitator is placed
horizontallyand is chargedwith 40 I. (equivalentto 5 cubic metersin the real agitator)of model
concrete.ne rotational speed of model agitator is set to 3 rpm, which is the speed at which a
real agitator usually discharges Besh concrete. rrhe number of rotations [revs] of the model
-21-
Spiralpitchangle
Model concrete --tA
Modelagitator
Outlet
Charging hoppe1
/
Chute
Rotationa1
axis
/
/
?
,
Bucket i
Blade
Digitalweigher
Fig.3 Spiral pitch angle
Fig.4
Zd
Experimental method for discharge efficiency
agitator required to complete the discharge is measured along with the weight of discharged
concrete. The discharge rate [l] is then calculated. The number of rotations divided by the
discharge is defined as the rotations to discharge a fixed quantity of concrete [rev/l]. A small
value means that the discharge ef6ciency is good. If the model concrete fails to fully discharge
after two rotations, we assume that the discharge is completed even if some model concrete
remains in the agitator.
The model agitator was also inclined at 12 degrees (with the outlet side raised) for further tests
of discharge efficiency.We comparedthe discharge efBciencyin the horizontallyconditionand
in the inclined condition, and investigatedthe innuence of the rate of concrete sliding over the
blade surface on the dischargeefficiency.
(2) hnuence of rotationalspeed
h this experimentwe use a the model agitator with a spiral pitch angle of 12.8 degrees. It is
placed horizontally and charged with 40 l of model concrete. As the model concrete is
dischargedat rotational speeds of 1.5, 2.0, 3.0, 4.0, and 6.0 rpm, we measure the time to hish
the discharge [s], the number of rotations [rev], and the discharged amount [l]. Using these
values, we clarify the relationship between time, number of rotations to discharge, and the
discharge ratio (the ratio of the amount discharged to the amount charged) to the rotational
speed. Incidentally, in this study, we represent the discharge efBciency by the number of
rotations or the dischargeratio, rather than by using the number of rotations to discharge a bed
quantityof concrete. h investigatingthe innuence of rotationalspeed, if the latter measurewere
to be used, the discharge efBciencywould be incorrectlyestimated, because, at high rotational
speeds, discharge ef6ciency may be low. Because that the total number of rotations needed to
discharge is large even if the number of rotations to discharge a fixed quantity of concrete is
small, and that the reduced concrete is many for that the many concrete adheres to the inner
surfaceof the drum.
3
. INVESTIGATION OF SIMILAmY
This approachis that of a reductivemodel.The aim is to make a reductivemodel that reflects
the real world, allowing the results for the reductive model to be applied to the real thing.
Therefore, we h our eyes upon two Theologicalconstants, the yield point and the plastic
consistency,that control concrete flow conditions.On this basis, we investigatehow closely the
model simulatesthe real world. Pi-numbers relatingthese rheologicalconstantsare the Bingham
number and the Hedstromnumber, which is the Bingham number multipliedby inertia. These
numbersare dehed as follows [5].
-22-
T
m
0
a_l
300
•`
F200
Flow time : 200s
•`O
U)
y)
E
8
100
O
-
a)200
Video Camera
Monitor
tttt
Q
c3
l50
#
4qOO
^
•`
n
I.
5
Flow time : 200s
E:
U
%
•`
O
A
Q
E
I .0
I•`
O
D-
I
nner cylinder
Outercylinder
Timera
Flowtime : loos
U)
D•`
IE)
C)
P
Flowtime : loos
0 .5
D
Video recorder
0 .0
Rotation-type
consistencygauge Unit : mm
Fig.5 Measurement of Theologyconstants
O .2
0.
1 .U
6
Volumeratio
Fig.6
Ryeology constzult Of model concrete
Tl
Bingham number
: Bi =
Hedstrom number : He =
(1)
PV
pTl2
(2)
FL
where, T : yield point p : plastic consistency
p : density v: velocity l: length
where the situationin questionis a now problemin a pipe, such as concretebeing transported
by a pump [4], the Bingham number is usually adapted as the main pi-number, since the
concrete has no free surface. However,in case of concrete flow in an agitator, as in this study,
we believe it is more appropriatethat the Hedstrom number be used as the main pi-number,
sincethe concretedoes have a free surface.
we thereforeinvestigatethe similarity of the model concreteto real concrete by calculatingthe
Hedstromnumbers. First, the Theologicalconstantsof some model concreteswith differentflow
times and volume ratios were measured from a video image by the oneiOint method using a
double-cylinder inner rotation type of consistencygauge. An oudine of this measurementis
shown in Fig. 5. We recorded the flow conditionsof the model concrete on its free surfacewith
a video tape recorder, and measured the angular velocities of tracer grains floating on the Bee
surfacefrom the video images.Using the torque and angularvelocitiesof tracer grains measured
in this experiment,we plotted the consistencycurve and calculated the plastic consistencyand
yield point diagrammatically.Here, to calculatethe Theologicalconstants, we used six kinds of
model concrete with flow times of loos or 200s and volume ratios of 0.6, 0.8, or 1.0 [6].
1
EoenscurltsteN=eSf.ouTd
To;1f.h6e.
fTueos!ansgt.ic(Eio:s!shtae,naCc7e,E
idTedd
tee,yieideEof:i
,Thysi::lthvealumeSdoef
model concrete.)
FL' = 100-200
I ' = 0.70-1.50
Poise
(3)
gf/cm2
-23-
The plastic consistency FL and yield point T Ofconcrete have been given differentvalues by
many workers using various methods.Accordingto Kikkawa' s measurements[7], the values for
concretewith a slump value of 10 to 15 cm and incorporatingwater gravel are as follows.
; : f7.o•`-1Tgf/oc1:e2
(4)
By substitutingvalue (4) into equation(2), the Hedstrom number He of real concrete can be
obtainedas follows.
H
e =
p x(1.4-1.9)Xl2
(250-
(5)
400) 2
= (1.19-2.24)X10-5x
pl2
rrhe relationshipsbetweenmodel concrete density and real concrete density,and betweenmodel
agitatorlength and real agitatorlengthare as follows.
p'=p/2,
l'=l/5
(6)
By substitutingvalue (3) and equation(6) into equation (2), the Hedstrom number He' of the
model concreteis found to be
H e'=
(p/2)X(0.70-1.50)X(l/5)2
(100- 200) 2
= (o.o75-0.140)X
10-5 x pl2
(7)
nus the ratio of the two Hedstrom numbers is about 16. When this Hedstrom number ratio
diverges from 1, the concrete sliding and adhesion phenomena in the model experiment are
different Bom the real situation. h short, this Hedstromnumber ratio is a measure of how well
the phenomena in the model experimentsimulatethe real phenomena.
4. DETERMINATION
OF MjCNG
CONI)ITIONS FOR THE MODEL CONCRETE
It is necessary to decide on mixing conditionsfor the high-polymer resin solution and the
artiBciallight aggregatesto ensure that the model concrete correspondsto real concreteof the
prescribedconsistency.It would be difBcultto h the mixing conditionsthroughslumptests, so
we need to use another type of consistencytest. We choose to Bx the mixing conditionsby
comparingthe dischargeefficiency of real concrete with vadous slump values and of model
concretemade under variousmixingconditions
with the number of rotations needed to
B
discharge a bed quantity of concrete. In
55DA
short, we aim to hd a correspondencein a
200,
I.0
5! B
limited physical property; that is, the flow
%a
l50,
i.O
conditionsof concretein the dischargeprocess.
a E3
If the number or rotationsneededto discharge
/loo,
1.0
.sa:
a fixed quantityof concreteare equal, we can
i A
200,
0.6
consider that the consistency of the model
200,
O.8
concrete mixed under a certain conditions
-: '!
l50,0.8
/
corresponds to that of fresh concrete with a
loo, 0.8
certainslumpvalue.
A
eq
d)
>
A
I-
-
•`
CTI
B3
2:
a
5
1 0
15
We investigated the discharge of a bed
Slump value (cm)
quantityof model concretemade with various
Bowtimes and various volumeratios. Figure 7 Fig.7 Number of rotations needed to discharge a
fixed quantity of various model concretes
shows the results, along with corresponding
-24
-
d6
d6B
1
O
OO
oo
C
Q
C) clO
Q0
QQ
_OQ
(a) 51.o.:ig?reat=i.2?Oo:
6
o,)51oolTL:ehreat:io2?
Oo:
8
QQQ
Q QO
Q
Fig.8 Appearanceof model conaete now in the section
direction
1',/'S,-
-x>'.I,
A d h e rin g
F re e su rfap c∼e w hi:wi;M"h IIl'st
i
e p 9 ris in g ;I._,T P g[?i
/A
aQ
Q
Q
0
O O
oQt}
QQq
aQQQc.iQ
Q OTIQ
C}
E)
0
Blade plane
n
/
LSe=QQ;bog
D
Q-c)
D
0 o
aO
Fig.9 Process of adhering and rising up
rop down
Fig.10 Rearward motion of concrete
slump values of the real concrete. The solid line in the Bgure shows rotation number for the
discharge of a fixed quantity of concrete for the real concrete and the real agitator, while the
data pohts representflow time and volume ratio for variousmodel concretes.Model concreteof
flow time 200s and volume ratio 0.8 corresponds to concrete with a slump value of 5.5cm.
Model concretewith a flow time of 200s and a volume ratio of 0.6 correspondsto concretewith
a slump value of 5.5cm.
h the subsequent discharge efficiency experiment, only these two types of model concrete are
used. Qualitatively,a model concrete with a long now time or a large volume ratio corresponds
to a low slump concrete.
5. FLOW CONDITIONS IN THE I)ISCMGE
PROCESS
We observedthe flow of model concrete during the dischargeprocess in detail, by naked eye
and with the video camera.This quicklydemonstratedthat the followingtwo phenomenahamed
the dischargeefficiencyof modelconcrete [5] :
1) The effectof the concreteslidingspeed over the blade surface
Figure 8 shows how the model concrete Rows across a section through the agitator. In
regionA, if the sliding resistanceof the concreteover the blade surfaceis high, the concrete
rises up as the agitator rotates. We call this effect "adheringand rising up". Figure 9 shows
how 17adhering
and rising up17takes place using a side view of the agitator. h region B in
Fig.8, the concrete flows down the slope made as the agitatorrotates. Concretewith a long
now time and large volume ratio slides slowly over the blade surface as a result of its low
nuidity, so it rises further up the drum. ne concrete which rises in this way flows back
toward the center of the agitator, because of the inclined rotational axis. As a result, the
dischargeefficiencyis lower. Figure 10 shows how the concrete flows backwardBom the
dischargeoutlet. During the discharge of concrete with a flow time of 200s and a volume
ratio of 0.8, i.e. low slump concrete,it was also observedthat the concreterose up near the
-25-
J^
•`
(
c
0 .8
•`.t-
_a
1&i
BS
B S
.u
b
Drop down
0.6
Flow time : Zoos
volume ra(io : 0. 8
0 .4
ed A
•` -
TC
b
L
B'!
-C
h
0.
B3
a a
ection A- A
S
2
Flow time : Zoos
Volume ratio : 0. 6
U
0.0
r-C
A
ill
l6
18
20
spiralpitch angle ( o)
Flow back
LuA
L
l2
Fig.12 Relationbetweenspiral pitch angleand
number of rotationsneededto discharge
a Bxed quantityof conaete
top of the agitatorand droppeddownunder
Lc
gravity. Clearly, if the sliding speed over
the blade surface is lower, the discharge
efdciency is lower.
3;
2) The effect of the sectional area of flow
route near the outlet
Near the oudet of the agitator, the inside of
the drum tapers because the drum is a
conoid. h this area, the blade spiral pitch
becomes smaller. TLele is also a blade
A
plane though which to charge the concrete.
As a result, the sectional area of the flow
Section C- C
route near the outlet is smaller than at any
Fig.ll Concrete flow neau:outlet
other position. Figure ll shows how the
concrete Rows near outlet. Though the
concrete in the center of the agitator is carried close to the outlet in large amounts,the
agitatoris unableto pass it throughthe outletbecauseof the smallcross-sectionalarea,and
the concreterises upwardbeforedroppingbackinside the agitator.
To improvethe discharge ef6ciency of a truck agitator, it is clearly importantto increase the
slidingspeed of the concreteover the bladesurfaceand to increasethe sectionalarea of the flow
route throughthe oudet.
6. EFFECT OF SPIRAL PITCH ANGLE
We measuredthe number of rotations needed to dischaqgea fixed quantity of concretefrom
model agitators in which the drums had blade spiral pitch angles ranging from 113 to 20.0
degrees.Figure 12 shows the result. Regardlessof the type of model concrete,the agitatorwith
the small pitch angle needed the least rotationsto dischargea fixed quantityof concrete.The
dischargeefficiencyof such a blade is consideredgood. When the blade spiral pitch angle is
large, the sectionalarea of the flow route near the outlet is greater,because the blade pitch is
large.TtLisalso has the effect of increasingthe distancemovedby the concretein one rotation
of the agitator.
h the case of an agitatorwith a spiral pitch angle of about 12 degrees,the number of rotations
to dischargea bed quantityof concreteis much affectedto the spiral pitch angle, but in case of
about 18 degrees,it is not much. It is the cause that the sliding speed of the concreteoverthe
blade is small because the large spiral pitch angle makes easy the incline of blade surface.In
short, a large spiral pitch angle has a positiveeffect on dischargeefBciencyin that the sectional
-26-
J•`
•`
LI.
0
tt
&&i
uu, B
3.
'i6
0
Flow time : 200s
Volume ratio : 0. 8
a
52 t!
.eBo
-
2.0
aJJ
0
>
-
0
B{!
I.
t<
0
Flow time : 200s
Volume ratio : 0. 6
B3
2: co
0
0
CTI
0.0
I 0
1 2
i 4
I 6
I 8
20
spiral pitchangle (.)
Fig.13
(a) Horizontal condition
Discharge efficiency in inclined condition
(b) hclined condition
Fig.14 Conditionof conaete during discharge
area of the now route is large, but it also has a negative effect becausethe blade surfacehas an
easy angle. nerefore, in case of an agitatorwith a larger spiral pitch angle, it is assumedthat
the discharge efBciencyis worse, and thereforean agitator probably exists of which the spiral
pitchangle is optimumfor dischargeef6ciency.
To confim this, we set up the model agitatoron an incline, and examinedit for the discharging
efBciency.rrhe result is shown in Fig. 13. The dischargeefficiencyin this inclined condition is
considerablyworse as comparedwith that in the horizontallycondition,even for the same spiral
pitch angle. Further,in the horizontal case, the spiral pitch angle giving the mium
numberof
rotations for a bed dischargeis more than 12 degreesPig. 12), while in the inclined case, it is
about 15 degrees (Fig. 13). This clariBes that if the angle of incline is larger, the spiral pitch
angle for making the least number of rotations to discharge a fixed quantity of concrete is
smaller. Figure 14 illustratesthe concrete position in the inclined and horizontalcases duhg
discharge. h the horizontal case, the Bee surface of the concrete is almost flat. On the other
hand, in the inclined case, it is steeply inclined such that the concrete adheres to the blade
surface and flows with the blade.
If we focus attention upon the concrete as it slides over the blade surface during the discharge
process, it appears that discharging the agitator in the inclined condition is equivalent to
dischargingan agitator with a larger spiral pitch angle in the horizontal condition; the sliding
speed of the concreteover the blade surface is smallerand the dischargeefBciencyis lower.
h the horizontalcondition, it is found that the most efficientspiral pitch angle for dischargeis
more than 20 degrees,but in the inclined conditionthe number of rotations needed to discharge
a fixed quantity of concrete Bom a 15-degree agitator is smallest. Thus it is shown that an
agitatorwith an opdmumspiral pitch angle exists.
h other words, increasing the sectional area of the now route near the outlet by makingthe
l20O
spiral pitch angle larger means causing a lower
sliding speed of the concreteover the blade surface,
so there is a trade-ofEeffect. As a result, the blade
Flow time : 200s
e, goo
Volume ratio : 0. 8
spiral pitch angle, which hfluences the discharge
0
ef6ciency, has an optimumvalue because of these .E3
two relations : the increase in discharge amount o 600
arising &om an increase in the sectionalarea of the 5
Flow t;me : 200s
flow route and the decrease resulting &om reduced ¥B 300
Volume ratio : 0. 6
slidingspeed over the blade surface.
A
O
5J)
A
O
7. EmCT
yi
2
OF ROTATIONAL SPEEI)
4
6
Rotationalspeed ofagitator (rpm)
Figure 15 shows the relationship between the Fig.15 Relationshipbetweenrotationalspeed
of agitatorand dischargetime
rotational speed of the agitator and the discharge
-27-
^
>
0 0
e 80
Flow time : Zoos
Volume ratio : 0.6
V
O
5J)
0
Ll
du
i3
A
•`g
60
T
-C
g
C
a
O
-
Flow time : Zoos
volume ratio : 0. 8
40
e
80
time
00Flow
: 2OOs
Volume ra(1o : 0. 8
O
bJ)
a
0
O
A
uEh
LI
6,
L4•`o 20
t•`
O
J=
e1.olTIem,eat=l.2?Oo:
6
70
E3
E
j
2:
90
6 0
0
,
2
4
2
4
6
6
Rotationalspeed ofagitator(rpm)
Rotationalspeed ofagitator(rpm)
Fig.16 Relationshipbetweenrotational speedand
number of rotationsneededto discharge
Fig.17 Relationshipbetween rotationalspeed
of agitatorand dischargeratio
time. The discharge time is short at high rotational speeds. Figure 16 shows the number of
rotations needed to make a discharge. Many rotations are needed to discharge when the
rotational speed is high, and as the rotational speed increases, the discharge becomes less
smooth. The concreteis pushed by the blade toward the outlet by sliding over the blade surface
or on the inner surface of the drum. Yet the drum itself rotates, so when the rotationalspeed is
high, the rotational speed of the drum is faster than the sliding of the concrete over the blade
surface. The concrete then adheres and moves with the rotation of agitator,failing to slide over
the blade surfacetowardthe outlet. Figure 17 showsthe relationshipbetween dischargeratio and
rotationalspeed. When the rotationalspeed is high, the dischargeratio is low and someconcrete
fails to be dischargedbecause it adheres to the drum or the blade.
h short, the discharge ratio falls if a high rotational speed is chosen to get a good discharge
speed, while the dischargespeed falls if a low rotationalspeed is chosento get a high discharge
ratio. To achieve a short dischargetime without a detrimentaleffect on the dischargeratio, it is
advisable to vary the rotational speed during discharge, e.g. use a high rotational speed until
most of the concreteis discharged,and then reduce the speed to get a high dischargeratio.
8. SUMh4ARY
h this study of the dischargeefBciencyof concretefrom an the agitator,we made the process
visible by using a model agitatorand investigatedthe influenceof blade spiral pitch angle and
drum speed on dischargeefficiency.The results were used to hd factors which influencethe
dischargeefBciency.Resultswere as follows:
(1) Observationsof the flow conditionof concreteduring the dischargeprocessdemonstratethat
the factors which have a detrimentaleffect on dischargeefficiency are the sliding of concrete
overthe bladesurfaceand the limitedsectionalarea of the Bowroute near the outlet.
(2) Measurementsof the numberof rotationsneededto dischargea fixed quantityof concretefor
various dischargeconditionsand blade spiral pitch angles clarifiedthat an agitatorwith a large
spiral pitch anglehas good dischargeef6ciencybecausethe sectionalarea of the flow route near
the oudet is larger. On the other hand, it had a negativeeffect in that the sliding speed of the
concretewas reducedbecause the angle of the blade is easier. h short,it was demonstratedthat
an opdmumspiralpitch anglefor the dischargeof concreteexists.
(3) An investigationof the innuence of rotational speed on dischargetime and dischargeratio
showed that the dischargeratio falls at higher rotationalspeeds, while the dischargespeed falls
at a low rotationalspeed.To obtain a short dischargetime without causingthe dischargingratio
to fall, it is advisableto change the rotational speed than to keep a constant rotational speed
throughoutthe discharge.
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References
[1] C.Hashimoto, M.Yoshida, J.Asaka, and Y,Tsuji, 'fSimilitudeof Dimensions of Experiment on
Concrete flow with Help of Visualization Teclmiques,'f Proc. of the JCI annual conference, Vol.
13-1, pp. 89-94, 1991 (in Japanese)
[2] A.Yasumoto, C.Hashimoto, K.Maruyama, and Y,Tsuji, "Visualization of Agitation of Fresh
Concrete hm Truck Agitator," Proc. of the JCI armual conference, Vol. 13J, pp. 107- 112,
1991 (in Japanese)
[3] C.Hashimoto, M.Yoshida, Y,Tsuji, and Y.Hayashi, "An Experimental Study on ef6ciency of
Discharging of Fresh Concrete from Truck Agitator," Proc. of the JCI annual conference, Vol.
14-1, pp. 107- 112, 1992 (in Japanese)
[4] C.Hashimoto, K.Maruyama, and K.Shimizu, "Study on Visualization Teclmique for Blocking
of Fresh Concrete Flowing in pipe," Concrete Library of JSCE, No. 12, pp. 139- 153, 1989
[5] I.Emori and D.J.Schuring, "rme theory and application of the model experiment,T7Sankaido,
pp. 84-85, 1988 (in Japanese)
[6] J.Murata and K.Okada, "ne newest concrete teclmology select books I, The rheology of the
&esh concrete, The elasticity and creep of concrete,TTSankaido, pp. 77-78, 1981 (in Japanese)
[7] K.Kikkawa, "Studies on the consistency equation of Besh concrete and its application,T7
doctoral thesis in the Department of Science and Engineering at Meijo university, 1987 (in
Japanese)
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