results and discussion

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MÉTODO PARA AVALIAR O DESEMPENHO DE CONCRETOS
REFRATÁRIOS APLICADOS POR PROJEÇÃO
(Evaluating the Shotcrete Performance of Refractory Castables)
R. G. Pileggi1, Y. A. Marques1, D. Vasques Filho1, V. C. Pandolfelli1,
N. Moreira2, S.C. Frasson2
1
Universidade Federal de São Carlos (UFSCar) / Departamento de
Engenharia de Materiais (DEMa)
Rodovia Washington Luis, km 235, São Carlos, SP, CEP: 13565-905,
Tel: (16) 260-8253, Fax: (16) 261-5404,
e-mail: [email protected] ou [email protected]
2
Saint Gobain / Brasil
ABSTRACT
The raising interest for the shotcrete application process is motivated by the
great installation rate and by the enhanced final properties of the sprayed materials.
Nevertheless, despite the high technology comprising the shotcrete equipments, the
development of sprayable compositions have been carried out through empirical
methods. Such technological gap probably reflects the lack of laboratorial techniques
able to evaluate the shotcrete performance of refractory castables. For this reason,
the main objective of the present work was the development of a novel
characterization method, which is based on castable rheometry, to simulate the
successive steps involved in the shotcrete process: mixing, pumping, spray shooting
and castable sticking / hardening on the surface. Additionally, three distinct
commercial
shotcrete
castables
were
evaluated
by
using
the
developed
characterization method.
INTRODUCTION
Sprayed concretes were originally developed for civil construction at the
beginning of the 20th century (1,2). High installation rates, low cost and good final
mechanical properties of the placed material are responsible for the currently
widespread use of this application process (1,2).
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The placing method consists of pumping either the dry powder (Gunning) or
the fluid concrete (Shotcrete) directly from the mixer to the pipeline nozzle, where a
spray flow is formed by the injection of high-pressure compressed air (both
techniques) and water (just Gunning). Immediately after reaching and covering the
surface, the projected concrete sudden loses its fluidity due to the action of cement
setting accelerators and/or coagulant additives also injected into the nozzle
(1-7).
Rebound losses and dust release to atmosphere are the major problems
associated to gunning
(1-2).
These difficulties are deeply diminished when the
concrete is pumped in the fluid state, thus justifying the growth of shotcrete
application (6).
The advantages associated to this method are well known, but only in recent
times the shotcrete of refractory castables became a common practice for the lining
of large areas or repairing damaged surfaces (6,7).
Despite the continuous evolution observed in the shotcrete machines
(1,2),
sprayable compositions are still being developed through empirical and semiempirical methods (6). Such discrepancy reveals the lack of characterization
techniques that simulates the shearing conditions during the shotcrete application
(6)
and the need for further research in this subject.
The main objective of the present study was to reduce this technological gap.
For that reason, a set of experiments based on castable rheometry was developed to
evaluate the shotcrete performance of refractory castables. Moreover, commercial
shotcrete compositions were evaluated using this new testing procedure.
Shotcrete Application Process
The shotcrete of refractory castables is a multistage application process
composed by four consecutive steps: mixing, pumping, spraying and consolidation.
The efficiency of this installation method is usually quantified by the rebound loss,
which is defined as the percentage of material that does not stick on the covered
surface (1-5).
It is usually accepted that only self-flowing castables (free-flow between 80
and 110 %)
(8)
are eligible for shotcrete applications because of their pumpability and
lower rebound due to the absence of large circular agglomerates (golf balls)
(7)
when
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sprayed. However, this statement is questionable, since opposite rheological
behaviors such as dilatancy and pseudoplasticity could also afford high-flow
compositions, but not necessarily be pumped or sprayed
(8).
Pileggi and Pandolfelli (8,9) demonstrated that the mixing and the pumping
performance of castables is deeply affected by the rheological behavior of these
materials. For instance, dilatant compositions are hardly mixed and are not suitable
for pumping. On the other hand, pseudoplastic castables with excessive water
content possibly segregate during pumping due to their very low matrix viscosity.
Nevertheless, both situations might be verified in self-flow castables.
In addition to the discussion about the rheological behavior, the action of
cement setting accelerators and/or coagulant additives is deeply influenced by the
castables chemical compositions (1-7).
The shotcrete capability and the effectiveness of the material hardening over
the covered surface are then consequence of all four consecutive steps (mixing,
pumping, spraying and consolidation) involved in the shotcrete technique.
EXPERIMENTAL SET-UP
Novel Multistage Characterization Technique
Taking into consideration the successive steps comprehending the shotcrete
application process, a multistage experimental set-up, comprising mixing, pumping,
spray shooting and consolidation characterization, was employed to evaluate the
performance of three distinct commercial shotcrete compositions (A, B and C).
General aspects about the castables, such as chemical composition,
polymeric fiber content, shotcrete additive (sodium silicate) and amount of water
required for mixing, were supplied by Saint-Gobain / Brazil (Table I).
MIXING: A rheometer
(8,9)
specifically developed to evaluate the rheological behavior
of castables was used for mixing the compositions. The torque profile and the time
elapsed for mixing 4 kg of each castable were on-line recorded. Castable mixing was
carried out at 33 rpm according to the following experimental set-up: (a) dry-powder
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homogenization (time: 180 s); (b) water addition and mixing up to the turning point,
and (c) castable homogenization after the turning point (time: 120 s).
Table I - Castables’ chemical composition, polymeric fiber content, shotcrete additive
(sodium silicate) and amount of water required for mixing.
composition
A
B
C
SiC + C (wt. %)
Al2O3
19.0
86.0
0.0
70.0
9.1
83.0
SiO2
7.0
2.6
13.0
CaO
Others
0.7
3.3
0.3
2.0
1.7
2.3
polymeric fiber (wt. %)
0.0
0.1
0.1
shotcrete additive (wt. %)
water content (wt. %)
1.0
7.0
1.0
6.5
1.0
6.0
Note: The compositions were supplied by Saint-Gobain / Brazil. Polypropylene fibers 6 mm long were
used in the compositions B and C.
In view of the successive steps involved in the shotcrete process, it is
reasonable to consider low mixing energy as a basic request for sprayable castables.
Such characteristic brings economical and technical benefits, such as the production
of highly homogeneous materials with no need of powerful and expensive mixers
and without promoting an undesired castable heating
(8,9)
(9),
and/or the degradation of
the polymeric fibers usually applied in sprayable compositions
(1,2,10).
PUMPING: The pumping of refractory castables is a demanding step where the material
flows inside pipelines, under high shear rates, for long distances and volume restricted
conditions (8).
In the present work, the free-flow value of each composition was measured
(ASTM C-860 adapted for self-flowing castables) just after mixing. In sequence, 4 kg of
each composition was subjected to two successive shearing cycles (between 2 and 75
rpm) performed in a rheometer where the castable was subjected to unrestricted and
volume restricted conditions (simulate the flow inside pipe lines) (8).
SPRAY SHOOTING: A laboratorial test specifically developed to characterize the
shooting performance of castables was recently proposed by Pileggi and
Pandolfelli (6). This method uses a rheometer to simulate the shear conditions during
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the castable application. Thus, it easily quantifies the impact of any specific factor
(additives, water content, rheological nature, etc.), on the shotcrete performance.
Nevertheless, this novel technique does not analyze the sticking tendency of the shot
sprayed materials or the strength increase of the consolidated layer. For this reason,
two alternative tests were proposed herein to improve the rheometer based
characterization method.
1. Sticking Test - After mixing, the conical device illustrated in Figure 1 (A) was
introduced inside the mixing bowl in order to set up a surface target. Also, the
planetary mixing paddle was replaced by another specifically designed for shooting
the castable at high-speed against the conical target.
(A)
Mixing bowl
Adhesion Cone
Shooting
paddle
Rough surface
Fluid castable
Polymeric screen
(B)
Revolution
speed (rpm)
75 rpm / 10 s
Additive
injection
33
20 rpm / 120 s
mixing
Sticking
test
Consolidation Time (s)
test
Figure 1 – (A) Schematic Drawing for the conical device employed in the sticking test;
(B) revolution speed set-up employed in the sticking (only until the 75 rpm / 10 s step)
and in the consolidation tests (75 rpm for10 s + 20 rpm for 120 s). Note: the polymeric
screen avoids the castable sliding over the internal surface of the cylindrical bowl.
For the sticking test, the previously prepared castable (2 kg) was additionally
homogenized at 33 rpm for 30 s. The shotcrete additive (1 wt. % of an aqueous
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solution composed by 0.4 wt. % sodium silicate and 0.6 wt. % of water) was injected
at the end of the homogenization step and the revolution speed was suddenly
increased to 75 rpm, held there for 10 s, and subsequently reduced to zero (blue path
in Figure 1 (B)). The rebound loss was defined as the weight percentage of nonadhered material.
2. Consolidation Test - In order to evaluate the reaction profile during the shotcrete
application process, the sodium silicate solution was injected into the castables (2 kg)
at the end of the mixing homogenization step. Just after the addition, the revolution
speed was suddenly increased to 75 rpm, held for 10 s, and subsequently reduced to 20
rpm and kept at this speed for 120 s to evaluate the torque increase at low shear
conditions (red path in Figure 1 (B)).
RESULTS AND DISCUSSION
MIXING: The results obtained (Figure 2) demonstrated that the fiber-free composition
(comp A) demanded the lowest mixing energy (ME) and the lowest torque at the
turning point (TTP), whereas, both fiber-containing systems (comp B and C) showed
superior values for these two parameters. Similar trend was reported by Salomão et
al.
(10),
who demonstrated that the mixing energy and torque at the turning point
scaled with the size and/or the content of the polymeric fibers within the castable.
14
ME (Nm.s) TTP (Nm)
comp A
comp B
comp C
12
2474.1
3904.0
2728.0
10.6
14.0
12.2
tTP (s)
292.65
190.21
243.49
Torque (Nm)
10
8
6
4
compBB
comp
comp
comp CC
2
comp A
comp A
0
0
100
200
300
400
dry-powder
wet mixing step
homogenization
500
Time (s)
Figure 2 – Mixing behavior of the compositions A, B and C. Note: the inset in the graph
presents the mixing energy required and the maximum torque at the turning point.
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Comparing these results to others described in literature (8-10), compositions A and
C could be classified as easy mixing systems. The large amount of highly irregular and
abrasive silicon carbide coarse grains in composition B probably explains its high mixing
energy.
PUMPING: Figure 3 shows the rheological behavior of the castables. Self-flowability
was only achieved for the fiber-free composition A mixed with 7.0 wt. % of water.
Compositions B and C displayed similar low fluidity suitable for castables applied
under vibration (8,9).
8
comp A
comp A / RV
comp B
comp B / RV
comp C
comp C / RV
7
Torque (Nm)
6
5
Free-Flow (%)
comp A
85.7
comp B
22.7
comp C
23.0
4
3
2
1
0
0
10
20
30
40
50
60
70
Revolution Speed (rpm)
80
90
100
Figure 3 – Rheological behavior of the compositions A, B and C measured through
shearing cycles (between 2 and 75 rpm) carried out without and with volume restriction
(RV). Note: orange area highlights the suitable torque range for pumping (8).
According to the traditional cone flow selection criteria, only composition A would
be selected for pumping and shotcrete applications
(7,8).
However, both compositions A
and C would be pumped due to their pseudoplastic behavior, which fitted into the ideal
torque range for pumping (8) under both shearing conditions (unrestricted and restricted
volume).
Regardless displaying pseudoplastic characteristics, composition B greatly
surpassed the ideal pumping range at high shear. Actually, such rheological behavior
does not inhibit the flow inside pipelines, but a high energy would be requested to
achieve this intent. In this case, the shotcrete performance would be negatively affected
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by the undesired outcomes related to excesses in the shearing energy, such as
excessive heating, irregular flow (plug-flow), segregation and structural damages (fiber
rupture, particles fracture) (6,8).
SPRAY SHOOTING: The essence of the shotcrete application process consists of
generating a compact castable layer by spray shooting the material onto the surface
to be covered. In practice, the conical geometry of the nozzle associated with the
injection of high-pressure compressed air and additives into the pumped material,
cause a high-speed spray flow against the surface, which sticks and consolidates
over the surface, as illustrated in Figure 4.
Figure 4 – Schematic drawing of the castable spray shooting and the material
consolidation onto the surface.
Ideally, the castable spray flow is composed by high-speed individual granules
(coarse aggregate covered by a matrix layer). In order to guarantee the granules
adhesion, it is reasonable to assume that the damping behavior of the matrix layer
must be able to dissipate all the kinetic energy associated to the granules when they
collide to the surface.
The energy dissipation is favored by reducing the granules size and/or
increasing its matrix / aggregate ratio. It is worth mentioning that the matrix layer
characteristics also control the adhesion of the granules to the surface, its
homogenization capacity and its consolidating nature.
Besides its elevated installation rate, undesired material losses (> 15 wt. %) is
commonly observed in the shotcrete process owing to the rebound losses and/or to
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the fragmentation of the first deposited layer when successive applications are
required
(1-7).
Problems associated to the consolidated material, such as creep strain,
sliding, detachment and lamination / cracking of the applied layer, are also usual
(1-7)
(Figure 5).
Ideal Consolidaton
Flow
Sliding
Detachment
Rebound
Fragmentation
Lamination
Cracking
Figure 5 – Schematic drawing from the distinct problems that may occur during the
shotcrete of refractory castables. Note: orange = ideally consolidated castable; blue =
distinct consolidation problems.
These flaws have usually been attributed to the inadequacy of the pumped
material to be sprayed
spray generation
(5).
(7)
and/or to some of the multiple parameters involved in the
However, the techniques conventionally applied to characterize
the shotcrete performance of refractory castables does not provide accurate
information about the phenomena involved in the spray shooting process, since they
are based only on post-application analysis or the matrix rheological evaluation
(6).
Large rebound losses were verified for the self-flowing pumpable composition
A with or without fiber addition, whereas compositions B and C actually displayed
good sticking features (Figure 6). These results confirmed the hypothesis that highfluidity is not directly related to the shotcrete performance. However, the fiber addition
unexpectedly increased the rebound loss in composition A.
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Taking into account that the shotcrete additive (sodium silicate) was the same
for all the compositions, it is possible to conclude that the chemical composition and
the physical characteristics of the tested materials deeply influenced the material
adhesion onto the surface.
60
Rebound Loss (wt. %)
50
40
30
20
10
0
comp A
comp B
comp C
comp A (fiber)
Figure 6 – Rebound loss obtained in the sticking test for the compositions A, B and
C. Note: sodium silicate (0.4 wt %) was the shotcrete additive. Polypropylene fibers
were also added to comp A at the same content found for comp B and C.
In theory, the first 10 s at 75 rpm simulates the high-speed spray shooting
process. Only the composition A (fiber) reacted instantaneously (Figure 7), resulting
in a positive initial reaction rate (Table II), estimated through the angular coefficient
calculated from the best linear fitting between torque and time really at 75 rpm in the
high speed step. The others systems remained almost unchanged in the first 10 s,
probably due to a balance between the effects promoted by the extra liquid added
and the coagulant action of the shotcrete additive.
The fiber addition accelerated the coagulant action promoted by sodium
silicate in the composition A, possibly due to the friction promoted by the fibers
(10).
This behavior is the main reason for the highest rebound loss accomplished by this
system, since it was previously demonstrated in literature that the rebound losses are
minimized when the additives react not so fast at this initial stage
(4).
High initial reaction rate hardens the material inside the nozzle, what restrains
the spray formation and leads to the generation of large agglomerates
(7).
Moreover,
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hardened matrixes can not damp the collision impacts between the granules and the
surface.
6
comp B
comp A (fiber)
comp A
comp C
5
33 rpm
75 rpm
20 rpm
Torque (Nm)
4
3
2
1
0
20
30
40
50
60 70 80 90100
Time (s)
Figure 7 – Reaction profile measured through the consolidation test for the
compositions A, B and C. Note: sodium silicate (0.4 wt %) was the shotcrete additive.
The red arrow point to the composition A (fiber) rupture under shearing. The reaction
rate was estimated through the angular coefficient calculated from the best linear
fitting between torque and time really at 75 rpm (gray area) in the high speed step.
Table II – Parameters calculated on the consolidation test: reaction rate; resting time;
maximum torque at the low speed step (20 rpm).
(20 rpm)
(75 rpm)
reaction rate resting time maximum torque
(Nm/s)
(s)
(Nm)
comp A
0.00
2.09
4.69
comp B
-0.04
4.17
1.01
comp C
-0.02
4.18
1.30
comp A (fibre)
0.22
0.65
4.15
Note: the reaction rate was estimated through the angular coefficient calculated from the best linear
fitting between torque and time at the 75 rpm step.
In order to produce a dense coating on the surface, the individual granules
must also show a time delay before starting their setting reactions (4). Such condition
ensures the binding and homogenization between the successive applied layers
without causing the material sliding and also great rebound.
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This time delay was indirectly measured through the resting time, herein defined
as the time elapsed to restart the torque increase at this stage (due to the additive
action), after reducing the revolution speed from 75 to 20 rpm in the rheometer.
According to this parameter, the composition A (fiber) showed the smallest
resting time, followed by the fiber-free one (comp A), thus further reinforcing their high
rebound loss. Both low-rebound compositions B and C displayed similar resting time.
The association of high initial reaction rate and low resting time may be
assumed as the driving force for increasing the rebound losses in the shotcrete of
refractory castables.
Regardless of the rebound, the consolidated material must also be
strengthened over the surface so as to avoid the creep strain promoted by its own
weight and allowing the thick coating layers installation. In the experimental set-up
proposed herein, the maximum torque at the final step (20 rpm / 120 s) describes the
after placing material’s consistency.
Composition A, with or without fiber addition, resulted in high maximum torque
values during this step. Oppositely, compositions B and C did not afford significant
maximum torque values, what could inhibit the installation of thick coating layers.
The fiber containing composition A developed an undesired rigidity, since its
structure was broken due to the shearing during the test (20 rpm). In practice, such
lack of plasticity may result in lamination or in the rupture of the consolidated layer by
impacts with the successive sprayed material.
In fact, a suitable behavior would have been a composition / additive
combination that would provide resting time similar to those presented by
compositions B and C, and final torque as that verified in composition A.
FINAL REMARKS
Through the new testing procedure proposed, it was shown that high-fluidity is
not the most important feature for a good shotcrete performance. Actually, low mixing
energy, and low torque values at high shearing rate under volume restricted conditions
are better requests for the shotcrete of materials.
According to the sticking and the consolidation tests, results showed that rebound
losses are reduced when the shotcrete additive does not cause high initial reaction rate
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and too short resting time. High maximum torque after consolidation would also be
required for placing thick layers.
Taking into account all these features, composition C displayed a good shotcrete
performance, but only for thin coating layers. Composition A resulted in large rebound
loss due to its high reaction rate, which was further increased with the fiber addition. On
the other hand, composition B requested a lot of energy to be mixed and was unsuitable
for pumping.
The multistage characterization technique developed to evaluate the shotcrete
performance of refractory castables was a further step in analyzing the mixing, the
pumping and the shooting behavior of these materials.
ACKNOWLEDGEMENTS
The authors are greatly thankful to the Brazilian research funding institutions
FAPESP and CNPq, and to Saint Gobain / Brazil for their support on this research work.
REFERENCES
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