Effect of pumping and spraying processes on the rheological properties and air content of wet-mix shotcrete with various admixtures

Construction and Building Materials 225 (2019) 311–323

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Effect of pumping and spraying processes on the rheological properties and air content of wet-mix shotcrete with various admixtures Lianjun Chen a,b, Guanguo Ma a, Guoming Liu a,b,c,⇑, Zhaoxia Liu a,b a State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266590, China b College of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, China c National Demonstration Center for Experimental Mining Engineering Education, Shandong University of Science and Technology, Qingdao 266590, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Effect of pump and spray process on

Effect of alternative thrust (shock) and shear on concrete flow in pipes.

rheology of concrete was investigated.  Mechanism of pump and spray process affecting concrete rheology was expounded.  Prediction models of the changes of concrete performance were built.  Influence degree of wet-mix shotcrete process on concrete rheology was analyzed.

Double-plunger pump

Pipe wall Alternating thrust Concrete plug

Lubrication layer

Shock

Aggregates

a r t i c l e

i n f o

Article history: Received 2 February 2019 Received in revised form 23 June 2019 Accepted 14 July 2019

Keywords: Wet-mix shotcrete process Concrete properties Influence Admixture

Shear zone

Plug zone

Shear

+ Cement paste

Reasonable distribution

a b s t r a c t Wet-mix shotcrete mainly includes two processes: pumping and spraying. In this study, to explore the effects of the pumping and spraying processes on the rheological properties and air content of concrete, a full-scale experimental system of wet-mix shotcrete was developed with the given work parameters of these processes. The rheological parameters, slump, and air content of the fresh concrete were measured before pumping, after pumping, and after spraying, respectively. In general, the air content decreased after pumping, whereas the flow resistance declined after pumping and then increased after spraying. These changes were linked to the shear and shock caused by the pumping or spraying process. Under the interference of the pumping and spraying processes, prediction models of the changes in concrete properties were developed. Finally, a one-way analysis of variance (ANOVA) was conducted to analyze the degree of influence of the pumping and spraying processes on concrete properties. This work could be beneficial for further improvement of the understanding of the overall wet-mix shotcrete material and processes. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding author at: State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, Shandong University of Science and Technology, Qingdao 266590, China. E-mail address: [email protected] (G. Liu). https://doi.org/10.1016/j.conbuildmat.2019.07.104 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

Shotcrete is a fast construction technique to place concrete in engineering projects such as repair of concrete structures, rock support, and slope stabilization. Therefore, the wet-mix shotcrete technology is attractive owing to its low dust concentration when

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compared with the dry-mix or semi-wet shotcrete [1–5]. Especially in the process of coal mining, pneumoconiosis caused by inhaling excessive dust is one of the main diseases [6–22], and thus, wetmix shotcrete has become increasingly popular because of the less dust it produces [23–31]. During the process of wet-mix shotcrete, fresh concrete is conveyed under pump pressure through pipes to the nozzle, where compressed air is introduced to project concrete onto the receiving surface. Wet-mix shotcrete comprises two processes: the pumping and spraying processes. It is assumed that the properties of fresh concrete (pumpability and shootability) may change under the actions of pump thrust and spray impact. Pumpability is the ability of concrete to be delivered through a hose or pipe system under pressure, and is related to flow resistance. Shootability is the ability of concrete to be shot, and is related to rebound and build-up thickness, among other factors. Feys et al. [32] stated that pump thrust could produce substantial shear in the fresh concrete. During the process of pumping self-consolidating concrete, the shear rate reached 30– 60 s 1 in the bulk concrete and more than 100 s 1 in the lubrication layer. Previous studies [33–35] have indicated that the shearing action affects the concrete properties. According to Feys et al. [36], the pumping process causes a change in the air-void systems and loss in the slump of fresh concrete. It is well known that the increase in air content would change the rheology of fresh concrete, accompanied by a decrease in the plastic viscosity [37,38]. Rust and Feys et al. [39,40] indicated that the effect of air content on the yield stress depended on the size of air bubbles in fresh concrete. Therefore, considering that fresh concrete is subjected to high pressure during the pumping and spraying processes, the properties of fresh concrete may change. More recently, numerous scientific studies have contributed to understand the flow process of self-consolidating concrete inside a pipe, evaluate the lubrication layer thickness [41,42], and predict the pumping pressure [32,43,44]. Regarding wet-mix shotcrete, there are very few studies about the influence of the pumping and spraying actions on its properties. Ginouse et al. [45] noted that understanding the spraying process was essential to improve the shotcrete quality by directly controlling the impact conditions. The same author investigated the impact of velocity distribution using a high-speed camera with a full-scale shotcrete test. Similarly, a single particle shooting setup was also designed to measure the incident particle velocity, and it revealed that the characterization of the material sprayed was related to the rebound and placement process [46,47]. Additionally, most of the existing coaxial cylinder rheology apparatuses encounter some problems when trying to measure the rheological properties of shotcrete. A low slump is necessary for the fresh shotcrete to assure that no slough or sag occurs, and the slump value may drop sharply after the spray operation, becoming close to zero. When measuring the rheological parameters of fresh shotcrete with a low slump, those abnormal voids built between the bulk concrete and the rheometer wall affect the measurement accuracy [48]. Kappiet et al. [49] used a gyratory compactor developed by an intensive compaction tester to measure the no-slump concrete workability. Wiering et al. [50] proposed a method for measuring the rheological properties of low flow concrete, but fresh concrete in tests was treated as a discontinuous body during the analysis. In brief, for the wet-mix shotcrete process, the main questions are how do the pumping and spraying processes affect the properties of fresh concrete and how do we determine their degree of influence. The corresponding knowledge is scarce. To answer these two questions, in this study, full-scale wet-mix shotcrete tests were conducted under fixed working conditions. A new type of rheometer was used to measure the rheological parameters of wet-mix shotcrete with varying workability, from high to low slump. The change trends of the rheological properties, slump,

and air content of fresh concrete after pumping and spraying were analyzed. 2. Experimental program 2.1. Materials 2.1.1. Cement A PO.42.5 ordinary Portland cement was used with a fineness of 3100 cm2/g and specific gravity of 3.14, which complies with the Chinese standard GB175-2007. The chemical characteristics are listed in Table 1. 2.1.2. Aggregates Natural river sand was used as fine aggregate. As coarse aggregate, crushed gravel with a maximum size of 10 mm was employed. The fineness moduli of sand and gravel were 2.66 and 5.70, respectively. The specific gravities of sand and gravel were 2.61 and 2.67, respectively. The water absorbed by the fine and coarse aggregate was considered to correct the mixing water. The gradation curves for the aggregates are shown in Fig. 1. 2.1.3. Admixtures 2.1.3.1. Fly ash. The fly ash employed in this study had a specific surface area of 300–500 m2/kg and a specific gravity of 2.34. The chemical characteristics are listed in Table 1. 2.1.3.2. Silica fume. Silica fume was used with a specific surface area of 16,000–30,000 m2/kg and a specific gravity of 2.21. The silica fume contained up to 95% of SiO2 and less than 1% of CaO. 2.1.3.3. Polypropylene fiber. Polypropylene fibers with a specific gravity of 0.93 were employed in this study. The fiber lengths were 6, 12, and 29 mm, respectively. Its melting point was 180 °C. The tensile strength and elastic modulus were more than 500 and 3850 MPa, respectively. 2.1.3.4. Chemical admixtures. A polycarboxylate-based water reducer and a triterpenoid saponin-based air-entraining agent were employed for the wet-mix shotcrete. The admixtures were represented as %, indicating the percentage of the admixtures relative to the binder content (in mass). 2.2. Mixture proportions To study in detail the influence of the pumping and spraying processes on the performance of fresh shotcrete, the number of experimental samples was increased as much as possible by changing the mix proportion of shotcrete. Two methods were used for changing the mix proportions: to increase the dosage of each component in the concrete mixtures gradually or to change the types of admixtures. As listed in Table 2, seven variables were used to change the mix proportions, including the water to bonding materials ratio (WC), fine aggregate to total aggregate percent (SR), silica fume (SF), fly ash (FA), water reducer (WR), airentraining agent (AE), and polypropylene fiber (FI). In brief, 69 groups of slump, air content, and rheological property tests were conducted in this study. These mixture components and mix proportions were referred to the previous experiments and the related literatures [1,2,4,45,51–60]. In this study, only the influence of a single factor on the concrete performances was considered. The compatibility effects of multiple factors on the concrete performances were not considered. An accelerator was not employed to avoid its possible effects on this investigation. The slump of fresh concrete before pumping varied from 1 cm (FI*29) to 15 cm

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L. Chen et al. / Construction and Building Materials 225 (2019) 311–323 Table 1 Chemical compositions of cement, fly ash, and silica fume (unit: %). SiO2

AI2O3

Fe2O3

CaO

MgO

SO3

Cement Fly ash Silica fume

19.5 51.5 >95

6.45 23.3 –

3.08 5.49 –

57.57 16 1<

1.21 1.2 –

2.01 2.14 –

40

60

80

100

2.3. Experimental methods

20

Fine agg. limits Coarse agg. limits Sand gradation Limestone gradation

0

Percentage passed by weight (%)

Raw materials

0

1 10 Sieve size (mm)

100

Fig. 1. Gradation curves of sand and gravel.

(WR1.2) in this test. Before pumping the concrete, cement mortar was pumped to create a lubrication layer inside the pipe wall in advance. Thus, low slump fresh concrete, of even less than 5 cm, could be pumped successfully.

2.3.1. Experimental procedure The experimental system of wet-mix shotcrete consists of a double-plunger pump, a mixer, pipes, and a sprayed wall, as shown in Fig. 2. Behind the pump, the first horizontal straight section was constructed with 20 m long steel pipes; then, the second section was installed with a 10 m hose for an easy and convenient operation. Both pipes were 50 mm in diameter. Pipe clamps were used for connecting the steel pipe with the hose. The pipe clamp could be opened for taking concrete samples after pumping. The work parameters of the wet-mix shotcrete were set as follows: pump pressure of 7 MPa; spraying distance from the sprayed surface, 1 m; added air pressure, 0.4 MPa; and spraying angle of 90°. Three different concrete samples were taken at three stages: before pumping, after pumping (or before spraying), and after spraying, as shown in Fig. 3. Once the samples were taken at each stage, the air content, rheology, and slump tests were carried out. Before pumping, a sample was directly taken from the concrete hopper; after pumping, a concrete sample was directly pumped into the measuring container of the rheometer or slump or air content measuring instrument. During spraying, the concrete was

Table 2 Mixture proportions of wet-mix shotcrete. No.

Variable

WC

Water (kg/m3)

SR (%)

Sand (kg/m3)

WC45-F WC*45 WC50 WC55 WC60 SR50 SR60 SR70-F SR*70 FI6-F FI*6 FI*12 FI*29 AE0.02 AE0.04 AE0.06 WR0.3 WR0.6 WR0.9 WR1.2 SF5-F SF*5 SF*10 SF*15 FA5 FA10 FA15

WC

0.45 0.45 0.5 0.55 0.6 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

198 198 220 242 264 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220 220

70 70 68 66 66 50 60 70 70 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

1100 1100 1080 1070 1050 900 1000 1100 1100 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000

SR

FI

AE

WR

SF

FA

FI (mm)

AEA (%)

WR (%)

SF (%)

FA (%)

0.3

0.3 6 6 12 29

0.3 0.3 0.3 0.02 0.04 0.06 0.3 0.6 0.9 1.2 0.3 0.3 0.3

5 5 10 15 5 10 15

Pump No Yes Yes Yes Yes Yes Yes No Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes

Note that ‘‘-F” means concrete pumping failure and ‘‘*” means modified fresh concrete with mixing water reducer to meet the pump requirements. During the tests, there were four instances of concrete pumping failure.

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Wall Shotcreting Pump+Mixer 3

2 Steel pipe

Hose

1

10 m

20 m 1 Samples before pump

2 Samples after pump

3

Samples after spray

Fig. 2. Pipeline and spray system of wet-mix shotcrete.

Fig. 3. Test samples for the rheology and air content tests before pumping, after pumping, and after spraying.

2.3.2. Air content and slump test The air content was measured by an LA-0316 type concrete air content measuring instrument. The slump value was tested with a slump cone. Two tests were conducted as per the method in the Chinese Standard GB/T50080-2016.

Problems such as segregation and structural breakdown could be avoided during measurement because one rotation in the measurement pot was enough for the eBT2 type rheometer. It was not necessary to measure twice at the same place in fresh concrete, and thus, this rheometer structure avoided the negative impacts caused by the low flowability of fresh concrete. Moreover, it provided the possibility to measure the compacting concrete after spraying.

2.3.3. Rheology test The rheological parameters of the fresh concrete were determined by using the eBT2 type rheometer purchased from Schleibinger Gerate Teubert u. Greim GmbH. The eBT2 rheometer includes a shaft decoder and two bending moment sensitive probes. A smart phone was used to control the tester over a Bluetooth interface. Laboratory studies have shown that the rheological behavior of fresh concrete is sufficiently described by the Bingham model [61,62]. A characteristic Bingham fluid presents a relation of torque (T) with rotation speed (N) such as T = g + h N, where g (Nm) and h (Ns) are parameters related to yield stress and plastic viscosity, respectively [63,64]. In this study, for convenience, we regarded ‘‘g (nm)” and ‘‘h (ns)” as flow resistance and torque viscosity, respectively. The flow resistance and torque viscosity were obtained according to the calculation from the momentum and rotated velocity in this rheometer [65,66]. The rules and methods of the eBT2 rheometer were discussed in [67–69].

2.3.4. Compressive strength test Before pumping, fresh concrete was directly cast into molds with size of 100 mm  100 mm  100 mm, and this was also done after the pump; for the sprayed concrete samples, the concrete was first sprayed into iron box molds with dimensions of 450 mm  350 mm  120 mm. After 1-day curing in a standard curing chamber with temperature of 20 ± 2 °C and 95% of relative humidity, the specimens were demolded. The large concrete slabs were subsequently cut into standard cube specimens with dimensions of 100 mm  100 mm  100 mm after 7-day curing. Finally, all concrete specimens were cured to 28 days under standard conditions. The uniaxial compressive strengths were measured using a WDW3100 computer-controlled electric universal test machine. According to the test results, for the concretes before pumping, the compressive strength varied between 16.7 (FI*29) and 37.24 MPa (SF*15%); for the concrete after pumping, the compressive strength varied between 18.5 (WC60) and 40.38 MPa

directly sprayed into the corresponding containers. Concrete samples taken for the rheology and air content tests are shown in Fig. 3.

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(SF*15%); and for the concrete after spraying, the compressive strength varied between 20.3 (FI*29) and 42.56 MPa (SF*15%). The rank of the average compressive strength was as follows: after spraying > after pumping > before pumping. This result was similar to those found in the literature [36,52], Feys et al. [36] indicated

315

that the changes in compressive strength were attributed to the changes in the surface area of cement particles. The actions of pumping and spraying allowed cement particles to be dispersed more uniformly and better hydrated, thereby increasing the compressive strength.

(a) Effect of pumping and spraying processes on slump

(b) Effect of pumping and spray processes on air content

(c) Effect of pumping and spraying processes on flow resistance

(d) Effect of pumping and spray processes on torque viscosity Fig. 4. Effect of pumping and spraying processes on slump, air content, and rheological parameters.

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3. Results and discussion The changes in concrete properties after pumping and spraying are plotted in Fig. 4. Note that ‘‘B-P,” ‘‘A-P,” and ‘‘A-S” mean ‘‘before pumping,” ‘‘after pumping,” and ‘‘after spraying,” respectively. 3.1. Influence of wet-mix shotcrete process on properties of concrete 3.1.1. Slump Fig. 4(a) shows that independently of the type of variable that affected the concrete slump, the trend was consistent. That is to say, the fresh concrete slump increased after pumping and declined sharply after spraying, and this result was similar to that in Feys’ report [36]. According to the analysis, the changes in slump were related to the operation of the double-plunger pump system and the lubrication effect of the cement paste. As shown in Fig. 5, during the process of pumping concrete in the pipes, the fresh concrete was pushed alternately by the double-plunger pump. On the

Double-plunger pump

one hand, the alternating thrust produced a shock that promoted the interaction between the concrete ingredients. On the other hand, the shear that occurred near the inner pipes further enhanced the mixing. The shear range depended on the flowability of concrete: the highest slump concrete had the largest shear zone, while the lowest slump concrete had the smallest or no shear zone [43,44]. These two phenomena of shock and shear increased the mixing degree of concrete materials, compensating the possible drawback caused by the poor mixing before pumping. In addition, the cement paste went through gaps between aggregates, enhancing the lubrication effect. As a result, the distribution of cement paste, sand, and gravel became more reasonable. Therefore, the flowability of concrete in the pipes increased, and thus, the slump of concrete became larger after the pump than that before the pump. From Fig. 6, it can be seen clearly that the fresh concrete after pumping became damper and smoother, whereas the concrete after spraying became more compact. In Fig. 6(c), some holes were introduced at the interface between the concrete plug and

Pipe wall Alternating thrust

Shear zone Concrete plug

Plug zone Lubrication layer

Shock Aggregates

Shear

+ Cement paste

Reasonable distribution

Fig. 5. Effect of alternating thrust (shock) and shear on concrete flow in pipes.

6.1 cm

Binary processing

9.3 cm 1.7 cm Pipe

(a) B-P

(b) A-P

(c) A-S

Fig. 6. Slump changes of SR60 concrete at B-P, A-P, and B-S.

Fig. 7. Schematic of double-plunger pump affecting air content of fresh concrete.

Holes

L. Chen et al. / Construction and Building Materials 225 (2019) 311–323

inner wall of the slump cone, which may be caused by the frustum of the cone structure. Fig. 6(c) was binarized to see the hole structure clearly. During the spraying process, the concrete was blown quickly onto the wall surface, forming the compacting concrete. The bonding between the ingredients in concrete became tight under the high-pressure force, and then the concrete flowability declined. Thus, the slump of concrete decreased after spraying. 3.1.2. Air content As can be seen in Fig. 4(b), the influence of the pumping process on air content was the same for concretes with different admixtures, and the air content declined to different degrees after pumping. Previous studies [70,71] have reported analogous results. However, the effect of the spraying process on air content was diverse. For WC, SR, and FI concretes, the air content increased after spraying. For FA and AE concretes, the air content decreased after spraying. When concretes had SF and WR with different dosage, the change in air content became complex. According to the analysis, the working process of a concrete pump could be divided into two parts: suction and thrust. The concrete pump consisted of two plunger actions; one sucked the materials into a cylinder while the other pushed the materials out from the other cylinder. As shown in Fig. 7, during suction of materials, the pressure in the fresh concrete system decreased and a loose material area was formed where small bubbles migrated, and thus, big bubbles were gradually formed or escaped into the surrounding air. During pushing of materials, the original large-size bubbles became many small bubbles under the pressure and dispersed into the fresh concrete system. After the concrete with small bubbles flowed into the pipes, because the pumping pressure dropped at the interval between two plunger actions, large bubbles would form from the small bubbles again. This phenomenon would be repeated until the fresh concrete flowed out of the pipe mouth. It is well known that bubbles can easily escape when fresh concrete is under no pressure. Therefore, with the thrust and suction of the concrete pump, the air content decreased after pumping. Beaupre [57] supposed that during pumping the air bubbles were lost by dissolution into the paste owing to the applied pressure. If the equilibrium between the surface tension of the paste and the internal pressure of an air void is considered, it seems that the small bubbles, compared to the large ones, would be easily lost. The distribution in size of the air voids before and after pumping affected the loss rate of air content in fresh concrete. With the changes in air content shown in Fig. 4(b), it was assumed that the effect of the spraying process on air content was linked to the initial air content of fresh concrete. For example, fresh concretes with WC, SR, FI, and SF have low air content before

317

pumping. Owing to the negative effect of pumping on air bubbles, the air content of fresh concrete after pumping becomes much lower than that before pumping. During the spraying process, some high-pressure air might be introduced into the concrete, leading to the increase in air content after spraying. Moreover, according to the surface topography of the sprayed concrete wall (Fig. 8), it was obvious that many small holes were formed on the shotcrete substrate because of the particle rebound and impact functions. Meanwhile, these holes might partially introduce air into the shotcrete substrate. On the contrary, the fresh concrete with initially high air content, such as concrete mixed with AE, WR, or FA, still had relatively high air content after pumping. During spraying, under the actions of different forces, such as the shear of high-pressure air, compacting pressure, collision between particles, and collision between particles and wall, numerous bubbles that were not broken during pumping were further broken and escaped when the concrete mixtures move towards the sprayed surface, resulting in the decrease in air content in the shotcrete substrate. In this phenomenon, although the spraying process reduced the air content, the air content value of these concretes with AE, WR, or FA after spraying was still higher than that in concretes with low initial air content. For example, WC55 concrete had 4.3% air content before pumping, whereas AE0.04% concrete had 6.1% air content after spraying in Fig. 4(b). Certainly, as mentioned above, some amount of highpressure air might be introduced into the concrete, but for these concretes with relatively high air content after pumping, the introduced air content was very small and could be neglected. Thus, the

Fig. 9. Correlation between flow resistance and slump.

Fig. 8. Surface topography of sprayed concrete wall with different hole types.

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Fig. 10. Comparison of rheological measurement before pumping and after spraying.

Z: A-S (Slump,

cm)

4

2

16 0

12

0

8

4 X: 8 B-P (Sl 12 um p, c m)

4

16

0

P AY:

c p, um l (S

m)

(b) Linear fitting of B-P vs A-P

(a) Three-dimensional distributions of slump

(c) Linear fitting of A-P vs A-S

(d) Linear fitting of B-P vs A-S

Fig. 11. Change trends in slump after pumping and spraying.

Table 3 Variance comparison of fitted equations. Property

R2 A-P vs B-P

A-S vs A-P

A-S vs B-P

Average of R2

Slump Air content Flow resistance Torque viscosity Average of R2

0.9254 0.9815 0.8073 0.9688 0.9207

0.8285 0.8297 0.7596 0.8690 0.8217

0.8183 0.8030 0.6352 0.7668 0.7558

0.8574 0.8714 0.7340 0.8682 –

L. Chen et al. / Construction and Building Materials 225 (2019) 311–323

pumping and spraying processes caused significant loss of air content for these concrete mixtures with initial high air content. In a previous study [72], it is stated that the spraying process causes a substantial effect on fresh concrete with high air content, and the author found that the average air content after spraying was approximately half of that after pumping. 3.1.3. Flow resistance Except for the concretes mixed with AE, the flow resistance of fresh concrete nearly declined after pumping and then increased after spraying, as shown in Fig. 4(c). This phenomenon was contrary to the change in slump. In this study, data on slump and the corresponding flow resistance were collected, and the relationship between flow resistance and slump is plotted in Fig. 9. The flow resistance decreased with increasing slump. It was determined that the rheological parameters were correlated to the slump [73–77]. According to the analysis, the pumping process increased the flowability of concrete in pipes because of the vibration and shear effects caused by the double-plunger pump (Fig. 5) in addition to the increasing lubrication effect of the cement paste. Thus, the flow resistance of fresh concrete declined after pumping, and this analysis was consistent with that proposed in Section 3.1.1. During the spraying process, owing to the compacting effect of the high-pressure air on the concrete substrate, the concrete became stiff. Besides, another explanation for the shooting stiffening that occurred during spraying was that a large quantity of air bubbles exploded when the concrete hit the receiving surface

[57]. Hence, the flow resistance increased after spraying. The flow resistance of concrete with AE kept increasing after pumping or after spraying, which could be explained by the fact that the pumping and spraying processes largely reduced the air content of concrete with AE. Based on the ‘‘ball effect” of air bubbles, the less the air content is, the lower the flowability. Thus, the loss of air content made the concrete stiff, leading to high flow resistance. 3.1.4. Viscosity As shown in Fig. 4(d), the torque viscosity declined slightly after pumping and then increased significantly after spraying. That is to say, the pumping process produced few effects on the torque viscosity of fresh concrete, whereas the spraying process caused substantial changes in torque viscosity. Under the forceful action of high-pressure air during spraying, the viscosity naturally increased because of the compacting concrete. During the measurement of rheological parameters, we observed the following distinction: a thin gap was generated owing to the rotation of probes in the concrete before pumping, as depicted in Fig. 10(a); and a larger gap with some large holes was formed owing to the stiffness of concrete after spraying, as shown in Fig. 10(b). This was the reason why the sprayed concrete had a very large torque viscosity. The eBT2 rheometer only rotated once, thus avoiding the effect of holes on the measurement. For the concrete with SF*15%, the probes could not rotate normally during the rheological measurement because of an excessively low flowability, and thus, we could not obtain its rheological parameters.

%)

8

Z: A-S (Air content,

6

4

2 0

X: B-

8 P (A ir c 12 ont ent ,% )

12 %) nt, e t n co

8

4 4 16 0

P( A: Y

r Ai

(a) Three-dimensional distributions of air content

(c) Linear fitting of A-P vs A-S

319

(b) Linear fitting of B-P vs A-P

(d) Linear fitting of B-P vs A-S

Fig. 12. Change trends in air content after pumping and spraying.

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According to this analysis, we assumed that the torque viscosity was related to the flowability. During pumping, the flowability of concrete became larger, and thus, the torque viscosity decreased for most concrete samples. During spraying, the spray process reduced the flowability, and then the viscosity increased. The degree of increase in concrete flowability determined the degree of increase in viscosity. After pumping, the increase in flowability was small; thus, the viscosity changed slightly with a downward trend. After spraying, the reduction in flowability was larger; hence, the viscosity changed significantly with an upward trend. However, Burns [51] found that if the pumping process caused less loss of air content of the concrete, the yield stress and plastic viscosity would increase after pumping owing to the increasing consistency of fresh concrete. If the action of pumping was very strong, the cement particles would be further dispersed, leading to the decrease in both yield stress and plastic viscosity. Feys et al. [36] thought that the pumping process might produce few effects on viscosity, or cause a slight increase in viscosity. The author thought that this was tightly correlated to the redispersion of the superplasticizer. 3.2. Change trends of fresh concrete properties after pumping and spraying To explore in detail the correlation of each concrete property between B-P, A-P, and A-S, the three-dimensional distributions of

the various property parameters are depicted in Figs. 11(a)–14 (a). Furthermore, linear fittings were conducted to predict the change trends of each property (slump, air content, flow resistance, and viscosity). Then, unusual data caused by mixing the higher or lower admixtures were screened and removed to improve the accuracy of the models. For the three-dimensional distributions, the X–Y face shows a performance correlation between B–P and A–P, which represent the influence of the pumping process; Y–Z shows a performance correlation between A-P and A-S, which represent the influence of the spraying process; and X–Z exhibits a performance correlation between B-P and A-S, representing the influence of both the pumping and spraying processes. All the variances of the fitting equations are listed in Table 3. According to the average of variance R2, for different properties, the fitting effect of air content was the best among all the properties, with the average of R2 reaching 0.8714, and the worst fitting effect was the flow resistance. From the point of view of the different processes, the fitting effect of ‘‘A-P vs B-P” was the best, with the average of R2 reaching up to 0.9207, whereas the worst was the entire process of ‘‘A-S vs B-P” owing to the superimposed effect of errors from both the pumping and spraying processes. Thus, the property prediction after pumping was relatively better than that after spraying. It can also be seen that the fitting effect in Fig. 11 (b)–14(b) was the best when compared to that in Figs. 11–14 (c and d), the fitting effect in Fig. 11(d)–14(d) was worst. The deleted

ance, Nm) Z: A-P(Flow reisit

2.0

1.6

1.2

0.8 0.0 B- P

X:

0.4 ( F lo wr

0.8 es i s 1.2 ta n ce , Nm )

1.2 m) 0.8 ce, N an 0.4 esist r w 0.0 (Flo P A Y:

(a) Three-dimensional distributions of flow resistance

(c) Linear fitting of A-P vs A-S

(b) Linear fitting of B-P vs A-P

(d) Linear fitting of B-P vs A-S

Fig. 13. Change trends in flow resistance after pumping and spraying.

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L. Chen et al. / Construction and Building Materials 225 (2019) 311–323

Ns) Z: A-S (Viscosity,

8

6

4 0 X: 2 B-P (Vi sco sity ,

4 Ns)

4 Ns) , ity 2 os c s Vi 0 P( A Y:

(b) Linear fitting of B-P vs A-P

(a) Three-dimensional distributions of viscosity

(c) Linear fitting of A-P vs A-S

(d) Linear fitting of B-P vs A-S

Fig. 14. Change trends in viscosity after pumping and spraying.

Table 4 F value of one-way analysis of variance (ANOVA) based on normalization. F value

Slump

Air content

Flow resistance

Torque viscosity

Average

WC SR FI AE WR SF FA Average

8.322298 19.99400 2.823733 11.03508 5.456638 1.536825 7.692024 8.122943

1.001991 10.02128 1.272152 71.73783 6.513096 0.174825 14.62704 15.04974

8.170050 11.89249 32.31372 10.6207 13.06496 – 26.16047 17.03707

8.178973 9.870144 6.139381 524.8004 512.8269 – 203.3749 210.8651

6.418328 12.94448 10.63725 154.5485 134.4654 0.855825 62.96361 –

data points in Fig. 11(b)–14(b) were less than those in Figs. 11–14 (c and d). Considering the linear law of fitting models, the main data points deleted were the concrete with more or less waterbinder ratio or water reducer or longer fiber. From this, we could see that the above fresh concrete had an unusual sensibility to the pumping and spraying processes. In general, most variances R2 in Table 3 surpassed 0.8, nearly approaching 1; thus, most models showed a better prediction for the slump, air content, and rheological parameters. It should be pointed out that all of the above prediction models were based on the specific working conditions of wet-mix shotcrete, and any changes in the operating parameters of shotcrete might cause changes in the prediction model. We would explore the effects of

different pumping and spraying conditions on the properties of wet-mix shotcrete in the next research project. 3.3. Influence degree of pumping and spraying processes on concrete performances To qualitatively explore the influence of wet-mix shotcrete on the performance of fresh concrete, ANOVA was employed using the SPSS software to conduct the analysis of F value, as listed in Table 4. The bigger the F value was, the more significant the influence of the holistic wet-mix shotcrete process on the concrete performances. In the process of analysis, the slump value, air content value, and rheological parameters obtained in the tests were

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dicting the change in air content was the best among all the properties; from the point of view of the different processes, the prediction of properties after pumping was relatively better than that after spraying. (3) The holistic wet-mix shotcrete process had the most significant effect on the torque viscosity. The influence of the holistic wet-mix shotcrete process on the concrete with an air entraining agent was maximum. The effect of the pumping process on air content was the maximum among all the properties. The maximum effect generated by the spraying process was on torque viscosity. Fig. 15. Comparison of influence between the pumping and spraying processes.

Acknowledgments normalized to ensure the fairness of comparison between various performance parameters. According to the average value in Table 4, for different properties of fresh concrete, the holistic wet-mix shotcrete process had the most significant effect on the plastic viscosity, and the effect on slump was the minimum. In terms of the mix proportion, the influence of the holistic wet-mix shotcrete process on the concrete with AE was maximum, whereas the minimum was the concrete with WC (excepting SF). Furthermore, to explore the effect of a single process, pumping or spraying, on the concrete properties, the difference in the average normalized value of each property between ‘‘B-P” and ‘‘A-P” was calculated, which represent the effect of the pumping process; the difference between ‘‘A-P” and ‘‘A-S” was also calculated, which represent the effect of the spraying process. The results are plotted in Fig. 15. The higher the absolute difference value was, the larger the effect of a single process on the concrete performance. It can be seen in Fig. 15 that the spraying process (green column) seemed to have larger impact than the pumping process (red column) on the whole. The effect of the pumping process on air content was maximum, and thus, the air content was greatly reduced. The maximum and minimum effects generated by the spraying process appeared on torque viscosity and air content, respectively. 4. Conclusions In this study, a test system of wet-mix shotcrete was established to explore the influence of the wet-mix shotcrete process (pumping and spraying) on the rheological properties and air content of the fresh wet-mix concrete. The working parameters of the pumping and spraying processes were given based on the routine practical experience. (1) The trend of the effect of the pumping and spraying processes on slump was nearly consistent for all shotcretes. The slump increased after pumping and decreased dramatically after spraying. The air content decreased at different degrees after pumping. However, the effect of the spraying process on air content was related to the initial air content of the fresh concrete. The flow resistance declined after pumping and then increased after spraying. The torque viscosity declined slightly after pumping and then increased significantly after spraying. The changes in most concrete properties were related to the operation of the double-plunger pump system and the lubrication effect of cement paste. (2) Three-dimensional distributions of the changes in each property were presented under the effect of the pumping and spraying processes. Prediction models of concrete performances, including flow resistance, torque viscosity, slump, and air content, were built under the effect of the pumping and spraying processes. The fitting effect for pre-

This study was funded by projects, such as the National Key Research and Development Plan of the 13th Five-Year Period [grant number 2017YFC0805203]; National Natural Science Foundation of China [grant number 51604163]; Shandong Key Research and Development Program [grant number 2018GSF116001]; Natural Science Foundation of Shandong [grant numbers ZR2019QEE007, ZR201801280006, ZR2019MEE115]; a Project of Shandong Province Higher Educational Science and Technology Program [grant number J15LH03]. Conflict of interest The authors declare that they have no conflict of interest. Statement of data availability The authors declare that the data in this paper is available. References [1] G. Liu, W. Cheng, L. Chen, Investigating and optimizing the mix proportion of pumping wet-mix shotcrete with polypropylene fiber, Constr. Build. Mater. 150 (2017) 14–23. [2] L. Chen, G. Liu, W. Cheng, G. Pan, Pipe flow of pumping wet shotcrete based on lubrication layer, Springerplus 5 (1) (2016) 1–14. [3] G. Liu, L. Chen, Development of a new type of green switch air entraining agent for wet-mix shotcrete and its engineering application, Adv. Mater. Sci. Eng. 2016 (2019) 1–10. [4] L. Chen, P. Li, G. Liu, W. Cheng, Z. Liu, Development of cement dust suppression technology during shotcrete in mine of China-a review, J. Loss Prev. Process Indust. 55 (2018) 232–242. [5] I. Galan, A. Baldermann, W. Kusterle, M. Dietzel, F. Mittermayr, Durability of shotcrete for underground support – review and update, Constr. Build. Mater. 202 (2019) 465–493. [6] G. Zhou, Y. Ma, T. Fan, G. Wang, Preparation and characteristics of a multifunctional dust suppressant with agglomeration and wettability performance used in coal mine, Chem. Eng. Res. Des. 132 (2018) 729–742. [7] T. Fan, G. Zhou, J. Wang, Preparation and characterization of a wettingagglomeration-based hybrid coal dust suppressant, Process Saf. Environ. Prot. 113 (2018). [8] N. Guanhua, D. Kai, L. Shang, S. Qian, Gas desorption characteristics effected by the pulsating hydraulic fracturing in coal, Fuel 236 (2019) 190–200. [9] G. Ni, Z. Li, H. Xie, The mechanism and relief method of the coal seam water blocking effect (WBE) based on the surfactants, Powder Technol. 323 (2018) 60–68. [10] X. Wang, Y. Zhang, B. Liu, P. Liang, Y. Zhang, Effectiveness and mechanism of carbamide/fly ash cenosphere with bilayer spherical shell structure as explosion suppressant of coal dust, J. Hazard. Mater. 365 (2019) 555–564. [11] G. Wang, M. Wu, R. Wang, H. Xu, X. Song, Height of the mining-induced fractured zone above a coal face, Eng. Geol. 216 (2017) 140–152. [12] G. Wang, W. Li, P. Wang, X. Yang, S. Zhang, Deformation and gas flow characteristics of coal-like materials under triaxial stress conditions, Int. J. Rock Mech. Min. Sci. 91 (2017) 72–80. [13] L. Zhen, Y. He, W. Wang, W. Cheng, X. Lin, Experimental study on the pore structure fractals and seepage characteristics of a coal sample around a borehole in coal seam water infusion, Transp. Porous Media 125 (1–2) (2018) 1–21. [14] X. Lin, Z.T. Wang, W. Gang, N. Wen, Z. Gang, W.M. Cheng, J. Xie, Technological aspects for underground coal gasification in steeply inclined thin coal seams at Zhongliangshan coal mine in China, Fuel 191 (2017) 486–494.

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