Determination of Lubrication Layer Thickness and Its Effect on Concrete Pumping Pressure

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Rong Deng *

, Tong Ye and Zhiwei Ye

materials

Article

Determination of Lubrication Layer Thickness and Its Effect on Concrete Pumping Pressure

Citation: Deng, R.; Ye, T.; Ye, Z. Determination of Lubrication Layer Thickness and Its Effect on Concrete Pumping Pressure. Materials 2024, 17, 5136. https://doi.org/10.3390/ ma17205136

Academic Editor: Carlos Leiva

Received: 14 September 2024 Revised: 13 October 2024 Accepted: 15 October 2024 Published: 21 October 2024

Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

School of Mechanical Engineering, University of South China, Hengyang 421100, China; yetong0805@163.com (T.Y.); yezhiwei0601@163.com (Z.Y.)

* Correspondence: dengrong20@163.com

Abstract: The flow of six kinds of fresh concrete under different flow rates and lubrication layer thickness (TLL) values in the horizontal pipe was numerically simulated. The influence of the TLL on the pressure per unit length (PL) was analyzed. It was determined that the formation of the lubrication layer (LL) significantly reduces the PL in concrete pumping. As the TLL increased, the PL decreased. However, the degree of reduction in the PL gradually decreased as the TLL increased. Relating the simulated PL with the experimental PL, the size of the TLL was obtained, which was between 1 and 3 mm. The minimum and maximum were 1.23 and 2.58 mm, respectively, and the average value was 1.97 mm. The strength (S24, S50), the size of the aggregate (A10, A20, A25), and the flow rate of pumping all affected the TLL. The type of fresh concrete and the flow rate of pumping significantly affected the PL, which impacted the TLL. However, the TLL also impacted the PL. Finally, this made the TLL change within a certain range. When PL > 14,000 Pa/m, 2 mm < TLL< 3 mm; on the other hand, 1 mm < TLL< 2 mm. Therefore, we can use CFD to simulate the flow of all types of concrete in the actual pumping pipeline with a TLL of 2 mm to obtain their pumping pressure and guide the actual construction.

Keywords: fresh concrete; pressure per unit length; horizontal pipe; simulation

1. Introduction

Concrete, as the most widely used engineering material, is extensively used in the con- struction of urban infrastructure, roads, bridges, and nuclear reactors. Pumping technology is a construction method used to complete the crucial tasks of concrete transportation and pouring, offering advantages such as speed, timeliness, quality assurance, and reduced labor consumption [1,2]. Especially for some large-scale reinforced concrete structures that use substantial amounts of concrete, high-rise buildings, narrow sites, and construction sites with obstacles, the concrete pumping technology is particularly effective [3–5]. Despite extensive experience with concrete pumping, several problems still occur during the actual construction process, including concrete segregation, pipeline blockage, and wear [6]. These problems greatly increase the pumping pressure and can even cause the pumping pipeline to rupture, disrupting the orderly progress of construction and compromising the strength and durability of hardened concrete. Concrete is a multiphase and multi-scale composite material. Its mechanical properties change with time, temperature, humidity, and stress state, showing the evolution between the elastic, viscous, and plastic phases [7,8]. Rheology is the study of the dynamics in the evolution of the viscoelastic–plastic behavior of concrete. It helps identify the changes in concrete during the fresh mixing stage by analyzing the interactions between the different phases in the slurry. Employing the rheology theory to study the rheological behavior of concrete, the pumping construction can be better guided. Thus, studying the rheological properties of fresh concrete within the pump pipe is crucial. This research holds significant value in predicting the pressure requirements for concrete pumping. In the process of pumping, pressure is the key parameter that determines the

Materials 2024, 17, 5136. https://doi.org/10.3390/ma17205136 https://www.mdpi.com/journal/materials

Materials 2024, 17, 5136

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efficiency of pumping. The main factors affecting the pressure during concrete pumping include rheological parameters, pumping flow rate, etc. Therefore, predicting the pumping pressure is crucial to ensuring a smooth and effective pumping process.

A thin layer of several millimeters may be formed on the pipe wall in the process of pumping; this is the lubricating layer (LL) [4,9,10]. The formation, size, and performance of the LL are strongly correlated with the pumping performance of fresh concrete and significantly impact the pumping pressure. It was shown that the rheological properties and thickness of the LL can effectively predict the pumping performance of concrete. The friction between the LL and the pipe wall, which is directly related to the composition of the LL, plays a crucial role in the pumpability of fresh concrete. As a result, the LL plays a key role in the flow process of concrete within the pipe, and with the increase in the TLL, the pipeline pressure gradually decreases [11–17]. At present, there are two main methods to characterize the LL. Through the use of tribology, the properties of the LL were described by Kaplan et al. [18]. The LL was described by other scholars as the relative slip between the concrete near the pipe wall and the pipe wall. The slip velocity was introduced to deal with the influence of the LL on the pumping [19–21]. It was agreed that shear-induced particle migration is the cause of the formation of the LL in the pipe wall during pumping [11,19,22]. The rheological properties of the LL were measured mainly using sliding-tube rheometers [20] and tribometers [21]. The sliding-tube rheometer was used to evaluate the performance of the LL during concrete pumping and to study its influence on the pumping performance of concrete. Numerous studies have shown that the composition and rheological properties of the LL are similar to those of the mortar in fresh concrete [10,23,24]. Le et al. [24] equated the measured rheological parameters of the mortar in fresh concrete to those of the LL to study the influence of the LL on concrete flow in the pump pipe through numerical simulation. It was shown that there is a good correlation between the experiment and the numerical simulation. Accordingly, the rheological properties of the LL could approximately be represented by measuring the rheological properties of mortar in fresh concrete. At present, ultrasonic velocity profiling (UVP) [23,25] and particle image velocimetry (PIV) [24] are mainly used to measure the TLL of fresh concrete during pumping. Studies showed that the TLL ranged from 2 mm to 8 mm and was influenced by the mix ratio of fresh concrete and the inner diameter of the pump pipe [26–29]. It was also believed that the TLL, ranging from 1 mm to 9 mm, is related to the volume of cement slurry, the water–cement ratio, and the content of superplasticizer [13].

The pumping pressure of fresh concrete is influenced by the LL. Thus, numerous schol- ars have proposed and established models for predicting the pumping pressure. Kaplan’s model [18] was more classic and closer to the actual situation among the various models considering the influence of the LL on the pumping performance of fresh concrete [30]. By comparing the shear stress of fresh concrete near the pipe wall to its yield stress, two models were established to predict the relationship between pumping pressure and flow. Meanwhile, the Kaplan model can also describe the influence of the properties of the LL on the pumping pressure of fresh concrete. Choi et al. [12] found that the rheological parameters of the LL were smaller than those of concrete and that its rheological properties significantly affected the pumping pressure. The formation of the LL was crucial to the pumping pressure. Without the formation of the LL on the pipe wall during the pumping process, the pumping pressure would greatly increase [31,32]. Pumping pressure would be significantly decreased with the increase in TLL [33]. Feys et al. [34] pointed out that the pressure of fresh concrete during pumping could be precisely evaluated by measuring the rheological properties and TLL combined with the rheological characteristics of fresh concrete. Choi et al. [10] simulated the pumping process of fresh concrete using CFD, considering the properties of the LL. The results showed that the numerical simulation could accurately predict the PL of fresh concrete during pumping. Chen et al. [17] also simulated the flow process of fresh concrete in the pipe using CFD and precisely estimated the pumping pressure required to form various TLL conditions on the pipe wall.

Materials 2024, 17, 5136

In conclusion, the pumping performance is significantly affected by the LL formed on the pipe wall during the pumping of fresh concrete. However, due to the lack of ap- propriate measurement methods, it is difficult to directly and accurately measure the TLL. Therefore, to precisely estimate the pumping PL to guide the actual construction more effectively, the numerical simulation of fresh concrete pumping, as well as considering3tohfe18 influence of the TLL and obtaining its size, is crucial. In this paper, the flow of six kinds of fresh concrete under different flow rates and TLL conditions in the horizontal pipe was

Concrete Grade C30

Water–Ce- ment Ratio 0.42

flowing, the flow velocity of the concreteτ i=s inτfl+ueηnγced by its plastic viscosity. (1) 0

τ is the shear stress, τ is the yield stress, η is the viscosity, and γ is the shear rate. Table 1. Content of each compo0nent of fresh C30 concrete.

The relationship curve between shear stress and shear rate is shown in Figure 1, where the

Concrete

Grade Ratio

C30 0.42

Cement Secondary Sand Fly Ash

300 90 900

Water-Reducing Agent

3.2

Water–Cement

Water

164

Stone

1080

In conclusion, the pumping performance is significantly affected by the LL formed

simulated. Firstly, the feasibility of CFD to simulate the rheology of fresh concrete using

on the pipe wall during the pumping of fresh concrete. However, due to the lack of

the Bingham rheological model was verified by the experiments and numerical simula-

appropriate measurement methods, it is difficult to directly and accurately measure the

tions of the slump test, L-box flow test, and V-funnel test. Then, the influence of the TLL

T . Therefore, to precisely estimate the pumping P to guide the actual construction more onLtLhe PL was simulated. Combining the simulationLPL and the experimental PL, the size

effectively, the numerical simulation of fresh concrete pumping, as well as considering the

of the TLL was obtained. The relationship between the actual size of the TLL and the PL was

influence of the T and obtaining its size, is crucial. In this paper, the flow of six kinds discussed. Finally, sLoLme important conclusions were given.

of fresh concrete under different flow rates and TLL conditions in the horizontal pipe was simulated. Firstly, the feasibility of CFD to simulate the rheology of fresh concrete using the

2. Materials and Methods

Bingham rheological model was verified by the experiments and numerical simulations of

2.1. Characteristics of Initial Materials

the slump test, L-box flow test, and V-funnel test. Then, the influence of the TLL on the PL

In this paper, the flow properties of fresh C30 concrete were tested and calibrated.

was simulated. Combining the simulation PL and the experimental PL, the size of the TLL

The concrete was supplied by a commercial concrete company. Its composition and pro-

was obtained. The relationship between the actual size of the TLL and the PL was discussed.

portions are shown in Table 1. C30 means the compressive strength of concrete is 30 MPa

Finally, some important conclusions were given.

after 28 days of curing. The flow behavior of fresh C30 concrete was assumed to be non- N2e.wMtoanteiarinalfsolalonwd iMngethoedBsingham law [35], which characterizes the yield stress and plas- ti2c.1v.isCchoasritayct[e3r5is,t3i6cs].oTfhIneitrihaeloMloagteircialsequationisshownasfollows:

In this paper, the flow properties of fresh C30 concrete were tested and calibrated. The

𝜏 = 𝜏0 + 𝜂γ (1) concrete was supplied by a commercial concrete company. Its composition and proportions

𝜏 is the shear stress, 𝜏 is the yield stress, 𝜂 is the viscosity, and 𝛾 is the shear rate. are shown in Table 1. C300 means the compressive strength of concrete is 30 MPa after

The relationship curve between shear stress and shear rate is shown in Figure 1, where

28 days of curing. The flow behavior of fresh C30 concrete was assumed to be non-

the influence of rheological parameters (yield stress and plastic viscosity) on the flow of

Newtonian following the Bingham law [35], which characterizes the yield stress and plastic fresh concrete is described. Fresh concrete remains stationary when the shear stress is less

viscosity [35,36]. The rheological equation is shown as follows:

than its yield stress. It immediately starts to flow once its yield stress is exceeded. Once

Secondary Water-Reducing

influence of rheological parameters (yield stress and plastic viscosity) on the flow of fresh

Water Cement Fly Ash Sand Stone Agent

concrete is described. Fresh concrete remains stationary when the shear stress is less than

164 300 90 900 1080 3.2

its yield stress. It immediately starts to flow once its yield stress is exceeded. Once flowing, the flow velocity of the concrete is influenced by its plastic viscosity.

Yield stress

Figure 1. Bingham fluid. Figure 1. Bingham fluid.

Plastic viscosity

shear rate

Table 1. Content of each component of fresh C30 concrete.

In the CFD simulation, the fresh concrete was regarded as an incompressible fluid. Throughout the flow process, the concrete was assumed to be isothermal, with the energy equation disregarded. The flow of the concrete was described using the Navier–Stokes

stress

Materials 2024, 17, x FOR PEER REVIEW 4 of 19

Materials 2024, 17, 5136

In the CFD simulation, the fresh concrete was regarded as an incompressible fluid.

4 of 18

Throughout the flow process, the concrete was assumed to be isothermal, with the energy

equation disregarded. The flow of the concrete was described using the Navier–Stokes equa-

tion. In the Cartesian coordinate system, the differential forms of the mass conservation

equation. In the Cartesian coordinate system, the differential forms of the mass conservation

equation and momentum equation were expressed by Equations (2) and (3), respectively.

𝜕𝜌 + ∇ ∙ (𝜌𝜈) = 0

𝜕𝑡 ∂ρ + ∇·(ρν) = 0

(2)

(3)

∂t

𝜕(𝜌𝜈) + ∇ ∙ (𝜌𝜈𝜈) = −∇𝑝 + ∇ ∙ (τ) + 𝜌𝑔 + 𝐹

𝜎𝑡 ∂(ρν)+∇·(ρνν)=−∇p+∇·(τ)+ρg+F σt

(3) where p is the static pressure on the fluid, ρ is the density, ν is the velocity vector, τ is the

where p is the static pressure on the fluid, 𝜌 is the density, 𝜈 is the velocity vector, 𝜏 is the stress tensor, and F is the generalized source term.

stress tensor, and F is the generalized source term. 2.2. Experiment

2.2. Experiment

Rheological properties mainly include the flow ability, filling ability, passing ability,

segregation resistance, etc. This measurement method mainly depends on its relationship

to workability. Additionally, factors such as cost, site conditions, and the advantages and

disadvantages of each test (e.g., economy, convenience, operability, and actual situation)

should also be considered. In this study, the slump, L-box test, and V-funnel tests were

used to calibrate the rheological parameters (yield stress and plastic viscosity) of fresh C30

concrete based on the CFD.

The slump test is mainly used to measure the flowability of fresh concrete. Owing to

simple equipment and operation, it is practical in laboratories and construction sites. In

simple equipment and operation, it is practical in laboratories and construction sites. In the

the slump test, the slump height H (mm) and expansion width L1 × L2 (mm) are key indi-

slump test, the slump height H (mm) and expansion width L1 × L2 (mm) are key indicators cators for assessing the flowability of fresh concrete. The basic device of the slump test is

for assessing the flowability of fresh concrete. The basic device of the slump test is shown

shown in Figure 2a. The dimensions are given in Figure 2b, with top and bottom diameters

in Figure 2a. The dimensions are given in Figure 2b, with top and bottom diameters of of 100 and 200 mm, respectively, and a height of 300 mm. H represents the slump height,

100 and 200 mm, respectively, and a height of 300 mm. H represents the slump height, and

and L represents the expansion width. The slump test was carried out several times, and

L represents the expansion width. The slump test was carried out several times, and the

the results are summarized in Table 2.

results are summarized in Table 2.

(a) Equipment (b) Related dimensions Figure 2. The equipment and related dimensions of the slump test.

Figure 2. The equipment and related dimensions of the slump test. Table 2. The experimental results of the slump test.

Number

245

510 × 600 510 × 600

Table 2. The experimental results of the slump test.

Number

H (mm)

2 3

5 Average

Average

245

230 240

235 245 239

460 × 510

240

235

245

239

460 ×459100× 530

480 × 500 490 × 530

230

NamNaeme

H(mm)

L ×L (mm) L1 ×L21(mm2)

The L-box flow meter is mainly used to evaluate the passing ability of fresh concrete, that is, the ability to cross dense steel bars. It consists of an L-shaped box made from steel plate, featuring a movable door for partitioning and a detachable steel mesh, as depicted in

500 × 550 480 × 500

488 × 538 500 × 550

488 × 538

Materials 2024, 17, x FOR PEER REVIEW 5 of 19

Materials 2024, 17, 5136

The L-box flow meter is mainly used to evaluate the passing ability of fresh concrete,

5 of 18

that is, the ability to cross dense steel bars. It consists of an L-shaped box made from steel

plate, featuring a movable door for partitioning and a detachable steel mesh, as depicted

in Figure 3a. In the L-box flow test, the flow index (Bm) is used to quantitatively describe

Figure 3a. In the L-box flow test, the flow index (Bm) is used to quantitatively describe the the flow performance of fresh concrete. Bm is defined in two ways: when the fresh concrete

flow performance of fresh concrete. Bm is defined in two ways: when the fresh concrete can

can flow to the rightmost end of the horizontal box, Bm = H2/H1; otherwise, Bm = (L1 − L)/L.

flow to the rightmost end of the horizontal box, Bm = H2/H1; otherwise, Bm = (L1 − L)/L. The parameters L1, L2, H1, and H2 are defined in Figure 3b. When −1 ≤ Bm ≤ 1, a larger value

The parameters L1, L2, H1, and H2 are defined in Figure 3b. When −1 ≤ Bm ≤ 1, a larger of Bm indicates better flowability of the fresh concrete. Multiple L-box flow tests were con-

value of Bm indicates better flowability of the fresh concrete. Multiple L-box flow tests were

ducted, with the results summarized in Table 3.

conducted, with the results summarized in Table 3.

(a) The equipment

Figure 3. The equipment and test dimensions of the L-box test.

105

0.381

(b) The dimensions of the Bm Figure 3. The equipment and test dimensions of the L-box test.

Table 3. The experimental results of the L-box test. Table 3. The experimental results of the L-box test.

concrete and is suitable for all grades of fresh concrete. The evaluation index is the flowing

time The(s)V,-wfuhninchelitsesdteifisnuesdedastothaessteimssethfreovmistchoesitoypaendingseogfretghaetivoanlvresuisntatinlcteheofrfersehsh v

coconnccreretetehaansdciosmsupilteatbeleyfeoxriatelldgrtahdeeVs-ofuf fnrnesehl. cTohnecrVet-efu. Tnhnelevteasltuawtiaosncianrdrieexdisotuhtesfleovwerianlg

titmimees,Twvi(tsh),twhehriechsuilstsdseufimnemdaarsiztehdeitnimTeabfrleom4.theopeningofthevalveuntilthefreshcon-

crete has completely exited the V-funnel. The V-funnel test was carried out several times,

Table 4. The experimental results of the V-funnel test. with the results summarized in Table 4.

H1 (mm)

L-L1

Number

Name Name H2 (mm) H2 (mm)

H1 (mm)

1 2 3 4 5

4Average

40 37 38 40 39 38.8

105 102

0.381 0.363

102

0.363

110

0.345

110

0.345

103

0.388

105 103105

0.372 0.307.388

Average

38.8

105 0.372 105 0.37

The V-funnel test is used to assess the viscosity and segregation resistance of fresh

Name

TNaubmleb4e.rThe experimental results of the V-funnel test.

Tv (s)

18.5

17.8

18.8

16.5

17.9 18.5 17.9 17.8

18.8

16.5

17.9

Number

5 1

Average 2

Name

Tv (s)

2.3. Simulation 4

The slump, L-box, and V-funnel tests were 3D modeled and meshed, and boundary conditions were setAavcecroargdeing to the actual situation. A two-phas1e7fl.9ow volume of fluid (VOF) model [37] was used to simulate the flow behavior of fresh concrete in these tests. The first and second phases were air and fresh concrete, respectively. The rheological model of fresh C30 concrete was characterized by the Bingham model, with a density of 2400 kg/m3. The rheological parameters were measured using the ICAR rheometer, as shown in Figure 4. The experimental procedure is not described in detail here. The rheological parameters were obtained by linearly fitting the torque and rotation speed to obtain the slope and intercept, which were calculated using the Reiner–Riwlin formula. The relationship between rotation speed, torque, yield stress, and plastic viscosity is shown in Figure 5.

logical parameters were obtained by linearly fitting the torque and rotation speed to ob-

Materials 2024, 17, 5136

tain the slope and intercept, which were calculated using the Reiner–Riwlin formula. The tain the slope and intercept, which were calculated using the Reiner–Riwlin formula. The

relationship between rotation speed, torque, yield stress, and plastic viscosity is shown in relationship between rotation speed, torque, yield stress, and plastic viscosity is shown in

Figure 5. Figure 5.

The simulated initial states of fresh concrete in the slump, L-box, and V-funnel tests The simulated initial states of fresh concrete in the slump, L-box, and V-funnel tests

are shown in Figure 6, where ‘0’ indicates only air, ‘1’ indicates only fresh concre6teo,fa18nd are shown in Figure 6, where ‘0’ indicates only air, ‘1’ indicates only fresh concrete, and

‘0–1’ represents a mix of both air and fresh concrete at any interface. ‘0–1’ represents a mix of both air and fresh concrete at any interface.

Figure 4. Measuring the rheological parameters of fresh concrete using the ICAR rheometer. Figure 4. Measuring the rheological parameters of fresh concrete using the ICAR rheometer. Figure 4. Measuring the rheological parameters of fresh concrete using the ICAR rheometer.

VV μμ

11 11

Rotation speed Ω Rotation speed Ω

(a) (a)

Shear rate γ’ Shear rate γ’

(b) (b)

Figure 5. Torque and speed are converted to rheological parameters: (a) curve of rotation speed and Figure 5. Torque and speed are converted to rheologiicall parameters:: (a) curve of rotation speed and

torque; (b) curve of shear stress and shear rate. torque; (b) curve of shear stress and shear rate. torque; (b) curve of shear stress and shear rate.

The simulated initial states of fresh concrete in the slump, L-box, and V-funnel tests

Materials 2024, 17, x FOR PEER REVIEW 7 of 19 are shown in Figure 6, where ‘0’ indicates only air, ‘1’ indicates only fresh concrete, and

(a) Slump

(b) L-box (c) V-funnel Figure 6. The initial distribution of fresh concrete in the flow tests.

‘0–1’ represents a mix of both air and fresh concrete at any interface.

Figure 6. The initial distribution of fresh concrete in the flow tests.

Duurrininggththeeppuumppininggpprroocceesss,,ththeefoforrmaatitoionnooffththeeLLLsisgignnifiificcaanntltylypproromootetessththee

ppuumppininggpprroocceesssooffththeefrferesshhccoonnccreretete..SSeeccrrieierruueettaal.l.[3[322]]sstatateteddththaattccoonnccrereteteccaannnoottbbee

ppuummppededwwithitohuotuthtehfeorfomramtiaotnioonf tohfethLeL LatLthaet tinhteerinfatceerfbaectewbeetnwtehencothnecrceotencarnedtethaendpipthee

wpaipll.eKwapll.aKnaeptlaln. [e1t8a]lf.o[1u8n]dfothuantdththeaLtLthheaLsLahthasicakntheiscskrnaensgsirnagnfgrionmg farpopmroaxpipmraotxeilmya1tetoly

51mtom5. Nmgmo.eNt galo. [e1t3a,1l.4[]1s3t,a1t4e]dsthaatetdthtehaTt thfeorTLdLiffoerrednifft ecorennctrectoenmcriexteurmesixvtuariessvbaertiweseebne- LL

tween 1 and 9 mm. It was also reported that the TLL ranges from 2 to 8 mm [26–29] or from 1 to 9 mm [13].

In this study, to determine the size of the TLL and its effect on the flow of fresh con- crete in pipes, various horizontal pipes were modeled with a diameter of 125 mm, a length of 2000 mm, and TLL values of 0, 1, 2, 3, 4, 6, and 8 mm. Then, they were meshed, and the

Torque T Torque T

Y Y

Shear stress τ Shear stress τ

τ0 τ0

Materials 2024, 17, 5136

7 of 18

S24 S50

Plastic viscosity (Pa.s) Yield stress (Pa)

0.5 8.0 15.0 300.0

1.3 25.0 11.0 100.0

0.8 10.0 12.0 200.0

2.0 30.0 12.0 80.0

1.0 5.0

2.5 50.0

t = 0 s

The experimental and simulation results of the L-box and V-funnel tests of fresh concrete are shown in Figures 8 and 9, respectively. The simulation results for the flow of fresh concrete in both the L-box and V-funnel were consistent with the experimental results at the same flow times. The average Bm value from multiple experiments was 0.37, while the simulated Bm was 0.39. The variation range of Tv obtained from multiple experiments was between 16.5 and 18.8 s, with an average value of 17.9 s. The simulated Tv was 18 s.

t =2 s t =4 s (a) Experiment

crete.

1 and 9 mm. It was also reported that the TLL ranges from 2 to 8 mm [26–29] or from 1 to 9 mm [13].

In this study, to determine the size of the TLL and its effect on the flow of fresh concrete in pipes, various horizontal pipes were modeled with a diameter of 125 mm, a length of 2000 mm, and TLL values of 0, 1, 2, 3, 4, 6, and 8 mm. Then, they were meshed, and the boundary conditions were set according to the actual working conditions. Finally, the flow process of fresh concrete in these pipes was simulated using CFD. The density of the central concrete was set at 2400 kg/m3. The properties of the LL are similar to those of the mortar of the pumped concrete [10]; thus, its density was set at 1600 kg/m3. The flow of six kinds of concrete with different rheological parameters under certain flow rates in the horizontal pipe was simulated. The rheological parameters of the center concrete and the LL in the horizontal pipe are shown in Table 5.

Table 5. Rheological parameters of fresh concrete and LL during pumping test [23].

Mixes

Strength Item LL Concrete LL Concrete LL

A25

Concrete

13.0 150.0

40.0 80.0

8 of 19

Aggregate Size A10 A20

Materials 2024, 17, x FOR PEER REVIEW 3. Results and Discussion

3.1. Comparison of Simulation and Experimental Results

The experimental and numerical simulation results of the final flow form of fresh

experiments was between 16.5 and 18.8 s, with an average value of 17.9 s. The simulated

concrete in the slump test are shown in Figure 7. The average H and L1 × L2 of the Tv was 18 s.

experiment are 240 and 488 × 538 mm, respectively. The simulated H and L1 × L2 are The experiments and simulations of the slump, L-box flow, and V-funnel tests

237 and 485 × 535 mm, respectively. As can be observed from the final flow form and the showed that the established CFD model using the Bingham model in commercial software

average H and L1 × L2, the simulation results were clearly close to the experimental results. ANSYS-Fluent v19 could well simulate the flow behavior and performance of fresh con-

Thus, the established model could well simulate the flow properties of fresh concrete in the

slump test.

(a) Experiment (b) Simulation Figure 7. The slump test.

Figure 7. The slump test.

experimental PL data from Choi et al. [23] and are shown in Figure 11. It was found that

Materials 2024, 17, 5136

(a) Experiment

Figure 7. The slump test.

t =2 s

(a) Experiment

(b) Simulation

8 of 18

t =0 s

t =4 s

t =0 s

t =2 s

(b) Simulation

t =4 s

Materials 2024, 17, x FOR PEER REVIEW

Figure 8. The flowing process of fresh concrete in the L-box test.

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Figure 8. The flowing process of fresh concrete in the L-box test.

t = 8 s t = 12 s

(a) Experiment

t = 18 s

t = 8 s t = 12 s

(b) Simulation

Figure 9. The flowing process of fresh concrete in the V-funnel test. Figure 9. The flowing process of fresh concrete in the V-funnel test.

3.2. Effect of TLL on PL

t = 18 s

The flow of S50A20 in a horizontal pipe with an inlet flow rate of 40 m3/h was simu- lated. When the size of the TLL was 0 mm, the yield stress and plastic viscosity of the fresh concrete were 80 Pa and 30 Pa.s, respectively. The simulated axial pressure contours and velocity in the pipe flow are shown in Figure 10. The axial pressure of the pipeline grad- ually decreased from the maximum pressure at inlet to 0 at outlet. The velocity of the concrete was highest at the center of the pipeline and lowest at the pipe wall. Moving from the center to the wall, the velocity of the concrete gradually decreased from the maximum to 0. Additionally, simulations were conducted for concrete pumping at a flow rate of 40 m3/h when the TLL was 0 mm. The simulation results for the PL were compared with the

Materials 2024, 17, 5136

9 of 18

The experiments and simulations of the slump, L-box flow, and V-funnel tests showed that the established CFD model using the Bingham model in commercial software ANSYS- Fluent v19 could well simulate the flow behavior and performance of fresh concrete.

3.2. Effect of TLL on PL

The flow of S50A20 in a horizontal pipe with an inlet flow rate of 40 m3/h was simulated. When the size of the TLL was 0 mm, the yield stress and plastic viscosity of the fresh concrete were 80 Pa and 30 Pa.s, respectively. The simulated axial pressure contours and velocity in the pipe flow are shown in Figure 10. The axial pressure of the pipeline gradually decreased from the maximum pressure at inlet to 0 at outlet. The velocity of the concrete was highest at the center of the pipeline and lowest at the pipe wall. Moving from the center to the wall, the velocity of the concrete gradually decreased from the maximum to 0. Additionally, simulations were conducted for concrete pumping at a flow rate of 40 m3/h when the TLL was 0 mm. The simulation results for the PL were compared with the experimental PL data from Choi et al. [23] and are shown in Figure 11. It was found

that the simulated results differed significantly from the experimental results when the

Materials 2024, 17, x FOR PEER REVIEW 10 of 19 pumped without the formation of an LL at the interface between the concrete and the

pipe wall. Therefore, to ensure the pumpability of fresh concrete in actual construction,

Materials 2024, 17, x FOR PEER REVIEW 10 of 19 TLL was 0 mm, being approximately three times larger. This means concrete cannot be

pipeline flow to reduce the effect of friction. Similarly, the influence of LL on the pumpa-

an LL with the appropriate thickness and stable state should be formed on the pipe wall

pipeline flow to reduce the effect of friction. Similarly, the influence of LL on the pumpa- bility of concrete should also be considered in the numerical simulation.

during the pipeline flow to reduce the effect of friction. Similarly, the influence of LL on the

bility of concrete should also be considered in the numerical simulation.

pumpability of concrete should also be considered in the numerical simulation.

(a) Distribution of axial pressure (b) Cross-section distribution of velocity

Figure 10. Simulation results of S50A20 pumping without LL.

70,000

70,000 60,000

60,000 50,000

50,000 40,000

40,000 30,000

30,000 20,000

20,000 10,000

10,000 0

S24A100

Measured results

Simulated results

Measured results

S24A20 S24A10

S24A25 MS2i4xAe2s0

S50A10

S24A25

Mixes

S50A20 S50A10

S50A20

Simulated results

Figure 11. Comparison of simulation and measured PL at flow rates of 40 m /h without LL.

Figure 11. Comparison of simulation and measured PL at flow rates of 40 m3/h without LL.

The pumping of S50A20 in a horizontal pipe with an inlet flow rate of 40 m3/h and a

The pumping of S50A20 in a horizontal pipe with an inlet flow rate of 40 m3/h and a TLL of 2 mm was simulated. The yield stress and plastic viscosity of the central-layer con-

TLL of 2 mm was simulated. The yield stress and plastic viscosity of the central-layer con- crete and the LL mortar were set at 80 Pa and 30 Pa.s and 12 Pa and 2 Pa.s, respectively.

crete and the LL mortar were set at 80 Pa and 30 Pa.s and 12 Pa and 2 Pa.s, respectively. The contours of pressure and velocity in the pipe flow were obtained and are shown in

The contours of pressure and velocity in the pipe flow were obtained and are shown in Figure 12. Compared to Figure 10, it was found that the formation of the LL significantly

PL (Pa/m) PL (Pa/m)

Materials 2024, 17, 5136

10 of 18

The pumping of S50A20 in a horizontal pipe with an inlet flow rate of 40 m3/h and a TLL of 2 mm was simulated. The yield stress and plastic viscosity of the central- layer concrete and the LL mortar were set at 80 Pa and 30 Pa.s and 12 Pa and 2 Pa.s, respectively. The contours of pressure and velocity in the pipe flow were obtained and are shown in Figure 12. Compared to Figure 10, it was found that the formation of the LL significantly reduces the pressure required for the flow of the fresh concrete in the pipe and the maximum speed of the central concrete. To ensure the pumpability of fresh concrete in actual construction, an LL should be formed on the pipe wall. Figure 13 presents a comparison between the simulated PL for different TLL values (0, 1, 2, 3, 4, 6, and 8 mm, respectively) and the measured PL for six types of concrete with different rheological parameters under specific flow rates. The simulated results showed the PL decreased with the increase in the TLL. However, the degree of reduction in the PL gradually decreased with the increase in the TLL. When the TLL was increased from 0 to 2 mm, the effect was more significant, reducing the PL. Especially, the formation of the LL on the pipe wall could largely reduce the PL, even if it is a thin layer. However,

Materials 2024, 17, x FOR PEERwRhEVenIEWthe TLL exceeded 4 mm, the influence of the continuous increase in the TLL on th1e1 of 19 PL was smaller. Combining the nonlinear fitted curve describing the simulated results

and the horizontal line expressing the experimental results, the value of the abscissa of

their intersection point was obtained, which represents the size of the TLL. For pumping 3 the size of the TLL. For pumping the S24A10, when the flow rate was 28, 40, and 50 m /h,

the S24A10, when the flow rate was 28, 40, and 50 m3/h, the obtained value of the TLL

the obtained value of the TLL was 1.54, 2.53, and 2.41 mm, respectively. For pumping the

was 1.54, 2.53, and 2.41 mm, respectively. For pumping the S24A20, when the flow

S24A20, when the flow rate was 29, 40, and 52 m3/h, the obtained value of the TLL was 1.23,

rate was 29, 40, and 52 m3/h, the obtained value of the TLL was 1.23, 1.8, and 1.53 mm,

1.8, and 1.53 mm, respectively. For pumping the S24A25, when the flow rate was 30, 40,

respectively. For pumping the S24A25, when the flow rate was 30, 40, and 50 m3/h,

and 50 m3/h, the obtained value of the TLL was 1.43, 1.42, and 1.38 mm, respectively. For

the obtained value of the TLL was 1.43, 1.42, and 1.38 mm, respectively. For pumping

pumping the S50A10, when the flow rate was 28, 40, and 50 m3/h, the obtained value of

the S50A10, when the flow rate was 28, 40, and 50 m3/h, the obtained value of the TLL

the TLL was 1.53, 2.37, and 2.39 mm, respectively. For pumping the S50A20, when the flow

was 1.53, 2.37, and 2.39 mm, respectively. For pumping the S50A20, when the flow

rate was 29, 40, and 50 m3/h, the obtained value of the TLL was 2.31, 2.3, and 2.31 mm,

respectively. For pumping the S50A25, when the flow rate was 29, 42, and 53 m3/h, the

obtained value of the TLL was 2, 2.58, and 2.39 mm, respectively. The size of the TLL was

obtained value of the TLL was 2, 2.58, and 2.39 mm, respectively. The size of the TLL

between 1 and 3 mm, and the minimum and the maximum were 1.23 and 2.58 mm, re- was between 1 and 3 mm, and the minimum and the maximum were 1.23 and 2.58 mm,

spectively. Their average value was 1.97 mm. The determined value of the TLL is shown in

respectively. Their average value was 1.97 mm. The determined value of the TLL is

Table 6.

shown in Table 6.

(a) Distribution of axial pressure (b) Cross-section distribution of velocity Figure 12. Simulation results of S50A20 pumping considering LL.

Figure 12. Simulation results of S50A20 pumping considering LL.

Simulation results of the flow rate = 28 m3/h Simulation results of the flow rate = 40 m3/h Simulation results of the flow rate = 50 m3/h Experimental results of the flow rate = 28 m3/h Experimental results of the flow rate = 40 m3/h Experimental results of the flow rate = 50 m3/h

PL (Pa/m)

Materials 2024, 17, 5136

the size of the TLL. For pumping the S24A10, when the flow rate was 28, 40, and 50 m3/h,

the obtained value of the TLL was 1.54, 2.53, and 2.41 mm, respectively. For pumping the

S24A20, when the flow rate was 29, 40, and 52 m3/h, the obtained value of the TLL was 1.23,

1.8, and 1.53 mm, respectively. For pumping the S24A25, when the flow rate was 30, 40,

and 50 m3/h, the obtained value of the TLL was 1.43, 1.42, and 1.38 mm, respectively. For pumping the S50A10, when the flow rate was 28, 40, and 50 m3/h, the obtained value of

Table 6. Determined results of TLL.

the TLL was 1.53, 2.37, and 2.39 mm, respectively. For pumping the S50A20, when the flow

Mixes 3 Measured Results [23] Determined Results

rate was 29, 40, and 50 m /h, the obtained value of the TLL was 2.31, 2.3, and 2.31 mm,

P TLL 3 respectively. For pumping the S50A25, when the flow rate was 29, 42, an(dmm5)3 m /h, the

Design Strength Aggregate Size L Flow Rate (m3/h) (Pa/m)

obtained value of the TLL was 2, 2.58, and 2.39 mm, respectively. The size of the TLL was

5294 28 1.54

between 1 and 3 mm, and the minimum and the maximum were 1.23 and 2.58 mm, re-

A10

7059 40 2.53

spectively. Their average value was 1.97 mm. The determined value of the TLL is shown in

Table 6.

S24

S50

8824 50 2.41

8824 29 1.23 10,588 40 1.8 13,529 52 1.53 11,176 30 1.43 13,529 40 1.42 16,471 50 1.38 11,176 28 1.53 14,706 40 2.37 18,824 50 2.39 15,882 29 2.31 21,765 40 2.3 25,882 52 2.31 19,412 29 2

A20

A25

A10

A20

A25

28,235 42 2.58

(a) Distribution of axial pressure

Figure 12. Simulation results of S50A20 pumping considering LL.

5000

02468

Figure 13. Cont.

(b) Cross-section distribution of velocity 35,294 53 2.39

Average value 1.97

TLL (mm)

(a) S24A10

11 of 18

PL (Pa/m)

aterials 2024, 17, x FOR PEER REVIEW Materials 2024, 17, 5136

12 of 19

12 of 18

40000 35000 30000 25000 20000 15000 10000

5000

02468

5000

Figure 13. Cont.

TLL (mm)

(b) S24A20

02468

TLL (mm)

Simulation results of the flow rate = 29 m3/h Simulation results of the flow rate = 40 m3/h Simulation results of the flow rate = 52 m3/h Experimental results of the flow rate = 29 m3/h Experimental results of the flow rate = 40 m3/h Experimental results of the flow rate = 52 m3/h

PL (Pa/m) PL (Pa/m)

Simulation results of the flow rate = 30 m3/h Simulation results of the flow rate = 40 m3/h Simulation results of the flow rate = 50 m3/h Experimental results of the flow rate = 30 m3/h Experimental results of the flow rate = 40 m3/h Experimental results of the flow rate = 50 m3/h

Materials 2024, 17, x FOR PEER REVIEW Materials 2024, 17, 5136

13 of 19

13 of 18

02468

Figure 13. Cont.

TLL (mm)

(d) S50A10

02468

TLL(mm)

(e) S50A20

PL (Pa/m) PL (Pa/m)

Simulation results of the flow rate = 29 m3/h Simulation results of the flow rate = 40 m3/h Simulation results of the flow rate = 50 m3/h Experimental results of the flow rate = 29 m3/h Experimental results of the flow rate = 40 m3/h Experimental results of the flow rate = 50 m3/h

Materials 2024, 17, x FOR PEER REVIEW Materials 2024, 17, 5136

14 of 19

14 of 18

120000
110000
100000

02468

TLL(mm)

(f) S50A25 FigFuirgeu1re3.1C3.oCmopmapraisriosnonooffssimimullationandmeeaasusurerdedPP.L.

The fitting line between the simulated PL and the measured PL when the TLL

Table 6. Determined results of TLL.

was 2 mm is shown in Figure 14. It could be found that the error between the fitting

line and the line of y = x was smaller. This indicated the simulation results were well Mixes Measured Results [23] Determined Results

correlated with the experimental results. In reference [24], a TLL of about 2 mm was

PL Flow Rate TLL

obtained by means of the particle image velocimetry technique, which is consistent

Design Strength Aggregate Size

with our conclusions. The effect of conc(Preat/emt)ypes on t(hme T/h) is shown in(Fmigmur)e 15.

Regardless of the size of the aggregate and the flow rate pumped, 1 mm < T < 2 mm

5294 28 L1L.54

for S24 concrete and 2 mm < TLL < 3 mm for S50 concrete. However, the value of

A10 7059 40 2.53

the TLL was related to the strength (S24, S50), the size of the aggregate (A10, A20,

conversely, 1 mm < T

< 2 mm. Relating Figure 15 to Figure 16, it was concluded 13,529 52 1.53

8824 50 2.41

A25), and the flow rate of pumping. The relationship between the PL and the TLL is

8824 29 1.23

shown in Figure 16; this relationship is similar to the effect of concrete types on the TLL.SI2t4was found that wAh2e0n the PL was l1a0rg,5e8r8than 14,0004P0a/m, 2 mm < TLL1<.83 mm;

that both the type of fresh concrete and the flow rate of pumping significantly affected

11,176 30 1.43

the PL . Then, the PL impacted the TLL . However, the TLL also impacted the PL .

A25 13,529 40 1.42

Finally, this made the TLL change within a certain range. The above findings also guide us in using CFD to simulate the16fl,o4w71of all type5s0of concrete in th1e.38actual

pumping pipeline with a T

of 2 mm to obtain their pumping pressure and guide the

A10

S50 A20

A25

Average value

11,176 28 1.53 14,706 40 2.37 18,824 50 2.39 15,882 29 2.31 21,765 40 2.3 25,882 52 2.31 19,412 29 2 28,235 42 2.58 35,294 53 2.39

1.97

actual construction.

Simulation results of the flow rate = 29 m3/h Simulation results of the flow rate = 42 m3/h Simulation results of the flow rate = 53 m3/h Experimental results of the flow rate = 29 m3/h Experimental results of the flow rate = 42 m3/h Experimental results of the flow rate = 53 m3/h

PL (Pa/m)

Materials 2024, 17, 5136

mm < TLL < 3 mm; conversely, 1 mm < TLL < 2 mm. Relating Figure 15 to Figure 16, it was

concluded that both the type of fresh concrete and the flow rate of pumping significantly

affected the PL. Then, the PL impacted the TLL. However, the TLL also impacted the PL. Fi-

nally, this made the TLL change within a certain range. The above findings also guide us

in using CFD to simulate the flow of all types of concrete in the actual pumping pipeline

with a TLL of 2 mm to obtain their pumping pressure and guide the actual construction.

40,000 35,000 30,000 25,000 20,000 15,000 10,000

5,000 0

5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 Measured PL (Pa/m)

Materials 2024, 17, x FOR PEER REVIEW

Figure 14. Comparison of simulation and measured pumping PL when TLL = 2 mm.

16 of 19

Figure 14. Comparison of simulation and measured pumping PL when TLL = 2 mm. 3.0

2.5

2.0

1.5

1.0

S24A10 S24A20 S24A25 S50A10 S50A20 S50A25

Mixes

Figure 15. The effect of concrete types on the TLL. Figure 15. The effect of concrete types on the TLL.

3.0

14,000

2.5

15 of 18

When the TLL is 2 mm

The fitting line y=x

TLL(mm) Simulated PL (Pa/m)

Materials 2024, 17, 5136

Mixes

Figure 15. The effect of concrete types on the TLL.

16 of 18

1.0

S24A10 S24A20 S24A25 S50A10 S50A20 S50A25

3.0

2.5

2.0

1.5

14,000

1.0

5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000

PL (Pa/m) Figure 16. The relationship between the PL and the TLL.

Figure 16. The relationship between the PL and the TLL. 4. Conclusions

In this paper, flow tests such as the slump test, L-box test, and V-funnel test on fresh C30 concrete were conducted experimentally and simulated numerically employing the CFD method. The flow of six kinds of fresh concrete under three groups with different pumping flow rates and different TLL values in the horizontal pipe was simulated. The main conclusions are summarized as follows:

The feasibility of simulating the rheological behavior and properties of fresh concrete employing the CFD method and the Bingham model was demonstrated through experi- ments and simulations of fresh concrete flow tests, such as the slump test, L-box test, and V-funnel test.

When the LL on the pipe wall was not considered, the simulation result of the PL was approximately three times higher than the experimental results. Conversely, the formation of the LL significantly reduces the PL. Therefore, to ensure the pumpability of fresh concrete in actual pumping construction, an LL must form on the pipe wall. The TLL significantly affects the PL. As the TLL increases, the PL decreases. However, the effect of increasing the TLL on reducing the PL gradually decreases. When the TLL increased from 0 to 3 mm, the reduction in PL was more pronounced. Especially, the formation of the LL could largely reduce the PL, even if it is a thin layer.

Relating the intersection point of the nonlinear fitted curve describing the simulated PL and the horizontal line expressing the experimental PL, the TLL for different flow rates for the six kinds of fresh concrete could be obtained. The values of the TLL ranged between 1 and 3 mm, with the minimum, maximum, and average values being 1.23 mm, 2.58 mm, and 1.97 mm, respectively. It was also found that the strength (S24, S50), aggregate size (A10, A20, A25), and pumping flow rate all affected the TLL. The mechanism of action was that the type of fresh concrete and the flow rate of pumping significantly affected the PL. Then, the PL impacted the TLL. However, the TLL also impacted the PL. Finally, this made the TLL change within a certain range. When PL > 14,000 Pa/m, 2 mm < TLL< 3 mm; conversely, 1 mm < TLL< 2 mm. Therefore, we can use CFD to simulate the flow of all types

TLL (mm)

Materials 2024, 17, 5136

17 of 18

References

of concrete in the actual pumping pipeline with a TLL of 2 mm to obtain their pumping pressure and guide the actual construction.

Author Contributions: Conceptualization, R.D. and T.Y.; data curation, T.Y.; formal analysis, Z.Y.; funding acquisition, R.D.; investigation, T.Y.; methodology, R.D., T.Y. and Z.Y.; project administration, T.Y. and Z.Y.; software, R.D. and T.Y.; supervision, R.D. and Z.Y.; validation, R.D., T.Y. and Z.Y.; visualization, R.D. and Z.Y.; writing—original draft, T.Y.; and writing—review and editing, all authors. All authors have read and agreed to the published version of the manuscript.

Funding: This work is supported financially by the NSFC Project (No. 51802148) and the Natural Science Foundation Project of Hunan Province (No. 2018JJ3435).

Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable.

Data Availability Statement: The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest: The authors declare no conflicts of interest.

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