### 2.1 Thickness of the Slip Layer

The performance of concrete pumping is governed by the characteristics of the slip layer formed between the piping surface and the concrete. Ngo et al. (2010) used various concrete mixtures according to the grading of coarse and fine aggregates and the cement type. The pumping test results supported that the constituents composing the slip layer are water, cement, and fine sand smaller than 0.25 mm. The slip layer thickness is proportional to the volume of the cement paste, the water-to-cement ratio, and the dosage of superplasticizer. The slip layer thickness also decreases with a higher amount of fine sand. The length and diameter of pipeline are also considered to change the thickness of the slip layer.

In spite of the variation of the slip layer thickness, it is generally accepted as roughly 2 mm thick (Jo et al. 2012; Choi 2013). A flow simulation conducted by Jo et al. (2012) and Choi (2013; Choi et al. 2013a, 2013b) showed that the thickness of the slip layer slightly depends on the length of the pipe, concrete mixture, and the piping diameter. However, the test results showed its variation is not large during the pumping process (Choi 2013). The velocity of concrete pumping is developed more than 90 % within the 2 mm-thick slip layer. Choi (2013) and Choi et al. (2014) installed a transparent pipe for the 170 m full-scale pumping test to take a direct measurement of the velocity distribution inside the pipe using an ultrasonic velocity profiler (UVP) (Met-flow 2002). The thickness of the slip layer, deduced from the velocity distribution, varies slightly depending on the mix-proportion of concrete even though its approximation could be still said to 2 mm.

### 2.2 Characterization of the Slip Layer Using a Tribometer

The first concrete tribometer developed by Kaplan et al. (2005) is comprised of a concrete container, a rubber seal installed on the bottom of the container to prevent concrete leakage, cylindrical rotating axis, and a motor connected through the cylindrical rotating axis. The equipment rotates the center cylinder immersed in concrete at a fixed rotation speed, and it measures the corresponding torque to calculate the rheological properties of the slip layer. Figure 1 illustrates the original concept of the tribometer and the process of pumping prediction using the tribometer. First, the rotation speed of the cylindrical rotating axis is set to five different speeds of 0.20, 0.55, 0.90, 1.25, 1.60 rps (revolution per second) to measure the corresponding torque. The flow layer formed between the cylinder surface and the concrete is considered the slip layer. The rheological property of the slip layer can be calculated based on the relationship between the torque and the angular velocity (converted from the rotating speed). The rheological property of the slip layer measured through the tribometer is used to calculate the pumping pressure and the flow rate. The pumping pressure calculation of the Kaplan’s model is shown by

$$ P = \frac{2L}{R}\left( {\frac{Q}{{3600\pi R^{2} k}}\eta^{t} + \tau_{0i}^{t} } \right), $$

(1)

where *P* is the pumping pressure (Pa), \( Q \) is the flow rate (m^{3}/h), \( L \) is the pipe length (m), \( R \) is the pipe diameter (m), \( k \) is the parameter optimally fitting the slope or the linearity between the torque and the angular velocity (N m s/rad), \( \tau_{0i}^{t} \) is the yield stress of slip-layer (Pa), and \( \eta^{t} \) is the interface viscous constant (Pa s/m). The pumping prediction of the model was verified through 180 and 230 m pipe length full-scale pumping tests (Kaplan et al. 2005).

Tattersall and Banfill (1983) and Chapdelaine (2007) refined the rubber seal installed on the floor of the container to prevent concrete leakage. The rubber seal in the original Kaplan’s tribometer causes additional torque at the point where the cylinder meets the rubber seal at the bottom which affects the torque measurement. Even small error on the measured torque can give unreliable determination on the rheological property of the slip layer because its viscosity is relatively small compared to that of concrete. Its verification was done by pumping tests at a laboratory level: 15 m-long pumping circuit, where Kaplan’s model for predicting the pumping pressure was used. However, its application on full-scale or in-filed pumping test has yet been reported.

Figure 2 illustrates the shape and dimension of the Chapdelaine’s tribometer. Chapdelaine’s tribometer, developed with the original Tattersall Mk-III rheometer (Tattersall and Banfill 1983), uses a hollow internal cylinder, and it has ribs inside to prevent rotation of the concrete inside the cylinder. However, the use of the hollow cylinder causes error on developing the slip layer and measuring the frictional force. In addition, concrete with very low fluidity could form a space between the rotating cylinder and the concrete, which could cause difficulties for accurately measuring the rheological properties of the slip layer.

Ngo et al. (2010) returned to the original form of tribometer as illustrated in Fig. 3, where it includes the rotating cylinder on the flat bottom surface of the container. Two separate measurements are taken at different depths of concrete filled in the container. This was done to eliminate the friction measurement value in the bottom surface of the cylinder. The first measurement is taken by filling the cylinder with concrete to the bottom of the rotating cylinder, and the second measurement is done by full filling. The rheological property of the slip layer can be calculated by using the difference between the first and second measurements. However, its verification through real-scale pumping test was not reported in the original proposition.

Kwon et al. (2013a, 2013b) developed a new tribometer to precisely measure the rheological property of the slip layer in conjunction with a pumping prediction algorithm. The Kwon’s tribometer was proposed by modifying the ICAR Rheometer, a commercially available rheometer to measure the rheological property of concrete. It has a vane with a rotating diameter of 130 mm and a height of 240 mm. Figure 4 illustrates the method of measuring the rheological properties of the slip layer using the Kwon’s tribometer and the process of pumping prediction. Taking the separate measurements at two different depth of concrete, similar to the Ngo’s procedure, allows to eliminate the shear rate concentration on the bottom surface and edges of the rotating cylinder. Numerous test results on various concrete mixtures provide the optimized depths of concrete, *h*
_{1} and *h*
_{2} to be tested. These are 200 and 300 mm. In addition, a precise calculation on the rheological property of the slip layer could be achieved by assuming the thickness of the slip layer. Note that the model using the aforementioned three tribometers does not consider the slip layer thickness for the calculation. Figure 5 illustrates the velocity profile inside the pipe during pumping, and three layers are formed: slip-layer zone, shearing zone in concrete, and plug flow zone in concrete (Kwon et al. 2013b). Kwon et al. (2013b) applied the assumption of 2-mm thick slip layer on an SIPM model, and then calculated the distribution of the rheological property of the slip layer. The flow rate by integrating the velocity distribution inside the pipe during pumping is finally given by

$$ \begin{array}{*{20}l} {Q = \int_{{R_{L} }}^{{R_{P} }} {2\pi rU_{S} } dr + \int_{{R_{G} }}^{{R_{L} }} {2\pi rU_{P1} } dr + \int_{0}^{{R_{G} }} {2\pi rU_{P2} } dr} \hfill \\ {\quad = 3600\frac{\pi }{{24\mu_{S} \mu_{P} }}\left[ {3\mu_{P} \Delta P\left( {R_{P}^{4} - R_{L}^{4} } \right) - 8\tau_{S,0} \mu_{P} \left( {R_{P}^{3} - R_{L}^{3} } \right)} \right.} \hfill \\ {\left. {\quad \quad + 3\mu_{S} \Delta P\left( {R_{L}^{4} - R_{G}^{4} } \right) - 8\tau_{P,0} \mu_{S} \left( {R_{L}^{3} - R_{G}^{3} } \right)} \right]} \hfill \\ \end{array} , $$

(2)

where \( Q \) is the flow rate (m^{3}/h), \( r \) is an arbitrary position in radial direction, \( U_{S} \) is the velocity profile within the slip-layer, \( U_{P1} \) is the velocity profile within the shearing zone, \( U_{P2} \) is the velocity in the plug flow zone, \( R_{P} \) is the radius of pipe, \( R_{L} \) is the distance from the center of the pipe to the slip-layer, \( R_{G} \) is the radius at which the shear rate starts, \( \Delta P \) is the pressure per unit length (Pa/m), \( \tau_{P,0} \) and \( \mu_{P} \) are the yield stress (Pa) and the viscosity (Pa s) of the concrete, respectively. The difference between \( R_{P} \) and \( R_{L} \) is the slip layer thickness. Verification of the pumping prediction method was accomplished through 350 and 548 m long full-scale pumping tests.

Choi (2013) proposed that the rheological property of the slip layer of pumped concrete can be possibly measured with the mortar acquired through wet screening of pumped concrete (using a 5 mm sieve). Figure 6 illustrates the process of rheological property assessment of the slip layer and pumping prediction proposed by Choi (2013). Brookfield DV-II viscometer (Brookfield Engineering Laboratories Inc. 2006) was used to measure the rheological property of the wet-screened mortar. A rotating cylinder of 8 mm diameter and 60 mm height was used in a container having a diameter of 35 mm. The volume of mortar sample was 0.5 L. The range of rotating speed was set in between 0.4 and 4.2 rps. The pumping prediction was done by using the model proposed by Kwon et al. (2013).

It should be noted that it is inconvenient to perform wet screening for every test, and the quality of the wet-screened mortar may be inconsistent. The assumption that the mortar has the same property of the slip layer has not been strictly verified. As mentioned previously, Ngo et al. (2010) presumed that the suspension of particles which are less than 0.25 mm composes the slip layer of the pumped concrete. Figure 7 compares the results of the pumping prediction based on the propositions by Kwon et al. (2013), Choi (2013) and Choi et al. (2014), where pumped concrete was assumed the same for all cases. The thickness of the slip layers was a control parameter showing variation in between 1 and 3 mm even though its average was still 2 mm. Both propositions used the same method for the pumping prediction, and then the results depend on each measuring method of the rheological property of the slip layer. From Fig. 7, it can be seen that the slip layer measurement method proposed by Kwon et al. (2013) is more appropriate compared to the method proposed by Choi (2013) and Choi et al. (2014).

Feys et al. (2014) modified Tattersall Mk-III Tattersall and Banfill (1983) to develop a tribometer. Figure 8 illustrates the shape and size of Feys’ tribometer. For convenience purposes, Feys et al. (2014) designed the end part of the inner cylinder in a conical shape. This tribometer was used to assess the rheological properties of the slip layer of highly-workable concrete and SCC (self-consolidating concrete). The measured value of the rheological property of the slip layer was applied to the pumping prediction model developed by Kaplan et al. (2005) for pumping prediction (Feys et al. 2015). Its verification was performed through a 30 m pipeline pumping test, where 125 or 100 mm-diameters of pipe were used. The comparison of the two different-sized pipe showed that 20 % reduction of pipe diameter caused the pumping pressure to double (Feys et al. 2016). However, similar to Chapdelaine’s tribometer and Ngo’s tribometer, verifications through a full-scale or in-field pumping test have not yet been performed.

### 2.3 Characterization of the Slip Layer Using a Pumping Pipe

Another method to measure the characteristics of the slip layer is done by using a pipe segment similar to the actual pumping pipe. Mechtcherine et al. (2014) developed an equipment called Sliding Pipe Rheometer (abbreviated by Sliper) to assess the rheological property of the slip layer. Figure 9 illustrates the components of the Sliper, test methods, and the process of pumping prediction. The pipe diameter of the Sliper is 126 mm. The top surface of the piston has a pressure sensor installed, which measures the concrete’s pressure and the pressure caused by the pipes at the same time. The pipe speed is measured using a distance transducer, which is related to the concrete’s flow rate during pumping. Weights could be added to increase the pipe’s slipping speed. The measured pipe speed and pressure are saved to generate a graph that illustrates the pressure-flow rate relationship, as shown on the bottom left image of Fig. 9. In the pressure-flow rate relation curve, the Y-intercept is directly related to the yield stress of the slip layer, and the slope is a function directly related to its viscosity. After measuring the rheological property of the slip layer through the Sliper test, the following equation could be used to predict the actual pumping (Mechtcherine et al. 2014).

$$ P = P_{y} + P_{v} + P_{H} = \frac{4L}{D}a + \frac{16 \cdot L \cdot Q}{{\pi \cdot D^{3} }}b + \rho \cdot g \cdot H $$

(3)

where \( a = \tau_{i0} \) is the equivalent of yield stress at the slip layer, \( b = \mu_{i} /e \) is the equivalent of effective viscosity at the slip-layer, \( Q \) is the flow rate, \( D \) and \( L \) are the diameter and length, respectively, of the pipe, \( e \) is the thickness of the slip-layer, \( \rho \) is the density of concrete, and \( H \) is the pumping height. The rheological property of the slip layer, \( a \) and \( b \), could be derived through the Sliper test. A desired pressure could be calculated by applying a desired flow rate \( Q \). The slip layer’s thickness cannot be measured through the Sliper test. Thus, the viscosity (\( \mu_{i} \)) and thickness (*e*) of the slip layer cannot be calculated. However, since the concrete flow condition in the Sliper test is assumed to be the same as the flow condition in the actual pumping process, the slip layer’s thickness is also assumed to be the same for both cases. Therefore, pumping can be predicted by calculating the parameter \( b = \mu_{i} /e \) based on the pressure-flow rate relationship without the need of the precise slip layer thickness.

The method proposed by Mechtcherine et al. (2014) considers only the friction on the surface of the pipe’s interior without concrete flow. In other words, it cannot examine the shear flow that occurs inside the concrete during the actual pumping process. Thus, this method is not suitable for predicting pumping for concrete mixtures with very low yield stress, such as highly flowable concrete.

Kim et al. (2014) developed a device to measure the internal frictional resistance. This device was installed in a 50 m horizontal pumping circuit to measure the frictional resistance of concrete mixture. Concrete pumping pressure was predicted based on the frictional resistance coefficient derived through the 50 m pumping test. The results showed that the predicted value and applied pressure value are highly correlated. Nevertheless, its application on predicting the pumpability of concrete is limited because it needs a full-scale pumping test.