Metrology Needs for Predicting Concrete 1 Pumpability

: 14 With the increasing use of pumping to place concrete, the development and refinement of 15 the industry practice to ensure successful concrete pumping is becoming an important need 16 for the concrete construction industry. To date, research on concrete pumping has been 17 largely limited to a few theses and research papers. The major obstacle to conducting 18 research on concrete pumping is that it requires heavy equipment and large amounts of 19 materials. Thus, developing realistic and simple measurement techniques and prediction 20 tools are a financial and logistical challenge that is out of reach for small research labs and 21 many private companies in the concrete construction industry. Moreover, because concrete 22 pumping involves the flow of a complex fluid under pressure in a pipe, predicting its flow 23 necessitates detailed knowledge of the rheological properties of concrete, which requires 1 new measurement science. This paper summarizes the technical challenges associated with 2 concrete pumping and the development in concrete pumping that have been published in 3 the technical literature and identifies future research needed for the industry to develop 4 best practices for ensuring successful concrete pumping in the field.


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Concrete pumping has become one of the most widely used approaches to place concrete. 10 Pumping enables the transport of concrete to forms and molds while increasing the speed 11 of delivery and allowing access to hard-to-reach areas. This is not a new technology as it 12 was first used in 1930, but its usage continues to grow due to an increase in demand for 13 super structures such as high-rise buildings and other tall structures. Consequently, the 14 optimization and development of prediction methods for concrete pumping is becoming a 15 crucial issue for the concrete industry. Since concrete pumping requires mixing trucks, 16 pumps and pipes, combined with a large amount of material and instrumentation, it is not 17 surprising that only a few theses and research papers exist on the topic. The goal to 18 develop realistic and simple measurement techniques and prediction tools is a challenge of 19 great importance for the concrete industry. 20 As concrete pumping involves the flow of a complex fluid under pressure in a pipe, 21 predicting its flow requires detailed knowledge of its rheological properties. However, the 22 proper characterization needed to predict flow is not easy to achieve because it involves 23 understanding a variety of factors such as dynamic segregation, the stability of entrained 24 air, the geometry of the pumping circuit, the dynamics of a slip-layer formed between the 1 bulk concrete and the pipe wall, and the relationship between the pressure and the flow 2 rate. In practice, this is colloquially referred to as the concrete 'pumpability'. Interestingly, 3 the American Concrete Institute (ACI) guide on terminology does not include a definition 4 of pumpability. 5 This paper identifies the dominant factors for a concrete to flow in a pipe in an effort to 6 define pumpability. The paper also summarizes the technical advances in concrete 7 pumping that have been published in the technical literature, which is used to identify gaps. 8 The resulting gaps are used to identify future research needed for the industry to develop 9 best practices for ensuring successful concrete pumping in the field. Pumping is increasingly utilized as an efficient and economical method to place concrete 14 in large projects while not compromising its desired performance. To ensure quality, it is 15 important that the fresh concrete properties are not significantly altered as it moves 16 through the pumping system (pump, pipes, etc.). This is not necessarily trivial in that 17 processes like segregation of aggregates can take place as the concrete is pumped. 18 Several attempts were made to develop metrologies to predict the flow of concrete in a 19 pipe. The most comprehensive state of the art report was prepared by Jacobsen et al. [1]. 20 They established one criterion for concrete flow in a pipe using the slump test for 21 concretes. Here, it was suggested that a slump range of 50 mm to 100 mm will provide 22 acceptable flow in the pipe; below that range the concrete will not flow in the pipe without 23 compromising the desired performance; above that range the concrete may not flow in the 24 pipe as desired. However, this criterion does not encapsulate the effects from factors such 1 as dynamic segregation or a slip-layer, which might play a dominant role in determining 2 the performance during pumping. Further, for concrete pumping, the shear rate is typically 3 around 10 s -1 to 100 s -1 , whereas for the slump test, it is only 1 s -1 or less [2]. Hence, 4 results from the slump test that is carried out in a flow regime different from that of 5 pumped concrete may not be relevant for predicting the ability of concrete to flow in a 6 pipe.

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A definition for concrete workability being suggested by Richtie [3] is composed of three 8 components: stability, ability to consolidate, and mobility. Each of these components has 9 associated materials properties/performance requirements as follows: 10 • The viscosity µ of a fluid is the ratio of the shear stress τ to the shear rate γ : This definition is convenient for Newtonian fluids, and certain non-Newtonian 4 fluids. In other cases, an engineering approach to the description of a fluid can simplify the 5 analysis. For instance if the fluid is approximated as a power law fluid, it can be described 6 by eq. (1): where K is the power law consistency index and n is the power law exponent. The 9 corresponding velocity profile in a circular pipe is then given by eq. (2) [4]: where v is the fluid velocity as a function of the radial position, r , in the pipe, Q is 12 the volumetric flow rate and p R is the pipe radius. The fluid power law consistency 13 index can be calculated using the following eq. (3) [4], which requires a pressure drop 14 measurement over a certain length: where P ∆ is the pressure drop, and L is the distance between the pressure sensors. The 17 exponent n and the factor K could also be determined via eq. (1) from rheological The shear rate at the wall surface is calculated using the following equation [4]: The local shear stress is: The eqs. (1) through (5) describe flow of a homogenous fluid in a pipe. However, concrete 5 is more a complex fluid because it contains aggregates with a wide range of sizes. These   Kaplan [9] reported that the flow of concrete in a pipe is mainly related to the viscosity of 6 the slip-layer and that its properties could be measured by tribometry. He found that the 7 correlation between the properties of the bulk material as measured in a rheometer and the 8 properties of the slip-layer was weak. Jacobsen et al. [10] showed by using colored 9 concrete that the velocity profile of the concrete resembled that of plug flow in the pipe 10 center, and non-moving slip-layer, similar to that shown in Fig. 1.

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Kwon et al. [11,12] measured the rheological properties of concrete before and after 12 pumping while monitoring the pressure and flow rate and found that while there was no 13 correlation between rheological properties of bulk concrete and flow rates, there was a 14 strong correlation between properties of the slip-layer and flow rates. Thus they deduced 15 that the slip-layer is the determining factor to predicting that concrete will flow in a pipe. 16 They also developed a tribometer that is a coaxial rheometer with a smooth bob made of 17 steel or covered with rubber to simulate the slip-layer of the pipe. 18 Ngo et al. [13] observed that the slip-layer is between 1 mm to 9 mm thick, by visualizing 19 the material flow in the rheometer. He analyzed the layer and found that it contained sand 20 with a particle size less than 0.25 mm. This would imply that there is a migration of coarse 21 aggregates from near the wall to the center of the pipe where the shear rate is lower than 22 that found near the walls. between the pressure of the pump and the flow rate of the material is linear: where 1 k and 2 k are two empirical parameters that depend on the material and other 7 experimental conditions. They concluded that the two parameters can be used to 8 characterize a specific mixture and the knowledge of these parameters for a specific 9 mixture and pumping circuit could be used as a quality control tool to ensure that the 10 applied pressure is sufficient to ensure the desired flow rate. self-consolidating concrete (SCC) requires a higher pressure, while the yield stress is 20 almost zero, but the plastic viscosity is higher than that for normal concrete. This could be 21 due to the slip-layer ( Fig. 1) that would require a higher shear stress at the same shear rate 1 due to the increased viscosity. Dynamic segregation is an additional factor that can influence concrete flow in a pipe. A 6 concrete can display no segregation while at rest, but undergo segregation during shearing. introducing the concrete in the pipe, so that the mortar would receive the coarse aggregates.

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It is important that this mortar have the correct rheological properties and suitable volume, 3 to prevent the coarse aggregates from separating from the concrete mixture. Moving the 4 aggregates that are in front of the concrete mixture would likely require a pressure that is 5 beyond the capability of the pump, due to the dry friction between the aggregates and the 6 walls. This will result in blockage of the pump. shearing layer is also difficult to characterize. Here it is believed that the rheological 1 parameters of viscosity and yield stress play a significant role. 4) Dynamic segregation 2 plays a major role in the distribution of the aggregates inside a pipe. 3 From this brief overview, the ability of concrete to flow in a pipe under pressure is 4 governed mainly by the slip-layer properties and the dynamic segregation. Thus, it could 5 be noted that tribology plays an essential role in predicting the concrete pumping. This 6 paper will, therefore, concentrate on this aspect of the flow of concrete in a pipe.    The measured torques and the applied angular velocities have the following relationship, The viscosity of the lubricating layer is related to the parameter, k , from eq. (9) and is 17 expressed as follows, Through the relationship between the torque and angular velocity of the tribometer, the rheological properties of the slip-layer could be determined. shearing layer should first be determined, as follows, where G R is the radius of the inner concrete (Fig. 1), and ,0 b τ is the yield stress of the 18 inner concrete. The shear rate of the inner concrete exists between G R and L R , and is 19 expressed by the following equation.
where pb µ is the plastic viscosity of the inner concrete. The inner region which has a 2 lower yield stress than the concrete has zero shear rate (plug flow).
The velocity is the integral of the shear rates from the wall to any position in the radial 5 direction, and is expressed by the following equations, Thus, the characteristic flow rate can be analytically determined using rheological  [21, 22,23]. This mechanism, as descried by Leighton et al. [22,23], assumes that 20 particles have a tendency to migrate away from region of higher shear rate to regions of 21 lower shear rate. Thus, as the higher shear rate is near the walls, particles would migrate 22 away from the wall of the pipe forming a slip-layer. The inhomogeneous distribution of the 23 particle concentration across a section of the pipe (radially) leads to spatially varying rheological properties in the suspension as they depend on the particle concentration. 1 Leighton et al. [22,23] suggested phenomenological models for particle migration in non-2 homogeneous shear flows that typically result from spatial variation in irreversible The general governing continuum equation of the shear-induced particle migration for the 10 Poiseuille flow is as follows [24], which describes the concentration of particles as a 11 function of radius and time: where φ is the particle concentration, t is the time, z u is the velocity component in 14 the flow direction, a is the particle radius, z is the flow direction, r is the radial 15 direction, η is the apparent viscosity of the concentrated suspension, and c K and K η 16 are dimensionless phenomenological constants. Here, the stress gradient is a driving force 17 to move particles toward the center of the pipe as described in the first term of the right 18 side in eq. (20). The increase of the particle concentration due to the migration may 19 increase the viscosity and the yield stress, which hinder the additional migration of the 20 particles as described in the second term of the right side in eq. (20). As a result, the 21 concentration of the particles inside the pipe is determined by the balance between the two 22 actions, namely, the migration due to the stress gradient and the hindrance due to the 1 increased viscosity. Through the analysis of the shear-induced particle migration, which is 2 one type of dynamic segregation, the formation of a slip-layer can be simulated and its 3 layer properties could be determined.

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An alternative approach for modeling suspension flow is called the "suspension balance 5 model" [25] in which the suspension is described as a continuum fluid whose dynamics is 6 described by the macroscopic mass, momentum, and energy balance equations. As in the 7 case of the particle diffusive model, this approach also predicts an increased particle which are not easy to clearly define.

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A second approach, based on eq. (20), is shear-induced particle migration (Fig. 3). This 11 continuum approach can account for particle migrations by modeling particle collisions in 12 highly sheared and/or highly concentrated zones that force particles to migrate from these The SPH approach could be utilized to study the following three flow scenarios to better 9 understand and predict the flow of pumped concrete. The integrated results from such simulations would provide insight into predicting the 5 successful flow of pumped concrete for many of the challenging flow scenarios found in 6 the construction industry. Costs can be reduced as fewer tests will be needed and optimal, 7 robust blends can be more easily formulated by the concrete producers.  2. In order to characterize the slip-layer, tribology tests were mainly investigated 21 using a tribometer which is a special coaxial rheometer whose bob is purposely 22 made with a smooth surface. Through the relationship between the torque and angular velocity of the tribometer, the rheological properties of the slip-layer can 1 be determined. 3. An analytical prediction of the flow rate and pumping pressure in a pipe was 3 obtained based on the assumption of three layers in a pipe.

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The critical research needs are also identified: properties of the matrix. 18 A suggested definition of pumpable concrete is:

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A property of a concrete, mortar or grout to flow through a pipe, for a given diameter 1 and length, that can be discharged with the desired performance, i.e., homogenous, non-2 segregated, and with the specified rheological properties needed for the application. 3 The definition of pumpability or the quantification of how pumpable a concrete is would  The present paper was mainly focused on a literature review and providing ideas on how to 8 characterize the flow of concrete and demonstrate the basic principles needed to analyze 9 the tribology. Thus, through a more specific investigation of tribology, the relationship 10 between the tribology and the pumpability as defined is needed to be examined. Dr. Ferraris and Dr. Martys gratefully acknowledge support from the CREME consortium.
14 We would like to thank Dr. Kenneth Snyder for his careful reading of this paper.