How do concrete rheology, tribology, ﬂow rate and pipe radius inﬂuence pumping pressure? Dimitri Feys a, * , Kamal H. Khayat a , Rami Khatib b a Department of Civil, Architectural and Environmental Engineering, Missouri University of Science and Technology, Rolla, MO, United States b Department of Civil Engineering, Universit e de Sherbrooke, Sherbrooke, QC, Canada article info Article history: Received 16 April 2015 Received in revised form 22 September 2015 Accepted 2 November 2015 Available online 6 November 2015 Keywords: Rheology Pumping Tribology Pressure Radius Concrete abstract Finding the critical factors that inﬂuence the pressure during pumping of concrete has been investigated for years. From ﬂuid mechanics, the relationship between pressure and ﬂow rate, radius or viscosity is known. In the practical guidelines for pumping of conventional vibrated concrete (CVC) the viscosity term is replaced by the concrete yield stress. However, recently, the inﬂuence of viscosity on pumping pressure has been reevaluated for self-consolidating concrete (SCC). In this paper, the inﬂuence of concrete rheology, tribology, ﬂow rate and pipe radius on pumping pressure are discussed, based on full-scale pumping tests. The concrete mixtures varied from pumpable CVC to segregating SCC. The inﬂuence of ﬂow rate and viscosity on pumping of concrete has been conﬁrmed. It is also shown that with a 20% decrease in pipe radius (from 125 to 100 mm), the pumping pressure can be roughly doubled. An increase in yield stress also increases the pumping pressure, but its inﬂuence is only visible when the viscosity is approximately constant. The total ﬂow resistance in the tribometer also appears to correlate well with the pumping pressure, proving that the developed trib- ometer mimics quite well the ﬂow of concrete in a pipe. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Concrete has been placed in formworks by means of pumping for decades. Although pumping of concrete can be regarded as a special case of pipe ﬂow ﬂuid dynamics, the main approach to study the behavior of concrete in pipes was practical and empirical of nature. Guidelines for proportioning pumpable concrete, for designing pipeline conﬁgurations, and best practices for placing concrete by pumping are based mostly on practical experience and trial and error approaches [1e4]. However, several studies on pumping of concrete were performed, focusing in most cases on predicting pumping pressure. In the earlier studies on pumping, friction was considered as the main physical aspect [5e8]. Concrete was assumed to slide in a pipe, and the frictionevelocity relation- ship was investigated. Later, the aspect of dynamic segregation of coarse aggregates was explored, which led to the deﬁnition of the lubrication layer: a micro-mortar layer that is formed near the pipe wall. Due to its lower yield stress and viscosity relative to concrete, shearing occurs in this micro-mortar layer. Friction is no longer assumed to dictate the ﬂow behavior of concrete, it is rather the complex variations in concrete composition and rheological prop- erties at different distances from the pipe wall that describe the ﬂow parameters of concrete in pipelines [9e12]. More recently, the velocity proﬁle near the pipe wall during pumping of concrete was visualized, clearly indicating the presence of the micro-mortar layer with lower rheological properties near the wall [13,14]. The thickness of this layer is approximately 1.5e2 mm for a variety of concrete mixtures and pumping conditions [13,14]. With the recent developments in assessing the thickness of the lubrication layer, pumping pressure in straight pipes can be calculated if the rheological properties of the micro-mortar and concrete are known. In another approach, developed in the early 2000's, the lubrication layer properties are reproduced in a trib- ometer [9e12]. This device is similar to a concrete rheometer, but while in a rheometer, the formation of the lubrication layer must be prevented, a tribometer has a smooth surface to provoke the dy- namic segregation of the coarse aggregate. Recently, while several tribometers were developed mainly for conventional vibrated concrete (CVC), the authors have proposed a design and calculation procedure to investigate the pumping characteristics of more ﬂowable concrete mixtures including self-consolidating concrete (SCC) [15]. * Corresponding author. E-mail address: feysd@mst.edu (D. Feys). Contents lists available at ScienceDirect Cement and Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp http://dx.doi.org/10.1016/j.cemconcomp.2015.11.002 0958-9465/© 2015 Elsevier Ltd. All rights reserved. Cement and Concrete Composites 66 (2016) 38e46