Mechanistic origins of concrete pumping: a comprehensive outlook and way forward

The construction of high-rise buildings and tunnels necessitates pumping concrete over long distances. Concrete pumping is a complex physico-chemical process that results in the formation of different layers with distinct characteristics inside the pipe. A thin cement paste/mortar layer formed at the concrete–pipe interface, called the lubrication layer (LL), is vital for facilitating concrete pumping. Engineering the pumping process involves understanding the origin and controlling the onset, magnitude and velocity of the LL. However, the formation, evolution and characteristics of the LL are still largely unexplored. This paper presents a comprehensive review of the mechanistic origins of the LL, its influencing parameters and measured data on LL thickness using various techniques to date. An attempt is made to find an overall link between different theories (shear-induced particle migration, wall depletion theory etc.) in terms of concrete rheology. The paper also provides a fundamental understanding of the different mechanisms involved in LL formation, with the help of different theories put forth by researchers in the field of particle rheology. Based on the insights derived from the discussions, recommendations are made for future research directions in concrete pumping.