Behavior of a fresh cement mortar under pipe flow

Abstract

Flow blockage during concrete pumping remains a poorly understood phenomenon, despites its regular occurrence in the field. In this talk, we discuss shear induced particle migration in mortars and concrete under pipe flow by studying a model mortar made with red glass beads and sheared in a circular pipe by flow-boundary oscillatory motion. Past research has shown that the migration of large sand particles and aggregates away from the pipe wall in concrete under pipe flow results in the creation of a lubricating layer, where the large particle content is decreased and the shear localized. This lubricating layer has been associated with a typically lower than expected pumping pressure in concretes. On the other hand, further migration of particles away from the wall and towards the center of the pipe results in their increased velocity, causing their accumulation at the concrete front. When the particle concentration becomes high enough, the flow becomes frictional, resulting in pipe blockage. In this work, we focus on shear-induced migration in a model mortar of relatively low yield stress, and we evidence migration not only close to the wall but across the whole pipe. An extensive rheological characterization of the mortar, whose properties evolve with time, fosters a discussion when comparing our results to those in literature.