On predictive modelling of yield stress increase in fresh cement paste

Abstract

AbstractFreshly mixed concrete is composed of cement paste as fluid‐like component and aggregates as solid particulate component. Development of yield stress in fresh mortar is significant for reliable and high‐quality concrete operation as too low or too high yield stress causes process‐ability challenges like the multi‐layer problem, aggregate flocculation or blockage. Traditional experimental tests on the evolution of yield stress are costly since a wide spectrum of different material and environmental conditions needs to be studied. Progressing hydration, thermal effects and mechanical loading/unloading may influence spatial distribution of the apparent yield stress. However, this local information is very difficult to obtain using traditional laboratory tests and virtual fresh concrete testing is therefore promising.In the context of a single‐component flow approach to homogenized modelling of the cement paste composition at the macro‐scale, the microstructural evolution affecting the development and distribution of apparent yield stress is taken into account by a thermo‐chemo‐mechanical phase‐field model bridging the micro‐macro length scales by using a Ginzburg‐Landau‐type free energy function. The phase‐field, that describes the smooth spatial transition between fluid‐like and solid‐like behaviour of cement paste in the setting phase, is governed by bond‐building chemical processes (hydration) and reversely‐acting mechanical effects (bond breakup). The resulting set of coupled nonlinear advection‐reaction‐diffusion equations are formulated in the Eulerian framework and discretized in space using the weighted residuals method and finite differences in time. Implementation of the predictive model is demonstrated using the FEniCS framework, together with numerical examples supporting model validation.