Analysis of the relationship between the L-box test and the yield stress of fresh concrete: an analytical approach

The objective of this work was to study fresh concrete flow in the L-box test, predict the final deposit shape after flow stoppage and estimate the yield stress. This is very useful for controlling concrete casting in real structures, detecting problems and developing solutions on time. Concrete rheometers used for measuring the rheological parameters of concrete are expensive, not always available and can lead to measurement disparity. To overcome these issues, an effective method is to analyse concrete flow phenomena physically and solve the problem analytically based on L-box measurements. In the first part of this work, fresh concrete was considered as a homogeneous yield stress fluid moving under the effect of its self-weight. By developing momentum equations for an incompressible fluid, a theoretical approach was used to study three probable final shapes depending on the concrete rheology. Expressions were established for predicting the final shape and the yield stress. In the second part, the results obtained were compared with those of previous models and good agreement was found, thus validating the proposed approach. This study provides a consolidation of rheometer results, allowing evaluation and perhaps a calibration to reinforce their validity.