L-Box Form Filling of Thixotropic Cementitious Paste and Mortar

Abstract

Rheological properties of cementitious pastes and mortar affect the casting, placement, and setting properties of fresh concrete. Fundamental rheological knowledge thus helps in predicting concrete flowability and workability. Empirical equations correlate actual rheological parameters based on physical material characteristics to workability tests. Still, these equations generally only take the dynamic yield stress of the material into account. This is not sufficient for thixotropic cementitious pastes or mortars, which possess structural buildup at rest. Workability predictions regarding the flow of concrete are thus more complicated with thixotropic materials. During form-filling in L-shaped formworks, the flow velocity of concrete slows down, wherefore rheological parameters change with time. At initial fast flow, high shear rates without structural buildup can be assumed. Dynamic yield stress and a steady state viscosity thus are proper parameters for empirical equations describing concrete flowability. During low shear rates, partial structural buildup takes place. Viscosity and yield stress increase due to agglomeration and affect the flowability of concrete tremendously. Rheological parameters of various cementitious pastes and mortars varying in their solid volume fraction and flowability were investigated in a vane-in-cup rheometer. The workability of the same mixtures was investigated by measuring the flow length in an L-shaped formwork. The effect of yield stress, viscosity, and thixotropic structural buildup on flow length was investigated. Subsequently correlations and discrepancies between flowability parameters and workability equations were analyzed.

The Slump Flow of Cementitious Pastes: Simulation vs. Experiments

“…where τ 0,A.R is the analytical yield stress in Pa, ρ p the density of the tested material in kg/m 3 , V the tested volume in m 3 , and R the final slump flow radius in m. Yield stress analysis with Equation (1) correlates with the rheological analysis from rheometric measurements for cement pastes with slump flow values > 200 mm; see e.g., [13,14]. The L-Box test and slump flow test are also commonly used as model setups in computational models.…”

Image‐based analysis of fresh concrete flow ‐ determining the correlation between flow behavior and rheological properties

“…The measures taken to reduce the environmental impact, however, also lead to a reduction in robustness of the concretes, e.g. by reacting more sensitively to fluctuations of the water content and the raw materials [8]. Therefore, it is necessary to develop new digital approaches for a realtime and continuous concrete characterization in order to improve quality control.…”