Measuring the thixotropy of conventional concrete: The influence of viscosity modifying agent, superplasticiser and water

The study sets out to evaluate the thixotropy of conventional concrete containing rheology modifiers using various methods of rotational rheometry. Five mixes were examined: the control, two mixes with viscosity modifying agent (VMA) and superplasticiser (SP) respectively, a mix with combined viscosity modifying agent and superplasticiser (VMA + SP) and the last mix with an increase in water content. These mixes were formulated to achieve concrete with varying rheological properties. Tests were carried out to examine the evolution of rheology parameters (static yield stress, dynamic yield stress and plastic viscosity), thixotropy index, shear thinning and thickening, flocculation properties, torque decay pattern (with a subsequent drop in apparent viscosity), and hysteresis loop. Results show that conventional concrete known to be a shear thinning and thixotropic material, can possess anti-thixotropic (rheopexy) and shear thickening properties depending on the condition pre-history, resting time, shearing rate and inclusion of rheology modifiers. These factors also influence the magnitude of the thixotropy but not necessarily the evolution of thixotropy. Inclusion of VMA in conventional concrete had only slight influence (both increasing and decreasing depending on the method of measurement) on its thixotropic behaviour while the inclusion of SP, VMA + SP and increased water considerably influenced concrete's thixotropic behaviour in a similar pattern. However, each rheology modifiers had its unique influence. VMA significantly improves the evolution of concrete's viscosity; inclusion of SP tends to result in a rheopectic concrete; addition of VMA + SP and pre-shearing of concrete may result in an unstable thixotropy, while SP can initially dominate the thixotropic behaviour of concrete containing both VMA and SP, VMA's influence may later surpass that of SP; more water in concrete tends to result in a lesser rate of thixotropic structure breakdown.