Workability design of fresh concrete based on numerical flow simulation

Author(s): Zhuguo Li
Paper category: Proceedings
Book title:4th International Symposium on Design, Performance and Use of Self-Consolidating Concrete
Editor(s): Caijun Shi & Zuhua Zhang, Hunan University, China
Kamal Henri Khayat, Hunan University, China
Missouri University of Science & Technology, USA
ISBN:978-2-35158-204-6
e-ISBN:978-2-35158-205-3
Publisher: RILEM Publications SARL
Publication year:2018
Pages:53-54
Total Pages:2
Language : English

Abstract: The use of various chemical and mineral admixtures results in the diversity of fresh concrete’s properties. Therefore, it becomes difficult to judge the workability only by the slump test or slump flow test. So far, a lot of studies on the rheological test method have been performed to try to evaluate the workability of fresh concrete, but since the workability is also related to the structural, construction and environmental conditions, the workability would not be properly judged if only based on the rheological test results. There are no doubts that designing workability of concrete by construction experiment is costly and labor intensive. Therefore, it is a good alternative to evaluate and design the workability of fresh concrete based on numerical flow simulation. At present, Bingham model is generally used to describe the rheological behavior of fresh concrete. Almost all the rheological tests are designed to measure the yield stress and plastic viscosity of Bingham model to evaluate fresh concrete. And Bingham model is commonly used as a constitutive model for the numerical flow simulation. However, fresh concrete is
composed of cement and aggregate particles, it is impossible that all the particles suspend in the mixing water without any contact. Particle contact results in an inter-particle friction. The inter-particle frictional resistance is dependent on the vertical pressure on the shear plane. Hence, the deformation resistance of fresh concrete is pressure-dependent, fresh concrete is exactly a kind of viscous granular material rather than a viscous fluid. The physical flocculation, dispersion and hydration of cement particles cause the change in the fabric of fresh concrete. It is considered that the change of fresh concrete’s fabric, during the rheological tests, results in that the flow curve of fresh concrete is nonlinear, and stress duration-dependent. Obviously, Bingham model is unable to describe the nonlinear, and normal stress-dependent characteristics of fresh concrete’s flow behaviors, and it can’t describe the change of rheological property with stress duration, i.e. the time-dependence. The Herschel-Bulkley model, which is polynomial and quadratic function, was proposed to describe the nonlinear characteristic, but other characteristics are not expressed and the parameters in the model are lack of physical meanings and test methods. Based on the assumption that yield stress and plastic viscosity increase with normal stress, a pressure-dependent model was proposed by Mori. Roussel proposed a thixotropic model to
describe the time-dependence of yield stress. However, these improved Bingham model can't describe simultaneously the nonlinear characteristic, time-dependence, and pressure-dependence, and some of the parameters in the models can’t be measured at present. Therefore, clarifying and modeling the rheological behaviors of fresh concrete are important issues for the workability design. Prof. Z. Li proposed a particle assembly model for fresh concrete and then clarified its viscosity mechanism. Furthermore, a yield model and a flow resistance model were gotten for high fluidity concrete through a series of theoretical analyses. And the relationship between the newly proposed flow model and Bingham model was investigated. In the lower shear rate range, the newly proposed flow model can be simplified as Bingham model. Based on the yield and flow models, the influencing factors of test results of Bingham constants, including height of sample, temperature, mean shear rate, and maximum shear rate, etc., were discussed quantitatively when describing high fluidity concrete with Bingham model.

Online publication : 2018
Publication type : full_text
Public price (Euros) : 0.00