Title:
Particle-Scale Mechanisms and Rheological Response of UHPC
Author(s):
Narayanan Neithalath
Publication:
Web Session
Volume:
Issue:
Appears on pages(s):
Keywords:
DOI:
Date:
3/28/2021
Abstract:
Ultra-high-performance concrete (UHPC) is generally proportioned using high fine particle contents (cement, supplementary cementing materials, and specialty fillers) and a very low water-to-powder ratio (w/p; ranging from 0.18-0.25). This necessitates the use of multiple chemical admixtures to ensure sufficient particle dispersion and flowability (e.g., for bridge-deck connections, where self-consolidating UHPCs are required). We discuss the development of economical UHPC mixtures that demonstrate > 150 MPa compressive strengths, with up to 50% (by mass) cement replacement materials, through control of particle packing and rheology at the paste scale. Packing is a function of particle sizes and their distribution, while sizes and distribution along with particle surface characteristics influence the paste rheology. Rheological characteristics of such low w/p pastes are critical since better rheology helps better dispersion of the grains, aiding in mixture placement as well as hydration in the presence of low amounts of water, and consequently better mechanical properties. We report the use of fundamental rheological parameters – yield stress and plastic viscosity, to select the UHP binders while small amplitude oscillatory stress growth experiments and extensional rheology are used to infer the particle-scale mechanisms at the paste level. It is noted that the storage and loss moduli for UHP binders are a few orders of magnitude lower than that of conventional pastes (w/c=0.40), even when the interparticle forces are expected to be more dominant because of the dense packing. The competing effects of the interparticle attraction that gives rise to solid-like response (yield stress, storage modulus) and the dispersing action of the high concentration of admixtures are discussed.