Rheological properties of engineered/strain-hardening cementitious composites (ECC/SHCC) and their 3D printability

Abstract

Ordinary cement-based materials used in construction often suffer from localized cracks under tension. To address this issue, flexible fiber reinforced cementitious materials, such as Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC), have been developed. SHCC offers excellent crack control and high ductility, making it suitable for resolving the conflict between steel reinforcement requirements and 3D concrete printing process. This study examines the impact of flexible fibers on the rheological properties of flexible fiber reinforced cementitious materials, considering fiber volume fraction and aspect ratio. Increasing the fiber volume fraction has been found to enhance static yield stress, equilibrium shear stress, and thixotropic index. In addition, increasing fiber length leads to an increase in static yield stress and thixotropic index, while a decrease in equilibrium shear stress. Additionally, this study highlights the importance of fiber orientation and dynamics. Shear-induced fiber orientation is crucial for 3D printing, affecting both printability and mechanical properties. A modified two-exponential thixotropy model is proposed to characterize the shear-induced fiber orientation. Furthermore, the morphologies of flexible fibers at different 3D printing stages are corresponded to the shear stress responses in the typical shear stress decay curve. The formation of fiber agglomerates causes significant challenges during continuous pumping and extrusion-based 3D printing. This study further explores the 3D printability of SHCC. Four groups of 3D printing outcomes are categorized to reflect 3D printability: (1) good 3D printability, (2) continuous and non-fractured filament with poor buildability, (3) fiber agglomerates clogging in the nozzle, and (4) fiber agglomerates clogging in the pump screw. Effects of fiber volume fraction, fiber length, water-to-binder (w/b) ratio, and nanoclay addition on the 3D printability of SHCC are explored. The formation of fiber agglomerates during continuous pumping and 3D printing could be related to the fiber instability under shear. A novel in situ method is proposed to quantify the fiber instability of SHCC using a rheometer, by calculating the fiber instability index (I_(fiber instability)) under shear. Additionally, a straightforward model is proposed to decompose the static yield stress of SHCC into matrix-matrix 〖(τ〗_(m-m)), fiber-matrix (τ_(f-m)), and fiber-fiber (τ_(f-f)) interactions. Fiber length is found to have a significant influence on the fiber-fiber interaction, while the rheological properties of cementitious matrix play a crucial role in the matrix-matrix and fiber-matrix interactions. Furthermore, τ_(f-f)/(τ_(m-m)+τ_(f-m)+τ_(f-f)) is suggested to evaluate and predict the 3D printability of various SHCC mixtures. Finally, this study investigates the tensile performance of 3D-printed SHCC. Experimental and theoretical studies of different printing patterns on the tensile performance and cracking control ability of 3D-printed SHCC are conducted. Results indicate that appropriately designing the pattern of 3D-printed SHCC can result in improved mechanical performance. These findings provide new insights into the design and manufacture of 3D-printed structures using flexible fiber-reinforced cementitious materials.