Study of various influence factors on fiber dispersion of strain-hardening cementitious composites (SHCC)

Abstract

3D concrete printing is reforming traditional concrete production industry. As a promising method, Strain Hardening Cementitious Composites (SHCC) is wildly adopted into 3D printing area which partially solves the problem between layer-by-layer 3D printing and rebar-adding methods. While SHCC provides structural toughness by fiber adding, low carbon emission and high environmental sustainability, there remains knowledge gap which has not been fully researched. This study establishes a comprehensive basic for fiber dispersion in SHCC and its impact for 3D concrete printing by experimental investigation and numerical analysis containing different factors. In the material preparation phase, the impact of water-binder ratio, nanoclay addition, and fiber effects are probed. It has been testified that water and clay have drastic impact on the paste viscosity, and fiber length changes the fiber deflection status. Consequently, water and nanoclay with different dosage and fiber with different length are added into the dry ingredient when mixing, and material-dependent fiber dispersion are experimentally investigated. In the fresh SHCC production phase, the effects of mixing parameters are investigated. Different fiber dispersion level in SHCC is obtained through different combination of mixing speed and time. The mixing volume also contributes to the diversity in fiber dispersion. A quantitative methodology has been proposed to establish relationship between fiber dispersion and mixing factors. Moreover, from the aspect of the paste viscosity modification caused by nanoclay and polycarboxylate ether superplasticizer (PCE). It has been testified that the viscosity of concrete mortar plays a crucial role in modifying fiber dispersion, and operating outside the suitable viscosity range renders all other parameters ineffective. Based on this result, the synergistic effects of mixing parameters and nanoclay are investigated to optimize the SHCC quality. It shows that low speed with medium mixing time combined with suitable amount of nanoclay addition and water-binder ratio to maintain the viscosity is beneficial for better fiber dispersion of SHCC. Further, 3D printability test is conducted after SHCC mixing to correlate the relationship among mix proportion, fiber volume fraction gap and 3D concrete printability. It shows that lower value of fiber volume fraction gap results in good printability while in large value, fractured filament or pump stop will happen. The successfully printed materials also undergoes uniaxial tensile test to testify its multi-cracking and strain-hardening performance. This research fills the research vacancy in 3D printing concrete regarding fiber dispersion of SHCC. It provides insights towards parameters affecting the fiber dispersion, and numerical methodology is proposed to quantify the fiber dispersion right after the mixing process, besides, the relationship between material proportion, fiber volume fraction gap and 3D concrete printability is also established.