X-Ray Tomography Insights into Early Drying Effects in 3D Concrete Printing (2024-09)¶

This study investigates an often overlooked phenomenon of early-age drying-induced shrinkage and microstructural damage in 3D-printed concrete filaments. Our study is based on soft permeable and stiff mortars, emulating the characteristics of 3D printed concrete filaments, including sample geometries, material mix design, material fresh properties, and drying conditions. Notably, our results underscore that drying-induced shrinkage, occurring within a couple of hours, like autogenous shrinkage, reaches around 1%. This early-age shrinkage poses a significant risk to geometrical accuracy and structural integrity, potentially resulting in warping and cracking if left unaddressed. At the material level, our investigation unveils the creation of drying-induced microstructural damages within the initial hours preceding hydration onset. In the stiff mortar, damage occurs similarly to high aspect ratio microcracks typically observed during hydration in sealed samples. Conversely, the soft model mortar exhibits unique damages named "cracklets," characterized by an aspect ratio of approximately ten and widths in the range of a few hundred micrometers. Remarkably, both types of damages manifest as early as one hour into the drying process, a critical insight often overlooked in existing literature. To elucidate the origin of these microstructural damages, an analysis of evaporation was conducted by measuring the drying rate, specifically during the first few thousand seconds. This analysis underscores the crucial roles played by the imposed evaporation regime (homogeneous drying or heterogeneous dry-front propagation) and the kinetics of capillary stress diffusion in the interstitial fluid. This study will help to optimize printing parameters and address structural vulnerabilities by limiting some microstructural defects that may lead to failure.