Understanding and mitigation of plastic shrinkage cracking in 3D-printed concrete elements

混凝土的3d打印有望带来建筑行业的突破，实现快速、经济、无模板和更少劳动密集型的建筑和结构制造。然而，在新技术能够普遍投入实践之前，有几个障碍需要克服。塑料收缩和相关的开裂是如此重大的挑战，因为它们不仅损害了印刷元件的耐用性、适用性和美观性，而且还可能损害形状稳定性并限制数字结构的几何精度。在3d打印的背景下，塑料收缩开裂风险变得特别明显的原因是：1)缺乏保护新鲜材料免受水分蒸发的模板；2)难以使用养护技术；3)与普通混凝土相比，使用的材料成分具有更高的浆料含量和更高的初始刚度。该项目的目的是全面了解塑性收缩变形的机制和相关的开裂，3d打印的混凝土构件和结构生产基于挤压选择性材料沉积技术。在此基础上，研究了缓解塑性收缩开裂的有效策略。这些主要目标将逐步实现以下目标：1)开发足够的实验方法来量化3d打印混凝土的塑性收缩和由此产生的开裂；2)开发准确的实验方法，以确定真实材料的性质，如体积模量、渗透系数、保水行为、抗拉强度和水化程度在挤压基选择性材料沉积的背景下；3)利用确定的材料特性进行塑性收缩和开裂模拟，并对开发的自由塑性收缩和约束塑性收缩模拟结果进行实验验证；4)在模拟和实验的基础上，对裂缝缓解方法进行了分析，并对其有效性进行了评估。缓解方法包括使用减缩外加剂（SRA）、高吸水性聚合物（SAP）、促进剂和固化剂。

The 3D-printing of concrete is expected to lead to a breakthrough in the construction industry, allowing the speedy, economic, formwork-free, and less labor-intensive fabrication of buildings and structures. However, there are several obstacles to be overcome before the new technology can be generally put into practice. Plastic shrinkage and the related cracking are such major challenges since they not only compromise the durability, serviceability, and aesthetics of printed elements, but as well they may compromise form stability and limit the geometrical precision of digital construction. The reasons for plastic shrinkage cracking risk’s becoming particularly pronounced in the context of 3D-printing are 1) lack of formwork to protect fresh material from water evaporation, 2) difficulty in using curing techniques, and 3) use of material compositions with increased paste content and higher initial stiffness in comparison to ordinary concrete. The aim of this project is to gain a thorough understanding of the mechanisms of plastic shrinkage deformations and the related cracking for 3D-printed concrete elements and structures produced by extrusion-based selective material deposition techniques. On this basis, efficient strategies to mitigate plastic shrinkage cracking are to be developed. These main objectives will be accomplished stepwise in achieving the following goals: 1) Developing adequate experimental approaches to quantifying plastic shrinkage and the resulting cracking in 3D-printed concrete; 2) Developing accurate experimental methods in determining genuine material properties such as bulk modulus, coefficient of permeability, water retention behavior, tensile strength, and degree of hydration in the context of extrusion-based selective material deposition; 3) Simulating plastic shrinkage and cracking by utilizing the determined material properties and experimental verification of the simulation results obtained using the developed setups for free and restrained plastic shrinkage; and 4) Analyzing cracking mitigation methods on the basis of the simulations and experiments and assessing their efficiency. The mitigation approaches will include use of Shrinkage Reducing Admixtures (SRA), Super Absorbent Polymers (SAP), accelerators, and curing agents.