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

混凝土的3D打印预计将导致建筑行业的突破，允许快速，经济，无模板和劳动密集型建筑物和结构的制造。然而，在这项新技术能够普遍付诸实践之前，还有几个障碍需要克服。塑性收缩和相关的开裂是这样的主要挑战，因为它们不仅损害印刷元件的耐用性、可维护性和美观性，而且它们还可能损害形状稳定性并限制数字构造的几何精度。在3D打印的背景下，塑性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）难以使用固化技术，以及3）与普通混凝土相比，使用具有增加的糊状物含量和更高的初始刚度的材料组合物。该项目的目的是深入了解基于挤出的选择性材料沉积技术生产的3D打印混凝土元件和结构的塑性收缩变形和相关开裂机制。在此基础上，提出了有效的控制塑性收缩开裂的措施。这些主要目标将逐步实现，以实现以下目标：1）开发适当的实验方法来量化3D打印混凝土的塑性收缩和由此产生的裂缝; 2）开发准确的实验方法，以确定真实的材料性能，如体积模量、渗透系数、保水性能、拉伸强度，3）通过利用确定的材料性质模拟塑性收缩和开裂，并对使用开发的自由和约束塑性收缩装置获得的模拟结果进行实验验证; 4）在模拟和试验的基础上，分析和评价了混凝土裂缝减缓措施的有效性。缓解方法将包括使用减缩剂（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.