Evolution of the Strength of Polymer Fiber-Reinforced SCC during Early Hydration

Abstract

Novel fabrication techniques for non-standard concrete structures rely on the interplay of robotic fabrication with the evolution of the material performance, mainly during early hydration. When the material transforms from a yield stress fluid to a cohesive frictional material, detailed knowledge on the evolution of the strength envelope becomes essential. This is even truer, as the dead weight acting on the previously fabricated zones increases with the size of the fabricated structures. A set of nonstandard mechanical tests for compression, tension and shear, particularly suited for measuring failure from early hydration states on, exhibits two distinct scaling regimes. The first one is characteristic for large local plastic deformations, followed by the transition to the second regime with material behavior dominated by crack growth. Finally we condense all test results to a limit surface in principal stress space and discuss its evolution with the advancing state of hydration. INTRODUCTION The strength evolution during early hydration is of increasing importance for new technologies in concrete fabrication such as robotic slip-forming of concrete pillars with variable cross-sections by flexible actuated formworks. It is significantly smaller than the structures produced and the material leaves the mold during the ongoing early hydration process, while it is still deformable and yet able to provide the strength necessary to sustain its own weight. The trade-off between deformability and strength during hydration restricts the time for active shaping. It is crucial to precisely predict the rheological properties of the material during the early phase of its transition from a complex yield stress fluid to a solid material. We focus on self-compacting concrete (SCC) with and without polymer fibers in an accelerated mixture. Two types of failure constrain the design window: (1) breakdown of the micro-structure due to motion of the formwork caused by the thixotropic nature of SCC, resulting in sudden local liquefaction and following structural collapse and (2) appearance of surface cracks due to forming, caused by the rapid increase in yield stress with respect to the current tensile strength. For such a demanding process, the strength evolution needs to be continuously monitored by means of simple on-line rheological measurements that are related to the known scaling behavior of the failure envelope for the SCC material during early hydration. To assess the early regime with mechanical tests at diverse stress states, “non-standard” mechanical tests were performed within the described bounds of liquefaction and fracture localization. This work presents results from 4 different test setups on SCC, followed by the discussion of the respective results and their condensation to a set of failure criteria evolving with the advancing state of hydration. CONCREEP 10 550