"Extrusion Behavior of Expanded Polypropylene Bead Cementitious Composites" 

Nicholas Christ, MSE PhD Candidate 

Advisor(s): Prof. John Howarter and Kendra Erk

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Lightweight and extrudable cementitious composites have seen a surge of both scientific and industrial attention in the past few years for applications in intricate building design, efficient material usage, and affordable housing. Many commercialization attempts have faced significant challenges with traditional cementitious mixtures, yielding a need for novel mixtures. In pursuit of these new materials, many studies have combined a variety of lighter-than-cement materials with traditional concrete ingredients. Polymer foams are one of the most successful options for creating lightweight composites; however, they struggle to bond to the cement matrix and often result in drastically reduced mechanical performance. Being highly compressible, these materials also pose a challenge to extrusion as their volume is pressure-dependent, leading to unexpected behavior in a pressure gradient-rich extrusion system. Using expanded polypropylene (EPP) beads in an ordinary Portland cement (OPC) matrix as a model system, this work explores the implications of compressible granular media as an additive for lightweight and extrudable cementitious composites. This is achieved through a three-pronged approach. The first prong entails thorough characterization of the EPP beads to acquire precise and accurate measurements for density and composition calculations, as well as to directly measure EPP bead responses to pressure changes. The second prong assesses the performance of finished EPP-OPC composites using flexural strength tests. The third and final prong makes use of a syringe pump and custom-built syringe to investigate the behavior of uncured EPP-OPC composite in a variety of extrusion conditions. A more detailed understanding of this basic EPP-OPC system’s extrusion behavior will enable development of new engineered formulations of lightweight and extrudable cementitious composites.