Mechanics Architected Cement-based Materials
There is an increasing interest in hierarchical design and additive manufacturing (AM) of cement‐based materials. However, the brittle behavior of these materials and the presence of interfaces from the AM process currently present a major challenge. Contrary to the commonly adopted approach in AM of cement‐based materials to eliminate the interfaces in 3D‐printed hardened cement paste (hcp) elements, this area of reseach focuses on harnessing the heterogeneous interfaces by employing novel architectures (based on bioinspired Bouligand structures). These architectures are found to generate unique damage mechanisms, which allow inherently brittle hcp materials to attain flaw‐tolerant properties and novel performance characteristics.
It is hypothesized that combining heterogeneous interfaces with carefully designed architectures promotes such damage mechanisms as, among others, interfacial microcracking and crack twisting. This, in turn, leads to damage delocalization in brittle 3D‐printed architectured hcp and therefore results in quasi‐brittle behavior, enhanced fracture and damage tolerance, and unique load‐displacement response, all without sacrificing strength. It is further found that in addition to delocalization of the cracks, the Bouligand architectures can also enhance work of failure and inelastic deflection of the architectured hcp elements by over 50% when compared to traditionally cast elements from the same materials.
Performance of Heterogeneous Additively Manufactured Materials
Fabrication of cement-based materials via additive manufacturing (AM) techniques has gained increasing interest. However, the processing-induced heterogeneities and interfaces represent a major challenge. The role of processing in creating interfaces and their characteristics requires understanding of the microstructure of 3D-printed hardened cement paste (hcp). In this research area we investigate the processing-microstructure relationship in 3D-printed elements (fabricated via direct-ink-writing), specifically, the microstructural features of architectured cement-based materials, including processing-induced heterogeneous patterns, interfacial regions (IRs), and pore network distributions with respect to the architectural patterns. A lab-based X-ray microscope (XRM) is typically used to perform X-ray micro-computed tomography (micro-CT) evaluations to explore the microstructural characteristics of 3-day old intact (i.e. not tested) 3D printed and cast specimens at two levels of magnification: 0.4X (in the order of 10s of microns) and 4X (in the order of microns). CT scans of printed specimens have revealed a patterned pore network and several microstructural features, including: (a) macropores (visible during printing), (b) micropores at interfacial regions (IRs), (c) accumulation of anhydrous cement particles near macropores, and (d) rearrangement of filaments away from their designed toolpath. In comparison, microstructural investigation of cast specimen at 4X scan revealed randomly distributed pores with no connectivity throughout the specimen. Micro-CT demonstrates suitability as a non-destructive technique for microstructural characterization of architectured cement-based materials. The processing is found to induce and to pattern heterogeneities such as IRs in materials. The architecture is demonstrated to play a role in controlling such heterogeneities and their directionality through the interface.
Low Carbon Concrete
The volume of aggregates in concrete is approximately 60–75%, and so the concrete performance is strongly affected by the aggregate’s properties, proportioning and packing arrangement. Optimized aggregate blends can provide concrete with improved performance and can be used to design concrete with lower cementitious materials content. Due to complexities in analytical description of aggregate packing, and irregularities in shape and texture, there is no universal approach to account for the contribution of aggregate’s particle size distributions and packing degree affecting the performance of concrete in fresh and hardened states. This research attempts to develop aggregate optimization techniques using modeling, experiment, and packing theories for aggregate blends.
Theoretical particle packing models (Aim and Toufar) were demonstrated capable of verifying and predicting the packing degree of the experimental data from compaction of concrete aggregates. The experimental, theoretical, and computational approaches are compared on the basis of the packing of the binary and ternary blends. The sequential packing algorithm demonstrated suitable to closely predict the particle size distribution and the packing degree of aggregate assemblies. The suggested experimental procedure with Vebe apparatus can be used to determine and verify the aggregate packing degree, reaching the compaction degree values predicted by theoretical and computational models. The correlation between the grading, packing of aggregates and concrete performance is developed and it was demonstrated that the aggregate packing can be used as a tool to optimize concrete mixtures, reduce the cement content, and yet improve mechanical strength. The grading techniques based on power curves and coarseness chart provide valuable information on expected performance and, therefore, can be effectively used to optimize the concrete mixtures.