The research objective of this Early-concept Grant for Exploratory Research (EAGER) award is to measure the fracture toughness of cement paste at submicron length scales using nanoindentation experiments with Focused Ion Beam (FIB) milled sample structures. Modeling fracture and plastic failure of cement and concrete materials often involves introducing the concept of microcracks, which propagate and eventually coalesce to cause material failure at larger scales. The small scale of these cracks and the volumes around them pose many challenges for measurements and modeling. New experimental techniques are required to quantitatively assess the quasi-brittle failure mechanisms at small scales that govern microcracking and eventually material failure. Research tasks will begin with development of experimental methods to create sample micro-structures using FIB milling. Micropillars, notched beams, and wedge-splitting geometries are will be tried as alternatives. Next, these sample structures will be tested using nanoindentation equipment to apply loads and measure displacements. This data will be analyzed to deliver strength and fracture toughness properties. Finally, sample data will be collected to build a preliminary statistical model of measured properties, focusing on the ability to measure material properties that are repeatable within each phase but distinguishable between phases.
The production of cement generates between 3-5 percent of total carbon dioxide emissions worldwide, but this could be significantly reduced if concrete is engineered to be more resistant to quasi-brittle failure. Damage and failure of concrete is often modeled by invoking the existence of microcracks in cement paste, but the properties of these microcracks and the material behaviors that lead to microcracks are not well understood. Concrete damage from extreme loading conditions or from durability issues including alkali-silica reaction, freeze-thaw cycles, reinforcement corrosion, and delayed ettringite formation that lead to microcracking may be understood and prevented if the origins of microcracking are understood. New knowledge about the fundamental origins of failure behaviors of will enable rational design of modified and new materials that will lead to more efficient use of natural resources and more sustainable and resilient structures. The research in this project represents an important first step towards this overall goal, and will enable future research to more broadly characterize the failure behavior of cement and concrete at small length scales. The techniques developed will be applicable to a wide range of other materials, including sedimentary rocks such as shale and carbonates, and biological materials such as bones, teeth, and shells.
