Over time, concrete structures deform under the weight they support, which ultimately leads to structural failure if not replaced. The concrete deformation is caused when the microscopic grains that bind the concrete together - grains known as calcium-silicate-hydrate - are compelled by the compressive force to move past each other. However, the mechanism that causes some concretes to deform more than others is not well understood. In this project, researchers are studying how a series of calcium-silicate-hydrates responds to compressive stress. For example, the team is examining the effect of adding aluminum, which is now being added to some cements as an environmentally friendly way to dispose of metal wastes. The researchers are using advanced X-ray-based imaging techniques to directly visualize how the grains respond to compression. The results of this project can be used to provide insights into concrete failure mechanisms and suggest remedies to increase durability towards more environmentally responsible infrastructure. As such, this project plays a crucial role in the forecasting and replacement of the nation?s aging civil infrastructure. The project is training both undergraduate and graduate students; an emphasis is placed on recruiting students from groups typically underrepresented in the sciences to join the team.
TECHNICAL DETAILS: An insidious decay mechanism of concrete is its creep under stress. The main binding phase, nanocrystalline calcium-silicate-hydrate (C-S-H), is implicated in the creep, but there is very little understanding of how these grains slide past each other. C-S-H grains comprise a number of calcium oxide sheets decorated with silica tetrahedra, where each sheet is separated by a thin layer of water molecules. Depending on the amount of silicon or if metals are present, the properties of the C-S-H can vary significantly. The team is using domestic synchrotron facilities to measure grain alignment (texturing) under uniaxial stress towards determining the mechanism(s) of grain slippage. The water environment in C-S-H is being studied with proton nuclear magnetic resonance relaxometry to determine how liquid- or ice-like it is. Finally, the strain in silicon - oxygen bonds under stress is being studied with uniaxial-stress Raman spectroscopy to determine if there are slip planes within the structure under uniaxial stress. The development of uniaxial-stress Raman spectroscopy will be useful for understanding similar phenomena in other nanocrystalline ceramic materials. The results of the project are useful for engineering models of structural integrity in cement-based structures.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.