The nanogranular origin of concrete creep : a nanoindentation investigation of microstructure and fundamental properties of calcium-silicate-hydrates

Abstract

With an annual per capita consumption of one cubic meter, concrete is the most manufactured material on Earth. But concrete subject to sustained load creeps, like chewing gum, at a rate that deteriorates the durability and lifespan of concrete infrastructure. While it is generally agreed that concrete creep originates from the complex viscous behavior of nanometer-sized building blocks of concrete, the calcium-silicate-hydrates (C-S-H), the origin of concrete creep is still an enigma and the creep properties of C-S-H have never been measured directly since C-S-H cannot be recapitulated ex situ in bulk form. This thesis develops a comprehensive nano-investigation approach to the assessment of the microstructure and the mechanical stiffness, strength and creep properties of the fundamental building block C-S-H. This is achieved by extending the realm of classical indentation analysis of homogeneous solids to highly heterogeneous, linear-viscoelastic, cohesive-frictional materials. Applied to and validated for a wide range of sub-stoichiometric cement pastes of different composition and processing conditions, the link between material composition, microstructure and nanomechanical stiffness, strength and creep properties of cement-based materials is assessed. It is found that C-S-H, exhibiting a unique nanogranular behavior, exists in (at least) three structurally distinct but compositionally similar forms (Low-Density, High-Density and UltraHigh Density) which are characterized by packings close to limit packing densities. It is found that at the nanoscale all C-S-H phases exhibit a logarithmic creep whose magnitude depends only on the packing of 5-nanometer sized particles and not on mix proportions, processing conditions, etc. Logarithmic creep is an intrinsic creep property of C-S-H.

(cont.) We suggest that the C-S-H creep rate (~1/t) is due to rearrangement of C-S-H particles similar to the compaction of vibrated particles, for which the free volume dynamics theory of granular physics provides a strong argument in favor of its justification. Finally, we show that the logarithmic creep measured by an indentation test in some minutes time at nanoscales is as exact as macroscopic creep tests carried out over years. This supports the simple idea that large time scales can be accessed by looking at small length scales, which is of great engineering importance.