Nanogranular origin of concrete creep

Abstract

Concrete, the solid that forms at room temperature from mixing Portland cement with water, sand, and aggregates, suffers from time-dependent deformation under load. This creep occurs at a rate that deteriorates the durability and truncates the lifespan of concrete structures. However, despite decades of research, the origin of concrete creep remains unknown. Here, we measure the in situ creep behavior of calcium-silicate-hydrates (C-S-H), the nano-meter sized particles that form the fundamental building block of Portland cement concrete. We show that C-S-H exhibits a logarithmic creep that depends only on the packing of 3 structurally distinct but compositionally similar C-S-H forms: low density, high density, ultra-high density. We demonstrate that the creep rate (Ϸ1/t) is likely due to the rearrangement of nanoscale particles around limit packing densities following the free-volume dynamics theory of granular physics. These findings could lead to a new basis for nanoengineering concrete materials and structures with minimal creep rates monitored by packing density distributions of nanoscale particles, and predicted by nanoscale creep measurements in some minute time, which are as exact as macroscopic creep tests carried out over years.C oncrete is the most-used construction material on earth. The annual worldwide production stands at 20 billion tons and increases per annum by 5%. However, the fundamental causes of concrete creep are still an enigma, and have deceived many decoding attempts from both experimental (1-3) and theoretical sides (4-8). In the United States alone, concrete creep is partly responsible for an estimated 78.8 billion dollars required annually for highway and bridge maintenance. Although it is generally agreed that the complex creep behavior of concrete materials is largely related to the viscoelastic response of the primary hydration product and binding phase of hardened Portland cement paste, the calcium-silicate-hydrate (C-S-H), the creep properties of C-S-H have never been measured directly. C-S-H precipitates when cement and water are mixed, as clusters of nanoscale colloidal particles (9, 10) that cannot be recapitulated ex situ in bulk form suitable for macroscopic testing. Over decades, therefore, concrete creep properties have been probed on the composite scale of mortar and concrete (11), with the conclusion that there are 2 distinct creep phenomena at play ( Fig. 1 A and B): a short-term volumetric creep and a long-term creep associated with shear deformation (3,7,12,13), with a creep rate evolving as a power function t Ϫn of exponent n between 0.9 and 1 (14). The assessment of this long-term creep is most critical for the durability of concrete structures, and requires, for a specific concrete composition and structural application, years of expensive macroscopic testing (11,14). After more than 40 years of research (4-8), basic questions persist regarding the physical origin of this logarithmic creep and its link with microstructure and composition.In this study, we investiga...

Microstructural Modeling of Early-Age Creep in Hydrating Cement Paste

“…The intrinsic creep properties of C-S-H are adopted from the experimental results of Vandamme andUlm (2009) andVandamme (2008). In their work, the in situ creep behavior of C-S-H, the nanocrystalline phase which forms the major part of hydrated cement paste was measured by means of a statistics on nanoindentation results.…”

“…2. Short-term nanoindentation measurements (Constantinides and Ulm 2007;Vandamme 2008;Vandamme and Ulm 2009) over some minutes at the scale of 0.1 μm, at which the C-S-H is assumed to be homogenous. In this study, the latter is used because it is more suitable for modeling of creep behavior over a time span from a few hours to a few days.…”

Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media

“…Nanoindentation studies on bonded colloidal crystals [162,163] and assemblies [ 164 ] or nanoparticles agglomerates [ 165 ] missed most of the particulate features. A few other works have investigated colloidal ensembles by ultrafast optoacoustic [ 166 ] or microcompression techniques [167,168].…”

Creep and drying shrinkage of a blended slag and low calcium fly ash geopolymer Concrete

“…The proportion of creep associated with each of these mechanisms is not yet understood despite extensive research over the last eighty years. Recent research relates the creep response of OPC to the packaging density distributions of calcium-silicate-hydrates [32]. The mechanisms of creep in fly ash geopolymer concrete are still to be determined and are likely to be different from those in OPC concrete.…”