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aUniversité Paris-Est, Ecole des Ponts ParisTech–UR Navier, 6-8 avenue Blaise Pascal, 77420 Champs-sur-Marne, France; and
bDepartment of Civil and Environmental Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139
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 (≈1/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.
Concrete 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−n 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 investigate the creep properties of C–S–H. This is achieved by means of a statistical nanoindentation technique (SNT), described and validated previously (15–17), which is most suitable for the in situ investigation of mechanical phase properties and microstructure of highly heterogeneous hydrated composite materials. Like the classical indentation technique, an indenter tip (here a 3-sided pyramid Berkovich tip) is pushed orthogonally to the surface of the cement paste, and both the load applied to the tip, and the displacement of the tip with respect to the surface are recorded. By applying continuum-based constitutive models to the resulting load-displacement curve, mechanical properties of the indented material are determined. Applied to heterogeneous and multiphase materials, the SNT then consists of carrying out a large array of such nanoindentation tests, and by applying statistical deconvolution techniques (15, 16) and micromechanical models to link microstructure to phase properties (17–19).
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