Presented by
Sharifa khan V.S.Naga Sai

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The nano-mechanical signature of Ultra High Performance Concrete by statistical nano indentation techniques
A b s t r a c t
Advances in engineering the microstructure of cementitious composites have led to the development of fiber reinforced Ultra High Performance Concretes (UHPC). The scope of this paper is twofold, first to characterize the nano-mechanical properties of the phases governing the UHPC microstructure by means of a novel statistical nanoindentation technique; then to upscale those nanoscale properties, by means of continuum micromechanics, to the macroscopic scale of engineering applications. In particular, a combined investigation of nanoindentation, scanning electron microscope (SEM) and X-ray Diffraction (XRD) indicates that the fiber- matrix transition zone is relatively defecting free. On this basis, a four-level multiscale model with defect free interfaces allows to accurately determine the composite stiffness from the measured nano-mechanical properties. Besides evidencing the dominant role of high density calcium silicate hydrates and the stiffening effect of residual clinker, the suggested model may become a useful tool for further optimizing cement-based
Engineered composites.
Introduction:
Over the last 15 years Ultra High Performance Concretes (UHPC) have become a vanguard product in industrial and structural applications thanks to outstanding properties, such as com- pressive strength of 150–200 MPa, tensile strength of 8–15 MPa with significant remaining post-cracking bearing capacity, and remarkable fracture energy of 20–30 kJ/m2 Commercial examples include Reactive Powder Concrete now integrated in the ®Ductal range
®BSI Ceracem, engineered and ultra-high strength and hybrid fiber-reinforced cement composites etc. The superior performance of these products has been achieved by tailoring their microstructures, that is by maximizing the packing density with very fine minerals, quartz powder and silica fume, and by enhancing the matrix toughness with an optimal fiber reinforcement. Nevertheless, the current knowledge of UHPC microstructure is still limited and mainly stems from image analyses which is rather qualitative than quantitative, experimental characterization of transfer properties], and X-ray diffraction (XRD) and thermo-gravimetric analysis (TGA) While the mechanical material improvement has been experi- mentally verified at the macroscopic scale (see Ref. for ®Ductal FM), the underlying mechanisms at the microscale that deliver this
Superior responses are still not well understood. In this work, we first apply a recently developed statistical nanoindentation technique (SNT) to an UHPC material in order to quantitatively characterize the UHPC microstructure [This method has been developed to extend the domain of application of classical nanoindentation from monolithic materials to multi-phase composites. Previously, classical nanoindentation techniques have been employed to study the local mechanical behavior of cement-based materials, such as the micro-mechanisms of creep in C–S–H phases or the interface transition zone in normal concretes reinforced by an optimal system of polymeric fibers In contrast, based on a large population of indentation tests, the SNT allows the in situ assessment of mechanical properties, volume proportions, packing density distributions of microstructural constituents, as well as the mapping of the micro- structural morphology. The application of this technique to cement- based materials led to the identification of the intrinsic properties of two characteristic morphological arrangements of Calcium Silicate Hydrates (C–S–H) in cement based materials, namely the High Density C–S–H (HD C–S–H) and the Low Density C–S–H (LD C–S–H) These properties were found to be independent of mix proportions and can be considered as intrinsic material properties of cement- based materials Notably, these invariant properties are quite unaffected up to a thermal treatment of 200 °C when dehydration processes take place The SNT has been also applied to study the anisotropic microstructure of bones and shales.
With tests carried out on Ductal G2FM [this paper focuses on the nano-mechanical signature of a wide class of UHPC materials and
On the role of the fiber–matrix interfaces in the homogenized properties. The paper is structured as follows: Following a brief introduction to the SNT method, we present results of an investigation of ‘real’ UHPC samples which were extracted from reference speci- mens cast in industrial conditions during the fabrication of a UHPC bridge deck The fiber–matrix interface zone is studied by line nanoindentation, scanning electron microscope (SEM) and X-ray diffraction analysis (XDR). The experimental results of this work are synthesized into a four-level micromechanics model that aims at linking microstructure and nano-mechanical properties to the macroscopic mechanical performance of UHPC materials.
2. Materials and methods
2.1. Material and sample preparation
To validate design concepts with innovative UHPCs, two 2-way ribbed UHPC deck slab segments (of dimensions 6.1 m × 2.5 m × 0.38 m, were cast in industrial conditions within the French MIKTI national R&D project Six beam specimens for bending tests were cast in the same conditions from the same batch (as described in the AFGC Recommendations A wide experimental campaign with static and dynamic tests was carried out on the bridge segment structure at LCPC Structures Laboratory and details can be found in For the
nanoindentation tests, cylinders with a diameter of 20 mm were cored out from the 50 mm thick beams, and then sliced in penny- shaped specimens of 50 mm-thick (At the time of the nanoindentation tests, the beam specimens had been kept in the 20 to 25 °C, 35 to 55% RH laboratory conditions for about 36 months. The material, which is commercially available by Lafarge, France, under the trade name ®Ductal G2FM was prepared at a water-to- cement ratio w/c between 0.19 and 0.21 using cement CEM I (chemi- cal composition in high-range water reducer, silica fume, quartz powder, silica sand and about 2.15% in volume of steel fibers. Compounds dimensions and densities are summarized in Silica fume is a by-product of the ferrosilicon alloy industries and has excellent pozzolanic properties. The steel fiber is the largest constituent with a diameter of 0.2 mm and a length of 12.7 mm, while the quartz sand is the largest granular material in the matrix with a diameter varying between 150 and 600 μm (In turn, the clinker and crushed quartz are the next to largest particle with an average diameter on the order of 10 μm.
At the 6th day after casting, the material was heat treated to improve strength and dimensional stability by applying, for 48 h, a temperature of 90 °C at a relative humidity of 90%.
The cored cylindrical specimens were sliced into samples of 5 mm thickness. The surface was polished with silicon carbide papers and diamond particles for 8 h according to a standard procedure which