A realistic understanding of the microstructure of freshly-mixed mortars (and concretes) is needed to provide a framework for visualizing and quantitatively modeling the patterns of cement hydration and space filling that are developed during hydration. In this paper the author discusses (a) postulated cement particle arrangements that have been used as starting assemblages in computer models of cement hydration, and their shortcomings; and (b) prospective experimental methods to assess the actual starting arrangements of particles in real freshly-mixed mortars and concretes. Scanning electron microscope investigations of fast-frozen fresh mortars suggest that the starting states assumed in current mathematical modeling for cement pastes are far from realistic. Several possible methods are discussed that may be capable of obtaining three-dimensional information on the actual starting states in fresh mortars or concretes.
Electron probe microanalysis (EPMA) is a promising method for the evaluation of various kinds of material transport and phase changes. It can be used for the accurate estimation of the diffusion coefficient in concrete and for the verification of advanced models describing the material transport of various elements because of its ability to measure elemental distributions with a high spatial resolution. Recently, a quantification method of element concentration by EPMA was introduced and a standard for the methodology including sample preparation was established by the JSCE. However, the device and methodology are still under development. Therefore, it is worthy to make a review of the application of EPMA for concrete. The basic configuration and principle of EPMA, characteristic examples of applications for the durability of concrete, techniques and problems related to methodology, and the future of this method are covered and discussed in this paper.
Ettringite (the AFt Ca6Al2(OH)12·(SO4)3·26H2O compound) and monosulfoaluminate (the AFm Ca4Al2(OH)12·SO4·6H2O compound) phases were synthesised by the coprecipitation method and studied by micro-Raman spectroscopy. Hydrogen bond network, involving sulfate anions, ensures the cohesion of the structure of these two compounds; the columnar struture of ettringite as well as the lamellar structure of monosulfoaluminate. Raman spectroscopy has been used as a probe to investigate mainly the inter-columnar and the inter-lamellar regions of the structures. Raman spectra allowed the characterization of the local environment of sulfate anions and the hydrogen bond networks. A significant asset brought by Raman spectroscopy is the ability to work on hydrated cement products without specific sample preparation; i.e. without a risk to dammage these hydrated compounds. Several examples of the possibilities brought by Raman spectroscopy in cement chemistry are given in this paper. A space group redetermination of the ettringite structure was performed on the basis of the point symmetry of sulfate tetrahedra. The inter-columnar region of ettringite was compared to the inter-lamellar region of monosulfoaluminate as these two structural parts contain the same species. The thermal behaviors at the beginning of the dehydration (up to 390 K) and the iron substitution in ettringite were also investigated.
Pore structural changes in hardened cement pastes, subjected to drying and wetting/drying cycles, were studied at micrometer and nanometer levels. Characterization techniques included Nuclear Magnetic Resonance (NMR), nitrogen and water vapor adsorption, mercury intrusion porosimetry (MIP) and under-water weighing. Coarsening of pore structure was observed with MIP and increase in the true density of C-S-H was suggested by the result of under-water weighing. Decrease in specific surface area due to drying was observed with nitrogen adsorption, and water vapor adsorption associated with Excess Surface Work (ESW) analysis suggested a development of cohesive structure in C-S-H. NMR confirmed polymerization of silicate anion chains. The drying-induced coarsening of pore structure is probably attributed to polymerization of silicate anion chains and development of cohesive structure in C-S-H.
Chemical analysis of cementitious material used in old structures yields empirical information concerning the long term-durability of concrete. Chemical analysis is also useful for scientific research on historical structures as a cultural heritage. Since the materials used in old structures have peculiarities deriving from the long time elapsed since the construction of the structures, it is necessary to take them into consideration for analysis. In this paper, guidelines for carrying out a systematic and effective analysis of old hardened cement even for small samples are proposed. Based on these guidelines, evaluation of a material such as hardened cement collected from actual old structures by X-ray fluorescent analysis, analysis of the acid-soluble part, powder X-ray diffraction and electron probe microanalysis were carried out, and the usefulness of the information obtained by these analyses is discussed.
In order to develop organic-fiber reinforced building materials with long-term stability and durability, the carbonation of autoclaved calcium silicate hydrate hardening bodies, prepared by low-temperature synthesis (150°C) in Ordinary Portland Cement (OPC)-γ-Ca2SiO4 (γ-C2S)-α-quartz systems, has been investigated. The CO2 absorption decreased remarkably as the water-to-powder ratio (w/p) of the samples decreased, and the carbonation of the samples was inhibited. In addition, this tendency became remarkable by the use of γ-C2S, and carbonation was fully inhibited in the case where the mass ratio of the starting materials OPC, γ-C2S and α-quartz was 1:4:5. It is estimated that only vaterite, which was based on residual γ-C2S after accelerated carbonation and had a lower density than calcite and aragonite, filled the pores and densified the sample.
A comprehensive numerical simulation system is proposed for solving the problem of steel corrosion in concrete related to deterioration of reinforced concrete structures in an environment contaminated by chloride ions. The distribution of the corrosion amount and the corrosion rate along the reinforced bar were calculated based on macro-cell circuit models consisting of micro-cell circuits. The models were quantified according to the results of exposure experiments under two environments, one under cyclic wetting and drying in a laboratory and the other in a splash zone located offshore. The comparisons on time-dependent half-cell potential, corrosion location and corrosion amount indicate qualitative coincidence between the experimental and numerical simulation results. In addition, based on the proposed system, numerical simulation of the macro-cell corrosion circuit between patched area and not-yet-patched area is reasonably achieved.
Fluid flow patterns in traditional rotational rheometers are generally well known and rheological parameters such as viscosity can be easily calculated from experimental data of single phase fluids and analytical solutions of the patterns. However, when the fluid is a suspension, where some of the particles are as large as 2 mm in diameter, these rheometers need to be modified. The distance between the shearing planes needs to be increased, which necessitates additional physical confinement of the fluid. This causes the flow pattern to be not analytically soluble leading to an inability to correctly compute the viscosity. This paper presents a modified parallel plate rheometer, and proposes means of calibration using standard oils and numerical simulation of the flow. A lattice Boltzmann method was used to simulate the flow in the modified rheometer, thus using an accurate numerical solution in place of the intractable analytical solution. The simulations reproduced experimental results by taking into account the actual rheometer geometry. The numerical simulations showed that small changes in the rheometer design can have a significant impact on how the rheological data should be extracted from the experimental results.
This paper discusses the mechanism appearing during fiber debonding in fiber reinforced cementitious composites with special emphasis on Engineered Cementitious Composites (ECC). The investigation is performed on the micro scale by use of a Finite Element Model. The model is 3 dimensional and the Fictitious Crack Model (FCM) and a mixed mode interface model are implemented. It is shown that the cohesive law for a unidirectional fiber reinforced cementitious composite can be found through superposition of the cohesive law for mortar and the fiber bridging curve. A comparison between the numerical and an analytical model for fiber pull-out is performed.
This paper investigates the nonlinear creep behaviour of concrete in compression and its relationship with cracking under uniaxial compression (cracks developing parallel to the loading direction). A physical model explaining the nature and the role of linear and nonlinear creep strains is presented, together with a failure criterion for concrete under sustained loads. The model assumes that all nonlinear creep strains are due to concrete micro-cracking. The soundness of this assumption is checked against the experimental results obtained by the authors and by other researchers. The proposed model is shown to fit quite well the experimental results, for various load patterns and concrete ages. The model also proves that the affinity hypothesis between linear and nonlinear creep strains (usually taken for granted in the design for stress levels below 70% of concrete strength in compression) is no longer valid when concrete fails under a sustained load, because of the unstable growth of cracking. Concrete response in these cases is analyzed in detail and a simplified but realistic approach for the evaluation of the failure envelope in compression is proposed.
As is well known, in the current design code, the shear strength of beams can be calculated based on the modified truss theory, which cannot take into account the effects of the top flange area of T-beams. Reported experimental data show that the top flange has an effect on the shear capacity of T-beams with shear reinforcement. To predict the shear capacity of T-beams more precisely, the effect of the concrete top flange area on the shear resisting mechanism must be clarified. Comparison of test results for rectangular and T-beams yielded insights into the shear resisting mechanism of T-beams. Verification and clarification of the shear resisting mechanism of T-beams were performed based on the 3D nonlinear finite element code (CAMUI). Finally, a simplified method for determining the failure criteria for shear of RC T-beams is proposed.
Full three-dimensional nonlinear finite element analysis based on the path-dependent non-orthogonal multi-directional crack model is applied to RC columns subjected to combined cyclic flexure/shear and torsion. This loading causes non-orthogonal crack-to-crack mechanistic interactions accompanying opening/closure and shear slip along crack planes. The analytical results show that the proposed methodology is able to simulate the highly nonlinear behavior of RC columns under cyclic loading, including not only load-carrying capacity but also complex restoring force characteristics and post-peak softening after peak strength. Sensitivity analysis is also carried out to examine the effectiveness of the transverse reinforcement, which simultaneously resists both transverse shear and torque moment. The current design procedure for determining the necessary and sufficient amount of web reinforcement is reviewed and the necessity of performance assessment is raised.