Dispersed, spherical particles of nano silica (NS) with controllable size have been synthesized using a metal alkoxide, tetraethoxysilane, as starting material and ammonia as base catalyst using sol-gel method. The size of particles and dispersivity were controlled by varying the solvent and temperature conditions of the reaction mixture. NS was synthesized using four different alcohols as solvent viz., methanol, ethanol, propanol and butanol at different reaction temperatures 25°C, 50°C, 70°C and 90°C. The results showed that the size of NS became larger and more uniform with the increase of the reaction temperature. The particle size of NS gradually increased with increasing carbon chain length of the solvent. Further, the compressive strength and setting time of cement pastes have been examined by addition of NS (-100 nm) of varied contents (0.25 %, 0.5%, 1.0%, 2.5% and 5.0%). The experimental results of these studies showed that the compressive strength of fresh cement paste was increased by increasing the content of NS up to 5% in cement paste. The setting time of fresh cement paste was decreased by increasing the content of NS. Characterization of synthesized NS particles were carried out using scanning electron microscopy (SEM) powder x-ray diffraction (XRD), 29Si MAS NMR, infra-red spectroscopy (IR) and microstructure of cement paste incorporated with NS were analyzed using SEM, XRD and thermogravimetric analysis (TGA) for morphological and mineralogical attributes.
An assessment of deteriorative effects on reinforced concrete structures is presented focusing mainly on the propagation of the reinforcement corrosion stage. It necessitates the utilization of relevant models and advanced simulation techniques to enable the probabilistic analysis of concrete structures and service life prognosis. Several analytical models for the initiation of concrete cracking, for crack width and for a decrease in the effective reinforcement cross-section are briefly presented. The described approach is applied to some illustrative examples showing the feasibility of predicting the development of reinforcement corrosion and its effect on the service life and/or reliability of the structure. The authors believe that such methodology supports the optimisation of design strategies as well as effective decision-making processes.
There have been many reports showing measured diffusion coefficients have a time dependency, but very few studies considering the mechanisms of that time dependency. The aim of this study is to investigate the variation of time dependency of diffusivity due to the type of cement and environmental condition using four parameters: the linear chloride binding capacity, unit cement content, porosity and pore structure factors. Concrete specimens with different phase compositions and pore structures were prepared using various cement-based materials and different mix proportions, and their apparent and effective diffusion coefficients Da and De were measured to know how they are influenced by the phase composition, pore structure and environmental conditions such as exposure temperature and marine tidal or splash zone environment. This paper verifies the applicability of analytical solutions of diffusion equations to the evaluation of Da with variations in surface chloride ion (Cl-) concentration depending on the immersion time. Over-time changes of Da and De are then discussed from the viewpoint of variations in influential factors depending on the mix proportions and environmental conditions. For the present study, specific focus is placed on the factors influencing Da including linear binding of Cl-, cement content and pore structure factors as well as tortuosity and constrictivity as typical pore structure parameters.
Greater confinement of the soil foundation may globally reduce the shear span length, which may raise the risk of shear failure of RC piles. On the other hand, soil confinement may improve the flexural ductility of the pile section locally. This study discusses these global and local impacts of soil confinement on the RC-pile mechanistic behaviors, experimentally and analytically. Small-scale mortar-based piles were newly produced as a mockup of real-scale reinforced concrete so as to obtain the consistent nominal shear strength by reducing the shear transfer along cracks to overcome the size effect. Then, the global impact of soil confinement on pile shear failure is reproduced inside the small-scale experimental devices with soil. The soil-pile interaction test also shows the local impact of soil confinement in terms of increasing the flexural ductility of the miniature pile section by suppressing cover spalling and local buckling of reinforcement. It is confirmed that the coupled local and global effects of soil confinement can be consistently taken into account in 3D computational simulation without reducing the degree of freedom for strain fields.
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