In this study, Author present a quantitative review of the mechanical performance, as well as the separation from the matrix and the recovery performance of surface modification coarse aggregate (SMCA) that was produced using aggregate, whose surface was modified using fine inorganic powder. Therefore, experiments were conducted to measure the compressive strength, microwave heating characteristics, distribution of the void volume, and chemical changes in SMCA concrete, as well as the recovery rate of the aggregate. The results of the experiments showed that reinforcing the interfacial transition zone, which is a weak part in concrete, by coating the surface of the original coarse aggregate with cement paste, can help suppress the occurrence of microcracks and improve the mechanical performance of the aggregate. Further, microcracking and the decomposition of hydrates were observed as a result of microwave heating. In other words, an increase in void size distribution and weakening of the hydrated cement paste led to the effective recovery of recycled coarse aggregate.
In 2009, a new methodology for the continuous monitoring of E-modulus of cement-based materials since casting was proposed, under the designation EMM-ARM (E-modulus Measurement through Ambient Response Method). This methodology is a variant to classic resonant frequency methods that allows continuous stiffness monitoring from the instant of casting. After the encouraging results obtained in the first applications of EMM-ARM to cement pastes, the present paper gives continuity to previous developments, through validations with additional experimental methodologies and extension to thermal activation testing. At first, a comparison is performed between the results of EMM-ARM and those obtained through: pulse velocity methods (both ultrasonic contact probes and bender-extender elements), penetration resistance (Vicat needle) and cyclic compression on cylindrical specimens. Afterwards, the possibility of studying the activation energy of the stiffness evolution on tests conducted at 20°C and 40°C is explored.
The present paper introduces a non-linear seismic response estimation method for reinforced concrete (RC) structures based on random vibration theory. Equivalent linear single-degree-of-freedom systems that have complex stiffness are adopted in order to approximate the displacement-dependent hysteretic characteristics of RC structures. The proposed method uses the transfer functions of the equivalent linear systems, root mean square responses, and peak factors. In contrast with existing approximation methods using response spectra, the proposed method allows individual evaluation of the power spectral densities and duration times of input motions. Earthquake responses estimated using the proposed method have been verified through comparisons with results of non-linear time history response analyses and two existing approximation methods. The proposed method has exhibited relatively high accuracy and has been found to be useful for response estimation of RC structures, considering frequency characteristics and duration effects separately.
To avoid the disadvantages caused by using silica fume, by using superfine cement (SC) to substitute silica fume, a new kind of ultra high performance concrete (SC-UHPC) was prepared and introduced. The influence of component types and dosages on the mechanical properties of SC-UHPC was investigated. The results show that 40% ground granulated blast furnace slag (GGBFS) or 10% fly ash (FA) & 30% GGBFS replacing SC are the most appropriate proportions to get high strength with satisfied workability and low cost. A suitable amount of defoaming agent (DA) in superplasticizer (SP) effectively reduced the void ratio in the UHPC. To optimize the strength and fluidity, the rational natural sand distribution with lowest clay particles amount can be used. The small sized steel fibers added into the mixture effectively improved the flexural behavior of SC-UHPC. Multiple nonlinear analysis show that, with sufficient calcium silicate hydrate (C-S-H gel) and enough mixture fluidity, the compressive strength of UHPC is closely related to the water to binder ratio and void ratio, and increases linearly with the incremental fiber amount. Microstructure analysis proved that the microstructure of SC-UHPC has ultra high density and homogeneity.
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