In service, cracks or microcracks are usually present in concrete as a result of several mechanisms, for example the drying shrinkage, thermal gradients, freezing-thawing cycles, alkali-aggregate reaction and external loading. It has been realized that cracking can significantly accelerate the ingress of chlorides into concrete since it provides preferential flow channels and allow more chlorides to penetrate. But it is also believed that cracking plays an important role on the penetration speed of chloride. The objective of this paper is to quantify the diffusion coefficient of chloride through cracks of concrete with different crack widths by means of the mesoscale modelling method based on the available experimental results from literatures. In the numerical models, the position and opening width of cracks are artificially prescribed based on geometrical layout of the samples used in test. Additionally, the Voronoi diagram technique is adopted to discrete the domain of a specimen in order to reduce the mesh bias. On the Voronoi diagram, a randomly distributed lattice network is constructed to represent the transport process of chlorides. The range of investigated crack width is from 20 to 600 μm covering the data in experimental program. The diffusion coefficients of chloride through cracks of different width, D cr , are numerically determined by the trial and error method. It is concluded that chloride can penetrate into cracks with a much higher speed than that in free water. When the crack width is lager than a critical value, D cr is determined as 10000 mm 2 /h and independent of the crack width.

For concrete structures exposed to salt environment, the microstructure and cracks play a crucial role in the ingress of chloride ions into concrete. In this study, concrete is simulated on the meso scale as a three-phase composite, i.e., aggregate particles, mortar and the interfacial transition zone (ITZ). Because of the advantages in predicting cracks behavior in concrete, Rigid Body Spring Model (RBSM) is employed to carry out the mechanical analysis to simulate the distribution and width of microcracks. And then, the truss network model is adopted to evaluate the chloride diffusivity of the cracked concrete. On the basis of the statistics analysis of diffusion coefficients of concrete and mortar determined experimentally, the diffusivity of ITZ is analytically clarified. The range of diffusion coefficient of ITZ estimated in this paper is approximately 3-16 times of that of mortar depending on the different assumed thickness, which agrees well with that of the previous experimental results. With the aim to validate the effect of microcracks on the diffusivity of concrete, a series of the chloride ions penetrating analysis is numerically carried out on the concrete specimen under different stress levels. The axial compressive and tensile loading conditions are investigated respectively and the effects of stress level on chloride diffusivity of cracked concrete are examined. Results indicate that the chloride diffusivity is significantly dependent on the stress level, but only considering the effect of cracks predicted by RBSM is not sufficient. So an empirical equation which can account for the microstructure variation of concrete under loading is proposed. With it, a reasonable estimation for chloride diffusivity of cracked concrete is achieved.

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Most traditional Var compensation-based voltage regulation methods are developed following the single-phase Volt-Var response rule. These methods typically have competent voltage regulation performance with balanced photovoltaic (PV) integration. However, certain randomness of single-phase rooftop PV installation may lead to significant PV power imbalance across three phases, especially in low voltage (LV) distribution systems. In such unbalanced situations, unintended inter-phase Volt-Var response which is ignored in the single-phase Volt-Var response rule will become significant and greatly challenge the effectiveness of the traditional methods on voltage regulation. This can further cause inverter saturation and consequently makes distribution systems vulnerable to overvoltage problems. In this paper, the mathematical equations of unbalanced three-phase Volt-Var response are first derived and analyzed to identify the strong MVE (mutual Var compensation effect) and the weak MVE. This analysis provides the theoretical foundation for the development of the proposed interphase coordinated consensus algorithm, which can successfully overcome PV imbalance-induced voltage regulation challenges (e.g. inverter saturation and network overvoltage), while does not need exact system parameters. The effectiveness of this method has been validated by time-series simulations with a real LV distribution system and recorded data.

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The reinforced concrete beam-column joint exhibits different properties under dynamic loading when compared with that under quasi-static loading, due to the effect of strain rate. However, the majority of previous studies are focused more on the rate effect of concrete and reinforcement, but less on beam-column joints. Based on the former considerations, the seismic behavior of 15 cruciform specimens subjected to various strain rates is studied in this paper, aimed at attaining a better understanding of the effect of strain rates on beam-column joints. In terms of the effect of different strain rates, the failure mode, carrying capacity, stiffness degradation, and energy dissipation of beam-column joints are discussed in detail. An empirical equation to predict the dynamic increase factor of horizontal shear carrying capacity of beam-column joints under different axial compression ratios and strain rates is also proposed through multiple linear regression analysis. Finally, four adjustments for the softened strut-and-tie model are made to get better predicting of the test results. It has been proved that predicted results by the improved softened strut-and-tie model are in good agreement with the test results.

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