Papers by Keyword: Long Term

Abstract: In the recent construction industry, Fiber reinforced polymers (FRPs) have been considered to be an innovative material to repair and strengthen damaged structures. It is because FRPs have many beneficial characteristics, such as corrosion resistance, a high tensile strength-to-weight ratio, non-conductivity and design flexibility. As a demand of FRPs has increased, many researches on behavior of the structures which were externally strengthened with FRPs have been conducted. However, researches on time-dependant behavior of the structures have not been conducted yet. In order to provide improved serviceability to reinforced concrete (RC) members, the behavior of the RC members strengthened with FRPs under sustained loads should be investigated. This paper presents a series of long-term experiments and deformation-recovery experiments. For the long-term experiments, three RC beams were fabricated and two of the beams were strengthened with a carbon fiber reinforced polymer (CFRP) plate and a glass fiber reinforced polymer (GFRP) plate respectively. The beams were placed under sustained loads for about 550 days. After the 550 days, all of the beams were unloaded for the measurement of deformation recovery. The deflection and strains of rebar and FRP reinforcements were measures for about 60 days. As the result of long-terms experiment, the beams strengthened with CFRP plate showed a better performance in terms of deflection and strains of rebar and CFRP plate. Moreover, the beam with CFRP plate showed a higher deformation recovery and residual strength than the other beams.

Abstract: The main objective of this paper is to review some models of medium-and-long-term wind speed distribution in wind farms, for example Gamma distribution, Log-normal distribution, Weibull distribution, Rayleigh distribution and Burr distribution. On the base of the Weibull distribution, some kinds of parameter estimation approaches are introduced. Meanwhile, the advantages and the disadvantages of various algorithms are analyzed and compared. The prospects of this research are put forward at the end of this paper.

Abstract: Long-term analysis of timber-concrete composite (TCC) structures is a challenging task owing to the time-dependent behaviour of timber, concrete and connections which are highly nonlinear under variable environmental conditions (i.e. temperature, humidity). In this paper an efficient numerical method that takes advantage of a finite element-finite difference (FE-FD) scheme is presented. The differential equations governing the long-term behaviour of TCC section under variable humidity are solved using the FD scheme and the differential equations governing the mechanical behaviour of the composite beam are solved by a FE formulation recast in the framework of force-interpolation concept. The comparison between experimental data and numerical results shows the sufficient accuracy of the proposed FE-FD model for capturing long-term behaviour of TCC members.

Abstract: This paper presents a finite element model for the linear and nonlinear analysis of time-dependent behaviour of concrete-filled steel tubular (CFST) arches. It is known when a CFST arch is subjected to a sustained load, the visco-elastic effects of creep in the concrete core will result in significant increases of the deformations and internal forces in the long-term. In this paper, a finite element model is developed using the age-adjusted effective modulus method to describe the creep behaviour of the concrete core. The finite element results of long-term displacement and stress redistribution agree very well with their analytical counterparts. The finite element model is then used to compare the linear and nonlinear results for the long-term behaviour of shallow CFST arches. It is demonstrated that the linear analysis underestimates the long-term deformations and internal force significantly and that to predict the time-dependent behaviour shallow CFST arches accurately, the nonlinear analysis is essential.

Abstract: The forecast of natural gas load is a comprehensive and difficult job. In this paper, the gas load forecasting with analysis of environmental resources based on gray model is discussed. The load data of some city from 2002 to 2011 is considered. The different experiments are finished to validate the method. From the experiments, it can be conclude that the gray forecasting model is suitable to short term forecasting. In addition, it can also be found that the oldest data has little influence on the future forecasting data.

Abstract: Experiments for studying the shrinkage of concrete cured under three conditions were designed, and the impact of different curing conditions on the 28-day shrinkage and 180-day long-term shrinkage of concrete was researched. The results are as follows: There are significant influences of different curing conditions on shrinkage of concrete; The variation rule of 28-day shrinkage of concrete is consistent with that of 180-day shrinkage of concrete, the shrinkage of the concrete cured under standard condition is less than that cured under indoor natural condition, and larger than that cured under totally enclosed condition; In addition to mixing polypropylene fiber and expansive agents to optimize the proportions of concrete mix in the manufacturing process of concrete, adequate curing, after placing, is quite necessary for effectively controlling the shrinkage development of concrete, reducing the risk of concrete cracking and improving the quality of concrete structure.

Abstract: Life assessment of Type 316H stainless steel (SS) is of great concern to industry due to the widespread use of this material in power plant components that often operate at a temperature of around 550 °C. An important issue to be understood and considered in the life assessment of 316H components is the creep deformation and crack growth behaviour of this material in short term and long term operation times. Due to the limited long term test data available in the literature (i.e. [1, ), a new technique have been recently developed to estimate uniaxial creep ductility trends and subsequently creep crack growth (CCG) behaviour of the material at low load levels [. In this new method the creep ductility trends have been estimated as a function of the applied stress normalised by the temperature dependent 0.2% proof stress of the material, σ/σ_0.2, to include the plasticity effects on the creep deformation behaviour of the material. These trends have been implemented in finite (FE) simulations to predict the CCG behaviour of 316H in intermediate and long term tests at 550 °C and provisional results are presented in [.

Abstract: With the continuous improvement of wind power penetration, the impact of the random fluctuation characteristics of wind power on the frequency control of the power system is growing. Currently, researchers began to study the methods of wind farms participation in frequency control to reduce the frequency adjustment pressure of other power plants and increase the wind power penetration. However, the existing simulation software for the short and long term frequency control of the power system is not so good. So in order to analyze the impact of load fluctuations or wind farm power fluctuations on system frequency control, this paper propose a frequency fluctuation simulation method based on the quasi-steady-state method.

Abstract: The long term behaviour of limestone cement concrete, stored in combined chloride and sulfate environment, was studied, taking into consideration the effect of both the limestone content of the cement used and the mineral admixtures addition. Concrete specimens of seven different compositions were prepared. Three of them were made from ordinary Portland cement and from two Portland limestone cements (limestone content: 15% and 35% w/w). The rest four compositions were prepared by substituting a certain amount of the limestone cement (15% w/w limestone content) with natural pozzolana, fly ash, blastfurnace slag or metakaolin. The specimens were immersed in four solutions of various sulfate and chloride contents and stored at 5oC. Visual assessment of the specimens and mass measurements took place up to 55 months. XRD analytical technique was used to identify thaumasite in the deteriorated parts of the specimens. Higher contents of limestone in cement and of sulfates in the storage solutions resulted in more intensive concrete damage. The use of mineral admixtures improved the behaviour of limestone cement concrete. After 24 months of exposure, chlorides delay the deterioration of limestone cement concretes caused by sulfates. After 55 months, the presence of chlorides led to a greater mass loss for them. The specimens containing mineral admixtures showed more intensive deterioration at 24 months when chlorides were present along with sulfates. Their mass seems to not be affected by chlorides. Fly ash was proved as the most efficient material to improve limestone cement concrete's performance, while concrete containing metakaolin suffered from significant damage after 55 months. XRD analysis showed that the damage observed was due to the formation of thaumasite.

Abstract: The study shows that geological disasters caused by mining is different from the natural one, which has a lagging effect and the persistence, and causing some epigenetic disasters. Some long term effect phenomena are described in detail, their causes are discussed, and some prevention and cure measures are also posed out in this paper.