Remnant creep based visco-elastic model for concrete creep analysis

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Finiteness of fixed equilibrium configurations of point vortices in the plane with a background flow

For a dynamic system consisting of n point vortices in an ideal plane fluid with a steady, incompressible and irrotational background flow, a more physically significant definition of a fixed equilibrium configuration is suggested. Under this new definition, if the complex polynomial w that determines the aforesaid background flow is non-constant, we have found an attainable generic upper bound $\frac{(m+n-1)!}{(m-1)!\,n_1!\cdots n_{i_0}!}$ for the number of fixed equilibrium configurations. Here, m = deg w, i0 is the number of species, and each ni is the number of vortices in a species. We transform the rational function system arising from fixed equilibria into a polynomial system, whose form is good enough to apply the BKK theory (named after Bernshtein (1975 Funct. Anal. Appl. 9 183–5), Khovanskii (1978 Funct. Anal. Appl. 12 38–46) and Kushnirenko (1976 Funct. Anal. Appl. 10 233–5)) to show the finiteness of its number of solutions. Having this finiteness, the required bound follows from Bézout's theorem or the BKK root count by Li and Wang (1996 Math. Comput. 65 1477–84).postprin

Chapter Development of Sustainable High-Strength Self-Consolidating Concrete Utilising Fly Ash, Shale Ash and Microsilica

With high flowability and passing ability, self-consolidating concrete (SCC) does not require compaction during casting and can improve constructability. The favourable properties of SCC have enabled its widespread adoption in many parts of the world. However, there are two major issues associated with the SCC mixes commonly used in practice. First, the cement content is usually at the high side. Since the production of cement involves calcination at high temperature and is an energy-intensive process, the high cement content imparts high embodied energy and carbon footprint to the SCC mixes. Besides, the exothermic reaction of cement hydration would cause high heat generation and early thermal cracking problem that would impair structural integrity and necessitate repair. Second, the strength is usually limited to around grade 60, which is considered as medium strength in nowadays achievable norm. With a view to develop sustainable high-strength self-consolidating concrete (HS-SCC), experimental research utilising fly ash (FA), shale ash (SA), and microsilica (MS) in the production of SCC has been conducted, as reported herein

Tension stiffening in concrete beams. Part 1: FE analysis

Although after cracking, concrete has negligible tension capacity, the intact concrete between cracks within the tension zone of a reinforced concrete beam can still develop significant tensile stresses to contribute to the flexural stiffness of the concrete beam. Such a tension stiffening effect in a flexural member is not quite the same as that in an axial member because the tensile stresses in a cracked flexural member are induced not only by the steel reinforcement-concrete bond but also by the curvature of the flexural member. In this study, the tensile stresses developed in cracked concrete beams are analysed using a finite-element (FE) model that takes into account the non-linear biaxial behaviour of the concrete and the non-linear bond stress-slip behaviour of the steel reinforcement-concrete interface. Based on the numerical results so obtained, a tensile stress block is proposed for section analysis of the moment-curvature curves of reinforced concrete beams at both the uncracked and cracked states. It will be shown in part 2 of this paper that the tensile stress block may also be used for member analysis of the load-deflection curves of concrete beams without resorting to FE analysis.published_or_final_versio

Tension stiffening in concrete beams. Part 2: Member analysis

Based on finite-element analysis of cracked reinforced concrete beams, a tensile stress block was proposed in part 1 of this paper. Here, the proposed tensile stress block is contrasted with existing ones developed by others to provide an overall review. The proposed tensile stress block is then applied to a section analysis of beams to evaluate the moment-curvature curves of typical beam sections. The section analysis results are compared with available experimental results and empirical curves given in design codes to demonstrate the validity of the tensile stress block. Subsequently, the proposed tensile stress block is applied to member analysis by first dividing the beam member into short segments and then applying section analysis to each segment to evaluate the load- deflection curves of typical beam members. The member analysis results are checked against finite-element analysis results and available experimental results to fine-tune the tensile stress block and verify the accuracy of the finetuned tensile stress block. Such member analysis could be extended to multi-level analysis of frame structures for their full-range load-deflection behaviour at the elastic, post-crack and post-peak states.published_or_final_versio

Modelling dowel action of discrete reinforcing bars in cracked concrete structures

AIP Conference Proceedings, vol. 1233 entitled: ISCM II and EPMESC XII : proceedings of the 2nd International Symposium on Computational Mechanics and the 12th International Conference on the Enhancement and Promotion of Computational Methods in Engineering and Science, Hong Kong-Macau, China, 30 November-3 December 2009Dowel action is one of the component actions for shear force transfer in cracked reinforced concrete. In finite element analysis of concrete structures, the use of discrete representation of reinforcing bars is considered advantageous over the smeared representation due to the relative ease of modelling the bond-slip behaviour. However, there is very limited research on how to simulate the dowel action of discrete reinforcing bars. Herein, a numerical model for dowel action of discrete reinforcing bars crossing cracks in concrete is developed. The model features the derivation of dowel stiffness matrix based on beam-on-elastic-foundation theory and the direct assemblage of dowel stiffness into the concrete element stiffness matrices. The dowel action model is incorporated in a nonlinear finite element programme with secant stiffness formulation. Deep beams tested in the literature are analysed and it is found that the incorporation of dowel action model improves the accuracy of analysis. © 2010 American Institute of Physics.published_or_final_versio

Research directions for high-performance concrete

The Conference program's website is located at http://hkie.cvd.annualconference.i-wanna.com/program.htmlSession 4: Construction Technology and Material SelectionThe continuous development of civil infrastructure has been imposing more and more stringent requirements on the concrete to be used. To meet with stringent requirements, the notion of high-performance concrete (HPC) processing high strength, high workability, high durability and high dimensional stability had been put forward in the last century. After about 20 years of development, a large variety of HPC has been produced. However, since the various performance attributes of HPC are often contradictory to each other and thus difficult to achieve simultaneously, the development of HPC has come to a bottle neck. To cope with such situation, a more scientific approach is needed. Here, we shall explain how conventional concrete technology could be reformed into modern concrete science by incorporating new theories on particle packing, water film thickness and particle interaction. We shall also introduce new technologies such as aggregate treatment, fillers and green concrete. It is our belief that with further advancement in concrete science and technology, even better HPC could be produced for more sustainable construction.postprin

Higher-Order, Polar and Sz.-Nagy’s Generalized Derivatives of Random Polynomials with Independent and Identically Distributed Zeros on the Unit Circle

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Shrinkage of Hong Kong granite aggregate concrete

In the design of many concrete structures, it is important to assess the amount of shrinkage that could occur because shrinkage could lead to extensive cracking or substantial prestress loss. A number of shrinkage models have been developed all over the world but they differ widely from each other, indicating that the shrinkage of concrete is dependent on the characteristics of the materials used and the local conditions. Early experimental studies in Hong Kong suggested that the shrinkage of Hong Kong concrete is considerably larger than that estimated using shrinkage models developed elsewhere. This is probably due to the local granite aggregate used. In order to resolve this problem and develop a shrinkage model for Hong Kong, a comprehensive testing programme using fibre-optic strain gauges to measure the shrinkage strain was launched. The shrinkage measurement lasted over 3.5 years and covered concrete mixes with and without pulverised fuel ash added and with paste volume varying from 30 to 40%. © 2010 Thomas Telford Ltd.published_or_final_versio

Radiation dose and cancer risk from pediatric CT examinations on 64-slice CT: A phantom study

Objective: To measure the radiation dose from CT scans in an anthropomorphic phantom using a 64-slice MDCT, and to estimate the associated cancer risk. Materials and methods: Organ doses were measured with a 5-year-old phantom and thermoluminescent dosimeters. Four protocols; head CT, thorax CT, abdomen CT and pelvis CT were studied. Cancer risks, in the form of lifetime attributable risk (LAR) of cancer incidence, were estimated by linear extrapolation using the organ radiation doses and the LAR data. Results: The effective doses for head, thorax, abdomen and pelvis CT, were 0.7 mSv, 3.5 mSv, 3.0 mSv, 1.3 mSv respectively. The organs with the highest dose were; for head CT, salivary gland (22.33 mGy); for thorax CT, breast (7.89 mGy); for abdomen CT, colon (6.62 mGy); for pelvis CT, bladder (4.28 mGy). The corresponding LARs for boys and girls were 0.015-0.053% and 0.034-0.155% respectively. The organs with highest LARs were; for head CT, thyroid gland (0.003% for boys, 0.015% for girls); for thorax CT, lung for boys (0.014%) and breast for girls (0.069%); for abdomen CT, colon for boys (0.017%) and lung for girls (0.016%); for pelvis CT, bladder for both boys and girls (0.008%). Conclusion: The effective doses from these common pediatric CT examinations ranged from 0.7 mSv to 3.5 mSv and the associated lifetime cancer risks were found to be up to 0.16%, with some organs of higher radiosensitivity including breast, thyroid gland, colon and lungs. © 2010 Elsevier Ireland Ltd. All rights reserved.postprin