No description available.

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Thesis (M. Phil.)--University of Hong Kong, 2001. / Includes bibliographical references.

Thesis--M. Phil., University of Hong Kong.

Thesis--M. Phil., University of Hong Kong, 1977.

A common problem faced by engineers nowadays is the restriction on structural member dimensions due to architectural and spatial concerns. Such restrictions have resulted in the use of high-strength concrete in vertical members to reduce sizes, the use of central core walls and peripheral columns to increase window areas, the use of prestressed concrete floors to increase spans, etc. Serviceability problems such as cracking may, however, arise in the long term if these problems have not received proper attention during the design stage. This paper addresses several major issues associated with this type of buildings. Firstly, the differential axial shortening between the core walls and columns caused by large differences in stress levels will induce additional stresses and strains in the horizontal structural members, which are not normally accounted for in the traditional design methods. Secondly, the post-tensioning of concrete floors gives rise to additional internal forces induced in several ways such as time-dependent effects, sequential construction, and secondary “P-δ” effects of the high-strength slender columns. Thirdly, the soil-structure interaction could induce significant additional deformations and stresses in the buildings, although they are not always taken into account properly especially when carrying out simple or preliminary designs. These issues are vital and should be carefully considered in regular structural analyses and designs. With the common practice that most of the designs of prestressed concrete building structures are sublet to prestressing specialists, common structural engineers seldom have the insight into the structural performance of these buildings. Furthermore, utilities for calculating steel relaxation, which is an important factor governing the behaviour of prestressed concrete buildings, and its interactions with other time-dependent effects of concrete are hardly found in popular commercial software packages developed for building designs. All of these problems present obstacles in the correct modelling of prestressed concrete buildings. In the light of this, a practical but accurate method of modelling steel relaxation using the equivalent creep in commercial packages, that are normally good at dealing with complicated geometry, has firstly been developed in this work. The accuracy and reliability of the method are examined by comparing the results with available numerical solutions. Good agreement is observed. Secondly, a series of studies have been carried out based on a typical prestressed concrete building to examine various effects on the structural performance. It is found that the most influential effect is the time-dependent behaviour. It induces extra column moments, differential axial shortening, losses of tendon stresses, and P-delta moments. The construction sequence and soil-structure interactions are also found to affect the structural performance but they are less critical compared with the time-dependent effects. Finally, a parametric study has been carried out to evaluate the likely ranges of time-dependent effects on the structural behaviour. / published_or_final_version / Civil Engineering / Master / Master of Philosophy

Due to increasingly stringent requirements on concrete performance and complexity of our infrastructures, concrete with not only high strength but also all-round high performance is called for. This puts forth the notion of the so-called high-performance concrete (HPC). However, one major hurdle in the development of HPC is that the technology is still based largely on empirical approaches. For further advancement of concrete technology into concrete science, it is advocated to adopt a more scientific approach, which can improve our understanding of concrete from a level of “know -how” to a level of “know-why”. The packing characteristics of solid particles have great influence on the performance of a concrete mix, but so far there is no generally accepted method of measurement. Herein, a new wet packing method was developed and applied to blended aggregates and concrete mixes. The results for blended aggregates revealed that whilst the packing density of coarse aggregate is only slightly higher under wet condition than dry condition, the packing density of a blended fine plus coarse aggregate is significantly higher under wet condition. Furthermore, the results for concrete mixes showed that the packing density is substantially higher and the filling effects of ultrafine supplementary cementitious materials are much better revealed under wet condition. Hence, the conventional dry packing method should be abandoned and replaced by the wet packing method. It is well known that the fresh properties of paste/mortar/concrete are governed mainly by the mix parameters: water content, packing density and solid surface area. However, these mix parameters vary simultaneously and therefore their individual and combined effects are difficult to evaluate. It has been found recently by others that the combined effects of these parameters may be evaluated in terms of the water film thickness (WFT), which has the physical meaning of the average thickness of water films coating the solid particles. Herein, it was proposed that besides the WFT, the paste film thickness (PFT) and mortar film thickness (MFT) should also have effects on the performance of mortar and concrete. Extensive tests and correlations of the measured properties of mortar to the WFT and PFT revealed that whilst the WFT is the single most important factor governing the properties of mortar, the PFT also has significant effects, especially on the cohesiveness and adhesiveness. Likewise, the results for concrete revealed that whilst the WFT and PFT have significant effects on the properties of concrete, the MFT also has significant effects, especially on the cohesiveness and passing ability. Based on the above findings, it may be concluded that the major factors governing the performance of concrete are the WFT, PFT and MFT. With the correlations of the various performance attributes of concrete to these factors so established, the door is open for the eventual development of a more scientific “three-tier mix design method” for HPC, by which the concrete mix is designed in three tiers: first the WFT of the paste portion, then the PFT of the mortar portion and finally the MFT of the concrete mix. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

The stress-strain relationship of concrete in flexure is one of the essential parameters in assessing the flexural strength and ductility of reinforced concrete (RC) structures. An overview of previous research studies revealed that the presence of strain gradient would affect the maximum concrete stress and respective strain developed in flexure. Previously, researchers have conducted experimental studies to investigate and quantify the strain gradient effect on maximum concrete stress and respective strain by developing two strain-gradient-dependent factors k3 and ko for modifying the flexural concrete stress-strain curve. In this study, the author established a new analytical concrete constitutive model to describe the stress-strain behavior of both normal-and high-strength concrete in flexure with the effect of strain gradient considered. Based on this, comprehensive parametric studies have been conducted to investigate the combined effects of strain gradient and concrete strength on flexural strength and ductility design of RC beams and columns with concrete strength up to 100 MP a by employing the strain-gradient-dependent concrete stress-strain curve using non-linear moment-curvature analysis. From the results of the parametric studies, it is evident that both the flexural strength and ductility of RC beams and columns are improved under strain gradient effect. A design value of ultimate concrete strain of 0.0032and anew equivalent rectangular concrete stress block incorporating the combined effects of strain gradient and concrete strength have been proposed and validated by comparing the proposed theoretical strength with the strength of 198 RC beams and 275 RC columns measured experimentally by other researchers. It is apparent from the comparison that the proposed equations can predict more accurately the flexural strength of RC beams and columns than the current RC design codes. Lastly, for practical engineering design purpose, design formulas and charts have been produced for flexural strength and ductility design of RC beams and columns incorporating the combined effects of strain gradient and concrete strength. / published_or_final_version / Civil Engineering / Master / Master of Philosophy

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