Thesis (M. Phil.)--University of Hong Kong, 2008. / Includes bibliographical references (p. 214-223) Also available in print.

Call number: LD2668 .T4 1954 D3

published_or_final_version / Civil Engineering / Master / Master of Science in Engineering

In environments of high moisture exposure, carbon fibre-reinforced polymer (CFRP) reinforcement is believed to be a good alternative to steel due to its non-corrosive properties. However the CFRP matrix experiences other types of changes, mechanical deterioration and transverse swelling, with exposure to moisture and sustained stresses. Specifically for structural applications, few studies have investigated the effects of moisture and stress-induced matrix-dominated changes to the CFRP-concrete bond when the CFRP rods are cast into concrete as prestressed reinforcement. Experiments were conducted to measure the transverse swelling and moisture uptake rates of stressed and unstressed CFRP rods immersed in water and concrete pore solution (CPS). CFRP rods were also embedded in concrete and immersed in water to observe whether the transverse swelling of the rods would cause the concrete to crack. Pull-out tests were conducted on stressed and unstressed CFRP rods cast in concrete cubes and immersed in water for up to 125 days. Six CFRP prestressed concrete prisms were fabricated, three were immersed in water for over 125 days and three were left in lab conditions before being tested in three-point bending. Moisture-induced transverse swelling did not appear to cause cracking of the concrete covering CFRP rods in water, contrary to the prediction of the thick-walled cylinder model, but possibly caused an increase in the CFRP-concrete radial contact pressure. The imposition of sustained bending stresses on wet CFRP rods was found to cause premature failure, believed to be a result of matrix softening and microcracking. These observations of swelling and matrix degradation were believed to affect the CFRP-concrete bond. The pull-out tests of unstressed CFRP rods indicated an increased likelihood for bond failure in the CFRP matrix with moisture exposure. This resulted in greater variations in the ultimate and residual bond stresses of the wet samples compared to the dry controls. For the prestressed pull-out samples, higher magnitudes of prestressing resulted in lower bond failure loads. Both outcomes are indicative of matrix weakening with moisture and stress exposure. However despite this, the three-point bending tests revealed no significant differences in behaviour between the wet and dry prestressed prisms. Although the wet prisms exhibited slightly greater variation in their post peak load behaviour compared to the dry prisms, in line with the observations from the pull-out tests. The work from this thesis reveals that despite observations of matrix deterioration and transverse swelling in CFRP rods exposed to moisture and stress, the effect of these changes to the overall behaviour of a CFRP prestressed concrete member are minimal. Any long-term effects are unlikely to cause significant changes to the behaviour of the member. However, the CFRP-concrete bond may be more variable in members exposed to moisture, which should be allowed for in the design process. Therefore CFRP is a promising alternative to steel reinforcement in high moisture environments, with good long-term durability.

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Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.

Thesis--M. Phil., University of Hong Kong, 1984.

Thesis (M. Sc. Eng.), University of Hong Kong. / Erratum slip inserted. Mimeographed.

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