Browsing by Subject "Concrete"

Now showing 1 - 7 of 7
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    Control of cracking in concrete by steel reinforcement : an examination of some of the mechanisms involved
    (1986) Wilson, Derek Leslie Skeeles; Kratz, Rolf Dietmar
    The techniques tried up till now to model the problem of cracking in reinforced concrete have been largely empirical. Many relationships have been identified between certain parameters and the occurrence and size of cracks. This thesis reviews these, reports on the current state of knowledge, and discusses some of the theories proposed. Because of the great variation and complexity of the materials and stress interactions involved, the problem has to date defied completely objective theoretical modelling. The finite element method provides a powerful new modelling tool for theoretical simulation of complex real problems. Recent developments on constitutive models for concrete make this method extremely attractive for use in this case. This thesis attempts to make use of these tools by carrying out some analyses of the cracking of reinforced concrete prisms stressed in tension. The method is found to be a viable way of examining mechanisms and effects which are not ordinarily visible in laboratory experiments. These mechanisms are discussed in the light of the experimental findings recorded in the literature.
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    The deformation properties of concrete with classified Lethabo fly ash
    (1990) Mukheibir, Pierre Victor; De Kock, M O
    It has become necessary to determine the magnitude of creep, shrinkage, elastic and thermal deformations of concrete as these characteristics determine the loss of prestressing in prestressed concrete and affect the deflections with time of large concrete sections. Much of the literature available on this topic has conflicting conclusions. In this research, the effect of fly ash was first investigated with regard to general concrete properties such as bleeding, early set, workability, mortar excess and compressive strength. Classified Lethabo fly ash and local Western Cape materials were used for this work. With the increase in the percentage fly ash present in the concrete mix, the water requirement was reduced in order to get the same workability. This characteristic reduced the amount of water available for bleeding. For a given C/W ratio the inclusion of fly ash in a concrete mix had no effect on the mortar excess. The early setting time was retarded for mixes with increasing percentages of fly ash. Higher cementitious material to water ratios were required for concrete with classified Lethabo fly ash than Ordinary Portland Cement mixes, to obtain the same 28 day compressive strength. The fly ash mixes had higher strength developments with time i.e. they have lower early strengths and higher long term strengths than OPC mixes for the same 28 day compressive strengths. Having developed a wide range of concrete mixes, the main investigation was done on specific deformation properties of concrete such as the elasticity, shrinkage, creep and thermal movement. The effect of different wet curing durations and testing ages on these properties were investigated. The elastic modulus was determined by both static and dynamic test methods. A relationship was established between the two methods to estimate the static modulus from the dynamic modulus, which was quicker to perform. In this thesis, the elastic modulus was not affected by the presence of fly ash. The elastic properties of the fly ash mixes was found to be similar to that of the OPC mixes of the same compressive strength. Similarly, the drying shrinkage and thermal movement were not affected noticeably by the presence of fly ash. The volume of aggregate was not a variable as it did not change when fly ash was added to the mix. An attempt was made to develop a test to determine the plastic shrinkage of an unrestrained sample. The effect of fly ash on the plastic shrinkage was not investigated fully. For the creep of concrete, it was established that mixes containing fly ash have lower creep factors than OPC concretes, although no clear trends were apparent for increasing percentages of fly ash. The effect of fly ash in pump mixes was also investigated and the same trends were apparent, although in general, the pump mixes had higher creep factors than the normal mixes. The curing of concrete is critical if good quality concrete is to be obtained. For all deformation properties, the longer a specimen was wet cured, the lower were the deformations. With longer wet curing, a larger volume of hydrated gel developed which gave higher compressive strengths and more rigidity within the matrix. The conclusion reached in this thesis was that the presence of classified Lethabo fly ash did not noticeably affect the deformation properties of the concrete for equivalent compressive strengths. Where some effects were noticed, the fly ash concretes displayed somewhat lower deformations.
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    Designed for durability in reinforced concrete structures, and associated economics
    (1997) Scott, Allan Nye; Alexander, Mark Gavin; Stevens, Alan J
    This dissertation outlines a model developed to allow for the comparison of various design options on a life-cycle cost basis for reinforced concrete structures. The model consists of two interlinked components: the first part of the model is technical, and can be used to estimate the service life of a structure within a specified environment based on a set of prediction models; the second part of the model is used to determine the economic implications of the various design options over a specified evaluation period. The use of a particular predictive model is subject to the environment in which it was calibrated. In many cases the models which are presented were developed overseas and as such their values may not be directly applicable to South African environments. The approaches and development of the models are however useful and if calibrated to particular South African environments could be of considerable benefit. A survey of consulting engineers was also conducted to determine common perceptions of various durability related issues. The results of the survey are presented in this project. The size of the sample was relatively small and as such it would be inappropriate to apply the results categorically to all engineers or organisations. The survey is of value however in that it identifies some areas of potential opposition to the concept of designing for durability and highlights other areas where its acceptance and implementation may be more favourable.
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    The durability of fly ash concrete in marine and softwater environments
    (1989) Mackechnie, James Ronald; De Kock, M O
    Concrete is attacked by aggressive agents in the marine and softwater environments which reduce the durability of concrete. To help lessen the effect of this aggressive attack, fly ash concrete has been recommended for use in these environments. The lower permeability, increased chemical resistance and higher long-term strength of fly ash concrete are expected to improve the concrete durability. In this research the effect of fly ash was investigated with regard, initially to general concrete properties such as bleeding, early set, workability, mortar excess and compressive strength. Lethabo field 2 fly ash and Western Cape materials were used for this work. Having developed a wide range of concrete mixes, further investigation was done into specific concrete properties such as the effect of different curing regimes, water absorption, permeability and freeze-thaw resistance. These properties are considered to have an influence on concrete durability. Comparisons were made between the concrete properties of Lethabo field 2, Lethabo classified and Matla classified fly ash concrete. The three types of concrete were tested for compressive strength, sorptivity (rate of water absorption) and density. At the same time, fly ash and OPC concrete samples were exposed to the marine and softwater environment for up to 10 months. Marine exposure was done in the submerged, tidal and spray zones in Table Bay. Softwater exposure was done at Constantia Nek and Steenbras Water Treatment Plants. The performance of concrete in the various exposure conditions was measured by compressive strength, sorptivity and density tests. Fly ash improved many of the properties of concrete, with fly ash concrete having better workability, higher long-term strength, reduced bleeding, lower sorptivity and reduced permeability than similar OPC concrete. Some of the properties of concrete were however worsened by using fly ash. Fly ash concrete had longer setting times, reduced resistance to freezing and thawing and was more adversely affected by dry curing than similar OPC concrete. Lethabo field 2 fly ash concrete had higher compressive strength and lower sorptivity than either Lethabo classified or Matla classified fly ash concrete. The long-term performance of Lethabo classified and Matla classified fly ash concrete was better than that of Lethabo field 2 fly ash concrete, with regard to compressive strength development and sorptivity reduction. Fly ash concrete performed well in both the marine and softwater environments. After 10 months of exposure in either marine or softwater conditions, fly ash concrete had higher compressive strength and lower sorptivity than similar OPC concrete. The good performance of fly ash concrete in the marine and softwater environment confirmed the ability of fly ash to improve many of the important durability properties of concrete. From this medium-term durability investigation it was found that Lethabo field 2 fly ash improved the performance of concrete in marine and softwater environments while fly ash, in general, improved many of the durability properties of concrete.
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    The effects of partitions on the vibration serviceability of concrete floors
    (2012) Mutombo, Christian Kabongo; Moyo, Pilate; Alexander, Mark Gavin
    The growing demand to construct long, slender floors with minimum supports for aesthetic and economic reasons especially in modern building developments has resulted in increased floor slenderness leading to vibration problems. As a result, vibration serviceability has become the governing design criterion for many of these new civil engineering structures. It is known that long span, slender floors possess lower natural frequencies and reduced damping leading to vibration serviceability problems. As vibration serviceability becomes a major concern in the design of concrete floors, investigations of the beneficial effects of non-structural elements to the vibration serviceability of floors are becoming increasingly important. The vibration serviceability of long span, slender concrete floors may be improved through the installation of non-structural elements such as partition walls and raised access floors. Little research exists into the quantification of the effects of various types of partitions on the vibration serviceability of concrete floors that support them. There are no guidelines available to designers which take into account the effects of partitions in the design of concrete floors for vibration serviceability besides the different damping ratios that are recommended in different codes for bare floors as well as floors with half- or full-height partitions. Therefore a research project was initiated with the objectives of investigating the effects of non-structural partitions on the vibration serviceability of concrete floors as well as proposing guidelines on how to account for partitions in the design of concrete floors for vibration serviceability. The research project consisted of modal testing on a prestressed pre-tensioned concrete floor slab. The excitation of the floor was generated by an electrodynamic shaker and instrumented impulse hammer. The electrodynamic shaker excitation was used mainly to extract the natural frequencies and mode shapes of the test floor. The instrumented impulse hammer excitation was used mainly for damping studies. Force-balanced QA 700 accelerometers with a sensitivity of 8 V/g mounted to base plates were used to measure the responses of the test floor. The Data Physics Signal Calc Mobilyser was used to acquire the data from the accelerometers. A personal computer was used to store, analyse and present the data. The curve fitting method in Vibrant Technologies ME'Scope was used to estimate the natural frequencies and mode shapes of the test floor. Modal 1.20 by Brownjohn (2009) was used to estimate the damping ratios of the test floor. Modal 1.20 uses the logarithmic decrement method to estimate damping. The tests investigated the effects of partitions on the vibration serviceability of concrete floors.
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    Railway Tunnels Management System in South Africa – Concrete Structural Elements
    (2019) Thako, Luba Jean-Pierre; Moyo, Pilate
    A reliable transportation network is key to economic development and social well-being of communities. Since 19th century, rail transport has provided the most efficient link between South African’s wealthy mineral heartland and the seaports. In developing this rail network, a considerable number of railway tunnels were built due to the topography of the coastal regions. These tunnels are ageing, their operational and environmental conditions are constantly changing. Thus, their management practice should be optimised to adequately respond to the needs of the organisations managing them and for them to continue providing a safe and sustainable service. Therefore, this study reviewed the current railway tunnels management practice in South Africa and proposed an approach to improve it, considering the existing structures management systems. Further, it focuses on the concrete structural elements and their related defects due to the harsh environments of these tunnels. Therefore, the Procedures to enhance tunnels management applied consisted in outlining the gaps uncovered in the current railway tunnels management practice in South Africa and enhance this practice considering the structures management systems reviewed. This is done by improving the components of each module and integrating them in the proposed railway tunnels management system in South Africa. First, it dealt with the inventory module, designed inventory forms for items and for concrete structural elements and their components to record, inspect and monitor. Second, it designed a Tunnel Inspection Programme that set the inspection intervals, the requirements for the inspection team members, the tools and techniques and procedures to apply. This programme specified also the method of evaluating the defects and the definition of the score and the relative score of the components of elements and the health of the tunnel. Third, it developed a Tunnel Monitoring System that targeted the most critical and vulnerable elements and set the techniques and tools to monitor them. Finally, it integrated all the modules in the system designed. As results, this research has proposed a computerised tunnel management system that enhances the current practice in South Africa. This arises from the scrutinised practice in light of the existing structures management systems reviewed. From the analysis of the existing inventory data on railway tunnels and on the heavy haul lines in South Africa, relevant information was obtained. Thus, an inventory module has been developed comprising tunnel inventory forms that classify the items to be recorded and inspected. This module also described the concrete structural elements and their specific components. A Tunnel Inspection Programme has been designed, specifying the frequency of inspections based on the condition of tunnels, their ages, and the unpredictability and the harshness of their environments. This programme has also promoted the relevant techniques to be applied to inspect concrete structural elements and the appropriate tools to be used. Additionally, the requirements for inspection team concerning the qualifications and experience of each member have been provided. This programme has also recommended the use of the current DER rating system, emphasising that the scope of this study refers to concrete structural elements that should all be rated and recorded. Additionally, it has designed a Tunnel Monitoring System specific to the most critical and vulnerable concrete structural elements of railway tunnels. This system includes a set of tunnel monitoring strategies, the setting up of the system and the sensory system. It finally integrated all these sub-systems into the main Railway Tunnel Management System in South Africa. In conclusion, this research proposes the integrated computerised railway tunnel management system for South Africa. It also set the “big picture” of the overall tunnel structures and the tunnels on the heavy haul lines currently managed by Transnet TFR in South Africa. Additionally, it sustains the current DER rating system and proposes its application to all the defects on the concrete structural elements, instead of the worst defect on the inspected element. As recommendations, the railway tunnels authority should adopt a monitoring system for each tunnel on the heavy haul lines. Also, the authority should make available information on all existing monitoring systems on railway tunnels and the most critical data collected. Moreover, the authority should make available the previous railway tunnels inspection files to be uploaded to the proposed system. Apart from this, we recommend a further comprehensive study to integrate the inspection of non-structural elements to the proposed system. Finally, we recommend to the management authority to organise a comprehensive study of the water leakage issues on railway tunnels to better understand and adequately respond to them.
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    The use of aggregate from demolition rubble in the making of ordinary and structural concretes
    (1987) Frick, Clayton; De Kock, Mike
    The aim of this thesis is to introduce the concept of recycling demolished concrete as aggregate which is then used in fresh concrete - to be known as "recycled concrete". Various aspects of concrete technology are covered and in this way recycled concrete is compared to conventional concrete. The work was performed in three phases, and it should serve as a guide to prospective users. Phase 1: Various recycled aggregates were tested according to standard specifications and were found to be satisfactory in most aspects. Recycled fine aggregate is very coarse though, and should be used with caution. The absorption and porosity of recycled aggregates should always be determined to enable their use in concrete. The specific gravity of such an aggregate should also be found to enable more accurate mix calculations. The highest compressive strengths normally possible for recycled concretes are between 56 and 71 MPa, but an average strength of 50 MPa should not be exceeded without thorough investigation, even though it is easily attainable. Phase 2: A wet-batching method of mix design was investigated and satisfactory recycled concretes were produced. Strength charts for such concretes are given. Methods of dry-batching are also presented, but are more complex than the wet-batch method. The water demand of recycled· fine aggregates was found to be considerably higher than for natural sands, and again the use of fine recycled aggregate should be carefully considered. Phase 3: The mechanical properties of recycled concretes were tested and Little difference found between recycled and conventional concretes. The compressive strengths were satisfactory and the elastic moduli sufficiently high, even though they were 15 to 20 percent Lower than those of corresponding dense concretes. The shrinkage of recycled concrete is comparable to that of conventional concrete, and the creep potential somewhat greater, although not excessivly so. The use of recycled coarse aggregate in both plain and structural concrete is then recommended as an alternative to the dwindling supply of natural aggregates. The use of recycled fine aggregate, however, is not recommended, although its use in Low-grade or mass concrete is condoned.