OpenUCT :: Browsing by Author "De Kock, M O"

Browsing by Author "De Kock, M O"

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Open Access
Anchorage bond in concrete bases
(1981) Vermaak, Willem Jacobus; De Kock, M O
Five series of tests were performed to investigate the anchorage bond stresses in concrete bases. In Series I to IV concrete base specimens were loaded and column starter bar strains measured for different supporting conditions of the base. Curves indicating the distribution of steel compressive stresses and bond stresses in the columns and base slabs are given. Results from these tests indicate that the conventional design approach towards bond stresses in concrete bases is conservative and that the allowable bond stresses in BS CP110 seem to be too low while the maximum stresses specified in ACI 318-71 and the CEB-FIP Recommendations seem to be more acceptable. In Series V pull-out tests were made on 60 concrete cube specimens subjected to biaxial normal pressure. The ultimate bond strength was found to increase with applied normal pressure in proportion to the square root of the normal pressure and square root of the concrete strength. A comparative finite element stress analysis was done on a typical concrete base specimen and the results were found to be in accordance with the results of the tests in Series I to IV.
Open Access
Aspects of vibration in bridge-girders
(1972) Weinheimer, Stefanie; De Kock, M O
A theoretical comparative study of methods for determining natural frequencies of vibration and their corresponding mode shapes is presented for the case of cantilevers, simply supported and continuous beam structures. Different methods are used, such as classical, energy and matrix methods. Effects of mass lumpings are investigated. A literature review is given for the general problem of dynamic behaviour of highway bridges, including the passage of heavy vehicles.
Open Access
Behaviour of reinforced concrete deep beams
(1985) Green, Jeremy Robert; Green, Jeremy Robert; De Kock, M O
Twenty five model beams were progressively loaded to failure in order to investigate the influence of the following variables on the behaviour of reinforced concrete deep beams : i) Concrete compressive strength ii) Reinforcement iii) Geometry. The model beams were all of 1500mm span, with a depth of 750mm. This span to depth ratio of 2 corresponds to the upper limit, to which the recommendations for deep beam design applies, as provided by many current codes of practice. Methods currently in use for the design of reinforced concrete deep beams were reviewed and compared. The experimental results were compared with the predictions of these design methods. This comparison revealed a large lack of agreement in the predictions of the cracking and ultimate strengths of deep beams.
Open Access
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.
Open Access
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.
Open Access
Fatigue and fracture of cement mortars containing fly ash
(1995) Taylor, Peter Clement; Tait, R; De Kock, M O
The aim of the work described in this thesis was two-fold; to investigate the effects of fly ash on the fatigue resistance of cement mortars when included as a partial cement replacement, and to seek to improve the understanding of cyclic fatigue crack growth mechanisms in cementitious materials. Mortar mixes were prepared with similar compressive cube strengths using a range of three fly ash contents from 0% to 25% (by mass of cement). Samples prepared using these mixes were tested in a double torsion facility under cyclic loading, and the rates of crack growth measured and recorded. These crack velocities were plotted against the applied stress intensities on log-log scales in so called V-K diagrams. An advantage of using the double torsion system was that the applied stress intensity was constant for constant load conditions and changing crack length, unlike many other configurations. However, the amount of scatter inherent in the system, and in testing cementitious materials, is large. This has meant that comparison between sets of data has had to be carried out on the basis of comparing the positions of clouds of data, rather than comparing the slopes of best fit lines. Another advantage of the OT system is that some of the parameters pertaining to the test can be changed whilst the test is in progress. This means that the effects of changing, say, load amplitude or cyclic frequency can be observed on the same specimen. Parameters that were considered in the test matrix. included the following: fly ash content, sample age, cyclic frequency and amplitude, maximum applied stress intensity, relative humidity and temperature of the environment, drying preparation of the sample and the type of fluid in which the samples were tested. The relative effects of all of these variables were compared in a series of V-K plots and trends were noted.
Open Access
Modelling of reinforced concrete slab deflections at service loading
(1994) Boting, Antony John; De Kock, M O
Deflection under service loading is an important aspect of reinforced concrete slab design. Under-design can cause large deflections which can be expensive to repair, if at all possible. Over-design can lead to material wastage and unnecessary dead load. Deflection is inversely proportional to the effective moment of inertia of the section under consideration. Cracks, which may or may not be present at the serviceability limit state, have a profound effect on the moment of inertia. Many Codes of practice approach the calculation of deflection in a conservative manner by using the cracked moment of inertia in deflection calculations and ignoring the effect of the concrete in tension. Two of the Codes reviewed make an attempt at including the stiffening effect of the concrete in tension. The theory in the CEB/FIP Model Code is used as a basis for the method that is developed to predict maximum deflections. This method proposes that the total maximum deflection is composed of two components: an elastic deflection and a deflection due to cracking. The elastic deflection for a beam is determined from elastic formulae that are developed from first principles for standard beam cases. The deflection due to cracking involves the cracking moment capacity of the beam, what portion of the beam is cracked, the formation of a hinge and the rotation of this hinge. One-way spanning slabs can be treated as broad, shallow-beams. Two-way spanning slabs are more complicated and to determine the load dispersion of a uniformly distributed load on such a slab, it is divided into five sets of orthogonal strips. The two outer strips do not carry any load. The three inner strips intersect at nine points or nodes. The deflection of each pair of orthogonal strips at each of the nine nodes must be equal. Deflection equations are set up in terms of an unknown portion of the load at each node. Since the full load at each node is known, the sum of the loads in the orthogonal directions must be equal to this full load. A matrix is set up and solved and the load dispersion at each node is determined. The equivalent load on a strip spanning through the region of maximum deflection is thus found. For the two orthogonal strips spanning through the region of maximum deflection, the average deflection is then taken. A computer program is written which incorporates the above approach. The program is then run for slab configurations that were tested in the laboratory and the results are compared. The results show that the proposed computational models over-predict slab deflections. As soon as the slab is clamped on more than one edge or if the aspect ratio increases above 1 then the results in the orthogonal directions differ by a large amount. The recommended improvements to these computational models are: - Increase the number of orthogonal strips and introduce torsion. This will also improve the continuity between strips spanning in the same direction. The tension stiffening factor has to be redefined. This will reduce the contribution of deflection due to cracking.
Open Access
Position of the Triassic-Jurassic boundary in South Africa and Lesotho : a multidisciplinary approach aimed at improving the chronostratigraphy and biostratigraphy of the Elliot Formation, Stormberg Group
(2016) Sciscio, Lara; Bordy, Emese M; De Kock, M O
The continental red bed succession of the main Karoo Basin in South Africa and Lesotho, the Elliot Formation (Stormberg Group, Karoo Supergroup), is a significant stratigraphic unit for the regional and global understanding of the Late Triassic - Early Jurassic evolution of terrestrial vertebrate faunas, however, the temporal resolution of its biostratigraphy is inadequate for detailed regional and global correlations. The main aim of this dissertation is to build a more comprehensive chronostratigraphic framework of the Late Triassic - Early Jurassic Elliot Formation by combining and constraining its biostratigraphy with new results obtained using magnetostratigraphic techniques. This dissertation presents magnetostratigraphic data from ten measured stratigraphic sections in the Elliot Formation across the main Karoo Basin in South Africa and Lesotho.
Open Access
The relationship between the compressive strength of cement mortar and the chemical composition and fineness of cement
(1993) James, S D; De Kock, M O
The purpose of this research was to determine a method whereby predictions of compressive strength could be made with regard to the chemical composition and fineness of Portland cement. Tests were performed on cement paste and mortar to determine which mixture would yield the most accurate, reproducible results. Paste was found to have a high degree of variation due to its mechanism of failure. Mortar, on the other hand, yielded reproducible and accurate results. In an attempt to prevent the high variations in the compressive strength of the paste, the paste was evacuated in a special evacuation chamber. The evacuation removed a large volume of air but did not alleviate the variations in the paste strength.
Open Access
The relationship between the compressive strength of cement mortar and the chemical composition and fineness of cement
(1993) James, S D; De Kock, M O
The purpose of this research was to determine a method whereby predictions of compressive strength could be made with regard to the chemical composition and fineness of Portland cement. Tests were performed on cement paste and mortar to determine which mixture would yield the most accurate, reproducible results. Paste was found to have a high degree of variation due to its mechanism of failure. Mortar, on the other hand, yielded reproducible and accurate results. In an attempt to prevent the high variations in the compressive strength of the paste, the paste was evacuated in a special evacuation chamber. The evacuation removed a large volume of air but did not alleviate the variations in the paste strength.

Browsing by Author "De Kock, M O"

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