http://hdl.handle.net/10539/11187 2013-06-19T21:14:29Z http://hdl.handle.net/10539/12781 Assessment of two cone penetration test based methods for evaluating the liquefaction potential of tailings dams. Torres Cruz, Luis Alberto The stability of tailings dams is of great importance to the mining industry. It is well known that soil liquefaction is one of the mechanisms that can compromise the stability of such structures. Given the difficulty of extracting undisturbed samples of any cohesionless material, the use of in situ tests to assess liquefaction potential has been intensely researched. The purpose of this work was to assess the applicability to tailings dams of two CPT-based liquefaction assessment methodologies, namely, the Robertsonbased and the Olson and Stark methodologies. Ten case histories were evaluated. When considering triggering of liquefaction, the Robertson-based and Olson and Stark methodologies correctly predicted the behaviour of four out of five and seven out of ten case histories, respectively. When considering the onset of flow failure, the Olson and Stark methodology correctly predicted the behaviour in four of seven case histories for which a post-triggering analysis was made. The results are useful in understanding the shortcomings of implementing these methodologies on TSFs and the limits of their predictive power. Interim research report. University of the Witwatersrand. Faculty of Engineering and the Built Environment. 2013-06-12T00:00:00Z http://hdl.handle.net/10539/11242 Reflections on future needs in concrete durability research and development Ballim, Y.; Alexander, M.G.; Beushausen, H.D; Moyo, P. There is no doubt that, over the past two decades, we have made enormous advances in the understanding and practice of concrete durability. Spurred by the often experienced early deterioration of reinforced concrete structures, with high capital investment for repair and rehabilitation, conceptions of design for durability have gained an increasingly higher level of importance in recent years. It is not difficult to recognise or to acknowledge the significant changes that have taken place in the field of concrete durability over the last 25 years. 2012-02-03T00:00:00Z http://hdl.handle.net/10539/11199 Effectiveness of the fi neness of two South African Portland cements for controllingearly-age temperaturedevelopment in concrete Graham, P.C.; Ballim, Y.; Kazirukanyo, J. In developing an assessment of the quantum and rate of heat evolution from hydrating cement, an important controllable variable is the fineness of grinding of the cement. This paper presents the results of a project in which two cement clinkers were used to produce cements with five different levels of fineness. These ten cements were then used to make concretes which were subjected to testing in an adiabatic calorimeter to determine the heat evolution characteristics. The results indicate that the effect of increasing fineness on the total amount of heat released during hydration is dependent on the mineralogy and crystal composition of the cement clinker. Also, the potential benefits of a so-called low heat cement can be lost if the cement is too finely ground. Based on simulations of temperature development using the different cement types tested, the results indicate that the fineness of grinding of cement is a more important parameter in the case of concrete elements with high cement contents but of moderate dimensions. In sections of larger dimension, coarse ground cements show lower levels of temperature development with lower thermal gradients. It is generally agreed that, for a given Portland cement, the rate and amount of heat liberated during the early stages of hydration (up to 28 days) is strongly influenced by the fineness of grinding of the cement (see review by Hooton et al 2005). This is intuitively correct since finer grinding means that the hydration reac¬tions between cement and water will proceed more vigorously at early ages and hence pro¬duce more heat and at a higher maximum rate. However, as Hooton et al (2005) show, opin¬ions vary in the literature as to the sensitivity of the relationship between cement fineness and early-age heat of hydration. Furthermore, when considering different cement types, the significance of the fineness of the cement has to be judged in relation to mineralogical parameters, such as the C3A, C3S and gypsum content, equally important factors in deter¬mining the heat characteristics of cement. 2011-04-01T00:00:00Z http://hdl.handle.net/10539/11198 The effects of supplementary cementing materials in modifying the heat of hydration of concrete Ballim, Yunus; Graham, Peter C. Supplementary cementing materials, such as ground granulated blastfurnace slag(GGBS), fly ash (FA) and condensed silica fume (CSF), are now routinely used in structural concrete. Used judiciously, these materials are able to provide improvements in the economy, microstructure of cement paste as well as the engineering properties and durability of concrete. They also alter the rate of hydration and can influence the time-temperature profile in large concrete elements. 2008-09-23T00:00:00Z http://hdl.handle.net/10539/11196 Deterioration Presentation Ballim, Y 2012-01-30T00:00:00Z http://hdl.handle.net/10539/11195 Opening Address to the 2nd International Conference on Concrete Repair, Rehabilitation and Retrofitting. Cape Town 24 November 2008. Ballim, Yunus Most people, whether consciously or unconsciously, are attracted to what I shall call, the “great human narrative” of transformation through journey. We find the narrative attractive because it appears prominently - as a leitmotif - in much of the way in which we construct our religious and cultural understandings of ourselves. The journey narrative speaks strongly to our sense of identity - both as belonging to a group (tribe, nation, human, etc) but also as individual (adult, academic, gardener, etc). We recognise these identities as having been formed through difficult intellectual, spiritual and often, physical journey. Importantly, we think of the personal as well as the shared journey as a process towards a better understanding of complexity – towards a more enlightened view of the world and our place in it. Opening Address to the 2nd International Conference on Concrete Repair, Rehabilitation and Retrofitting. Cape Town 24 November 2008. 2004-11-24T00:00:00Z http://hdl.handle.net/10539/11194 Early-age Thermal Characteristics of Clinker Cements in Relation to Microstructure and Composition: Implications for Temperature Development in Large Concrete Elements Ballim, Y.; Graham, P.C. This paper presents an assessment of the heat response of nominally similar cement clinkers from a range of cement production facilities in South Africa. Clinker samples were collected at nine cement plants and cements were prepared by grinding each clinker with a uniform quality of gypsum. XRF and optical microscope techniques were then used to characterise each clinker and cement in terms of chemical composition and cement compound morphology. The thermal response of concrete due to hydration of cement is a predominant factor in the potential for early-age cracking of large concrete elements. An anal¬ysis of this cracking potential requires an ability to quantify both the amount of heat that is evolved by the cement as well as the rate at which this heat is evolved [1]. Both these parameters are strongly influenced by the chemical and mineralogical composition of the cement, insofar as it affects the kinetics of the hydration reac¬tions of cement. Furthermore, clinker morphology has been shown [2] to influence the compressive strength and, by inference, the hydration development of cement. Clearly, an ability to estimate the thermal response of cement in concrete, based on a knowledge of clinker characteristics would be of assistance to mass concrete 2004-01-01T00:00:00Z http://hdl.handle.net/10539/11192 A Maturity Approach to the Rate of Heat Evolution in Concrete Ballim, Y.; Graham, P.C. This paper discusses the use of the concept of maturity as a means of combining the effects of time and temperature in describing the rate of heat evolution from hydrating cement in concrete. The proposed maturity approach allows the rate of heat evolution determined from an adiabatic test to be expressed in a form which is independent of the starting temperature of the test. This relationship can then be directly used in a time-temperature prediction model which requires a solution of the Fourier equation for heat flow. Early-age cracking as a result of temperature induced stresses can be a serious problem in mass concrete structures or in concrete structural elements in which a high cement content concrete is used. These stresses are induced by temperature differences in the concrete as a result of the heat liberated by hydrating cement. A strategy that is aimed at controlling or limiting such cracking must include a reliable determination of the space-time distribution of temperature throughout the concrete element under consideration. 2003-06-03T00:00:00Z http://hdl.handle.net/10539/11191 A maturity approach to the rate of heat evolution in concreate Ballim, Y.; Graham, P.C. Early-age cracking as a result of temperature induced stress can be a serious problem in mass concrete structural elements in which a high cement-content concrete is used. 2003-06-03T00:00:00Z http://hdl.handle.net/10539/11190 Modelling the cooling of concreate by piped water Myers, T.G.; Fowkes, N.D.; Ballim, Y. Piped water is used to remove hydration heat from concrete blocks during construction. In this paper we develop an approximate model for this process. The problem reduces to solving a one-dimensional heat equation in the concrete, coupled with a first order differential equation for the water temperature. Numerical results are presented and the effect of varying model parameters shown. An analytical solution is also provided for a steady-state constant heat generation model. This helps highlight the dependence on certain parameters and can therefore provide an aid in the design of cooling systems. Large concrete structures are usually made sequentially in a series of blocks. After each block is poured it must be left to cool and shrink for a period depending on its size, but typically for around 1 week, before the next block is poured. The reason for the delay is that the mixture of cement and water, which constitute the binding agent of the concrete, results in a series of hydration reactions that generate heat. 2009-03-25T00:00:00Z