Condition assessment of a Johannesburg skyscraper
- Ekolu, Stephen, Van Ede, Willem F.
- Authors: Ekolu, Stephen , Van Ede, Willem F.
- Date: 2014
- Subjects: Skyscrapers - Evaluation
- Type: Book chapter
- Identifier: uj:5045 , ISSN 9781614994664 , http://hdl.handle.net/10210/13565
- Description: This paper presents a condition assessment conducted on a Johannesburg skyscraper. It had become apparent that concrete spalling occurred in areas of the top 11m of the Southern and Western facades of the reinforced concrete skyscraper constructed in the 1970’s. However, the extent of the problem was unknown. The only information available were two photographs showing spalled concrete and severely corroded reinforcement bars. Some literature search pertaining to the structure was conducted after which, visual inspection and mapping of spalled areas and cracks were conducted. This was followed by a cover meter survey, a half-cell potential survey, and the removal of 20 (twenty) cores for laboratory testing. It was found that the typical problems associated with carbonation and insufficient cover resulted in the corrosion of reinforcement which then led to the spalling observed.
- Full Text:
- Authors: Ekolu, Stephen , Van Ede, Willem F.
- Date: 2014
- Subjects: Skyscrapers - Evaluation
- Type: Book chapter
- Identifier: uj:5045 , ISSN 9781614994664 , http://hdl.handle.net/10210/13565
- Description: This paper presents a condition assessment conducted on a Johannesburg skyscraper. It had become apparent that concrete spalling occurred in areas of the top 11m of the Southern and Western facades of the reinforced concrete skyscraper constructed in the 1970’s. However, the extent of the problem was unknown. The only information available were two photographs showing spalled concrete and severely corroded reinforcement bars. Some literature search pertaining to the structure was conducted after which, visual inspection and mapping of spalled areas and cracks were conducted. This was followed by a cover meter survey, a half-cell potential survey, and the removal of 20 (twenty) cores for laboratory testing. It was found that the typical problems associated with carbonation and insufficient cover resulted in the corrosion of reinforcement which then led to the spalling observed.
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Effect of surface treatments on abrasion and permeation properties of clay concretes
- Solomon, Fitsum, Ekolu, Stephen
- Authors: Solomon, Fitsum , Ekolu, Stephen
- Date: 2014
- Subjects: Reinforced concrete , Concrete construction , Floors, Concrete - Design and construction
- Type: Article
- Identifier: uj:5066 , ISBN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13637
- Description: The resistance of concrete surfaces to various types of wear can be a defining performance characteristic in certain applications. Some concrete structures are required to be abrasion-resistant; among them are dams, canals, roads, floors, footpaths, parking lots, and paths in parks. The work reported in this paper is based on a laboratory study of the concrete-surface treatments (CSTs) consisting of dry shake and screeding. Four control concrete mixtures were prepared with ordinary Portland cement (OPC), having a cement content (CC) of 350 kg/m3 at water-cementitious ratio (w/cc) = 0.70, 0.75; and 280 kg/m3 CC of w/cc = 0.80, 0.85. Other concrete mixtures were then prepared by substituting OPC in the control mixtures with 10, 20, 30, and 40% local raw clay. Compressive strength, abrasion resistance, and air permeability of the surface-treated concretes were measured at the ages of six months, while drying shrinkage was monitored for up to year. Drying shrinkage of concrete reduced with decrease in w/cc ratio and increased with increase in raw clay content. Among all the mixtures, the control concrete of w/cm ratio = 0.70 and the claycement concrete containing w/cm=0.80 at 70% OPC/30% raw clay showed the least and highest shrinkage increments respectively. Abrasion resistance of concrete, measured by the rotating-cutter method, increased with increasing clay content and decreasing compressive strength. However, abrasion performance results of clay-cement concrete mixtures with CSTs were generally higher. Air permeability of the concretes was measured and its relationship with the depth of abrasion of the concrete was examined
- Full Text:
- Authors: Solomon, Fitsum , Ekolu, Stephen
- Date: 2014
- Subjects: Reinforced concrete , Concrete construction , Floors, Concrete - Design and construction
- Type: Article
- Identifier: uj:5066 , ISBN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13637
- Description: The resistance of concrete surfaces to various types of wear can be a defining performance characteristic in certain applications. Some concrete structures are required to be abrasion-resistant; among them are dams, canals, roads, floors, footpaths, parking lots, and paths in parks. The work reported in this paper is based on a laboratory study of the concrete-surface treatments (CSTs) consisting of dry shake and screeding. Four control concrete mixtures were prepared with ordinary Portland cement (OPC), having a cement content (CC) of 350 kg/m3 at water-cementitious ratio (w/cc) = 0.70, 0.75; and 280 kg/m3 CC of w/cc = 0.80, 0.85. Other concrete mixtures were then prepared by substituting OPC in the control mixtures with 10, 20, 30, and 40% local raw clay. Compressive strength, abrasion resistance, and air permeability of the surface-treated concretes were measured at the ages of six months, while drying shrinkage was monitored for up to year. Drying shrinkage of concrete reduced with decrease in w/cc ratio and increased with increase in raw clay content. Among all the mixtures, the control concrete of w/cm ratio = 0.70 and the claycement concrete containing w/cm=0.80 at 70% OPC/30% raw clay showed the least and highest shrinkage increments respectively. Abrasion resistance of concrete, measured by the rotating-cutter method, increased with increasing clay content and decreasing compressive strength. However, abrasion performance results of clay-cement concrete mixtures with CSTs were generally higher. Air permeability of the concretes was measured and its relationship with the depth of abrasion of the concrete was examined
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Factors that influence friction in tendons of unbonded post-tensioning systems
- Dundu, Morgan, Ward, Michelle
- Authors: Dundu, Morgan , Ward, Michelle
- Date: 2014
- Subjects: Tendons (Prestressed concrete) , Corrosion and anti-corrosives , Steel, Structural - Protection
- Type: Article
- Identifier: uj:5059 , ISSN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13621
- Description: The coefficient of friction in an unbonded post tensioning system is influenced by many factors, and these factors include the thickness of lubricant or grease present in the system, clearance between the strand and the plastic sheath, surface characteristics of the strand and plastic sheath. In order to determine the effect of these factors, a series of friction tests were performed on two different diameters of strand, namely; 12.7 mm and 15.24 mm diameters. Through a regression analysis, it was found that the frictional force decreases with increase in the thickness of the grease and that friction increases with increase in the mass of the strand. The amount of friction was found to be dependent on the surface characteristics of the strand and plastic sheath, clearance between the plastic sheath and the strand and the extrusion process of the plastic sheath.
- Full Text:
- Authors: Dundu, Morgan , Ward, Michelle
- Date: 2014
- Subjects: Tendons (Prestressed concrete) , Corrosion and anti-corrosives , Steel, Structural - Protection
- Type: Article
- Identifier: uj:5059 , ISSN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13621
- Description: The coefficient of friction in an unbonded post tensioning system is influenced by many factors, and these factors include the thickness of lubricant or grease present in the system, clearance between the strand and the plastic sheath, surface characteristics of the strand and plastic sheath. In order to determine the effect of these factors, a series of friction tests were performed on two different diameters of strand, namely; 12.7 mm and 15.24 mm diameters. Through a regression analysis, it was found that the frictional force decreases with increase in the thickness of the grease and that friction increases with increase in the mass of the strand. The amount of friction was found to be dependent on the surface characteristics of the strand and plastic sheath, clearance between the plastic sheath and the strand and the extrusion process of the plastic sheath.
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Infrared thermography as a diagnostic tool for subsurface assessments of concrete structures
- Scott, Matthew, Kruger, Deon
- Authors: Scott, Matthew , Kruger, Deon
- Date: 2014
- Subjects: Thermography , Testing concrete in structures , Infrared radiation - Congresses , Infrared - thermography
- Type: Article
- Identifier: uj:5060 , ISSN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13623
- Description: This paper presents on-going research into the application of infrared (IR) thermography as a means of diagnosing the presence of delaminations in concrete structures. Infrared thermography, as a diagnostic tool, has demonstrated the potential to detect and visually display areas of delamination in concrete structures, offering a feasible supplement to the traditional techniques used for delamination surveys. However, the thermal gradients that develop in the concrete, and that are essential for the detection of the delaminations, are the result of the prevailing ambient conditions that surround the structure. Depending on the nature of these conditions, certain delaminations may be more visible than others while some may not be visible at all. Solar radiation has a significant effect on these thermal gradients and consequently, the temperature contrasts that develop between the areas of delaminated and intact concrete. This paper addresses the effect that solar radiation has on the ability of infrared thermography to detect and reveal areas of delamination in concrete structures. This effect has been studied by quantifying the temperature contrasts that developed at the surface of a concrete panel into which delamination type defects were intentionally embedded at different depths.
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- Authors: Scott, Matthew , Kruger, Deon
- Date: 2014
- Subjects: Thermography , Testing concrete in structures , Infrared radiation - Congresses , Infrared - thermography
- Type: Article
- Identifier: uj:5060 , ISSN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13623
- Description: This paper presents on-going research into the application of infrared (IR) thermography as a means of diagnosing the presence of delaminations in concrete structures. Infrared thermography, as a diagnostic tool, has demonstrated the potential to detect and visually display areas of delamination in concrete structures, offering a feasible supplement to the traditional techniques used for delamination surveys. However, the thermal gradients that develop in the concrete, and that are essential for the detection of the delaminations, are the result of the prevailing ambient conditions that surround the structure. Depending on the nature of these conditions, certain delaminations may be more visible than others while some may not be visible at all. Solar radiation has a significant effect on these thermal gradients and consequently, the temperature contrasts that develop between the areas of delaminated and intact concrete. This paper addresses the effect that solar radiation has on the ability of infrared thermography to detect and reveal areas of delamination in concrete structures. This effect has been studied by quantifying the temperature contrasts that developed at the surface of a concrete panel into which delamination type defects were intentionally embedded at different depths.
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Palm kernel incinerated ash as low cost concrete component
- Agbenyeku, Emem-Obong Emmanuel, Okonta, Felix Ndubisi
- Authors: Agbenyeku, Emem-Obong Emmanuel , Okonta, Felix Ndubisi
- Date: 2014
- Subjects: Lightweight concrete , High strength concrete , Incineration
- Type: Article
- Identifier: uj:5058 , ISBN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13620
- Description: The incorporation of commercially available fly ash (FA) as low cost material in concrete has long been established. Large quantities of FA are utilized for research and field applications because of the pozzolanic characteristics. One of many pozzolanic materials is palm kernel incinerated ash (PKIA). The abundance of PKIA as an agricultural waste material in West Africa obtained by the incineration of palm kernel husk and shells in milling boilers paved way for this study. Specimens containing 50%PKIA were cast and cured both in water and air and their compressive strength and shrinkage behaviours were investigated with and without the addition of superplasticizer (hydroxylated carboxylic (HC) acid) in comparison with controlled specimens containing 100%OPC. Results showed that the strength development in PKIA green concrete at all ages of curing; 7,14 and 28 days were lower than OPC concrete. On the addition of superplasticizer, strength of 36.9N/mm2 was observed for specimens with superplasticizer as against 31.7N/mm2 for specimens without superplasticizer on the 28day. The water curing method produced the best results while the predicted shrinkage strain of the green concrete in accordance to ACI 209R-92 standard was higher than the control specimens.
- Full Text:
- Authors: Agbenyeku, Emem-Obong Emmanuel , Okonta, Felix Ndubisi
- Date: 2014
- Subjects: Lightweight concrete , High strength concrete , Incineration
- Type: Article
- Identifier: uj:5058 , ISBN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13620
- Description: The incorporation of commercially available fly ash (FA) as low cost material in concrete has long been established. Large quantities of FA are utilized for research and field applications because of the pozzolanic characteristics. One of many pozzolanic materials is palm kernel incinerated ash (PKIA). The abundance of PKIA as an agricultural waste material in West Africa obtained by the incineration of palm kernel husk and shells in milling boilers paved way for this study. Specimens containing 50%PKIA were cast and cured both in water and air and their compressive strength and shrinkage behaviours were investigated with and without the addition of superplasticizer (hydroxylated carboxylic (HC) acid) in comparison with controlled specimens containing 100%OPC. Results showed that the strength development in PKIA green concrete at all ages of curing; 7,14 and 28 days were lower than OPC concrete. On the addition of superplasticizer, strength of 36.9N/mm2 was observed for specimens with superplasticizer as against 31.7N/mm2 for specimens without superplasticizer on the 28day. The water curing method produced the best results while the predicted shrinkage strain of the green concrete in accordance to ACI 209R-92 standard was higher than the control specimens.
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Sulphate corrosion behaviour of high volume slag concrete
- Ekolu, Stephen O., Ngwenya, Adam
- Authors: Ekolu, Stephen O. , Ngwenya, Adam
- Date: 2014
- Subjects: Concrete - Mixing - Congresses , Concrete - Corrosion , Alumina cement
- Type: Article
- Identifier: uj:5063 , ISBN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13626
- Description: Concrete mixes of water-cementitious ratio (w/c’s) = 0.45, 0.5, 0.65 were used to prepare 100 mm cubes and 75 x 75 x 285 mm prisms. Moderately high alumina slag was incorporated in proportions of 50 or 70% and the specimens immersed in sodium sulphate or magnesium sulphate solutions of 50 g/L as SO4. Emphasis in the study was to assess sulphate resistance using concrete samples and other non-conventional methods. Expansion measurements, mass loss and corrosion attack or physical deterioration of concrete were monitored for 69 weeks. It was found that the results of expansion, mass gain and corrosion were consistent for Na2SO4 solution, showing improved sulphate resistance with increase in ground granulated blast-furnace slag (GGBS) incorporation. The expansion observations under MgSO4, while showing increased resistance with use of GGBS, gave conflicting results between expansion on one hand and, mass gain and corrosion on the other with the most severe corrosion occurring in concrete containing the high 70% GGBS. The corrosion test method appears to give a more realistic indicator of physical damage under MgSO4 attack compared to expansion measurement.
- Full Text:
- Authors: Ekolu, Stephen O. , Ngwenya, Adam
- Date: 2014
- Subjects: Concrete - Mixing - Congresses , Concrete - Corrosion , Alumina cement
- Type: Article
- Identifier: uj:5063 , ISBN 9781614994657 , ISSN 9781614994664 , http://hdl.handle.net/10210/13626
- Description: Concrete mixes of water-cementitious ratio (w/c’s) = 0.45, 0.5, 0.65 were used to prepare 100 mm cubes and 75 x 75 x 285 mm prisms. Moderately high alumina slag was incorporated in proportions of 50 or 70% and the specimens immersed in sodium sulphate or magnesium sulphate solutions of 50 g/L as SO4. Emphasis in the study was to assess sulphate resistance using concrete samples and other non-conventional methods. Expansion measurements, mass loss and corrosion attack or physical deterioration of concrete were monitored for 69 weeks. It was found that the results of expansion, mass gain and corrosion were consistent for Na2SO4 solution, showing improved sulphate resistance with increase in ground granulated blast-furnace slag (GGBS) incorporation. The expansion observations under MgSO4, while showing increased resistance with use of GGBS, gave conflicting results between expansion on one hand and, mass gain and corrosion on the other with the most severe corrosion occurring in concrete containing the high 70% GGBS. The corrosion test method appears to give a more realistic indicator of physical damage under MgSO4 attack compared to expansion measurement.
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