Masters Degrees (Civil Engineering)

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The characterisation of compressed earth blocks stabilised with cement and agro-industrial residues
(Stellenbosch : Stellenbosch University, 2016-03) Malherbe, Danielle; Boshoff, William Peter; De Villiers, Wibke; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: The construction industry is renowned for its detrimental impact on the environment due to the significant resource and energy consumption, as well as the large volumes of carbon dioxide emissions. The increased awareness raised for the conservation of the environment led to the introduction of the sustainable development concept. The South African government aims to address the national housing deficit of 2.3 million units through the development of sustainable human settlements and in addition, reduce the carbon emissions through the implementation of a carbon tax. A building material capable of sustaining thermal comfort within a structure will contribute towards a reduction in the total volume of energy consumed throughout a building’s life cycle and the sustainable development of the construction industry. This creates the need for the development of an alternative building material with minimal environmental impact and an excellent thermal performance, compared to conventional masonry units. The compressed stabilised earth block was selected as a viable alternative, however its use is hindered by the paucity of knowledge and specifications pertaining to these masonry units. This study investigates specific properties of compressed stabilised earth blocks with the intention of making an earnest contribution towards the development of this building material and contribute to the further development of technical specifications, which also forms the main objective of this investigation. The stabilising materials added to the soil mixtures used to produce these masonry units were varied in both quantity and type to investigate the influence thereof on the properties measured. These properties include the compressive strength, Young’s modulus, fracture energy, tensile splitting strength, material density, porosity, thermal conductivity, water absorption and drying shrinkage. Some of the compressed stabilised earth blocks had a compressive strength in excess of the minimum recommended by SANS 10249. The Young’s modulus values, fracture energies and tensile splitting strengths measured for the various compressed stabilised earth blocks were relatively low compared to conventional masonry units, while noteworthy thermal conductivity values were obtained. These masonry units exhibited undesirable water absorption and shrinkage, which should be accounted for when using compressed stabilised earth blocks for the construction of sustainable human settlements in South Africa. The stabiliser content and type, along with the particle packing arrangement of each soil mixture within the block press chamber, had the most significant effect on the measured properties of compressed stabilised earth blocks. The secondary objective of this investigation is to assess the potential of South African sugar cane bagasse ash, a residue produced during the processing of sugar canes, as a supplementary cementitious material. This was done by determining the strength activity index of mortar cubes of which a portion of the cement was partially replaced by unprocessed sugar cane bagasse ash. Based on results obtained, unprocessed sugar cane bagasse ash, as obtained from South African sugar mills, may not be classified as a supplementary cementitious material. The effect of additional processing of the ash on its strength activity index was investigated by grinding the ash using a swing mill. This showed promising results and therefore additional processing methods by which the pozzolanic reactivity of sugar cane bagasse ash can be enhanced, should be investigated and standardised. The properties of compressed stabilised earth blocks in which the cement was partially replaced by unprocessed sugar cane bagasse ash, was compared to an equivalent block in which the cement was partially replaced by fly ash. The fly ash blocks had enhanced properties compared to that of sugar cane bagasse ash blocks, which correlated to the reduced pozzolanic reactivity of sugar cane bagasse ash. The properties of both these types of compressed stabilised earth blocks were found to be comparable to that of compressed stabilised earth blocks manufactured from soil mixture stabilised solely with cement. The knowledge gained throughout this investigation justifies the classification of compressed stabilised earth blocks as a potential building material for the development of sustainable human settlements in South Africa. However, the application of these masonry units within the South African construction industry should be supported by extensive and focussed investigative studies.
Construction in in-situ cast flat slabs using steel fibre reinforced concrete
(Stellenbosch : Stellenbosch University, 2011-12) Jarrat, Robert; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Fibre reinforced concrete (FRC) transforms concrete from a characteristically brittle material to one with a post-crack tensile residual capacity. Its application in industry has varied over the past of which the tensile properties have generally been used in the form of crack mitigation. More recently, the introduction of steel fibres has broadened this scope to structural applications in which the resisting tensile stresses that develop within a steel FRC (SFRC) element can be rather significant. This thesis reviews the existing practices and design models associated with SFRC and the suitability of its implementation as the sole form of reinforcement in in-situ cast flat slab systems. As a material SFRC is dependent on a number of factors which include the fibre type and volume, fibre distributions, element size, as well as the support and applied load conditions. Thus, its performance can be considered rather variable in comparison to conventional concrete should the incorrect practices be implemented. In order to adequately define the material characteristics, it is necessary to use test procedures that accurately reflect on the intended structural application. As a result a number of test procedures have been developed. In addition to this, the post-crack material performance is associated with a non-linear behaviour. This attribute makes the design of structural SFRC elements rather difficult. In an attempt to simplify this, existing design models define stress-strain or stress-crack width relations in which assumptions are made regarding the cross-sectional stress distribution at specified load states. This thesis takes on two parts in defining the suitability of SFRC as the sole form of reinforcement in flat slab systems. The first is a theoretical investigation regarding the micro and macro scale material performance of SFRC, the practices that exist in defining the material properties and its application in structural systems (particularly suspended slab systems), and a breakdown of the existing design models applicable to strain softening deflection hardening SFRC materials. The second part is an experimental program in which the fresh state and hardened state material properties of specified SFRC mix designs defined through flow and beam testing respectively. These properties are then implemented in the design and construction of full scale flexural and punching shear test slabs in an attempt to verify the theory applied. The investigation reveals that the use of SFRC significantly improves the ductility of concrete systems in the post-crack state through fibre crack bridging. This ductility can result in deflection hardening of flat slab systems in which the redistribution of stresses increases the load carrying capacity once cracking has taken place. However, the performance of large scale test specimens is significantly influenced by the construction practices implemented in which the material variability increases as a result of non-uniform fibre distributions. The results indicate that the load prediction models applied have potential to adequately predict the ultimate failure loads of SFRC flat slab systems but however cannot account for possible non-uniform fibre distributions which could result in premature failure of the system.
Design method development and software implementation for fibre-reinforced concrete slabs-on-ground
(Stellenbosch : Stellenbosch University, 2017-12) Mudge, Frederik Jacobus; Van Rooyen, Gert Cornelis; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Ground-supported concrete slabs are common structural elements, used for a multitude of purposes. In industrial flooring applications, slabs-on-ground (SOG) are often subjected to severe loads, concentrated at points or acting over extended areas. Adequate reinforcement of such slabs is essential to obtain sufficient load capacity and to guarantee serviceability of a slab throughout its lifetime. Synthetic fibre reinforcement has been shown to be effective in increasing the tensile strength and toughness of concrete slabs-on-ground. It increases the load capacity of slabs without requiring procurement of costly steel-mesh, the labour associated with installing it, or major alterations to concrete mix design. Although extensive research has been carried out to analyse and predict the performance of synthetic-fibre reinforced concrete (SynFRC) slabs-on-ground, no universally accepted design guideline exists. Similarly, no computer-based design packages that facilitate the analysis and design of such slabs are available. In this study a comprehensive set of algorithms is developed for the analysis and design of SynFRC ground-supported slabs. It includes an algorithm that can optimise any given slab design in terms of cost. The proposed algorithms are based on an extensive review of relevant academic and industrial literature pertaining to SynFRC, slabs-on-ground and their associated design approaches. Long term settlement and the bearing capacity of soil are not accounted for. The reaction of soil to slab loading is included by means of a modulus of subgrade reaction, k. The yield-line approach to assessing point load capacities is adopted, while elastic methods are employed to analyse the effect of line- and uniformly distributed loads on the structure. A software prototype that implements the algorithms and provides a user friendly interface is developed using the Java programming language. It includes various features which aid the process of modelling a slab, such as the generation of the most adverse wheel loads within a traffic zone. To ensure the validity of all algorithms and their implementation, a series of unit tests and validations are carried out. It is concluded that the proposed algorithms and software prototype operate successfully and yield useful results.
Developing a low embodied carbon-content concrete with conventional concrete properties
(Stellenbosch : Stellenbosch University, 2019-04) Diekmann, Michael Andreas; Boshoff, William Peter; Combrinck, Riaan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: In an ever-developing world, the use of concrete as a construction material, and cement as a main constituent thereof, is at a historical peak and set to increase even further in future. At the same time, the world is confronted with environmental challenges partly due to greenhouse gas emissions, to which the production of cement is a large contributor. In order to decrease the emissions and ensure greater sustainability of the concrete industry, it is therefore critical to reduce the cement content in conventional concrete. This reduction in cement content can however not sacrifice the quality of the concrete, in terms of certain properties that conventional concrete exhibits. It is therefore the main objective of this study to develop a low cement-content concrete, and as such a low embodied carbon-content concrete, with conventional concrete properties. Three approaches of achieving this can be defined. Firstly, cement in concrete can be replaced by more environmentally friendly supplementary cementitious materials (SCM) or fillers. Furthermore, the water requirement of concrete can be reduced in order to achieve a lower cement content by, secondly, using superplasticisers or, thirdly, optimising particle packing. This study establishes reference mixes using the first approach, before separately using the latter two approaches to lower the water requirement of the former mixes at a constant slump and water/binder ratio. The three approaches are finally combined in order to establish what are termed the “optimised” mixes in terms of cement content, with conventional concrete properties being the aim. The concrete properties that all mixes are evaluated for include rheological properties, setting time, compressive strength, permeability as part of durability and the equivalent carbon dioxide (CO2e) emissions. Furthermore, certain indices showing the efficiency of use of cement and the CO2e emissions due to the mixes in terms of compressive strength are determined. It was found that the replacement of various fractions of cement showed a pronounced reduction of CO2e emissions, while resulting in mixes with conventional properties. The inclusion of superplasticisers improved the rheological properties of these mixes and further reduced the emissions of the mix, by significantly reducing the cement content. However, this decreased the compressive strength of the mixes. The optimisation of particle packing improved all the measured properties. The combination of all three approaches resulted in mixes with improved rheological properties, as well as a 40% to 60% decrease in the emissions due to the concrete. The compressive strength was negatively effected and halved compared to the reference mix. However, certain mixes still showed better efficiency indices than the reference mixes, meaning they used less cement and CO2e emissions to develop strength. With regard to cement content, they could indeed be termed the “optimised” mixes.
The durability of natural sisal fibre reinforced cement-based composites
(Stellenbosch : Stellenbosch University, 2015-03) De Klerk, Marthinus David; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: The building industry is responsible for a substantial contribution to pollution. The production of building materials, as well as the operation and maintenance of structures leads to large amounts of carbon-dioxide (CO2) being release in the atmosphere. The use of renewable resources and construction materials is just one of the ways in which the carbon footprint of the building industry can be reduced. Sisal fibre is one such renewable material. Sisal fibre is a natural fibre from the Agave Sisalana plant. The possibility of incorporating sisal fibre in a cement-based matrix to replace conventional steel and synthetic fibres has been brought to the attention of researchers. Sisal fibre has a high tensile strength in excess of polypropylene fibre and comparable to PVA fibre. Sisal fibre consists mainly of cellulose, hemi-cellulose and lignin. The disadvantage of incorporating sisal fibre in a cement-based matrix is the degradation of the composite. Sisal fibres tend to degrade in an alkaline environment due to changes in the morphology of the fibre. The pore water in a cement base matrix is highly alkaline which leads to the degradation of the fibres and reduced strength of the composite over time. Sisal fibre reinforced cement-based composites (SFRCC) were investigated to evaluate the durability of the composites. Two chemical treatments, alkaline treatment and acetylation, were performed on the fibre at different concentrations to improve the resistance of the fibre to alkaline attack. Alkaline treatment was performed by using sodium hydroxide (NaOH), while acetylation was performed by using acetic acid or acetic anhydride. Single fibre pull-out (SFP) tests were performed to evaluate the influence of chemical treatment on fibre strength, to study the fibre-matrix interaction and to determine a critical fibre length. A matrix consisting of ordinary Portland cement (OPC), sand and water were used for the SFP tests. This matrix, as well as alternative matrices containing fly ash (FA) and condensed silica fume (CSF) as supplementary cementitious material, were reinforced with 1% sisal fibre (by volume) cut to a length of 20 mm. The OPC matrix was reinforced with untreated- and treated fibre while the alternative matrices were reinforced with untreated fibre. Alternative matrices containing varying fibre volumes and lengths were also produced. Three-point bending- (indirect), direct tensile- and compression tests were performed on specimens at an age of 28 days to determine the strength of the matrix. The remainder of the specimens were subjected to ageing by extended curing in water at 24˚C and 70˚C respectively and by alternate cycles of wetting and drying, after which it was tested at an age of 90 days from production to evaluate the durability of the fibre. An increase in fibre volume led to a decrease in compressive strength and peak tensile strength. The optimum fibre length at a volume of 1% was 20 mm for which the highest compression strength was recorded. The combination of alkali treatment and acetylation was the most effective treatment condition, followed by alkali treatment at low concentrations of sodium hydroxide. At higher concentrations of sodium hydroxide, a significant reduction in strength was recorded. The addition of supplementary cementitious materials also proved to be effective in mitigating degradation, especially in the cases where CSF was used. FA proved to be less effective in reducing the alkalinity of the matrix. However, the use of FA as fine filler resulted in higher strengths. Specimens manufactured by extrusion did not have superior mechanical properties to cast specimens. The conclusion was made that the use of sisal fibre in a cement-based matrix is effective in providing ductile failure. Chemical treatment and the addition of supplementary cementitious materials did improve the durability of the specimens, although degradation still took place.
The effect of macro-synthetic fibres on the drying shrinkage cracking behaviour of concrete slabs on grade.
(Stellenbosch : Stellenbosch University, 2015-12) Du PLessis, Louise; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Slabs on grade are the most commonly used floors for the ground floor of structures without basements. These slabs are made from the generally used construction material called concrete. Concrete is known for its relatively high compressive strength, but because of its comparatively low tensile capacity the concrete cracks easily under tensile forces. Cracks in the concrete slabs on grade reduce the serviceability and durability of the floor. The use of synthetic fibre reinforced concrete (SynFRC) in slabs on grade has shown major advantages particularly in controlling the extent and size of cracks that are formed due to drying shrinkage. An investigation is done to determine the effect of locally produced macro-synthetic fibres on the drying shrinkage cracks in slabs on grade. This is done thought a large scale test that compared the drying shrinkage cracking of polypropylene fibre reinforced concrete (PPFRC) with that of conventional concrete. The results of the large scale tests are then compared with that of a mathematical finite element method (FEM) model created in a finite element program called Diana. The different material properties of the PPFRC that are needed for the FEM analyses are obtained through flexural three point beam tests, wedge splitting tests, compression tests and shrinkage tests. The effect of the fibres are tested in terms of the different spacings between saw cut joint, the different fibre dosages, the slab thickness and the degree of friction between the concrete and the sub-base under the concrete. It is found that the use of macro-synthetic fibres results in an increase in the number of cracks between the saw cut joints, especially if the joint spacing is increased. Although there is an increase in the number of cracks, the use of fibres does consequently lead to a decrease in the crack widths. The use of macro polypropylene fibres in slabs on grade will thus result in an increase in the number of cracks, but with smaller crack widths, especially with an increase in spacings between the joints.
Establishment of performance-based specifications for the structural use of locally available macro-synthetic fibres
(Stellenbosch : Stellenbosch University, 2015-03) Odendaal, Courtney Megan; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: FRC (Fibre-reinforced concrete) has become a common form of secondary and even primary reinforcing in some applications throughout the world. In South Africa, the structural applications are limited primarily to steel fibres while cheaper, lighter and more durable synthetic fibres have been side-lined due to low stiffnesses. The purpose of this research project is to investigate the behaviour of synthetic fibre-reinforced concrete (SynFRC) using fibres which are locally available in South Africa, and to propose a performance-based specification and test method for the use of these fibres. In order to achieve this, single fibre pull-out tests were performed on four locally available polypropylene fibres. It was found that the average bond stresses of the fibres are influenced primarily by the fibre cross sectional shape, longitudinal geometry and surface treatment, and secondarily by the aspect ratio. The W/C ratio had little effect on the single fibre performance of non-treated fibres, but appeared to have a slight effect on the single fibre performance of the surface treated fibre. From the experimental results, the highest fibre bond stress will be generated by using a fibre with an X-shaped cross section, longitudinal crimping and applying a surface treatment to this fibre. It also appears that the bond stress distribution for flat fibres is close to uniform, while the bond stress distribution for non-flat crimped fibres has a high mechanical interlock component at the surface end. Macro-mechanical performance tests were performed by means of the BS EN 14651 (2007) three point beam bending test and the ASTM C1550 (2012) Round Determinate Panel Test (RDPT). These tests were selected following a thorough literature review. The RDPT was found to be more consistent and able to identify trends which the three point beam bending test could not. In addition, the three point beam bending test’s most popular output, the Re,3 value tended to be misleading with varying W/C ratios, and it is recommended that the equivalent flexural tensile strength be used instead if the three point beam bending test is used. The macro-mechanical testing showed that increasing the fibre dosage did increase post-cracking performance. The flat fibres’ performance was significantly better than that of the non-flat fibres, and also increased at a faster rate with increasing fibre dosage. The post-cracking performance decreased with increasing W/C ratios and increasing aggregate sizes. The macro-mechanical performance was inversely proportionate to the single fibre performance. The macro-mechanical performance decreased with increasing fibre bond stress, and increased with increasing equivalent diameter, which equates to fewer fibres in a set volume of fibres. Finally, basic principles were developed from the data. These principles were used to predict the RDPT and three point beam bending test performance parameters based on fibre dosage, single fibre properties (bond stress and equivalent diameter), W/C ratio and aggregate size from the available data. The principles can be further refined with more experimental data.
Generic model for predicting the performance of macro-synthetic fibre reinforced concrete for industrial flooring applications
(Stellenbosch : Stellenbosch University, 2017-03) Bester, Hermanus Lambertus; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: The versatility and ready availability of concrete has ensured that this material will continue to be one of great and increasing importance for all types of construction (Domone, 2010). Due to its low tensile strength in comparison with its compressive strength, unreinforced concrete suffers from brittle failure in uni-axial or flexural tension. This drawback can be compensated for by the addition of fibres to the concrete in its fresh state to provide ductility to the brittle concrete matrix. The primary objective of this study is to create generic models which can be implemented to predict the post-cracking performance of Macro-Synthetic Fibre Reinforced Concrete (MSFRC), specifically for industrial flooring applications. To develop the generic models, an extensive background study on FRC is conducted to identify possible factors influencing the performance of MSFRC. Concrete compressive strength, coarse aggregate size, coarse aggregate volume, fibre dosage, and mixing time of MSFRC in its fresh state are identified as the possible influencing factors. Research hypotheses are stated and investigated to determine which of the factors identified have a significant influence on the post-cracking performance of MSFRC, specifically for an experimental macro-synthetic fibre supplied by CHRYSO. Generic linear models are derived to predict the residual flexural tensile strength of MSFRC at specific crack mouth opening displacements (CMOD) and are based on the macro-mechanical tests performed according to EN 14651 (European Norms, 2007). It is concluded that fibre dosage is the only identified factor indicating a significant influence on the residual flexural strength of MSFRC. It is also concluded that the limit of proportionality (LOP), which corresponds to the maximum stress between a CMOD of 0 − 0.05 mm, is only influenced by the compressive strength. Single-fibre pull-out tests (SFPOT) are performed to investigate the effect of compressive strength on the single-fibre performance of the CHRYSO macro-synthetic experimental fibre in its virgin and premixed fibre state. An increase in the performance is evident for the premixed fibres and can be attributed to the mixing process, causing a roughening of the fibre surface and ultimately increasing the fibre-matrix bond characteristics. It is established that compressive strength does not affect the single-fibre performance of the fibre in its virgin state. However, an increase in the performance of the premixed fibres is evident for a decrease in compressive strength, with the explanation of this phenomenon being unclear. Simple- and multiple regression analyses are performed to statistically identify the factors that have a significant effect on the performance of MSFRC and to derive linear models predicting the performance parameters. The regression analyses are based on the obtained macro-mechanical results. As from the visual inspection of the macro-mechanical results, the regression analyses concluded that fibre dosage is the only factor that has a significant effect on the residual flexural tensile strength of MSFRC, and compressive strength as the only factor that influences the LOP. Therefore, the models predicting the performance parameters associated with the residual flexural strength of MSFRC are based on the influence of fibre dosage, and the model predicting the LOP is based on the effect of compressive strength. The models can further be refined with additional experimental data, incorporating a Model Factor (MF) that takes account of additional variation experienced in the construction industry and determining partial material factors (m) to derive suitable design values.
Investigating the tensile creep of steel fibre reinforced concrete
(Stellenbosch : Stellenbosch University, 2012-03) Mouton, Christiaan Johannes; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Research in concrete has advanced to such an extent that it is now possible to add steel fibres to concrete in order to improve its durability and ductility. This led to a research group in Europe, FIB, who has provided guidelines to designing Steel Fibre Reinforced Concrete (SFRC) structures. They have found that it is possible for SFRC beams in flexure to be in static equilibrium. However, the time-dependent behaviour of SFRC has not been researched fully and it requires further investigation. When looking at a concrete beam in flexure there are two main stress zones, the compression zone and the tension zone, of which the tensile zone will be of great interest. This study will report on the investigation of the tensile time-dependent behaviour of SFRC in order to determine how it differs from conventional concrete. The concrete has been designed specifically to exhibit strain-softening behaviour so that the material properties of SFRC could be investigated fully. Factors such as shrinkage and tensile creep of SFRC were of the greatest importance and an experimental test setup was designed in order to test the tensile creep of concrete in a simple and effective manner. Comparisons were be made between the tensile creep behaviour of conventional concrete and SFRC where emphasis was placed on the difference between SFRC specimens before and after cracking occurred in order to determine the influence of steel fibre pull-out. The addition of steel fibres significantly reduced the shrinkage and tensile creep of concrete when un-cracked. It was however found that the displacement of fibre pull-out completely overshadowed the tensile creep displacements of SFRC. It was necessary to investigate what effect this would have on the deflection of SFRC beams in flexure once cracked. Viscoelastic behaviour using Maxwell chains were used to model the behaviour of the tensile creep as found during the tests and the parameters of these models were used for further analyses. Finite Element Analyses were done on SFRC beams in flexure in order simulate creep behaviour of up to 30 years in order to determine the difference in deflections at mid-span between un-cracked and pre-cracked beams. The analyses done showed that the deflections of the pre-cracked SFRC beams surpassed the requirements of the Serviceability Limit States, which should be taken into account when designing SFRC beams.
Investigation into the Effect of Fibre Geometry on the Performance of Macro Synthetic Fibre Reinforced Concrete
(Stellenbosch : Stellenbosch University, 2016-03) Lerch, Jean Oliver; Boshoff, William Peter; Van Rooyen, Algurnon Steve; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Unreinforced concrete has the inherent shortcomings of low tensile strength and low strain capacity at fracture (ACI Committee 544, 2002). In order to overcome these shortcomings, fibres can be added to the fresh concrete with the aim to introduce ductility to the brittle concrete matrix. Synthetic fibre reinforced concrete (FRC) gained popularity over the past years (Bolat et al., 2014), finding its primary application in ground supported slabs. The purpose of this research is to improve the general understanding of macro synthetic FRC on the single-fibre and the macro-mechanical level. Special attention is given to fibres with various geometric properties such as fibre length, profile and fibre shape. Single-fibre pull-out (SFPO) experiments have been conducted on macro synthetic fibres with various embedment lengths. Fibres were premixed prior to embedment into the fresh concrete paste matrix to investigate in-service conditions. The effect of premixing was especially noted for flat fibres, indicating an increase in interfacial bond exceeding 100 % in contrast to virgin unmixed fibres, depending on the embedment length. Embossed fibre profiling proved to be the most efficient fibre geometry, providing the highest interfacial bond with the surrounding paste matrix. It is generally accepted that a high interfacial bond is a good indication for the overall performance required in typical macro synthetic FRC applications. Additionally, time dependent pull-out (TDPO) experiments have been conducted on single embedded fibres. It was found that premixing has a significant influence on the TDPO performance, withstanding sustained loads considerably longer than unmixed fibres. Embossed fibre geometries revealed substantial resistance against sustained loads, undergoing very little displacement, being representative for small time dependent crack openings in contrast to non-embossed fibre geometries. Non-embossed fibre geometries typically exhibited considerable pull-out displacement, demonstrative for large crack openings. Small time dependent crack opening is a property desired when structural soundness is required. Macro-mechanical tests were performed in order to establish parameters required in the structural design aspect of synthetic FRC, as these type of tests represent the in-service conditions of macro synthetic FRC. It was found that the embossed fibre profile indicated the highest performance followed by that of the flat fibre type. Robust macro-mechanical performance is required for the development of economic macro synthetic FRC elements and a reduced eco-footprint. In addition, macro-mechanical experiments have been conducted on macro synthetic FRC subjected to prolonged mixing times. It has been established that prolonged mixing typically decreases the post-cracking performance of macro synthetic FRC. Therefore, mixing time has a significant influence on the structural performance of macro synthetic FRC. It has been recognised that the best overall structural performance is achieved by embossed fibre geometries. In addition, the mixing stage was found to have a significant influence on the fibre performance in the hardened state, especially for flat fibres. Depending on the type of macro synthetic FRC application, longer fibre lengths are required for higher levels of deformation, while shorter fibre lengths revealed adequate performance for lower levels of deformation. Furthermore, TDPO experiments revealed concerning behaviour of nonembossed polypropylene macro synthetic fibres.
An investigation into the use of low volume - fibre reinforced concrete for controlling plastic shrinkage cracking
(Stellenbosch : Stellenbosch University, 2012-03) Maritz, Jaco-Louis; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Plastic shrinkage cracking (PSC) in concrete is a well-known problem and usually occurs within the first few hours after the concrete has been cast. It is caused by a rapid loss of water from the concrete, either from the surface through evaporation or through absorption by dry subgrade or formwork in contact with the concrete and results in an overall reduction in concrete volume. If this volume reduction or shrinkage is restrained, plastic shrinkage cracks can occur. Plastic shrinkage cracks create an unsightly appearance on the concrete surface which reduces the quality of the concrete structure. These cracks also develop weak points in the concrete which can be widened and deepened later on by drying shrinkage and thermal movement. As a result harmful substances may enter the cracks causing accelerated concrete deterioration. These cracks may also expose the steel reinforcement causing it to corrode more aggressively. Consequently, the aesthetic value, serviceability, durability and overall performance of the concrete will be reduced. Therefore it is important to consider methods of limiting PSC. One of these methods is the addition of low volumes of polymeric fibres to concrete to reduce PSC. However, the application of this low volume fibre reinforced concrete (LV-FRC) is not clearly understood since there is a lack of knowledge and guidance available for the use of LV-FRC. The objective of this study is to gain a full understanding of PSC behaviour in conventional concrete and LV-FRC by investigating the effects of evaporation and bleeding as well as the effect of various fibre properties on PSC. The following significant findings were attained: A basis for a crack prediction model in conventional concrete was developed using the average differences in cumulative evaporation and cumulative bleeding to create a crack prediction value (CPV). This preliminary model showed that there exists a certain CPV range (-0.2 to 0.4 kg/m2 for this study) where a slight decrease in the CPV results in a significant PSC reduction. It also showed that if the CPV falls outside this range, varying the bleeding or evaporation conditions will have very little effect on the PSC. A study on the fibre properties in LV-FRC showed that there exist certain limits to the fibre volume, length and diameter where a further increase or decrease in value will have no or little effect on reducing PSC. It also showed that the effect of the fibres depend on the level of severity of PSC. The knowledge gained from this investigation can serve as a basis for the design of a model that can predict the risk of PSC in conventional concrete and specify preventative measures needed to reduce this risk. It also provides information that can be used to develop guidelines for the effective use of LV-FRC.
The mechanical and volumetric behaviour of sisal fibre reinforced concrete blocks
(Stellenbosch : Stellenbosch University, 2013-03) Coetzee, Gerrit; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Natural fibre reinforced concrete (NFRC) is a type of concrete that has become of particular interest in recent years, due to its potential for being used as a sustainable and economically viable building material. Natural fibres are often cheap and widely available in developing nations. Sisal is one such fibre predominantly grown in Brazil and has been identified as having the potential to be commercially cultivated in Southern Africa. The durability of sisal fibres in a cementitious environment tends to be adversely affected due to the high alkalinity of pore water and the presence of calcium hydroxide. This research dealt with the use of sisal fibre reinforced concrete (SFRC) blocks. It focused on the mechanical and volumetric properties of blocks with varying fibre and condensed silica fume content (CSF). Two different SFRC blocks were produced (solid and hollow) using an average fibre length of 10 mm. Two matrix types were used: one using a 70:30 cement:fly-ash ratio and another using a 60:30:10 cement:fly-ash:CSF ratio by weight. Samples of each matrix type were prepared with 0, 0.5 and 1% fibre content by volume. Hollow blocks were tested for compressive strength and capillary water absorption, while solid blocks were tested for compressive strength, flexural strength, capillary water absorption, dimensional stability, drying shrinkage, density, total water absorption and void content. All tests were performed on samples with an age of 28 days. Solid block compressive tests were also performed on samples with an age of 7 days. The hollow blocks had significantly lower average compression strength than the solids, but an increase in fibre content caused a slight increase in strength. For solid blocks, it was found that the addition of natural fibres decreases the strength, although a partial substitution of cement with CSF, in conjunction with fibres, did increase the strength relative to blocks without CSF. The flexure strength was also lowered somewhat by the addition of fibres, but an increase in ductility was noted, although not quantified. The addition of CSF to fibre-containing blocks led to an increase in capillary water absorption, but a decrease in absorption through immersion. This shows that the addition of CSF does significantly alter the pore system of a cementitious matrix reinforced with natural fibres. Also, the dimensional stability increased with the addition of CSF and fibres. The same can be said for drying shrinkage. Even though an increase in fibre and CSF caused samples to shrink more under drying, they were more stable under cycles of wetting and drying. It was concluded that the addition of fibres to a matrix had a detrimental effect on strength, although ductility did increase. The volumetric properties of concrete were also adversely affected by the addition of fibres, although dimensional stability was improved. The partial substitution of cement with CSF did improve many of the mechanical and volumetric properties of samples containing sisal fibre.
Mechanical properties of fly ash/slag based geopolymer concrete with the addition of macro fibres
(Stellenbosch : Stellenbosch University, 2014-12) Ryno, Barnard; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Geopolymer concrete is an alternative construction material that has comparable mechanical properties to that of ordinary Portland cement concrete, consisting of an aluminosilicate and an alkali solution. Fly ash based geopolymer concrete hardens through a process called geopolymerisation. This hardening process requires heat activation of temperatures above ambient. Thus, fly ash based geopolymer concrete will be an inadequate construction material for in-situ casting, as heat curing will be uneconomical. The study investigated fly ash/slag based geopolymer concrete. When slag is added to the matrix, curing at ambient temperatures is possible due to calcium silicate hydrates that form in conjunction with the geopolymeric gel. The main goal of the study is to obtain a better understanding of the mechanical properties of geopolymer concrete, cured at ambient temperatures. A significant number of mix variations were carried out to investigate the influence that the various parameters, present in the matrix, have on the compressive strength of fly ash/slag based geopolymer concrete. Promising results were found, as strengths as high as 72 MPa were obtained. The sodium hydroxide solution, the slag content and the amount of additional water in the matrix had the biggest influence on the compressive strength of the fly ash/slag based geopolymer concrete. The modulus of the elasticity of fly ash/slag based geopolymer concrete did not yield promising results as the majority of the specimens, regardless of the compressive strength, yielded a stiffness of less than 20 GPa. This is problematic from a structural point of view as this will result in large deflections of elements. The sodium hydroxide solution had the most significant influence on the elastic modulus of the geopolymer concrete. Steel and polypropylene fibres were added to a high- and low strength geopolymer concrete matrix to investigate the ductility improvement. The limit of proportionality mainly depended on the compressive strength of the geopolymer concrete, while the amount of fibres increased the energy absorption of the concrete. A similar strength OPC concrete mix was compared to the low strength geopolymer concrete and it was found that the OPC concrete specimen yielded slightly better flexural behaviour. Fibre pull-out tests were also conducted to investigate the fibre-matrix interface. From the knowledge gained during this study, it can be concluded that the use of fly ash/slag based geopolymer concrete, as an alternative binder material, is still some time away as there are many complications that need to be dealt with, especially the low modulus of elasticity. However, fly ash/slag based geopolymer concrete does have potential if these complications can be addressed.
Model for predicting creep of cracked steel fibre reinforced concrete
(2018-03) Pike, Leo Mallett; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: The investigation of the creep of cracked fibre reinforced concrete has gained momentum over the past few years. However, there is still no proposed model of how to include the additional creep caused by the pull-out behaviour of fibres in the structural design of Steel Fibre Reinforced Concrete (SFRC) structures. The purpose of this study was to create a preliminary design model that included this additional fibre pull-out behaviour. The Age Adjusted Effective Modulus (AAEM) method, used in the Fédération Internationale du Béton (fib) Model Code (2010), was used as a base model for predicting long-term deflections. To reach this goal, time-dependent experimental investigations were performed at two levels, namely the macroscopic level and structural level. At the macroscopic level, uniaxial tensile creep tests were performed on cracked fibre reinforced specimens. At the structural level, flexural creep tests were performed on cracked reinforced concrete and SFRC beams, as well as cracked reinforced beams with a combination of fibres and steel bar reinforcing. This was performed to determine the experimental long-term deflections of each type of beam. A sustained stress level of 40% of the cracking tensile and flexural strengths was used. The results of the uniaxial tensile creep tests were used to calculate the rate of fibre pull-out for the fibre reinforced specimens. These rates were used to calibrate an additional fibre pull-out factor that could be included in the AAEM method. A damage model for each beam type was developed to include the effect of pre-cracking. The long-term deflection results of the conventionally reinforced beams were used to verify the current fib Model Code’s AAEM method. It was found that the adapted AAEM method could accurately predicted the long-term deflections of both cracked SFRC and combined reinforced beams.
Moment redistribution behaviour of SFRC members with varying fibre content
(Stellenbosch : Stellenbosch University, 2012-03) Mohr, Arno Wilhelm; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Steel fibre reinforced concrete (SFRC) is the most prominent fibre reinforced concrete composite that was engineered to enhance the material’s post-cracking behaviour. In certain situations it is utilised to replace conventional reinforcement and considered to be more cost-efficient. The purpose of this research is to characterise the moment redistribution behaviour of a statically indeterminate SFRC structure with varying volumes of fibres, with the focus on the development of the moment redistribution accompanied by the rotation of the plastic hinges at the critical sections in the structure. The material properties were characterised with a series of experimental tests. The compression behaviour was obtained with uniaxial compression tests while the uniaxial tensile behaviour was obtained with an inverse analysis performed according to flexural test results. These properties were utilised to derive a theoretical moment-curvature relation for each SFRC member which supplied the basis for the characterised moment-rotation behaviour and the finite element analyses (FEA) performed on the statically indeterminate structure. Experimental tests were conducted on the statically indeterminate structure in laboratory conditions to validate the theoretical findings. For the different SFRCs the material properties in compression were similar, while it resulted in an increased tensile resistance with an increase in the volume steel fibres. The theoretical momentcurvature and moment-rotation responses also indicated an increased structural capacity and member ductility with an increase in the volume fibres. From the finite element analyses the computational moment redistribution-plastic rotation relations were obtained. It was found that the final amount of moment redistribution decreased with an increase in the fibre volume, but that the rotational capacity increased. It was found that the experimental moment-curvature and moment-rotation results correlate well with the theoretical predictions. Also, unexpected structural behaviour was observed, but the issue was addressed with applicable computational analyses which confirmed the possible causes. It was concluded that the computational moment redistribution approximations were reasonably accurate. A parameter study indicated that the crack band width differed among the different SFRC members.
The performance of locally produced supplementary cementitious materials when incorporated in concrete
(Stellenbosch : Stellenbosch University, 2016-03) Alexandre, Vital Jorge; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: The production of cement has a strenuous impact on the environment. Nonetheless, it is required by the construction industry as a key parameter for socio-economic development. Supplementary cementitious materials (SCMs) are a group of materials that can be used to partially replace cement as a binder in concrete mixtures, two of which includes slag and fly ash (FA). These materials are obtained as waste products from iron smelting and coal combustion processes, respectively. These materials have the capability to generate hydration products similar to that of cement when used. The current study’s main objective was to investigate the performance concrete containing slag and fly ash, produced in South Africa, as a means to reduce the dependency of cement as the only binder used in concrete. The goal of the study was to establish to what degree each of the materials can replace the cement in a typical concrete mix and the impacts thereof. Experiments were conducted on concrete samples made from a series of mixes with a constant binder content and water-to-binder ratio. Cement was replaced on a mass basis, with limits defined by the typical construction replacements: slag at 25, 50 and 75% and FA at 15, 25 and 35 %. The performance is based on: reactivity of the SCM, fresh state, mechanical properties and durability. Setting time was found to be sensitive to the SCM quantity and reactivity, however it was accelerated by more reactive materials and low replacement level. In addition, a wider SCM-particle span increased the bleeding capacity and reduced the bleeding rate. FA was found to increase the plastic settlement of concrete more when compared to slag based concrete, with the maximum plastic settlement occurring at 25 % FA content. In addition, the unrestrained plastic shrinkage of all mixes was significantly greater than that of the reference, yet decreased with increased SCM content. The compressive strength of SCM based concrete was lower than the reference at early ages and improved with curing age. At 91 days the control and a few SCM-based concretes had similar compressive strength of approximately 62 MPa. Moreover, the indirect tensile strength per unit compressive strength of the SCMs based concretes were higher than that of the control, signifying the improvement of the interfacial transition zone. The addition of SCMs also significantly improved the concrete microstructure. Additionally, the durability performance of the SCMs based concrete was better than or equivalent to that of the reference. The chloride resistance of slag-based concrete was four times lower than fly ash concrete or the control mix. The knowledge from the current study shows that SCMs can be used, to a great extent, to replace cement in the construction industry. The early age properties may require attention, yet, in the end, the final product of SCM-based concrete is found to be superior to that of the control. Hence, the use of slag and fly ash, as a binder replacement, does provide a solution to reduce the environmental impact of cement production.
Plastic shrinkage cracking in conventional and low volume fibre reinforced concrete
(Stellenbosch : Stellenbosch University, 2012-12) Combrinck, Riaan; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Plastic shrinkage cracking (PSC) is the cracking caused by the early age shrinkage of concrete within the first few hours after the concrete has been cast. It results in unsightly surface cracks that serve as pathways whereby corroding agents can penetrate the concrete which shortens the expected service life of a structure. PSC is primarily a problem at large exposed concrete surfaces for example bridge decks and slabs placed in environmental conditions with high evaporation rates. Most precautionary measures for PSC are externally applied and aimed to reduce the water loss through evaporation. The addition of a low dosage of polymeric fibres to conventional concrete is an internal preventative measure which has been shown to reduce PSC. The mechanisms involved with PSC in conventional and low volume fibre reinforced concrete (LV-FRC) are however not clearly understood. This lack of knowledge and guidance leads to neglect and ineffective use of preventative measures. The objective of this study is to provide the fundamental understanding of the phenomena of PSC. To achieve the objective, an in depth background study and experiments were conducted on fresh conventional concrete and LV-FRC. The three essential mechanisms required for PSC are: 1→ Capillary pressure build-up between the particles of the concrete is the source of shrinkage. 2→ Air entry into a concrete initiates cracking. 3→ Restraint of the concrete is required for crack forming. The experiments showed the following significant findings for conventional and LV-FRC: PSC is only possible once all the bleeding water at the surface has evaporated and once air entry has occurred. The critical period where the majority of the PSC occurs is between the initial and final set of concrete. Any preventative measure for PSC is most effective during this period. The bleeding characteristics of a mix have a significant influence on PSC. Adding a low volume of polymeric fibres to concrete reduces PSC due to the added resistance that fibres give to crack widening, which increases significantly from the start of the critical period. The fundamental knowledge gained from this study can be utilized to develop a practical model for the design and prevention of PSC in conventional concrete and LV-FRC.
Plastic shrinkage cracking in conventional and low volume fibre reinforced concrete
(Stellenbosch : University of Stellenbosch, 2011-03) Combrinck, Riaan; Boshoff, William Peter; University of Stellenbosch. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Plastic shrinkage cracking (PSC) is the cracking caused by the early age shrinkage of concrete within the first few hours after the concrete has been cast. It results in unsightly surface cracks that serve as pathways whereby corroding agents can penetrate the concrete which shortens the expected service life of a structure. PSC is primarily a problem at large exposed concrete surfaces for example bridge decks and slabs placed in environmental conditions with high evaporation rates. Most precautionary measures for PSC are externally applied and aimed to reduce the water loss through evaporation. The addition of a low dosage of polymeric fibres to conventional concrete is an internal preventative measure which has been shown to reduce PSC. The mechanisms involved with PSC in conventional and low volume fibre reinforced concrete (LV-FRC) are however not clearly understood. This lack of knowledge and guidance leads to neglect and ineffective use of preventative measures. The objective of this study is to provide the fundamental understanding of the phenomena of PSC. To achieve the objective, an in depth background study and experiments were conducted on fresh conventional concrete and LV-FRC. The three essential mechanisms required for PSC are: 1→ Capillary pressure build-up between the particles of the concrete is the source of shrinkage. 2→ Air entry into a concrete initiates cracking. 3→ Restraint of the concrete is required for crack forming. The experiments showed the following significant findings for conventional and LV-FRC: PSC is only possible once all the bleeding water at the surface has evaporated and once air entry has occurred. The critical period where the majority of the PSC occurs is between the initial and final set of concrete. Any preventative measure for PSC is most effective during this period. The bleeding characteristics of a mix have a significant influence on PSC. Adding a low volume of polymeric fibres to concrete reduces PSC due to the added resistance that fibres give to crack widening, which increases significantly from the start of the critical period. The fundamental knowledge gained from this study can be utilized to develop a practical model for the design and prevention of PSC in conventional concrete and LV-FRC.
Quantifying the cracking behaviour of strain hardening cement-based composites
(Stellenbosch : Stellenbosch University, 2012-03) Nieuwoudt, Pieter Daniel; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
ENGLISH ABSTRACT: Strain Hardening Cement Based Composite (SHCC) is a type of High Performance Fibre Reinforced Cement-based Composite (HPFRCC). SHCC contains randomly distributed short fibres which improve the ductility of the material and can resist the full tensile load at strains up to 5 %. When SHCC is subjected to tensile loading, fine multiple cracking occurs that portrays a pseudo strain hardening effect as a result. The multiple cracking is what sets SHCC aside from conventional Reinforced Concrete (RC). Conventional RC forms one large crack that results in durability problems. The multiple cracks of SHCC typically have an average crack width of less than 80 μm (Adendorff, 2009), resulting in an improved durability compared to conventional RC. The aim of this research project is to quantify the cracking behaviour of SHCC which can be used to quantify the durability of SHCC. The cracking behaviour is described using a statistical distribution model, which represents the crack widths distribution and a mathematical expression that describes the crack pattern. The cracking behaviour was determined by measuring the cracks during quasi-static uni-axial tensile tests. The cracking data was collected with the aid of a non-contact surface strain measuring system, namely the ARAMIS system. An investigation was performed on the crack measuring setup (ARAMIS) to define a crack definition that was used during the determination of the cracking behaviour of SHCC. Several different statistical distributions were considered to describe the distribution of the crack widths of SHCC. A mathematical expression named the Crack Proximity Index (CPI) which represents the distances of the cracks to each other was used to describe the crack pattern of SHCC. The Gamma distribution was found to best represent the crack widths of SHCC. It was observed that different crack patterns can be found at the same tensile strain and that the CPI would differ even though the same crack width distribution was found. A statistical distribution model was therefore found to describe the CPI distribution of SHCC at different tensile strains and it was established that the Log-normal distribution best describes the CPI distribution of SHCC. After the cracking behaviour of SHCC was determined for quasi-static tensile loading, an investigation was performed to compare it to the cracking behaviour under flexural loading. A difference in the crack widths, number of cracks and crack pattern was found between bending and tension. Therefore it was concluded that the cracking behaviour for SHCC is different under flexural loading than in tension.
Quantifying the environmental dimension of sustainability for the built environment : with a focus on low-cost housing in South Africa
(Stellenbosch : Stellenbosch University, 2012-03) Brewis, Chandre; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil engineering.
ENGLISH ABSTRACT: Sustainability is difficult to achieve in a world where population and economic growth leads to increased production of greenhouse gases, resource depletion and waste generation. Today, the environmental dimension of sustainability, which is more commonly known as the natural environment, and the construction industry are two terms often mentioned together. In Europe, 12.4 % of greenhouse gas emissions are induced by the construction and manufacturing industry (Maydl, 2004). Also, 50 % of the resources extracted are used in the construction industry and more than 25 % of waste generated is construction and demolition waste. In South Africa, the building sector accounts for approximately 23 % of the total greenhouse gas emissions (Milford, 2009). Furthermore, 60 % of investment is made in the residential sector where 33 % of the building stock is the focus of the government’s Housing Programme. It is seen that the construction industry significantly impacts the natural environment and the aim should be to reduce this negative impact. Within the local residential sector, the low-cost housing sector presents potential when it comes to sustainable improvements. Each of the three spheres of sustainability, namely economy, natural environment and society, plays a crucial role in this sector. Various studies have been done on the economical and social fields, but little information exists on the impact low-cost houses have on the environment. A need arises to scientifically quantify the environmental impact hereof, therefore it is chosen as the focus of this study. Various methods in order to determine the environmental impact of the built environment exist globally, but they tend to be complex, are used in conjunction with difficult to understand databases and require expensive software. A need for a local quantification method with which to determine the environmental impact of the built environment, more specifically low-cost housing, has been identified. A simple and easy-to-use analysis-orientated quantification method is proposed in this study. The quantification method is compiled with indicators related to the local conditions; these include Emissions, Resource Depletion and Waste Generation. The end objective is to provide the user with an aggregated total value called the Environmental Impact Index to ease comparison of possible alternatives. The quantification method is developed as a mathematical tool in the form of a partial Life Cycle Assessment which can aid in objective decision making during the conception and design phase of a specific project. Note that only the Pre-Use Phase of the building life cycle is considered during the assessment, but can be extended to include the Use Phase and End-of-Life Phase. The proposed method has the capability of calculating and optimising the environmental impact of a building. Regarding low-cost housing, different housing unit designs can be compared in order to select the best alternative. The quantification method is implemented for two low-cost house design types in this study. Firstly, the conventional brick and mortar design is considered whereafter a Light Steel Frame Building is viewed as an alternative. The model implementation demonstrates that the model operates in its supposed manner. Also, Light Steel Frame Building housing units are shown to be worth investigating as an alternative to the conventional brick and mortar design but should be confirmed with a more accurate Life Cycle Assessment.

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