Browsing by Author "Combrinck, Riaan"

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    Cracking of Plastic Concrete in Slab-Like Elements
    (Stellenbosch : Stellenbosch University, 2016-03) Combrinck, Riaan; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept of Civil Engineering.
    ENGLISH ABSTRACT: The cracking of plastic concrete involves two cracking types namely: plastic settlement cracking which is caused by differential settlement of the concrete and plastic shrinkage cracking which is caused by evaporation of free concrete pore water. These cracks are mainly a problem for slab-like elements exposed to conditions with high evaporation rates and typically occur within the first few hours after the concrete has been cast. The early occurrence of these cracks greatly reduces the durability and service life of a concrete structure. These cracks remain a problem in the construction industry even though there are several effective, but mostly neglected, precautionary measures. The reasons these cracks remain a problem are due to the complex nature of the cracking as well as the lack of a unified theory or model that can account for all the complexities involved. With this in mind, this study aims to fundamentally understand both plastic settlement and plastic shrinkage cracking in slab-like elements individually and combined as well as to determine the tensile material properties of plastic concrete. Once the cracking is fundamentally understood the final objective is to develop a model that can simulate the cracking of plastic concrete using a finite element method approach. The fundamental understanding of these cracks was obtained by conducting various tests on different mixes at various climates and in various moulds. The tests showed that both crack types can occur separately, where plastic settlement cracking occurs first in the form of multiple cracks at the surface as well as shear induced cracks beneath the surface, followed by plastic shrinkage cracking in the form of a singular, well defined crack. In addition, a significant deviation from the individual cracking behaviour was observed when combining these cracks, highlighting the shortfall of most available literature where these cracks are seldom researched in tandem. From all the tests, six different cracking behaviours were identified depending on the potential severity for each cracking type. The test also showed worryingly that both these cracks can be present internally without being visible at the concrete surface where they act as weak spots for future crack growth. The practically challenging tensile testing of plastic concrete was conducted with a newly built direct tensile test setup, which provided stress-strain curves that were used to determine the tensile material properties of plastic concrete such as: Young’s modulus, tensile strength, strain capacity and fracture energy. This included tests at different temperatures as well as cyclic tests. The results showed that the tensile material properties develop significantly faster, the greater the ambient temperature surrounding the concrete as well as the resilient nature of a still plastic concrete which proved to be capable of withstanding cyclic loading without failure, while a solid but still weak concrete could not. The tensile material properties together with the measured strains of plastic concrete were combined to provide both an analytical and numerical estimation of the cracking behaviour of plastic concrete. The analytical estimation was more simplistic and required a few crude assumptions, while the numerical estimation used finite element methods to create a model that accounted for the major complexities involved such as time-dependency of material properties and anisotropic volume change of plastic concrete. Both the analytical and finite element model gives adequate representation of the cracking behaviour for extreme climates but not for normal climates, with the size discrepancy between the interior and surface cracks during experiments as well as the relaxation of stresses in plastic concrete being provided as the main reasons for the poor correlation. The finite element model was further used to conduct a parameter study, where the settlement and shrinkage strains were shown to govern the size of the final crack, while the material properties only influence the time of crack onset and rate of crack widening. Finally, the finite element model was successfully applied to a large scale example of a concrete slab, indicating that the model can be a helpful tool to simulate the cracking of plastic concrete without the need to perform timely experiments.
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    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.
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    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.