Browsing by Author "Nieuwoudt, Pieter Daniel"

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    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.
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    Time-dependent Behaviour of Cracked Steel Fibre Reinforced Concrete: from Single Fibre Level to Macroscopic Level
    (Stellenbosch : Stellenbosch University, 2016-03) Nieuwoudt, Pieter Daniel; Boshoff, William Peter; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The addition of steel fibres to a concrete matrix is known to improve the material’s post crack mechanical behaviour under short term loading conditions. However, limited information is available on the material’s post-crack behaviour under long-term loading, particularly under sustained uni-axial tensile loading. The purpose of this research study is to investigate and quantify the time-dependent Crack Mouth Opening Displacement (CMOD) behaviour of cracked Steel Fibre Reinforced Concrete (SFRC) under sustained uni-axial tensile loading, and to develop a mathematical model which is able to simulate the time-dependent crack width opening behaviour of cracked SFRC under sustained uni-axial tensile loading. To reach this goal, experimental investigations were performed at two levels, namely macroscopic level and single fibre level. At the macroscopic level, the short term mechanical properties of SFRC were investigated by performing compressive and uni-axial tensile strength tests. To investigate the long term mechanical properties of cracked SFRC, sustained uni-axial tensile load tests were performed at stress levels ranging from 30 % to 85 % of the residual tensile strength. To understand the mechanisms causing the time-dependent CMOD as well as the factors that can influence the behaviour, single fibre pull-out rate tests and single fibre sustained load tests were performed on hooked-end steel fibres. The SFRC showed significant toughness and energy absorption capacity after cracking, both under compression and uni-axial tensile loading. The sustained uni-axial tensile load results showed that the time-dependent CMOD increases with the applied sustained stress level. Over the measured time period of 240 days none of the tested specimens fractured even for stress levels as high as 85 % of the residual tensile strength. Significant variability was found in the results at each load level and it was concluded that the variation in the crack plane fibre count for each specimen is one possible reason for the variability at each load level. The single fibre pull-out rate results showed significant rate sensitivity for the slip at maximum pull-out force. This rate effect is induced by the interface between the hooked-end of the fibre and the surrounding matrix. The fibre embedment inclination angle and the geometry of the mechanical hooked-end of the fibre have been found to significantly affect the pull-out behaviour. The single fibre sustained load results showed that the pull-out due to a sustained load is dependent on the applied load level. The pull-out due to a sustained load is referred to as pull-out creep and is defined as the relative movement between the fibre and the matrix interface under sustained loading. It is found that the pull-out creep is induced by the localised compression of the surrounding matrix by the hooked-end of the fibre. The mechanisms responsible for the pull-out creep are therefore believed to be similar as the creep for bulk normal concrete under compression. A constitutive model was developed based on the theory of rheology to simulate the single fibre pull-out creep behaviour of an aligned hooked-end steel fibre. The model was then generalised to simulate the CMOD of cracked SFRC under sustained uni-axial tensile loading by assuming a uniform fibre distribution over the crack plane. It was found that the orientation of the fibres at the crack plane was a relevant factor that affects the time-dependent CMOD. The generalised model was able to simulate the CMOD under various applied sustained stress levels (30 % to 85 %) with relative high accuracy.