Browsing by Author "Khan, Mohammed Yaseen"

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    The tensile material properties of plastic concrete and the influence on plastic cracking
    (Stellenbosch : Stellenbosch University, 2018-03) Khan, Mohammed Yaseen; Combrinck, Riaan; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: During the plastic state of concrete, two forms of volume change exist, namely: plastic settlement which refers to the gravitational settlement of solid particles, which in turn displaces bleed water to the surface of the concrete, as well as plastic shrinkage which occurs directly after plastic settlement and can be explained as the rapid removal of water from the capillary pores of concrete, due to ongoing evaporation. However, a change in volume is only detrimental to the concrete body if restrained, for example by reinforcing steel embedded within the concrete body. Any hindrance or resistance of the free volume change in plastic concrete induces tensile stresses and or strains in the concrete element. Crack formation is expected to occur if the tensile stress and strain induced within the concrete is greater than the tensile strength or strain capacity of the concrete. Another often overlooked factor that also influences the plastic cracking potential of concrete is relaxation. However, performing relaxation tests on plastic concrete presents great difficulty. Due to this, literature on the tensile and relaxation behaviour of plastic concrete is scarce and therefore a significant knowledge gap exists on the influence of tensile properties on the cracking behaviour of plastic concrete. In light of this, the main objectives of this study are to investigate the tensile behaviour and relaxation properties of plastic concrete as well as the rheological influence on these properties. Once the tensile properties are fundamentally understood, the influence of a viscosity modifying agent on the cracking behaviour of plastic concrete is addressed. Lastly the influence of initial curing on the cracking behaviour of plastic concrete is also investigated. Investigation into the tensile properties and relaxation behaviour of plastic concrete was carried out using a direct tensile testing machine, on specimens at hourly intervals up to after the final setting time of the concrete. The tests showed that the tensile strength of plastic concrete increases exponentially from the initial setting time of the concrete, while a significant reduction in strain capacity was observed between initial and final setting times. Furthermore, results indicate that the relaxation behaviour of concrete is dependent on the rate of hydration, with maximum relaxation potential occurring during the stiffening phase of concrete and reducing significantly as the concrete enters the setting and hardening phases. Multiple loading results showed the resilient nature of a still plastic concrete which is capable of withstanding multiple loading cycles. Capillary pressure measurements during tensile tests revealed that the mechanism behind relaxation is the negative capillary pressure build-up induced by the mechanical tensile strain. The addition of the viscosity modifying agent (VMA) significantly reduced the tensile strength of the concrete during the setting and hardened phases. Furthermore, relaxation tests indicate that the addition of VMA increased both the relaxation potential, as well as the ability to complete multiple loading cycles, compared to the reference mix. The cracking behaviour after the addition of the VMA, displayed an increase in crack area. The lower tensile strength, the increase in slump and the slightly retarded setting time is believed to be the cause of this observation. Initial curing results, indicate that curing applied before air entry, relieves a greater amount of stress build-up and therefore a larger reduction in crack area, compared to curing applied before the build-up in capillary pressure. The addition of a VMA resulted in a larger stress reduction, compared to the reference mix. Furthermore, results indicate that applying curing procedures only once is not sufficient in preventing plastic cracking. However, if this is the only option, curing applied just before point of air entry has greater benefits in terms of crack area reduction compared to curing applied before the build-up in capillary pressure.