Investigation on rheology of cement paste and concrete at high temperature

The objective of this research is to explore the important mechanisms that control the rheology of cement paste and concrete incorporating various chemical admixtures and subjected to high temperature and prolonged mixing time. The thesis also aimed at formulating recommendations for the effective use of chemical admixtures in high temperature environments. In order to achieve these objectives, the rheological properties of portland cement pastes incorporating various chemical admixtures were first investigated at different temperatures using both the conventional flow technique and the oscillatory shear technique. The results indicated that current technical data for chemical admixtures need to be validated for hot weather conditions; admixtures proven effective in mild weather may become ineffective at high temperature. For example, some water reducing and retarding admixtures acted as accelerators at high temperature, yet they have been recommended by manufactures for concrete in hot weather conditions.;The coupled effects of mixing time and ambient temperature on the rheology of portland cement paste incorporating superplasticizers were also investigated. Results suggested that polycarboxylate-based superplasticizers should be used at a dosage close to the saturation in order to ensure adequate rheological behaviour at high temperature and prolonged mixing time, while melamine sulfornate- and naphthalene sulfonate-based superplasticizers should be used at dosages beyond the saturation level.;The insights gained through these studies were then applied to study the rheology of fresh concrete incorporating various superplasticizers and subjected to prolonged mixing under high temperature. The test results indicated that the Bingham constants of concrete are significantly affected by changes in temperature, mixing time and admixture dosage. The relationship between the yield stress and plastic viscosity of concrete was investigated to develop a rational means for the objective assessment of the rheology of concrete mixtures in hot weather.;The coupled effects of the ambient temperature and mixing time on the compressive strength of concrete incorporating superplasticizers were also investigated. The results indicated that the compressive strength of concrete mixtures is temperature, mixing time and superplasticizer type dependent.;Furthermore, new equations were developed using genetic algorithms (GA) to predict the Bingham parameters (yield stress and plastic viscosity) of cement paste, the yield stress of concrete, and the oscillatory yield stress of cement paste considering various parameters including the ambient temperature, mixing time and superplasticizer type and dosage. The results indicated that the computed rheological parameters for cement paste and concrete compared well with corresponding experimental data. The GA equations thus developed captured well the effects of the various test parameters.;Keywords. rheology, cement paste, concrete, high temperature, mixing time, superplasticizer, yield stress, viscosity, thixotropy, compressive strength, slump loss, genetic algorithm, oscillatory rheology, modeling.