Effect of Heating Rate on the Thermal Properties of High Strength Mortar

• Author / Creator

  Bingyu Xie

• Concrete is the most common building material in use in terms of per capita consumption. As demand increases continuously, the change in thermo-physical properties of concrete under sustained elevated temperatures has drawn attention. Although concrete is not combustible and possesses better fire resistance compared to other building materials, it is still limited when exposed to elevated temperatures. Explosive spalling for instance, is the most severe and critical mode of failure. The occurrence of explosive spalling in high strength concrete is especially of concern, due to the superior water tightness. The internal microstructure is further affected by the rate of heating, which in turn has a bearing on the thermal constants in higher strength mixtures. Therefore, the present study undertakes to examine how the thermal conductivity of cement-based materials is affected by different heating rates.

  Two mortar mixtures, one designed for normal strength and the other, designed for high strength, were prepared for evaluatingthe role of heating rate on their thermal constants. Samples were extracted for test under three different heating rates ranging from 1°C/min to 10°C/min to a soaking temperature ranging from 200°C to 400°C. The thermal constants were measured using a hot disk thermal analyser system after approximately 30 min soaking at each target temperature. In order to understand the behavior of cementitious mortar under different heating rates, it is necessary to investigate the corresponding change in microstructural parameters and their impact on thermal properties. This study finds that the microstructure of high strength mortar is more sensitive to the heating rate compared to that of the normal strength mortar. Large cracks are observed in images at heating rates of 10°C/min for both mixtures, but images of high strength mortar show higher crack intensity. The porosity of both normal and high strength mortar increases with an increase in the temperature of exposure. However, the porosity drops for an increase in the heating rate. Additional image processing on back scattered electron images was made to evaluate the box count based fractal dimension. For both mixtures, the fractal dimension of the pores increases with an increase in the temperature. This study finds that the fractal dimension increases with heating rate. However, this trend was more pronounced at lower temperatures of exposure, so that the fractal dimension becomes less conspicuous at higher temperature of soaking. The specific heat of mortar mixtures is mostly dependent on the presence of water at lower temperatures. However, the large cracks caused by thermal degradation may cause the specific heat of high strength mortar to decrease with an increase in the heating rate when exposed to temperatures beyond 300°C.

• Subjects / Keywords

  • thermal conductivity
  • porosity
  • pore size distribution
  • mortar
  • fractal dimension
  • heating rate
  • specific heat

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-rbyq-cz88

• License

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