Thermal properties of concrete with different Swedish aggregate materials

University essay from Lunds universitet/Avdelningen för Byggnadsmaterial; Lunds universitet/Väg- och vattenbyggnad (CI)

Abstract: Earlier studies show that different concrete aggregates have different thermal properties, and from this an idea to optimize thermal properties of concrete was developed. The purpose of this master thesis is to investigate thermal properties of concrete so additional costs and negative environmental impacts can be minimized. Measurements of thermal properties of different Swedish aggregates from existing quarries were investigated. It will also study if the thermal properties affect ther thermal expansion of concrete. The experimental study in this work was done in laboratory conditions. Preparation studies that were made before casting concrete were sieving aggregates, density of aggregates and moisture content. While the concrete was fresh standard tests such as slump, air-content and density of fresh concrete were made. When the concrete was 28 days old compressive cube strength and thermal expansion measurement were executed. The thermal properties (thermal conductivity, volumetric heat capacity and thermal diffusivity) were measured on aggregates and concrete cubes with the instrument Hot Disk 2500-S. The results show that the minerals quartz and magnetite have significantly different thermal properties than the other studied aggregates/concretes. The aggregate/concrete with high amount quartz mineral has high thermal conductivity and diffusivity. The aggregates/concrete with magnetite has high thermal conductivity and volumetric heat capacity. The other aggregates and concretes have similar thermal properties. The thermal expansion coefficients have similar values (12.6-15.5∙ 1 /⁰C) for different aggregates, and it was not possible to see if there was a correlation between them and the thermal properties. Thus was the value of thermal expansion coefficient in a range that is normal for concrete. The thermal shock resistance is dependent of the thermal conductivity, thermal expansion coefficient, fracture strength and elastic modulus. The concretes with high thermal conductivity also had high thermal shock resistance whereas so such concrete should better resist rapid temperature changes. The conclusions that can be drawn from this work are that the thermal properties of the aggregates will be reflected in the properties of the concrete. For both thermal conductivity and heat capacity the values for the dry concretes (RH 10-30%) were about 40-70% of the values of the aggregates. Note that this conclusion only applies for thermal conductivity and heat capacity and does not apply on thermal diffusivity, since diffusivity is the ratio of conductivity and capacity it will be similar in the concrete. In this study only two aggregates significantly affected the thermal properties: quartz has high thermal conductivity and diffusivity and magnetite rock has high thermal conductivity and heat capacity. Another conclusion from this study was high thermal conductivity also give high thermal shock resistance, where the concrete resist rapid temperature changes better.

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