چكيده به لاتين
Concrete structures are subject to the effect of thermal changes during their lifetime. In response to temperature rise, concrete tends to elongate, and in case of its constraint, it causes development of cracks throughout the concrete. Reduction of the coefficient of thermal expansion (CTE) of concrete helps in reducing the cracks caused by thermal changes. In this research, by making concrete and cement paste containing various mineral admixtures such as silica fume, metakaolen and slag, the effect of mineral admixtures with changes in the structure of concrete and cement paste pores on CTE at different ages has been examined. The results indicated that CTE of reference concrete had a descending trend up to 60 days, and diminished by 12%, after which it remained constant. On the other hand, in the slag-containing concrete, and opposite trend was observed, where after 60 days, the descending trend started and declined by 10% up to 120 days. For the concrete containing silica fume and metakaolin, at the very early ages, it had 14% less CTE as compared to reference concrete, while with the ageing of the sample, its CTE dropped by around 8%. Across all Samples, reduction of CTE was associated with lowered total porosity, but there is no strong correlation between the total porosity and the coefficient of thermal expansion concrete and cement paste. Also, the relationship between the thermal expansion coefficient of concrete and paste with compressive strength, as well as its relation to total porosity, there was no strong correlation between compressive strength and thermal expansion coefficient. Also, with investigation of the distribution of pore size, it was observed that when pore diameter decreases, CTE declines, such that a strong relationship has been observed between the median diameter of pores and CTE. Considering the dependence of concrete CTE on aggregates and cement paste, a model is presented based on CTE of cement paste and packing density to estimate concrete CTE. Also, in order to predict coefficient of thermal expansion over the lifetime of concrete, a model based on the coefficient of thermal expansion of concrete at 7 days of age is presented to estimate it at the desired age.
