Thermal effects on concrete properties

Abstract

The expansion of nuclear energy as a future power source in South Africa, and the use of concrete for the containment vessels, has substantially increased the need for research in the concrete field. The production and development of environmentally friendly construction materials raise concerns about structural fire safety and thermal behaviour, as nuclear radiation shields face high levels of external heat and there is limited research on the performance of these materials at high temperatures. Therefore, the effect of elevated temperatures on the properties of concrete containing recycled aggregates, admixtures as well as blended cements is of importance in the design of concrete structures, such as concrete radiation shields. Operating temperatures of nuclear power plants range between 285°C – 650°C, depending on the reactor type. Other application such as outer shells of industrial chimneys or stacks and structures in metallurgy and chemical industry workshops, can also benefit from research on the thermal behaviour of concrete at high temperatures. This study highlights the notable influence of aggregate type on the performance of concrete subjected to elevated temperatures. It is well known that heating concrete to elevated temperatures causes shrinkage of the hardened cement paste as well as thermal expansion of the aggregates. This can cause microcracking within the concrete, leading to degradation of the Interface Transition Zone (ITZ) between the aggregate and hardened cement paste, resulting in a reduction in the concrete strength and stiffness. The study proofed that concrete exposed to elevated temperatures in service should preferably contain aggregate with a low coefficient of thermal expansion. Concrete exposed to 350°C retained more than 64% of its original strength, while concrete exposed to 500°C can retain more than 70% of its original strength after recovery as a result of rehydration when exposed to water. The order of preference of natural aggregate type for concrete exposed to elevated temperatures (up to 500°C) is felspathic (andesite, dolerite), granitic (granite, felsite) and calcareous (dolomite). Furthermore, high paste volumes (> 400 l/m3) show noticeably more deterioration in strength after exposure to elevated temperatures. It is therefore recommended that the use of concrete mixtures with excessive paste volumes or cement contents should be avoided. The use of SCMs, such as fly ash, showed higher strength deterioration compared to pure Portland cement concrete. This was attributed to the disruptive effects of the cement paste shrinkage opposed by aggregate expansion for concrete with a compact microstructure. RAC can compete with concrete made with aggregates from conventional quarries, not only under normal temperature conditions but also after exposure to high temperatures. Structural concrete can easily be manufactured where 100% coarse aggregate and 30% fine aggregate is replaced with RCA. It was hypothesised that aggregate that contain elements and minerals that decompose at relatively low temperatures, would place less stress on the surrounding cement paste, thus reducing the damage caused to the ITZ by the thermal expansion of the aggregate. The study established that the mass loss of aggregate obtained from thermogravimetric analysis (TGA) might give an indication of the performance of concrete exposed to elevated temperatures, especially considering dry compressive strength as well as mass loss of the concrete. The results indicate that it would be possible to limit the extent of thermal damage to concrete by selecting aggregates with limited (at least 1%) but not excessive (less than 2%) mass loss at the exposure temperature. The study demonstrated that degradation of concrete due to temperature exposure is not only caused by the thermal expansion of the aggregates but also by the mass loss of aggregates. To limit the damage caused to concrete by exposure to elevated temperatures, there seems to be a balance required between the thermal expansion of the aggregate and the reduction in stress caused by the aggregate degradation as indicated by mass loss of the aggregate at the specific exposure temperature.