Creep Properties of Alkali-Activated Concrete

Concrete is an ageing viscoelastic material exhibiting both elastic (instantaneous) as well as viscous (time-dependent) deformation under loading conditions (either external or internal). There is a limited number of studies focused on the time-dependent response of alkali-activated concretes (AACs) under loading/unloading conditions. Creep of AAC is a complex phenomenon, which is influenced by exposure conditions of the material, including the loading magnitude, temperature, relative humidity, thermal and drying histories; as well as chemical composition and phase assemblages (e.g., type and amount of reaction products) present in the cementitious matrix. AAC has shown very vibrable creep behaviors, due to different raw materials and processes using during their production. Creep studies on room temperature cured slag-based AAC usually show high creep; however, creep studies of different AACs, including fly ash-based and fly ash-slag-blended, indicate that elevated temperature curing could be a suitable mitigation strategy for reducing creep. This is associated with the development of a more mature microstructure in the material, due to an accelerated reaction kinetics and a consequent increase in strength and lower creep. However, applying a curing temperature above 80 °C causes thermal defects and cracks which increases the creep. For most aluminosilicate-based AACs that produced with fly ash, metakaolin and their blends with a small amount of ground granulated blast furnace slag, the recommended curing method is to use thermal curing at about 60 °C. In addition, curing time and initial loading time are also important. It must be noted that because of the complexity of raw materials properties and mix proportions, there is no universal method for all types of AACs. The existing creep prediction models for Portland cement-based concretes cannot be transferred and adopted in AACs directly due to the distinct nature of hydration products. Therefore, more studies investigated the creep at both small size and full-scale of AACs are urgently needed.