Autogenous shrinkage induced stress of alkali activated slag and fly ash concrete under restraint condition

Autogenous shrinkage is an important engineering property for construction materials since it can induce internal tensile stress and consequent micro- or macro- cracking of the concrete. Alkali activated slag and fly ash (AASF), as a promising alternative to ordinary Portland cement (OPC), has shown many interesting properties such as high early age strength, good durability and fire resistance, but it also shows high autogenous shrinkage in the meantime, which hinders a wider application of this eco-friendly binder material. However, high autogenous shrinkage doesn’t necessarily mean high stress of the material under restraint condition, since there are creep and associated stress relaxation happening simultaneously. Therefore, the creep of AASF needs to be considered in order to better understand the stress induced by autogenous shrinkage. In this study, the autogenous shrinkage induced stress of AASF concrete (with a liquid/binder ratio of 0.5) under restraint condition is measured by Thermal Stress Testing Machine (TSTM). The free autogenous shrinkage of a twin specimen is measured by Autogenous Deformation Testing Machine (ADTM). The elastic modulus of AASF concrete is tested on prisms. The creep coefficient of AASF concrete is calculated based on the stress, free autogenous shrinkage and elastic modulus. It is found that the creep coefficient of AASF concrete is much higher than that of OPC concrete, and creep plays an important role in relaxing the shrinkage induced stress and thus reducing the cracking potential of AASF concrete. Please click Additional Files below to see the full abstract

Mechanical properties and creep behaviour of an alkali-activated concrete

Alkali activated materials (AAMs) have received great interest for production of eco-friendly concrete as an alternative for OPC-based concrete. AAMs usually have comparable or better mechanical properties and durability compared to that of OPCs, and at the same time help in reduction of CO2 emissions and utilization of industrial wastes. In general, AAMs have a high resistance to very high and low temperatures and show a rapid strength gain which makes them an excellent option for rapid construction. As a potential structural material, AAMs face some obstacles for application in construction including the lack of knowledge on long-term performance and stability as well as the lack of standardized procedures for production and design alkali activated concrete. The problem of control and increase of the material performance is the main challenge for AAMs researchers. Despite the recent interest and intensive research on the performance of AAMs, the time-dependent deformation and creep of these materials have not been fully investigated yet. The creep phenomenon depends on many environmental (like temperature and humidity) and structural (the mix design and curing conditions) parameters and therefore it is expected have differences in AAMs compared to OPC. In this work, the authors present a preliminary experimental investigation of this phenomenon in concrete made of AAMs. The presented experimental data, including mechanical properties and creep tests results, are a step towards understanding of the creep mechanism in these materials. The test results shows the increase of compressive strength, splitting strength, and Young’s modulus during the first 30 days of curing. From 30th to 90th days, the mechanical properties have insignificant changes. The creep tests are performed for five months. The creep curves are similar to OPC and show a creep coefficient of about 5.5 for total creep and 4 for basic creep