To enable wide structural application of alkali activated concretes (AACs) as a sustainable alternative to Portland cement concretes (PCCs), it is vital to understand their interaction with steel reinforcement. Even before applying mechanical loading, significant internal loads (strains), originating from cement hydration, changes in temperature and (internal) relative humidity, can be imposed and will affect reinforcement-concrete interaction. Herein, early-age and long-term induced steel strains have been measured on embedded reinforcement bars instrumented with distributed fiber optical sensors (DFOS) in reinforced concrete beams. A GGBFS-based alkali activated concrete (S-AAC) was compared to a reinforced CEM III/B-based concrete (denoted as S-PCC). Both concretes have been fog-cured for 28 days and successively exposed to 20 °C and 55 % relative humidity for a duration of 1 year. By combining bonded and unbonded DFOS strain measurements, mechanical steel strains induced by autogenous and drying shrinkage could be decoupled from temperature effects. DFOS allowed to detect local strain peaks and microcracks and determine microcrack widths along the length of reinforcement bars. Autogenous shrinkage developed rapidly in S-AAC due to the fast polymerization reaction of GGBFS and negligible early-age expansion, leading to 3 times larger shrinkage at 28 days compared to S-PCC. Yet, unlike S-PCC, S-AAC showed a slower development of shrinkage induced deformations than plain S-AAC, indicating that a significant portion of shrinkage in S-AAC is creep. In addition, autogenous shrinkage induced deformations led to microcracking in S-AAC, contributing to a lowered development of steel deformation. Under drying, S-AAC showed a rapid increase in microcrack width without further significant increases in the mean steel deformation. Microscopic images confirmed the presence of more pronounced microcracking in S-AAC compared to S-PCC. These findings are relevant to gain understanding of the long-term behavior of reinforced AACs.

Although alkali activated concretes (AACs) are promising for reducing the carbon emissions of concrete, in order to enable their wide application it is vital to understand their long-term behaviour. Herein, we report the development of mechanical properties of a ground granulated blast furnace slag (GGBFS)-based AAC and a binary fly ash (FA) /GGBFS-based AAC exposed to 55% relative humidity and 20 °C up to the age of 5 years. For comparison, two ordinary Portland cement (OPC) concretes were monitored for 3.5 years. For the GGBFS-based AAC, after an initial decrease within the first 6 months the elastic compressive modulus stabilized, while its tensile splitting strength continued to decrease for the tested period of 5 years. The binary AAC showed a continuous decrease in its tensile splitting strength for 5 years and a reduction in its compressive strength after 2 years. No decreases in mechanical properties were observed in OPC-based concretes. To reveal underlying mechanisms, additional analyses were performed. Permanent degradation was observed in both AACs; the binary AAC mainly suffered from carbonation, and the GGBFS-based AAC showed microcracking. These cracks were probably caused by drying shrinkage and drying-induced chemical changes. Based on the measured mechanical properties of AAC, crack widths and stiffness of reinforced AAC beams under bending were analytically evaluated and compared to experiments. Decreases in bending stiffness and increases in crack width were observed for reinforced AAC beams tested at later ages. A bimodular approach is proposed to predict the reduction of bending stiffness in the studied AACs over time. These findings are relevant to understand serviceability limit states of reinforced AACs.

A promising solution for reducing the carbon footprint of concrete is the use of alkali-activated concretes (AAC). Before this material can be widely applied, its long-term behaviour needs to be understood, especially since some studies reported a decrease of mechanical properties over time. Similarly, Prinsse et al. reported decreasing mechanical properties, especially elastic modulus and flexural and splitting tensile strength for the studied slag-based AAC (S100) and the blended slag- and fly-ash-based AAC (S50) up to the tested age of 2 years. They hypothesized that these decreases could be only temporarily. To test that hypothesis, this study continued to monitor the mechanical properties of both AACs up to the age of 5 years. As a reference, two OPC-based concretes (OPCC), with different strength classes, are monitored up to the age of 3.5 years. In addition, the internal structures of the concretes are assessed for carbonation and internal micro cracking. S100 shows stabilization of the elastic modulus and the compressive strength, whereas the tensile splitting strength continued to decrease up to 5 years. This is attributed to a combination of carbonation and drying, since the microscopic analysis showed increased porosity around the ITZ and in the carbonated region. In addition, S50 shows an ongoing decrease of all tested mechanical properties, which is attributed to carbonation. No decreases in mechanical properties are found for OPCC.