Early-age and long-term deformations in reinforced slag-based alkali activated concrete

Abstract

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.