Efforts are being made globally to reduce CO2 emissions, particularly within the construction industry, which is a major contributor. Cement production alone is responsible for around 6% of global anthropogenic CO2 emissions. To address this, alternative materials to Ordinary Portland Cement (OPC) are being explored, such as alkali-activated concrete (AAC), which uses industrial by-products like granulated blast furnace slag (BFS) and fly ash (FA). These by-products are activated using alkaline solutions, leading to AAC, which exhibits comparable or superior mechanical properties, alongside enhanced chemical, acid, and fire resistance. However, despite its potential, AAC has not yet seen widespread adoption due to limited understanding of its long-term behavior, particularly in larger structural components.

This study focuses on alkali-activated slag-based concrete (AAS), which has shown changes in material properties when exposed to drying, with reductions in elasticity modulus, flexural strength, and tensile splitting strength over time. The impact of these changes on the bond behavior between reinforcement and concrete is unclear, yet bond strength is crucial for structural integrity. The main aim is to evaluate how curing conditions, curing age, and reinforcement type influence the bond strength of AAS, with an emphasis on the effects of drying on material properties.

Pull-out tests were conducted to assess the bond strength, where the reinforcement is pulled from the concrete cube while measuring the applied tensile force and rebar displacement. Specimens were cured under different conditions: some under standard moisture conditions, while others were exposed to drying for up to 84 days. The tests examined two types of reinforcement: steel and prestressing strands, and compared the results to conventional concrete (CC) as a reference. A preliminary study addressed optimal test conditions, with a focus on pull-out failure, as it provides a more ductile failure mode compared to the brittle splitting failure, offering more reliable data on bond capacity.

The optimal specimen configuration was determined to be one with an unbonded concrete layer above the bonded region, which helped achieve pull-out failure. Additionally, the level of confinement during testing was optimized to prevent splitting failure, which occurs when concrete cracks due to excessive radial tensile stresses.

The main experiment revealed that bond strength in CC remained stable even under drying conditions, whereas AAS specimens showed a decline in bond strength over time, likely due to microcracking induced by shrinkage. However, the results were inconsistent, suggesting that drying may not have a definitive effect on bond strength. Prestressed strands showed weaker bond strength compared to steel reinforcement, which is attributed to differences in bond transfer mechanisms and concrete tensile strength. The smoother surface of prestressed strands relies more on friction, which is less effective in AAS due to its reduced tensile strength.

Further analysis using Distributed Fiber Optic Sensing (DFOS) showed that internal strain measurements aligned with theoretical values in some cases, though inconsistencies arose near the boundaries of the bonded region. This highlights the need for further refinement of the testing setup.

Finally, the pull-out test results were compared to semi-empirical models and code standards. These models were generally conservative, especially for AAS specimens exposed to drying, which showed reduced bond strength below the minimum requirements set by standards like AS 01 and ACI 318. While the results for moisture-cured AAS aligned with the Harajli bond behavior model, the reduced bond strength in drying-exposed AAS could lead to unsafe designs if not accounted for in structural design standards.