Assessment of test methods for characterizing the hydrophobic nature of surface-treated High Performance Concrete

•Silane/siloxane based surface water repellent treatment for High Performance Concrete (HPC).•Comparison between untreated, treated and naturally weathered specimens.•Direct and indirect methods to evaluate hydrophobic performance.•In case of HPC, indirect methods reach their limits.•Direct methods IR spectroscopy and Energy Dispersive X-ray Analysis show promising performance.

The durability of concrete and concrete structures is significantly influenced by environmental conditions, resulting in e.g. the ingress of water and incorporated aggressive agents, with the latter being responsible for initiating and progress of deterioration processes within the material system. In order to reduce the ingress of water, water repellent agents are applied onto the concrete surface (surface treatment), with its performance strongly depending on the agent itself, the mode of application, environmental conditions etc. For the assessment of the performance of surface-treated concrete, standard test methods, such as Karsten-Tube penetration test, contact angle measurement, hydrophobic quality test, as well as more refined test methods, such as Infrared Spectroscopy (IR), moisture sorption analysis, and scanning electron microscopy coupled with elemental X-ray analysis (SEM/EDS) are used. These methods are commonly employed for the assessment of surface-treated ordinary concrete, leaving the question on their applicability to material systems exhibiting a denser microstructure, such as High Performance Concrete (HPC), unanswered. Within this experimental study, the aforementioned test methods are critically reviewed regarding their potential application to HPC, finally providing recommendations for evaluating the performance and durability of water-repellent surface treatment when applied to HPC.