High-resolution optical pH imaging of concrete exposed to chemically corrosive environments

Major types of chemical concrete degradation such as carbonation, leaching and acid attacks are strongly associated with decreasing internal, surface and/or external pH. Thus, a precise pH determination is crucial for the assessment regarding the degree of corrosion and corresponding development of prevention strategies. Conventional pH measurement methods for concrete, such as pH visualization via color change of phenolphthalein indicator, pH analyses of extracted pore solutions and surface pH characterizations using flat surface electrodes have proven to hold significant limitations and inadequacies. This contribution presents the application of sensor foils based on luminescent, pH sensitive dyes for quantification and imaging of the spatial distribution of surface pH of concrete within a pH range between 9.35 and 12.35. For this purpose, an imaging technique called time-domain dual lifetime referencing (t-DLR) was used. High-resolution pH images of concrete samples exposed to carbonation and biogenic acid corrosion were successfully recorded and consequently compared to the state-of-the-art methods for pH assessment on concrete-based construction materials.