Image analysis for determination of cement content in concrete to improve accuracy of chloride analyses

The chloride content Ccl expressed as %Cl by weight of cement is important in condition assessment of reinforced concrete structures. Whereas standardized procedures determine Ccl in concrete powder, the cement content Cm is generally assumed equal to the mix design or an experience-based constant value. This work shows in concrete with maximal aggregate diameter 32 mm, Cm exhibits significant variability in 50 mm diameter cores, because the specimens are too small to be representative of bulk concrete. In such specimens, Cm might differ from the bulk cement content by a factor of up to 2. Thus, a reliable determination of Ccl in terms of %Cl by weight of cement requires the analysis of both Ccl and Cm in a concrete specimen. A procedure based on colouring the cement paste, scanning the specimen surface, and image analysis allows the practically non-destructive determination of Cm with good accuracy.