Nondestructive assessment of reinforced concrete structures based on fractal damage characteristic factors

Nondestructive damage assessment of civil engineering structures has become a focus of increasing interest for recent decades. Its core is to extract effective damage characteristic information capable of reflecting structural damage status. In this study, fractal theory is adopted to extract the fractal damage characteristic factors of a reinforced concrete structure by characterizing its surface-crack distributions. The concentrated and even load spaces are generalized as applicable spaces for employing fractal-to-structural damage assessment. As demonstrated in the damage assessment of reinforced concrete beams under four-point bending and aged crossbeams of an operation bridge in a sluice, the surface-crack distributions of reinforced concrete structures exhibit monofractal character in the concentrated load space, and multifractal character in the even load space. The physical damage interpretations of the extracted monotractal and multifractal damage characteristic factors in the respective load spaces are then established by analyzing the correlations between the monotractal dimension and the natural frequency, and between the multifractal singular spectrum and the average carbonized depth and residual material intensity, respectively. The closely linear fitting relationships between the fractal quantities and traditional damage characteristic factors indicate that the fractal (i.e., monofractal and multifractal) quantities can serve as viable and novel damage characteristic factors in the online damage assessment of concrete structures. It is significant that the proposed fractal damage characteristic factors overcome some disadvantages of traditional damage characteristic factors in practical applications, and they extend the technique of fractal into the meaningful damage assessment of reinforced concrete materials.