Applicability Analysis of Assessment Methods for Morphological Parameters of Corroded Steel Bars

Corrosion of a steel bar is one of the principal reasons for deterioration of a concrete structure and is caused by concrete carbonation and/or chloride intrusion. In concrete carbonation, corrosion tends to be generalized; while in chloride intrusion, it becomes more localized^1,2. No matter what the causes are, corrosion cracks the concrete cover from radial expansion of corrosion products, deteriorates the bond between a steel bar and its surrounding concrete, penetrates the bar surfaces, and decreases the bar cross-sectional area considerably^3,4.

Due to the non-homogeneity of structural concrete and variations in the service environment, corrosion of a steel bar occurs randomly over its surface and along its length with great uncertainty. Contrary to the generalized uniform corrosion caused by concrete carbonation, the pitting corrosion caused by chloride intrusion causes attack penetration. Furthermore, it causes the residual section of a corroded bar to vary considerably among the bar surface and length. As a result, the bar strength and bar ductility decrease. Extensive research has been performed to study the effects of corrosion on mechanical properties of a steel bar^{5,6,7,8,9,10,11,12,13,14,15}. However, less attention has been given to the measurement methods of morphological parameters and corrosion characteristics of steel bars.

Some researchers have used mass loss to evaluate the amount of corrosion of a steel bar^5,10,11,14. However, this method can only be used to determine the average value of the residual sections and cannot measure the distribution of the sections along its length. Zhu and Franco have improved this method by cutting a single steel bar into a series of short segments and weighing each segment to determine variations of the areas of the residual sections along its length^13,14. However, this method causes extra loss of the steel material during the cutting and cannot touch the minimum residual section of the corroded bar exactly, which dominates its bearing capacity. A Vernier caliper is also used to measure the geometric parameters of a steel bar^14,15. However, the residual section of a corroded bar is very irregular, and there is always a significant deviation between the measured and actual sectional dimensions of a corroded bar. Based on Archimedes' principle, Clark et al. adopted the drainage method to measure the residual sections of a corroded bar along its length, but displacement of the bar was manually controlled without significant accuracy in this case¹¹. Li et al. improved this drainage method by using an electric motor to automatically control the displacement of a steel bar and measure results more accurately¹⁶. Finally, over the past few years, with the development of 3D scanning technology, this method has been used to measure the geometric dimensions of a steel bar^17,18,19,20. Using 3D scanning, the diameter, residual area, centroid, eccentricity, moment of inertia, and corrosion penetration of a steel bar can be precisely acquired. Although researchers have used these methods in different experimental settings, there has not been a comparison and evaluation of the methods with respect to their precision, suitability and applicability.

Corrosion, particularly pitting corrosion, compared to generalized corrosion, not only changes the mechanical properties of corroded bars but also decreases the residual bearing capacity and service life of concrete structures. More accurate measurements of morphological parameters of corroded steel bars for the spatial variability of corrosion along bar length are imperative for more reasonable assessments of bar mechanical properties. This will help evaluate the safety and reliability of reinforced concrete (RC) structures damaged by corrosion more precisely^{21,22,23,24,25,26,27,28,29}.

This protocol compares the five discussed methods for measuring the geometry and amount of corrosion of a steel bar. A single, 500 mm long and 14 mm in diameter, plain round bar was used as the specimen and subjected to accelerated corrosion in the lab. Its morphology and level of corrosion were carefully measured before and after using each method, including mass loss, a Vernier caliper, drainage measurements, 3D scanning, and X-ray micro computed tomography (XCT). Finally, the applicability and suitability of each were evaluated.

It should be noted that the ribbed bars embedded in concrete, not the plain bars exposed to air, are commonly used in concrete structures and subjected to corrosion. For ribbed bars, the Vernier caliper may not be as easily applied. Because these bars corrode in concrete, their surface penetration is more irregular compared to bars exposed to air¹¹. However, this protocol is geared towards the applicability of analysis of different measurement methods on the same bar; therefore, it uses a naked plain bar as the specimen to eliminate the influence of ribs and concrete non-homogeneity on morphological parameter measurements. Further work on the measurement of corroded ribbed bars using other methods may be carried out in the future.