Applicability Analysis of Assessment Methods for Morphological Parameters of Corroded Steel Bars
Summary
This paper measures the geometry and the amount of corrosion of a steel bar using different methods: mass loss, calipers, drainage measurements, 3D scanning, and X-ray micro-computed tomography (XCT).
Abstract
The irregular and uneven residual sections along the length of a corroded steel bar substantially change its mechanical properties and significantly dominate the safety and performance of an existing concrete structure. As a result, it is important to measure the geometry and amount of corrosion of a steel bar in a structure properly to assess the residual bearing capacity and service life of the structure. This paper introduces and compares five different methods for measuring the geometry and amount of corrosion of a steel bar. A single 500 mm long and 14 mm diameter steel bar is the specimen that is subjected to accelerated corrosion in this protocol. Its morphology and the amount of corrosion were carefully measured before and after using mass loss measurements, a Vernier caliper, drainage measurements, 3D scanning, and X-ray micro-computed tomography (XCT). The applicability and suitability of these different methods were then evaluated. The results show that the Vernier caliper is the best choice for measuring the morphology of a non-corroded bar, while 3D scanning is the most suitable for quantifying the morphology of a corroded bar.
Introduction
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 localized1,2. 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 considerably3,4.
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 bar5,6,7,8,9,10,11,12,13,14,15. 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 bar5,10,11,14. 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 length13,14. 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 bar14,15. 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 bar17,18,19,20. 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 precisely21,22,23,24,25,26,27,28,29.
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.
Protocol
Representative Results
Discussion
Figure 6A and 6B show that the measured diameters of the non-corroded bar specimen do not vary significantly along its length. The maximum difference between the measured diameters along the bar length is only about 0.11 mm with a maximum deviation of 0.7%. This indicates that the geometry of a non-corroded bar can be well evaluated using a Vernier caliper. However, the measured diameters at different angles of the same cross-section differ consistently and considerably from…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The authors at Shenzhen University greatly acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51520105012 and 51278303) and the (Key) Project of Department of Education of Guangdong Province. (No.2014KZDXM051). They also thank the Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil Engineering at Shenzhen University for providing testing facilities and equipment.
Materials
| Supplies | |||
| Plastic ruler | Deli Group Co.,Ltd. | No.6240 | |
| white paint pen | SINO PATH Enterprises.,Ltd. | SP-110 | |
| Tube with Branch | Customized-made | ||
| Measurement cylinder | Beijing Huake Bomex Glass Co., Ltd. | ||
| 500mL Beaker | Beijing Huake Bomex Glass Co. , Ltd. | CP-201 | |
| sandpaper | Shanghai Noon Decoration Material Co., Ltd. | P04 | |
| white developer | SHANGHAI XINMEIDA FLAW DETECTION MATERIAL CO., LTD. | FA-5 | |
| Reagents | |||
| epoxy resin adhesive | Hunan Baxiongdi New Material Co., Ltd. | DY·E·44 | |
| epoxy hardener | Hunan Baxiongdi New Material Co., Ltd. | DY·EP | |
| HCl | Dongguan Dongjiang Chemical Reagent Co., Ltd. | AR-2500ml | |
| saturated lime water | Xilong Chemical Co., Ltd. | AR-500g | |
| Equipment | |||
| Digital electronic scale | Kaifeng Group Co., Ltd. | Model JCS-0040 | |
| Digital vernier caliper | Shanghai Measuring & Cutting Tool Works Co., Ltd. | Model ST-089-229-090 | |
| Cutting machine | Robert Bosch GmbH | TCO2000 | |
| 3D reconstructed X-ray microscope | XRADIA | Model MICROXCT-400 | |
| 3D scanner | HOLON Three-dimensional Technology(Shenzhen) Co.,Ltd. | Model HL-3DX+ | |
| Electromechanical Universal Testing Machine | MTS SYSTEMS (China) Co., Ltd. | Model C64.305 |
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