Concrete Rebar Research Articles

Effects of Different Inhibitors on the Corrosion Mitigation of Steel Rebar Immersed in NaCl-Contaminated Concrete Pore Solution

The corrosion of steel rebar embedded in concrete under marine conditions is a major global concern. Therefore, it needs a proper corrosion mitigation method. Various types of corrosion inhibitors are used to mitigate the corrosion of steel rebar in chloride-contaminated concrete; however, selecting the appropriate inhibitor and determining its optimal concentration remains a concern. Therefore, in the present study, three types of inhibitors—calcium nitrite (CN: Ca(NO2)2), N,N′-dimethyl ethanol amine (DMEA: (CH3)2NCH2CH2OH), and L-arginine (LA: C6H14N4O2) in three different concentrations, i.e., 0.3, 0.6 and 1.2 M—were compared with a control (without inhibitor, i.e., blank) sample to determine the optimum concentration of the inhibitor for corrosion resistance performance evaluation of reinforcement bars immersed in 0.3 M NaCl-contaminated concrete pore (NCCP) solution for various durations. The corrosion resistance properties were assessed using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) with immersion duration, and potentiodynamic polarization (PDP) after 168 h of exposure. The results showed that the CN inhibitor performed exceptionally well (corrosion inhibition efficiency greater than 97%) in terms of corrosion resistance. However, due to its hazardous nature and its ban in the U.S. and European Union, CN cannot be used in construction. In comparison, while DMEA showed some effectiveness, LA performed better and is also eco-friendly. The corrosion resistance efficiency of samples containing 0.6 M LA remains above 97% even after 168 h of immersion in the NCCP solution. This efficiency is consistent throughout the entire immersion period, from 1 h to 168 h. Therefore, it is recommended that LA be used as a corrosion inhibitor for steel reinforcement bars instead of CN, particularly in chloride-contaminated concrete, as it is both effective and safer than CN.

Image-Based Analysis Method for Detecting a Rebar in Concrete Using Short-Time Fourier Transform (STFT)

Image-Based Analysis Method for Detecting a Rebar in Concrete Using Short-Time Fourier Transform (STFT)

Bond behavior of confined rebar in recycled brick aggregate concrete using bending pullout test

The pertinence of recycled brick aggregate (RBA) in concrete structures largely depends on their bonding ability with the rebars. Considering the practical detailing of concrete structures where main rebars are usually confined by stirrups, this study conducted bending pullout tests on 12 beam specimens to study the influence of concrete confinement on the bond behavior of rebar in RBA concrete. The investigated variables include RBA%, rebar diameter (d), embedment length (le), and the ratio of concrete cover to rebar diameter (c/d). Bond stress-slip relationships were acquired, analyzed, and average bond strengths were compared to the AS 3600, CEB-FIP codes, and existing MacGregor's model. The average bond strength of confined specimens was nearly double that of the unconfined specimens. A progressive increase in bond strength was noticed with the presence of RBA regardless of confinement. The confined specimens also exhibited higher deformability, ductility, and toughness values than those of the unconfined beams. At 15% RBA content, the maximum toughness value of the confined specimen was 3078.1 J, while the unconfined specimens reached a value of 278.1 J, only. Similarly, the ductility of the confined beam was found in a range of 1.21–1.60 whereas the value is only 1.04–1.10 for the unconfined cases. Moreover, the confined specimens exhibited a significant number of cracks during their failure, while the unconfined specimens experienced a split type brittle failure. Nevertheless, these findings may contribute to the further development of the bond behavior of rebar in RBA-based concrete structures and thus play a role in the sustainability of construction materials.

Bond between single and bundled high-modulus ribbed GFRP bars with short embedment lengths and concrete

The bond between glass fiber-reinforced polymer (GFRP) rebar and concrete is one of the most important parameters in the design of GFRP-reinforced structures. This study investigates the bond behavior of single and bundled high-modulus ribbed GFRP rebars with short embedment lengths in concrete. To achieve this, a total of 48 pullout specimens were constructed and tested. The effect of various parameters, including GFRP rebar embedded length, the concrete block dimensions, GFRP surface characteristics, presence of transverse reinforcement, and bundling of rebars on the bond behavior of high-modulus ribbed GFRP rebar was studied. The experimental results indicated that the failure mode changed from pullout to splitting failure as the embedment length increased. The presence of transverse reinforcement, in many cases, changed brittle splitting failure to partial splitting failure followed by pullout failure. The variation in rebar surface characteristics resulted in differences in the bond-slip behavior of GFRP rebar in concrete. The experimental findings illustrated that rebar stress at failure for specimens with bundled GFRP was generally higher than that of corresponding individual bar with approximately same cross-sectional area. This observation suggests that the equivalent area method may serve as a conservative approach for evaluating the embedment length of bundled ribbed GFRP rebars. Furthermore, the adhesion between ribbed GFRP rebars and concrete was quantified for various bar sizes. Finally, the experimental results were employed to refine and calibrate existing bond-slip models of ribbed GFRP rebar to concrete.

Corrosion Initiation on Reinforcing Steel with Mill Scale in Mortar

Corrosion of reinforcing steel rebar is the main cause of loss of the long-term reliability of concrete structures. When the rebar is corroded, the rust induces cracks and detachment of the cover concrete. The mill scale, a thick oxide film, is formed on the rebars when the rebar is hot rolled. The role of mill scale on the corrosion resistance of rebars in concrete has not been clarified, yet. This study studied the corrosion behavior of rebars with mill scale in mortar.The samples are commercial rebars with 19 mm diameter, which have a mill scale of several tens of micrometers in thickness. The rebars without mill scale prepared by the electrolytic polishing were used as the comparison samples. A Hyperbaric-oxygen accelerated corrosion test (HOACT) [1] was carried out using the rebars embedded in the mortar with a 5.5 mm cover thickness. To prompt the breakdown of passive film on the rebars, 2.1 M NaCl solution was mixed with the mortar. In the HOACT, pressurized oxygen gas is supplied to the sample, and the oxygen reduction reaction on the steel surface is enhanced, leading to the steel's corrosion acceleration. The oxygen pressure was 0.6 MPa and the test period was 14 days. The corrosion of the rebar with a mill scale was more significant than that without a mill scale. This result shows that the mill scale reduces the corrosion resistance of the steel rebar in concrete [2].To investigate the effect of the mill scale on the corrosion resistance of the rebar, anodic polarization measurements of the rebars with and without the mill scale were carried out in a saturated Ca(OH)2 solution containing Cl-. The pitting potential of the rebar with mill scale was more noble than that of the sample without mill scale. However, when the mill scale was scratched, the pitting potential was significantly shifted to the less noble direction, and it was below that of the rebar without the mill scale. These results show that the mill scale improves the corrosion resistance of the rebar in concrete, in contrast, the corrosion resistance of the rebar with a defective mill scale becomes even worse than that of the rebar without the mill scale [2].[1] K. Doi, S. Hiromoto, H. Katayama, E. Akiyama, J. Electrochem. Soc., 165(9), C582-C589 (2018).[2] K. Doi, S. Hiromoto, T. Shinohara, K. Tsuchiya, H. Katayama, E. Akiyama, Corrosion Science, 177, 108995 (2020).

Research Progress in Corrosion Behavior and Anti-Corrosion Methods of Steel Rebar in Concrete

The corrosion of steel rebars is a prevalent factor leading to the diminished durability of reinforced concrete structures, posing a significant challenge to the safety of structural engineering. To tackle this issue, extensive research has been conducted, yielding a variety of theoretical insights and remedial measures. This review paper offers an exhaustive analysis of the passivation processes and corrosion mechanisms affecting steel rebars in reinforced concrete. It identifies key factors such as chloride ion penetration and concrete carbonization that primarily influence rebar corrosion. Furthermore, this paper discusses a suite of strategies designed to enhance the longevity of reinforced concrete structures. These include improving the concrete protective layer’s quality and bolstering the rebars’ corrosion resistance. As corrosion testing is essential for evaluating steel rebars’ resistance, this paper also details natural and accelerated corrosion testing methods applicable to rebars in concrete environments. Additionally, this paper deeply presents an exploration of the use of X-ray computed tomography (X-CT) technology for analyzing the corrosion byproducts and the interface characteristics of steel bars. Recognizing the close relationship between steel bar corrosion research and microstructural properties, this paper highlights the pivotal role of X-CT in advancing this field of study. In conclusion, this paper synthesizes the current state of knowledge and provides a prospective outlook on future research directions on the corrosion of steel rebars within reinforced concrete structures.

Detecting Reinforced Concrete Rebars Using Ground Penetrating Radars

A new algorithm is developed to automatically detect rebar locations and diameters of reinforced concrete structures using the ground penetrating radar technique. The study uses two-way travel time and biquadratic equations to formulate electromagnetic wave speed in reinforced concrete structures where hyperbolic signatures are approximated. Leveraging an established algorithm, a computer code has been developed to offer automated analysis of ground-penetrating radar data obtained from survey grids. Four reinforced concrete slabs were designed, fabricated, and tested to validate the developed evaluation approach. The proposed methodology demonstrates outstanding signal processing proficiency and reliably and effectively identifies rebar information.

Bond behavior of deformed rebar in concrete containing recycled brick aggregate and waste rubber

The reuse of aggregates and crumb rubber (CR) in concrete is gaining interest globally for reducing the adverse effects on the environment and maintaining construction sustainability. The bond strength of reinforcing rebars in such concrete is one of the key issues to be addressed before their practical application, particularly in structural elements. Pull-out tests were performed on 80 specimens to assess the bond performance of deformed rebars in rubberized recycled brick aggregate concrete (RRBAC). After investigating the strength properties of RRBAC, pull-out tests were performed on two sizes of cylindrical specimens. RBA%, CR%, rebar diameter (d), embedment length (le), and concrete cover-to-rebar diameter ratio (c/d) were among the test variables in this bond behavior investigation. Bond stress-slip relationships were obtained, examined, and average bond strengths were compared with the available codes. Concrete split failure was observed in 79% of the tested specimens, whereas rebar fracture was not seen in any of the tests. The bond strength was found to be decreased with the inclusion of RBAs and CRs though relative improvement was observed for higher percentage RBAs. Additionally, regression analysis was performed on the experimental data to produce a closed-form equation for forecasting the bond strength for RRBAC. With a strong coefficient of determination (R2 = 0.97) and low values of error terms, the predicted bond strengths were seen to be a good agreement with the test results.

Experimental study on bond performance of staggered lap rebars in ultra-high performance concrete (UHPC)

Ultra-high performance concrete (UHPC) has received extensive attention worldwide due to its ultra-high mechanical properties and durability. The bond performance between steel bars and UHPC is crucial for the design of Reinforced-UHPC structures. To study the bond performance of overlapping steel bars in UHPC, a total of 13 groups of 39 pull-out tests were conducted with overlapping lengths, cover thickness, steel bar diameter, and clear spacing of steel bars as influencing variables in this study. According to the results, when the lap length increased from 4 times the bar diameter (4db) to 5db and 6db, the maximum bond strength decreased by 10% and 20%, respectively. Moreover, with an increase in the cover thickness from 25 mm to 35 mm and 45 mm, the maximum bond strength increased by 6% and 14%, while the residual bond strength increased by 15% and 48%, respectively. When the diameter of the reinforcing bars increased from 16 mm to 20 mm and 25 mm, the ultimate load capacity increased by 50% and 70%, respectively, while the bond stress decreased by 16% and 27%. Additionally, the bond strength showed less sensitivity to the clear spacing of steel bars, with a decrease of 3% and 10% in bond strength when the clear spacing increased from 1.5db to 2db and 3db, respectively. The ductility of the specimen decreases when the bar diameter or the clear distance between bars is too large. A theoretical calculation formula for splitting bond strength was established, which yielded accurate results. Furthermore, a bond-slip model was fitted based on the test results, providing reference for finite element analysis of steel bar-UHPC structures.

Assessing the effect of sodium nitrite as corrosion inhibitor against the corrosion of steel rebar in alkali-activated concrete

The aim of this study is to investigate the effectiveness of corrosion inhibitor against chloride-induced corrosion of steel rebar in fly-ash (FA) and GGBS (ground granulated blast furnace slag) and Nanofine-based alkali-activated concrete. The Alkali-Activated Concrete (AAC) was prepared from FA, ground granulated blast furnace slag (GGBS), and Nanofine with Sodium hydroxide (NaOH) and Sodium Silicate (Na2SiO3) solutions. The concentration of NaOH varies to 10 M, 12 M, and 14 M, and all the specimens were subjected to accelerated corrosion, to perform the non-destructive test after the corrosion of the steel rebars. Fourier Transform Infrared (FTIR), scanning electron microscope (SEM), Energy dispersive X-ray spectroscopy (EDS), and Electron Backscatter Diffraction (EBSD) were used to examine the microstructure of AAC in the presence of Nano-Fine GGBS (NF), Sodium Chloride (NaCl), and NaNO2 and to compare the strength and corrosion properties with AAC and OPC. The ratio of sodium silicate to sodium hydroxide was taken as 2.5 and the solution-to-binder ratio was taken as 0.5. The compressive strength and ultrasonic pulse velocity (UPV) test of the Nano-fine based alkali-activated concrete increases as compared to the control concrete. The increase in the dosages of corrosion inhibitor and nano-fine increases the compressive strength of the control concrete and alkali-activated concrete. The half-cell potential of the control concrete and nano fine admixed with alkali-activated concrete mixes with different molarities is uncertain. The increases in the dosages of corrosion inhibitors reduce the occurrence of corrosion in the steel reinforcement rebars.

Techno-Eco-Efficiency Assessment of Using Recycled Steel Fibre in Concrete

The steel industry is one the three biggest producers of carbon dioxide and it is experiencing technical challenges due to the gradual decrease in the quality of iron ore. Steel is extensively used in the construction industry for structural applications like steel components, while steel fibres are intensively used as additives to concrete in order to improve its performance. It is thus important to consider the use of recycled steel as a replacement for virgin steel in order to address the aforementioned environmental consequences. This paper applies the eco-efficiency framework to determine the economic and environmental implications of the use of recycled fibre in concrete as a replacement for virgin steel. A number of concrete mixes were considered that used virgin, recycled, and treated recycled rebar in concrete. The eco-efficiency framework, which uses a life-cycle assessment approach to calculate the environmental and economic values of concrete mixes in order to determine the portfolio positions of these concrete mixes, was used for comparison purposes and to establish the eco-efficient option(s). Whilst the recovery and recycling process is energy-intensive, the use of recycled steel fibre in reinforced concrete has been found to be eco-efficient and deliver the same level of mechanical performance compared to that obtained using virgin steel fibre. Treating steel fibre could improve its technical performance, but it was found to increase both costs and environmental impacts and was therefore identified as not being eco-efficient.

BASIC METHODS OF MONITORING OF CORROSION PROCESSES OF CEMENT STRUCTURES AND THEIR IMPLEMENTATION FOR PROJECTING OF POSSIBLE MINERALOGICAL CHANGES IN MATRIX COMPOSITION: REVIEW

Long time service of concrete constructions is accompanied by continuous impact of environment on its structural integrity and capability to carry project capacity of constructions. This influences are able to change not only chemical composition of cement matrix but itself activates corrosion processes that have place to occur on the joints of metal rebar and cement matrix in reinforced concrete (RC). Chemical initiation in the reinforced concrete begins due to the migration of water with parts of molecular oxygen in the thickness of concrete via porous structure of material. The same can be said about the injected compounds of either SO4 2- or Clcomponents that both can be present either in environment or concrete itself due to the modifications of slurry. Such occurrences usually accompanied by changes of pH of the environment that is crucial for the electrolytic environment to appear. Therefore, the process of the reduction of water/ proton may begin at rebar surface wich is the cause not only of the reinforced concrete rebar corrosion but also are reason of accelerated cement matrix degradation. Also, must be noted, that steel corrosion in RC accompanied by changes in current flow. Thus, by understanding the chemical processes and changes that occur in cement matrix of concrete and basics of steel corrosion initiation under the influence of aggressive solutions, the list of non-destructive methods of monitoring (NDM) based on electrical methods of monitoring are reviewed. Different approaches using electrical methods of corrosion monitoring has shown that the use of any of the technics either measurements of the corrosion potential or concrete resistivity or polarization resistance provides different results in mapping of areas with high corrosion risks, monitoring of the heterogeneity of concrete and insights on transport phenomena (e.g. water and salts ingress) in the material. Moreover, advances in potential monitoring without connection to the rebar as non-destructive measurements have shown their effectiveness. Nevertheless, colligation of the results of both numerical and NDM methods is necessary for assortment the results to provide a better data of the worktime of RC structures.

Performance degradation of steel rebar considering corrosion spatial variability in concrete

The corrosion of steel bars is the main issue affecting the durability of reinforced concrete structures, which is nonuniform owing to the material properties and external environmental effects. In this study, the performance degradation of steel rebars in concrete is estimated considering corrosion spatial variability. With the aid of a three-dimensional measurement technique, a finite element model based on the scanning result was established to examine the corrosion effect on the tensile behaviour of the corroded bars, which was validated with the experimental results and digital image correlation observation. To assess the deformation behaviour of the corroded steels, a semi-analytical model was developed based on the cross-sectional area distribution and material properties. Subsequently, the spatial weight loss distributions with different degrees of corrosion were generated using a random field model. The model parameters were determined by combining engineering judgement and estimation of the experimental results. Finally, probabilistic model of degraded performance was established using the deterministic assessment and simulated steel weight loss distribution.

Microstructural Analysis of Corrosion Products of Steel Rebar in Coral Aggregate Seawater Concrete

Microstructural Analysis of Corrosion Products of Steel Rebar in Coral Aggregate Seawater Concrete

Coconut coir dust extract as a novel green corrosion inhibitor for carbon steel in the chloride-contaminated concrete pore solution

The potential of coconut coir dust to inhibit corrosion, especially the pitting of rebars in concrete was tapped by electrochemical techniques. Electrochemical impedance spectroscopy and linear polarization results show that both deionized water and ethanol extracts of coconut coir dust can slow down corrosion development by forming a protective film even after long-term exposure to chloride and the latter performs better and is more stable. Cyclic potentiodynamic polarization results further show that ethanol extract at a concentration of 0.1 g/L can significantly delay the initiation of pitting. This inhibitor has exhibited the highest inhibition efficiency, which can be sustained at over 90 % even after six months of immersion in solutions with high chloride concentration, but increasing the concentration of the inhibitor does not result in any additional improvement in its efficiency. Critical chloride concentration for corrosion initiation prediction and corrosion resistance evaluation was also discussed.

Deep learning-based extraction and quantification of features in XCT images of steel corrosion in concrete

The corrosion of rebars in concrete has been regarded as one of the most predominant factors for the degradation of reinforced concrete (RC) structures. The accurate predictions of corrosion damages are the basis for the safety assessment of in-service RC structures and the enhancement of new structure designs. X-ray computed tomography (XCT), known for its non-destructive capabilities, has been widely utilized to understand the corrosion process inside the concrete, offering three-dimensional (3D) visualization and aiding in the identification of the parameters in the predictive model of steel corrosion. This study uses U-Net, a deep learning network, to detect various phases of steel corrosion evolution in concrete, including the identification of rebar, corrosion products, pores, cement mortar, and corrosion-induced cracking. A galvanostatic accelerated corrosion test was conducted to simulate rebar corrosion within the mortar. The XCT test was performed, producing a series of XCT images that revealed the corrosion-induced cracks in the concrete. This set of images formed a comprehensive training database for the U-Net model. The performance of the model was significantly improved by data augmentation within the training dataset, thereby enabling it to recognize corrosion-induced cracking and pores. The U-Net model proved effective in accurately segmenting the XCT images into different phases, demonstrating a high accuracy rate of 99.18% on the validation dataset. A mean Intersection over Union (IoU) of 82.1% and a mean F1 score of 89.1% are observed on the test dataset, showing superior segmentation performance on XCT images. This significant advancement has automated the process of visualizing and 3D reconstructing the steel corrosion process inside the concrete. Concurrently, it has enabled the quantification of the volumetric expansion coefficient of corrosion products. The findings reveal spatial and temporal variations in this coefficient due to the migration of the corrosion product through the cracks.

Bond Behavior of Recycled Tire Steel-Fiber-Reinforced Concrete and Basalt-Fiber-Reinforced Polymer Rebar after Prolonged Seawater Exposure

The integration of basalt-fiber-reinforced polymer (BFRP) rebars into concrete design standards still remains unrealized due to limited knowledge on the performance of the rebars in concrete, particularly in terms of bond durability in harsh conditions. In this work, we investigated the bond durability characteristics of BFRP rebars in fiber-reinforced self-compacting concrete (FRSCC) structures. To this aim, a number of 24 FRSCC pullout specimens reinforced with either BFRP rebar or glass-fiber-reinforced polymer, GFRP, rebar, which is a commonly used type of FRP, were fabricated. Half of these specimens were submerged in simulated seawater for a two-year span, while the other 12 similar specimens were maintained in standard laboratory conditions for comparative purposes. Subsequently, all 24 specimens underwent monotonic and fatigue pull-out tests. The exploration in this study focused on investigating the influence of the environmental condition, reinforcement type, and loading type on the bond stress versus slip relationship, maximum bond stress, and failure mode of the specimens. Based on the results obtained and by adopting the durability approach of industry standards for prediction of the bond retention of FRP-reinforced concrete, the bond strength retention between BFRP/GFRP and FRSCC after 50 years of exposure to seawater was estimated. The outcomes of the study are expected to enhance engineers’ confidence in the use of FRP, especially BFRP, for constructing durable and sustainable reinforced concrete structures in aggressive environments.

CONSTRUCTION MATERIAL WASTE AT THE “APARTMENT X” PROJECT IN BEKASI, WEST JAVA, INDONESIA

The implementation of a construction project activity is a lengthy process that is fraught with numerous issues and challenges. The presence of a lot of material waste is one of them. Waste material in the form of concrete rebar was also discovered at the “Apartment X” project in Bekasi, West Java. This research was carried out through interviews, questionnaire distribution, and observation. The findings of the research revealed that the largest percentage of concrete rebar waste in several columns work is D19 rebar waste with a percentage of 18.35%; then the largest percentage in partial wall of STP work is D16 rebar waste with a percentage of 1.29%. Carelessness and worker mistakes, followed by lack of workforce skills, are the factors that influence the occurrence of concrete rebar waste at the “Apartment X” project in Bekasi, West Java, the contractor should have a good quality control system and supervise the workers involved in concrete rebar properly in order to minimize errors that may result in the occurrence of excessive waste. The contractor should also ensure that all parties involved with concrete rebar, from the engineering side to the field implementer, have the capacity to carry out the work that will be assigned.

Analysis of the bond behavior of a GFRP rebar in concrete by in-situ 3D imaging test

Computed tomography combined with mechanical tests offers completely new insight into the behavior of concrete samples under stress. Particularly the development of new fiber reinforcement materials for concrete elements requires appropriate material models and thus for investigating the interior of the concrete structure. In 3D image data obtained by computed tomography, local structural changes within the sample due to mechanical loading can be observed without further altering the sample. We applied this state-of-the-art approach to a concrete core with an embedded glass fiber reinforced polymer rebar under increasing forces applied to pull out the rebar. In this paper, authors describe a novel in-situ setup for non-destructively 3D imaging during the pull-out test. Conducting the pull-out test leads to the formation of local pore volume changes along the rebar. These pore volume changes are not only visualized but quantified analytically based on the images. Interpreting these volume changes, we derive a novel method for calculating strain and normal stresses in the rebar. Our new method captures the detailed distribution of the bond stresses between rebar and concrete and consequently describes the bond behavior more accurate. It turns out that the observed bond behavior cannot be explained completely by commonly assumed material laws. This emphasizes the need for further, more extensive research combining 3D imaging, mechanical testing, and quantitative image analysis.

Influence of rubber geometrical characteristics on the corrosion behavior of rebar in rubberized concrete

Reinforced concrete containing rubber is widely used in building engineering, roads and other fields due to good vibration damping and energy dissipation of rubber. Unlike ordinary concrete, the corrosion-induced cracks of rubberized concrete may differ significantly from that of ordinary concrete due to the irregularity of rubber morphology. In this paper, the 3D mesoscale model of rubberized concrete containing rebars is developed, and the corrosion behavior affected by rubber geometrical characteristics is studied by numerical simulation and experiment. Chloride ion and stray current are selected as the main simulation factors, and the corresponding experiment is conducted through applying constant current. On the basis, the influence of different rubber contents, rubber shapes and rubber sizes on corrosion-induced cracks of rubberized concrete is considered. The main conclusions are as follows: (1) The rust is centered on the rebar and develops around in a forked shape. Rubber particles accumulated in concrete can delay the damage development to some extent. (2) The damage range of the longitudinal sections of rebars is close, and the damage degree is the largest in the section near the center of rebars. (3) The complete rust layer shows an oval shape with a large upper part and a small lower part. The maximum rust layer thickness is obtained near the position of 80 mm or 85 mm.