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.

1. Testing the Specimen and the Manufacturing Process
<ol>
	<li>Acquire a 500 mm long, 14 mm diameter plain steel bar (grade Q235) for the manufacturing of the test specimens.</li>
	<li>Polish the surface of the bar using a fine sandpaper to remove the mill scales on the surface.</li>
	<li>Cut the bar at 30 mm and 470 mm from its left end, as shown in Figure 1, using a cutting machine.</li>
	<li>Measure the weights of the three bar specimens, using a digital electronic scale.</li>
	<li>Measure the diameters of the three specimens using the five methods described in step 2, and record the results of the non-corroded bar specimens.</li>
	<li>Corrode the 440 mm bar specimen using the electrochemical method, as detailed below:
	<ol>
		<li>Cover 70 mm of each end firmly with insulation tape. Attach an electrical wire to one end of the 440 mm bar specimen.</li>
		<li>Mix an adhesive with a hardener in a 1:1 proportion to make epoxy resin. Apply the epoxy resin on the insulated 70 mm ends of the bar specimen uniformly to protect both ends from corrosion.</li>
		<li>Place the 440 mm long bar specimen into a plastic water tank that contains 3.5% NaCl as an electrolyte and a copper plate as a cathode.</li>
		<li>Connect one end of the bar specimen as a node to the positive pole and the cathode copper plate to the negative pole of a direct current (DC) power supplier, respectively, to set up an electrical circuit for the accelerated corrosion of the bar specimen.</li>
		<li>Switch on the DC power supply to apply a constant direct current of 2.5 &#956;A/cm2 onto the bar specimen for the whole period of corrosion.</li>
		<li>Switch off the current to terminate the corrosion process when the amount of corrosion of the bar specimen reaches the anticipated level of corrosion, as estimated using Faraday&#8217;s law.</li>
		<li>Place the above corroded bar specimen into a 12% HCl solution tank for 30 minutes to remove the corrosion products from its surface. Immerse the acid-cleaned bar specimen into a saturated lime water tank for neutralization and further clean using tap water.</li>
		<li>Dry the above cleaned corroded bar specimen in air. Mark its surface for the measurement.</li>
	</ol>
	</li>
	<li>Measure the morphological parameters and corrosion amount of the corroded bar specimen. 
	NOTE: Cleaning does affect the mass loss of a corroded steel bar. Different types of acid solution and the different times of immersion in the acid solution would cause different amounts of mass loss. In this test, however, no comparison was made between different cleaning techniques, For consistency, the cleaning process follows the China National Standard for test methods of long-term performance and durability of ordinary concrete30.</li>
</ol>
2. Measurement Methods and Procedures
<ol>
	<li>Mass loss method
	<ol>
		<li>Place an electronic scale on a horizontal platform and zero it.</li>
		<li>Place the polished bar specimen before corrosion horizontally onto the electronic scale and take a reading from the scale as the mass of a non-corroded steel bar m0 (g).</li>
		<li>Place the cleaned bar specimen after corrosion horizontally onto the electronic scale and take a reading from the scale as the mass of the corroded steel bar mc (g).</li>
		<li>Calculate the amount of corrosion of the bar using an equation of Qcor= (mc-m0)/m0x100%.</li>
		<li>Calculate the average area of the residual section of the corroded bar specimen using an equation Asc=As0(1-Qcor), where, As0 is the area of a non-corroded steel bar.</li>
	</ol>
	</li>
	<li>Vernier caliper method
	<ol>
		<li>Mark the surface of the bar specimen along its length in 10 mm intervals from the left end of the bar using a marker pen, as shown in Figure 1.</li>
		<li>Move the Vernier scale of the caliper to its original position. Make the two jaws touch each other and line up the two zero lines of the Vernier and main scales. Then push its zero button to zero the Vernier scale.</li>
		<li>Place the Vernier caliper across the diameter of the bar specimen. Move the Vernier scale to make its two jaws touch the bar surface gently. Measure the diameter of the bar specimen at the marked section and at the given angle.</li>
		<li>Repeat step 2.2.3 four times to measure the bar diameters at the marked section and at angles of 0&#176;, 45&#176;, 90&#176; and 135&#176;, respectively, as shown in Figure 2.</li>
		<li>Average the above four measured diameters and take it as the representative diameter Di (mm) of the bar specimen at the marked section.</li>
		<li>Calculate the cross-sectional area of the bar specimen at the marked section using an equation Ai=pDi2/4 (mm2).</li>
		<li>Repeat steps 2.2.3 to 2.2.6 for all the marked sections of the bar specimen to measure the distribution of its cross sections along its length after corrosion.</li>
	</ol>
	</li>
	<li>Drainage method
	<ol>
		<li>Set up the electromechanical universal testing (EUT) machine, as shown in Figure 3.</li>
		<li>Place a glass container under the head of the EUT machine and pour tap water into the container until the water level reaches the outlet.</li>
		<li>Place a 200 mL beaker on the platform of an electronic scale right below the outlet of the glass container.</li>
		<li>Clamp one end of the bar specimen using the head of the EUT machine vertically.</li>
		<li>Switch on the EUT machine to move its head down slowly until the other end of the bar specimen just touches the top surface of the water in the container.</li>
		<li>Take the initial reading of the electronic scale as Mi.</li>
		<li>Run the EUT machine to move the bar specimen down into the water in the container at a rate of 1.0 mm/min.</li>
		<li>Take the final reading of the electronic scale as Mi+1 for the mass of the water that has been discharged from the container due to the 10 mm displacement of the bar specimen into the water in the container.</li>
		<li>Assume the cross-section of the 10 mm displaced bar specimen is uniform, calculate the cross-sectional area of the h=10 mm displaced bar using the equation of Ai= (Mi+1 - Mi)/(&#961;h), where (Mi+1 - Mi ) is the measured mass of the water discharged from the container for the 10 mm displaced bar specimen. &#961;=1,000 kg/m3 is the density of water.</li>
		<li>Repeat steps 2.3.6 to 2.3.9 for each 10 mm long displaced bar specimen until displacing the whole length of the bar into the water to measure the distribution of bar cross sections along its length.</li>
	</ol>
	</li>
	<li>3D scanning method
	<ol>
		<li>Spray white developer on the surface of the bar specimen and dry it in air. Place it horizontally onto the platform of a 3D scanner, as shown in Figure 4.</li>
		<li>Calibrate the position of the bar specimen on the platform of the 3D scanner by randomly making white small dots on label paper for the 3D reconstruction of the bar specimen.</li>
		<li>After launching the 3D scanner and the corresponding data extraction software, scan the bar specimen along its length and collect the corresponding scanned data via the 3D scanner. Use manufacturer&#8217;s instructions.</li>
		<li>Develop the spatial model of the bar specimen using the software and collect the relevant date files.</li>
		<li>Place the developed spatial model data of the bar specimen and two self-compiled MATLAB programs in the same folder of a computer.</li>
		<li>Run the first MATLAB program on the developed spatial model data of the bar specimen to generate the relevant MAT file. Save the obtained MAT file in the same folder.</li>
		<li>Run the second MATLAB program on the above obtained MAT file to generate the relevant morphological data of the bar specimen, including sectional area, moment of inertia, polar moment of inertia, eccentric distance etc.</li>
	</ol>
	</li>
	<li>XCT method 
	NOTE: After the four measurements on the 440 mm long bar specimen, the fifth measurement was done on the 30 mm long bar specimens using the XCT method due to its bar length limitation.
	<ol>
		<li>Cut a 30 mm bar specimen from both ends of a 500 mm long steel bar and from the 440 mm long corroded steel bar, as shown in Figure 1. Use them as the non-corroded and corroded bar specimens, respectively.</li>
		<li>Place the bar specimens onto the rotatable platform of the XCT instrument, as shown in Figure 5. Close the door of the XCT instrument. The bar specimen is sandwiched between the radioactive source and the signal receiver of the XCT instrument.</li>
		<li>Run the XCT operation software installed on a computer to set up shooting parameters. Adjust the bar specimen to the shooting position.</li>
		<li>Set up the pixel size and magnification factor in the &#8220;image control&#8221; table of the XCT instrument operation software.</li>
		<li>Run the XCT instrument by clicking the Start button to scan the bar specimen. Collect the scanned data of bar specimen.</li>
		<li>Run the software package on the above scanned data to produce the geometric parameters of the bar specimen accordingly.</li>
	</ol>
	</li>
</ol>

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The authors have nothing to disclose.

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...

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&#39; 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.

<table>
	<tbody>
		<tr>
			<td>Supplies</td>
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			<td></td>
			<td></td>
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			<td>Plastic ruler</td>
			<td>Deli Group Co.,Ltd.</td>
			<td>No.6240</td>
			<td></td>
		</tr>
		<tr>
			<td>white paint pen</td>
			<td>SINO PATH Enterprises.,Ltd.</td>
			<td>SP-110</td>
			<td></td>
		</tr>
		<tr>
			<td>Tube with Branch</td>
			<td>Customized-made</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Measurement cylinder</td>
			<td>Beijing Huake Bomex Glass Co., Ltd.</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>500mL Beaker</td>
			<td>Beijing Huake Bomex Glass Co. , Ltd.</td>
			<td>CP-201</td>
			<td></td>
		</tr>
		<tr>
			<td>sandpaper</td>
			<td>Shanghai Noon Decoration Material Co., Ltd.</td>
			<td>P04</td>
			<td></td>
		</tr>
		<tr>
			<td>white developer</td>
			<td>SHANGHAI XINMEIDA FLAW DETECTION MATERIAL CO., LTD.</td>
			<td>FA-5</td>
			<td></td>
		</tr>
		<tr>
			<td>Reagents</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>epoxy resin adhesive</td>
			<td>Hunan Baxiongdi New Material Co., Ltd.</td>
			<td>DY&#183;E&#183;44</td>
			<td></td>
		</tr>
		<tr>
			<td>epoxy hardener</td>
			<td>Hunan Baxiongdi New Material Co., Ltd.</td>
			<td>DY&#183;EP</td>
			<td></td>
		</tr>
		<tr>
			<td>HCl</td>
			<td>Dongguan Dongjiang Chemical Reagent Co., Ltd.</td>
			<td>AR-2500ml</td>
			<td></td>
		</tr>
		<tr>
			<td>saturated lime water</td>
			<td>Xilong Chemical Co., Ltd.</td>
			<td>AR-500g</td>
			<td></td>
		</tr>
		<tr>
			<td>Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Digital electronic scale</td>
			<td>Kaifeng Group Co., Ltd.</td>
			<td>Model JCS-0040</td>
			<td></td>
		</tr>
		<tr>
			<td>Digital vernier caliper</td>
			<td>Shanghai Measuring &#38; Cutting Tool Works Co., Ltd.</td>
			<td>Model ST-089-229-090</td>
			<td></td>
		</tr>
		<tr>
			<td>Cutting machine</td>
			<td>Robert Bosch GmbH</td>
			<td>TCO2000</td>
			<td></td>
		</tr>
		<tr>
			<td>3D reconstructed X-ray microscope</td>
			<td>XRADIA</td>
			<td>Model MICROXCT-400</td>
			<td></td>
		</tr>
		<tr>
			<td>3D scanner</td>
			<td>HOLON Three-dimensional Technology(Shenzhen) Co.,Ltd.</td>
			<td>Model HL-3DX+</td>
			<td></td>
		</tr>
		<tr>
			<td>Electromechanical Universal Testing Machine</td>
			<td>MTS SYSTEMS (China) Co., Ltd.</td>
			<td>Model C64.305</td>
			<td></td>
		</tr>
	</tbody>
</table>

applicability analysis of assessment methods for morphological parameters of corroded steel bars

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.

Figure 6 shows the diameters of the 500 mm long non-corroded bar specimen at angles of 0&#176;, 45&#176;, 90&#176;, and 135&#176; for each section along its length measured using Vernier calipers. The bars were then cut into three parts, as shown in Figure 1.
Figure 7 presents the cross-sectional areas of the non-corroded bar specimens alon...

Watch this Scientific Journal Video about Applicability Analysis of Assessment Methods for Morphological Parameters of Corroded Steel Bars at JoVE.com

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

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).

The irregular and uneven residual sections along the length of a corroded steel bar substantially change its mechanical properties and significantly ...

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6.3K Views.  Shenzhen University. 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).