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Measurement Methods of Geometrical Parameters and Amount of
Corrosion of Steel Bar
Dawang Li a,b, Ren Wei a,b, Yingang Du c,*, Xiaotao Guana,b, Muyao Zhoua,b
a Guangdong Province Key Laboratory of Durability for Marine Civil Engineering, Shenzhen, 518060,
China
b Department of Civil Engineering, Shenzhen University, Shenzhen, 518060, China
c Department of Engineering and the Built Environment, Anglia Ruskin University, Chelmsford CM1 1SQ,
United Kingdom
E-mails: [email protected] (Dawang Li); [email protected] (Ren Wei);
[email protected] (Yingang Du); [email protected] (Xiaotao Guan);
[email protected] (Muyao Zhou)
Highlights
Five methods were used to measure bar geometrical parameters and amount of corrosion.
The results using 3D scanning and XCT match well and more precise than other methods.
3D scanning is most suitable for measuring geometrical parameters of a corroded bar.
Vernier caliper is the best option for measurement of a non-corroded bar.
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Key Words:
Weight loss, Vernier caliper, Drainage, 3D scanning, XCT method, Corroded bar
Abstract
This paper aims to evaluate the applicability and suitability of the different methods, including
weight loss, vernier caliper, drainage method, 3D scanning and XCT methods in the
measurement of geometric parameters and amount of corrosion of a steel bar. A single 400mm
long and 14.11mm diameter steel bar was measured first as non-corroded specimen before an
accelerated corrosion of its 300mm long middle part took place. This was followed by the
measurement and evaluation of the geometrical parameters of the same bar specimen within
its 300mm long corroded part and 30mm non-corroded part at its right end using different
methods. The results show that the geometrical parameters of a corroded bar measured using
3D scanning and XCT methods well matched each other and much more precise than those
using weight loss, vernier caliper and drainage methods. 3D scanning is the most suitable
method to measure the geometrical parameter of a corroded bar. Vernier caliper is the best
option for measuring the geometrical parameter of a non-corroded bar.
1. Introduction
Corrosion of steel bar is one of the major reasons for the deterioration of concrete structures
that are widely used in our society. It not only causes cracks on concrete surface and even
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spalling of concrete cover, but also decreases the effective areas of a steel bar and, in
particular, reduces its strengths and ductility significantly [1-3]. As a result, the load-bearing
capacity and service reliability of a concrete structure deteriorate substantially, which has ever
been a concern for the owners and users of the existing concrete structures [4-6].
It has been well recognized that the corrosion of a steel bar initiates on its circumferential
surface and penetrates bar surface very irregularly afterwards. This results in the uneven
residual sections along the length of a corroded bar, which in turn dominates the mechanical
properties of a corroded bar and the safety of a deteriorated structure. Therefore, a precise
measurement of the geometrical parameters and amount of corrosion of a corroded bar is
crucial for the assessment of safety and reliability of a deteriorated structure.
Various methods, including weighing loss, vernier caliper, drainage method, 3D scanning and
XCT methods, etc. have been attempted to measure the geometrical parameters and amount of
corrosion of a corroded bar. Among these methods, weight loss method is one of the most
popular method for the measurement of amount of corrosion of a steel bar [6-9]. However,
weight loss method can only measure the average value of the residual section of a corroded
bar [7-9].
In fact the load-bearing resistance and deformation capacity of a corroded bar depends on its
minimum residual section and the distribution of its residual section along the length of the
bar, respectively [1]. Accordingly Zhu and Francois cut the whole length of a corroded bar
into a number of 10mm to 20mm small segments before measured their weight loss for the
purpose of reflecting the variation of the residual area along its length and approaching the
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called minimum residual section [10, 11]. However, cutting of a corroded bar not only causes
a loss of its mass and some section, but also potentially misses some minimum residual
section. Therefore it still cannot evaluate the geometric feature of a corroded bar precisely.
Zhu, Francois and Torres-Acosta used the vernier caliper to measure the diameter and the
pitting depth of a corroded bar for estimation of its residual area and mechanical properties
[10, 11 and 12]. However, due to the irregular corrosion pitting and residual section, the
deviation of the measured results is inevitable. On the basis of the Archimedes' principle that
buoyant force on an object that is submerged in water is always equal to the weight of the
water it displaces, Du et al set up an apparatus and used drainage method to measure the
variation of the residual section of a corroded bar along its length qualitatively[1]. However, in
their apparatus, the movement of the steel bar was manually controlled and therefore it could
not define the amount of corrosion qualitatively. Over the past few years, with the
development of 3D scanning technology, the 3D scanning has been used to describe the
surface morphology of a corroded bar, including the diameter, area, morphology, depth of
pitting, centroid and inertia moment of a cross section [13-17]. However, the majority of
publications have just focused on how to acquire the measured data from a steel bar specimen,
few of them were devoted to the applicability and suitability of the different methods for
measuring the characteristics of a steel bar in different conditions [18]. In fact, different
measurement methods have their own test principles, accuracy and applicability. In particular,
so far less significant comparison and validation have been made between different methods
that have applied to the same specimen under the same corrosion condition.
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Hence, this paper aims to evaluate the applicability and suitability of the different methods,
including weight loss, vernier caliper, drainage method, 3D scanning and XCT methods in the
measurement of geometric parameters and amount of corrosion of steel bar. A single 400mm
long and 14.11mm diameter steel bar was taken as a non-corroded specimen and measured for
its surface feature before an accelerated corrosion of its 300mm long middle part took place.
This was followed by the measurement and evaluation of the geometrical properties of the
same bar specimen within its 300mm long corroded part and 30mm non-corroded part at its
right end. The results measured using different methods show that the geometrical parameters
of a corroded bar measured using 3D scanning and XCT methods well match each other and
much more precise than those using weight loss, vernier caliper and drainage methods. 3D
scanning is the most suitable method to measure the geometrical parameter of a corroded bar.
Vernier caliper is the best option for measuring those of a non-corroded bar.
2. Experimental work
2.1. Specimen and corrosion tests
A 14.11mm diameter plain bar in grade of Q235 was used for the test specimen. The steel bar
is 400mm long in total and has 300mm length in its middle to be corroded, as shown in Figure
1.
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50 300 50
Electrical tape and epoxy resin Central axis
200 50 100 150 250 300 350 400 (Unit:mm) 0 355 385
Non-corroded sample
240 270
Corroded sample
x
Wire30 30
Figure 1. Schematic drawing of the non-corroded bar specimen.
The steel bar in grade Q235 has a minimum yield strength of 235MPa, ultimate strength of
370 and elongation of 20%, as specified in China’s National Standard – GB/T11253-2007
[19]. The geometric parameters and self-weight of the steel bar before its corrosion were first
measured along its length and taken as the benchmark of non-corroded bar specimen.
Afterwards, the same steel bar was subjected to an accelerated corrosion test under an
impression of 2.25mA/cm2 direct current and taken as the corroded bar specimen. Before
corrosion, both 50mm long ends of the steel bar specimen were covered using the electrical
insulation tape and epoxy resin to protect them from corrosion. Namely only the 300mm long
middle part of the bar specimen was subjected to corrosion, as shown in Figure 1. After the
amount of corrosion of the steel bar reached the anticipated level of corrosion, as predicted
using Faraday’s law, it was cleaned using acid solution and tape water, before dried in air. The
weight of the corroded steel bar was measured using a scale for its weight loss, before it was
painted in white for the further measure at a spacing of 10mm along the length of the corroded
bar specimen as shown in Figure 2.
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200 50 100 150 250 300 350 400 (Unit:mm) 0 355 385 240 270
x
Figure 2. Photo of the corroded bar specimen
2.2. Measurement methods
Five different methods were used to measure the geometric parameters and corrosion mount of
above specimen, namely, weight loss method, vernier caliper, drainage method, 3D scanning
and XCT methods for both non-corroded and corroded specimens, as detailed blow.
2.2.1. Weight loss method
It is assumed that weight loss of the corroded bar took place only within its 300mm long
middle corroded part. Therefore, the amount of corrosion was determined by Equation 1.
Qcor=W 0−W 1
W 0×100 %
Equation 1
Where, Qcor is the amount of corrosion of a steel bar (%), W 0 is the weight of the non-corroded
bar prior to its corrosion, W 1 is the weight of the same steel bar after it was corroded, cleaned
in acid solution and dried in air.
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Accordingly the average cross-sectional area and penetration depth of the corroded steel bar
can be calculate by Equations 2 and 3,
A sc=A s 0(1−Qcor )
Equation 2
xsc=ds 0 (1−√1−Qcorr) Equation 3
WhereAsc and xsc are the average cross-sectional area and penetration depth of the corroded
bar,A s0 and ds0 are the initial cross-sectional area and diameter of the same bar specimen prior
to its corrosion.
2.2.2. Vernier caliper method
A digital vernier caliper was used to measure the original diameter of the non-corroded bar
specimen and the residual diameter of the 300mm long corroded bar. The vernier caliper has a
maximum deviation of 0.01mm. 31 sections of the bar specimens at a spacing of 10 mm along
their length were marked, as showing in Figures 1 and 2, and were measured for their residual
diameters using the caliper. For each cross section of the bar specimen, four readings were
taken at the angles of the 00, 450, 900and 1350 in circumferential direction of the bar section, as
shown in the Figure 3. Among the four readings, both maximum and minimum readings were
picked up and averaged for nominal diameter of bar specimen, which, in turn, is used for the
calculation of cross sectional area and other geometrical parameter of the bar specimens.
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Figure 3. Measurement of bar diameter using vernier caliper.
2.2.3. Drainage method
As shown in Figure 4, a new apparatus was set up and used to measure the original area of the
non-corroded bar and the residual area of the 300mm long corroded bar [20]. This apparatus
uses a motor to control the vertical movement of the bar specimen with a maximum deviation
of 0.01mm. An electronic scale was used to measure the weight of the water excluded from
the container with an accuracy of 0.1g. On the basis of the measured weight of the excluded
water, the displacement of the bar specimen in the water container and density of 7.85g/cm3 of
steel material, the average sectional area of the bar specimen was calculated.
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Figure 4. Apparatus of drainage method.
2.2.4. 3D scanning method
Figure 5 shows the apparatus of measuring the geometrical parameters of a bar specimen using
3D scanning method. As shown in the Figure 5(a), the white-painted specimen was placed on
the working platform, which slowly moves along the length of the bar specimen through the
scanning area. After one side of a bar specimen has been scanned, it was rotated for scanning
of another. The scanning data was acquired and processed via the Geomagic software, as
shown in Figure 5 (b). Finally, the scanned model file was dealt with via self-compiled
MATLAB package for the purpose of producing the geometrical parameters, such as the
residual area, diameter, centroid, eccentricity, moment of inertia and corrosion penetration,
etc. of bar specimens.
Bar
specimen
Motor control system for the bar displacement
Electronic scale
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(a) Apparatus of 3D scanning (b) Window screen of Geomagic software
Figure 5. Measurement using 3D scanning method
2.1.5. XCT method
A three-dimensional X-ray image microscope was used in the research with a scanning
accuracy of 0.028mm. Because of the limit of the dimension of a specimen that can be
scanned using the XCT instrument, however, only two 30mm long bar specimen, as shown in
Figure 6, were used as the corroded and non-corroded specimens for XCT measurement. Both
30mm long bar specimens were cut off from the same bar spacemen shown in Figure 2 at the
distances of 240mm and 355mm from its left end for the corroded and non-corroded
specimens, respectively. As shown in Figure 7, the 30 mm long bar specimen was scanned by
the XCT apparatus, before the image data was processed via AVizo software package to
produce the geometric parameters of the bar specimens .
3D scanner
Bar specimen
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(a) non-corroded bar segment at x=355mm (b) corrosion bar segment at x=240mm
Figure 6. Bar specimens for measurement using XCT method
Figure 7. Apparatus of XCT method.
3. Results and Discussion
3.1. Measured results of non-corroded bar specimen
The cross sectional area of the 400mm long non-corroded bar specimen that were measured
using weight loss, vernier caliper and drainage methods are shown Figure 8. It is clear that the
three measured areas of the bar specimen are overall consistent along the length of the bar.
However, there were some ups and downs in the results measured using the drainage method.
Rotating stage
X-radiographic source
Receiver
CCD Camera
CCD摄像头Bar specimen
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This may be caused by the surface tension of water, the bond forces between water and bar
surface, the accuracy of electronic scale and surface friction of water container.
Figure 8. Measured cross sectional areas of the non-corroded bar segment specimen.
In addition to the above measured results along the whole length of 400 mm of the bar
specimen, the right end of the 30 mm long non-corroded bar segment at the distance of
x=355mm from its left end after the corrosion of its 300mm middle part, as shown in Figure 2,
was also measured using all the above different methods with the measured diameters
summarized in Table 1 and the measured sectional areas shown in Figure9.
Table 1. The diameter of a non-corroded bar segment at the distance of x=355mm
Diameter(mm) Caliper method XCT method 3D scanning method
Maximum 14.43 14.22 14.60
Minimum 13.79 13.59 13.54
Deviation 0.64mm 0.63mm 1.06mm
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Figure 9. Measured sectional areas of the 30mm long non-corroded bar segment at x=355m
It is obvious that the data measured using the different methods were consistent and
comparable to each other. Therefore, theoretically they all can be used to determine the
geometrical parameters of a non-corroded bar. However, taking the cost and efficiency into
consideration, the vernier caliper method is much more convenient and economical than other
four methods and hence it mostly fit for the purpose of measuring the diameter of a non-
corroded bar.
3.2. Corroded reinforced specimen
Figure 10 shows the geometric mode of the 300 mm long corroded bar specimen using 3D
scanning method. Figure 11 presents the typical cross sections of the corroded bar measured
using XCT method.
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Figure10. Geometric model of 300mm long corroded bar using 3D scanning method.
Figure 11. Typical cross sections of the corroded bar using XCT method.
Figures 10 and 11 show that the corrosion very irregularly penetrates the circumferential
surface of a steel bar and leaves its cross section no longer circular one. In particular, Figures
10 and 11 indicate that both 3D scanning and XCT methods can precisely define the
geometrical parameters and visually demonstrates the geometrical features of a corroded bar,
compared with the other three methods, i.e., weight loss, vernier caliper and drainage method.
Figures12 and 13 report the residual cross sectional areas of the corroded bar along the lengths
of the whole 300mm and the localized 30 mm, respectively. It should be noted that due to the
limit to the size of a specimen, XCT method was only applied to the local 30mm long
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specimen that was cut off from the 300mm long corroded bar at the distance of 240mm from
the left end of the bar specimen, as shown in Figure 2.
Figure 12. Residual cross sectional area of the whole 300mm corroded bars.
Figure 13. Residual cross sectional area of the local 30mm long corroded bar segment.
Figures 12 and 13 show that the overall trend of the residual sectional areas of the corroded
bar specimens that were measured using all the four or five different methods varies
consistently. In particular, the residual cross sectional area measured using 3D scanning
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method was very close to those measured using XCT method. This also well matches the area
of 96.9mm2 calculated from the measured weight loss.
This indicates that the 3D scanning can accurately measure the surface morphology of a
corroded steel bar. However, the residual areas measured using vernier caliper and drainage
method varied discretely, compared with those using 3D scanning and XCT methods. The
measurement data using vernier caliper and drainage method is quite different from those
using the 3D scanning and XCT methods with some difference left between the caliper
method and drainage method, especially in the serious corrosion section. This is because that
the residual cross section of the corroded bar no longer remains its smooth circular cross-
section but with some irregular pit on its surface. One cannot use a vernier caliper to measure
the actual residual diameter of a corroded bar precisely and calculate its residual area
reasonably. For the drainage method, the irregular surface of a corroded bar might increase the
bond force and surface tension of water and cause more discrete measurement data.
Apart from the above residual cross sectional area, 3D scanning method can also create more
specific information, such as penetration depth, sectional eccentricity and moment of inertia of
a corroded bar, as shown in showed in Figures 14 to 16.
Figure 14 indicates that the penetration depth varies substantially and irregularly along the
length of a corroded bar. This is well consistent with the variation of the residual sectional
area, as shown in Figure 12. The maximum penetration depth of about 6.5mm in Figure 14 left
the minimum residual area of about 64mm2 in Figure 12 at the same section. This is also
comparable with the calculated penetration of 3.01mm based on weight loss measurement.
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It should be pointed out that because of the irregular penetration depth and the residual area
along the length of a corroded bar, the centroid of the actual residual section of a corroded bar
changes and therefore an eccentricity of its residual irregular section from its original non-
corroded circular section occurred, as shown in Figure 15. Accordingly the moment of inertia
of the residual section of a corroded bar varies along the length of the bar, as shown in Figure
16. These information can only be obtained using 3D scanning method and is very useful in
the analysis and calculation of mechanical properties of a corroded bar and in particularly of
corroded-damaged concrete structure.
Figure 14. Penetration depth of the corroded bar using 3D scanning
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Figure 15. Eccentricity of the residual section of a corroded bar using 3D scanning
Figure 16. Moment of inertia of the residual section of the corroded bar using 3D scanner
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In addition to the above valuable measurement data of the corroded bar specimen, the actual
cost of using 3D scanning method on the measurement of geometric parameters of a corroded
bar is much lower than using the XCT method. As a result, it can be concluded that the 3D
scanning is the most suitable method to measure the parameters of corroded bar.
4. Conclusions
Based on the above results and discussions, the following conclusions can be drawn:
1) Compared with other four methods, the vernier caliper is the most suitable for the
measurement of the geometrical parameters of a non-corroded steel bar.
2) Drainage method can qualitatively reveal the variation of the cross section of a steel bar
along its length. Due to some affecting factors, water absorption of bar surface and surface
tension of water, etc. however, this method cannot define the features of a steel bar precisely.
3) XCT method can accurately measure the residual cross-sectional area of a steel bar.
However, the size and configuration of a steel bar for using this method is limited and very
rigid. Hence, XCT cannot be widely used in the engineering practice.
4) The residual sectional area of a corroded bar measured using 3D scanning and XCT method
well match each other.
5) Compared with the XCT method, the 3D scanning method can precisely define the
geometrical parameters of both non-corroded and corroded steel bar. It also has low cost, high
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efficiency, high precision and so on and hence is suitable for the measurement of the
geometrical parameters of a corroded steel bar.
Acknowledgments
The authors at Shenzhen University greatly acknowledge the financial supports from the
National Natural Science Foundation of China (Grant No. 51520105012 and 51278303). 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.
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