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2014 全國技專校院學生實務專題製作競賽 研究成果報告書
Behavior of natural and laboratory accelerated corrosion
參賽類群：英文代碼 F 土木與建築 類群 編號：(由主辦單位編列)
中華民國 103 年 03 月 21 日
Behavior of natural and laboratory accelerated corrosion
Abstract
In this fast growing gen eration, buildings play an important role around us.
But for now, the old treaty seems did not be established so well, furthermore they did not notice the importance of corrosion of steels to the building. This project is going to use different methods of corrosion, find out the difference of strength, elements from the corrosion bar and the substance.
Discuss which method can simulate the situ problem, and then we can do more research in the future. From this project, we also acquire the method to simulate the near seaside corrosion like semi-soaked method and some parameter. The datum shown us there is almost the same chemical element bet ween t he s i t u s teel bar and l ab acceleration, but the corrosion level of semi-soaked method is more serious.
Compare the result between semi-soaked and total-soaked; we can say that the mechanic property of semi-soaked method decrease more and the corrosion level are h i g h e r. T h e r e f o r e w e c a n s a y t h e semi-soaked method is more conservative t o t h e n e a r s e a s i d e c o r r o s i o n .
Key words ： Semi-soaked, total soaked, SEM-analysis, Tensile test, Sea wind effect.
1. Research background and Motivation As the time passed by, people will get old. Not only human beings, but also the buildings we lived with. The stability、
strength decreases as well. In Taiwan, there are amount of short reinforced concrete buildings. The speed of deteriorate of our building especially fast. According to the statistics of Taiwan buildings, there are
around 37 percents of structures has the problem; their buildings are over 30years which belongs to the over-aged building.
Which means there are 37 percent of them are over-aged, after decades later, the rate of the old building will raise to 56 percent.
Some important places which like hospital 、 school gym、police office and township offices included. Those belong to shelter for emergency evacuate and usually crowd of people. Those places play a very important role in relief and social safety. For that reason, nowadays our structure design terms still need more attention.
The motivation is to acquire the most economic and safety structure design.
However, when we are doing assess of our design specifications; we did not put steel corrosion and the seismic behavior into our consideration as a factor. However, in reality, this part usually becomes the most critical. Problems of corroded RC buildings usually arise because of environment and poor structural design. As the concrete cover steels been set up for a long time.
There must some deterioration problem or other phenomenon, the problems also comes with corrosion、strength decreasing.
Because corrosion makes the steel bars dilatation, destroy the bond strength. Make the steel out of the concrete or flake of the concrete. Besides, it is hard to find the specimen in situ, so we hope to find the method which closest the situ corrosion and make it easier to do. Then we can predict the life cycle more accurate.【Figure1】
shows the corrosion state and exposed steel
bars of the structure, from the concrete
2014 年立台灣科技科大學學生張修齊實務專題製作競賽暨成果
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surface we can observe there are some steels out of the protect level. Even flake the protect cover. Then we could know the importance of the corrosion, it will decrease the seismic performance function of our structure.
(a) Corroded structure 2. Research Objective
Because of the corrosion by time the stability of our building may not as well as we design before. Especially the short R.C building, so we start to concern the corrosion of steel bar inside. We know that the strength of the R.C building is provided by the estimated array steel and the concrete. But how can we estimate the situation after we build the building. The following are the things we are trying to figure out:
1. Reconnaissance of corrode structures and sampling naturally corroded steel bars.
2. Compare the corroded bars from natural corrosion with accelerated corrosion by tensile test and rust SEM analysis.
3. Literature review
I use the Accelerated Corrosion method which by the corrosion in the transverse reinforcement was accelerated using an electrochemical method by imposing an electrical current to the reinforcement.
Corrosion was controlled to occur only in the steel net area in the different situation.
7The other theory of predicting the corrosion level is the equation modify by Ching-Hung Yeh
1from the Faraday’s
corrosion level calculation. The steel corrosion level test in his paper was modifying for the Taiwan steel. He did compare the corrosion level; weight loss and the actual corroded beam by rescale it.
Through the test he figure out the coefficient of the Faraday’s formula is 76.40%.1As both oxygen and moisture should also be present to provide a corrosion cell, a system was devised to reproduce the in-situ corrosion. Therefore, we change the specimen from vertical to horizontal combination of sponges and soak in salt water. It served two purposes: (1) the specimens corroded in different environment, and (2) it provided convenient access to observe each state and all faces for inspection and servicing.
54. Approaches
There are three kinds of my specimens, one is we put the specimen into the mixed chlorine water accelerate the corrosion in lab, the other one is covered by the wet sponge with the chlorine water and male it corrosion, after that we compare them with the situ sample. For that case I also do the reconnaissance. We use 100ton universal testing machine with NDI machine to do the tensile test and SEM-analysis to compare the elements in the steel bar.
4.1 Reconnaissance of corroded structures Location1:Haiwan Xincheng(located in New Taipei City Sanchi area)
This is the first place we visited. I also
use the grinder to get some samples for the
SEM analysis and the other tests. For that
case we prepared hammers and fabric
glovers, blinkers…and the other tools
provide getting hurts. Figure 【b】 shows
the exposed steel bar and Figure 【c】 is the
procedure of sampling.
Figure (b)
Figure (c)
Location2: Jinshan Bridge No. 40
This bridge connect with downtown Jinshan and Provincial Highway No.2. The bridge also have a severe corrosion problem which shown in Figure【d】. Furthermore I also find the sea wind effect of the bridge. The function of sea wind effect means when the structure of facing sea wind side will be corroded more which shown in Figure【e】.
Figure (d)
Figure (e)
Location3: Shimen Eighteen Wang Kung Temple
It is the discharge port of the Nuclear Power Planet. The point of the bridge is that was made by two different materials which shown in Figure【f】. Therefore we can find two different corrosion phenomenon in one bridge. Besides it was only be done for 20 years. However there are several places with corrosion included the pitting in the steel structure shown in Figure【g】 and longitude cracks and the repairing of the deck in R.C structure shown in Figure【h】
and Figure【i】. I think there are three reasons of pitting, included small dust hit, sea wind and the splash of the sea water. In Figure 【 h 】 I also found the styrofoam between the column and protect cover. I guess the function of the Styrofoam is to strength the protect cover. However the effect still needs time to prove.
Figure (f)
2014 年立台灣科技科大學學生張修齊實務專題製作競賽暨成果
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Figure (g)
Figure (h)
Figure (i) Conclusion of Reconnaissance:
The near sea side structures have damaged by wind effect or other factor, so we need to strengthen the facing or built a shelter to improve. The second problem is the 4 cm protect cover of main beam in Wang Kung Temple and 10 cm protect cover of main column in Wang Kung Temple are not enough because of the sea water spray, besides that they also install the 10 cm Styrofoam as the second cover.
But the function still need time to indicate.
4.2 Accelerated corrosion experiment I use two different methods to simulate the natural corrosion situation shown in Table 【 a 】 and the dimension of the specimen shown in Figure【j】
Table (a)
Figure (j)
The following steps are the procedure that I made the specimen, as we got the concrete we have to do the property test first in order to make sure all the material are in the standard which shown in Figure
【k】. After that I grouting the concrete into
the model shown in Figure【l】 and stay one
day for concrete curing and put them in the
water tank and connect with the D/C power
shown in Figure【m】.
Figure (k)
Figure (l)
Figure (m)
4.2.1 Accelerated corrosion steps Place the specimen in 65 cm
45cm
30cm acid-proof water tank which filled with salt water. Use D/C power connect with the specimen which surround by the stainless steel. Anode with the wire and negative electrode with the stainless steel which shown in Figure 【n】and after few day that we estimate with the Faraday’s corrosion formula we will get the corrosion result shown in Figure【o】
Figure (n)
Figure (o)
After the corrosion procedure I took them out of the water tank remove the steel net and Styrofoam and mark the cracks shown in Figure【p】 and dig the steel bar out of the specimen to do the mechanical test shown in Figure【q】.
Figure (p)
Figure (q)
2014 年立台灣科技科大學學生張修齊實務專題製作競賽暨成果
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I also use AUTO CAD draw the crack of each face shown in Figure【r】 .
Figure (r)
From the test, we can find the following behaviors:
1. Most of the cracks are straight, and parallel to the bar.
2. Most cracks are focus on one face, some faces do not have cracks
4.3 Tensile test
The corroded steel bars were obtained from real building in Taiwan. In order to know the residual strength of the corroded steel bars, we did the tensile test. The coefficient of the 100ton tensile machine shown in Figure【s】 is 20, so we need to do the unit transforms to get the stress we need which is shown in formula.1. The strain was compare to the initial displacement which is shown in formula.2. After that we also derive the reduction factor of the corroded bars and do compared with the naturally corroded bars.
Figure (s)
I use the tensile machine to acquire the strain-stress diagram and do the weigh lose table shown in Figure【t】 and Table【u】
Figure (t)
From Figure【t】we can obtain the mechanic properties from our artificial corroded steel bars. By this table we can say that the corrosion of semi-soaked specimen is more serious than the total soaked one. When the weigh lose level is higher which means the probability of the cross section will be damage more and decrease the strength.
Table (u)
From Figure【t】and the mechanic properties of natural corroded steel bars form Yu-Chen Ou and Yudas Tadeus Teddy Susanto
8-Mechcanical behavior of corroded steel bars we can obtain the reduction factor which shown in Table【c】and plot in Figure
【v】. The red marked number means the natural corroded slope and the black one is slope of artificial corroded one. If the slope inclines more which means the corrosion speed is faster. In this slope we can find the relation between the strength and strain reduction and the corrosion weight loss.
Table (c)
Figure (v) 4.4 SEM-analysis
The results of the Scanning Electron Microscope / Energy-Dispersive X-ray Spectroscopy (SEM/EDS) were summarized in Table【d】. The SEM/EDS test was conducted to investigate the percentage of chemical elements of the naturally corroded steel bars. The samples were taken from various naturally corroded
steel bars from two bridges; Bridge No. 40.
Table (d) Note:
1. Lab accelerated corroded steel bars with sponge which half-soaked in the salt solution
2. Lab accelerated corroded steel bars with steel net which soaked in the salt solution
3. Un-corroded steel bars taken from specimen without sponge
4. Naturally corroded steel bars taken from Bridge No. 40
5. Rust taken from naturally corroded steel bars (rebar was located near 18 Wang-Kung Bridge; rebar was taken from deserted RC block)
6. Rust taken from naturally corroded steel bars located near Bridge No. 40
7. Rust taken from naturally corroded steel (from Bridge No. 40)
8. Rust taken from naturally corroded steel bars (from Bridge No. 40)
After the analysis we can find that the content of the oxygen is from 16% to 20%
and our artificial method is above 20%, so we can say it is a conservative result for the corroded element analysis.
5. Conclusion
We can observe from Table【b】 that Semi-soaked sponge specimen corrosion is more serious than Total-soaked specimen.
Furthermore the semi-soaked specimen
dilated more and make the protect cover
lose the function. From SEM-analysis I
found the semi-soaked specimen had more
Oxygen than the total-soaked one. Compare
to the naturally corroded bar the oxygen of
which is 16~20%, both accelerated methods
are acceptable in terms of oxygen content in
the rust. In Figure【v】 the tensile properties,
i.e., yield strength, ultimate strength, elastic
2014 年立台灣科技科大學學生張修齊實務專題製作競賽暨成果
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