summary:
Concrete durability is a key factor affecting the performance of mixed concrete in corrosive environments. The purpose of this research project is to infer the nature of the variables that control the aggressiveness of the environment.
Both the air and humidity sensor and the corrosion sensor were pre-cast in the area where the bridge pier was submerged by water. The protective layer of this area was repaired in 1999.
After 3 years, the corrosion sensor is still working well. No changes were recorded between the positive and negative electrodes, indicating that chloride did not penetrate into the concrete on a large scale. The chloride ion penetration data proves the low chloride content of concrete. This is not surprising, because the concrete quality of this project is very high, so there is no danger of corrosion in the protective layer in a short time.
introduction:
The main purpose of this study is to evaluate the number of freeze / thaw cycles and their impact on concrete exposed to seawater, and to comprehensively evaluate the overall performance of high-performance concrete in harsh environments against frost resistance and chloride ion Period of use.
The structure sample selected in this research is located in the west of Iceland. The structure was casted in 1999 with a protective layer surrounding each pier, see Figure 1. The first bridge pier was repaired in 1998, and the second bridge pier was repaired in 1999. The protective layer is made of high-performance concrete, which is selected according to Gudmundsson and Wallevik (2002) standard for selecting concrete binder and pre-mixing. At the same time, the third bridge pier was repaired with self-compacting concrete in 2002.
Figure 1a Appearance of the bridge Figure 1b Protective layer cast in 1999
The total span of the bridge is 512 meters
The authors described the aging process of the original concrete in other works (see Gudmundsson, 1997 for details). In short, the damage to concrete is the result of a combination of freeze / thaw cycles and seawater erosion.
The type of concrete used in the restoration was selected according to the standards of Gudmundsson and Wallevik, 2002. Based on the test results of frost resistance test and chloride ion permeability test, we selected a ternary blended binder in the concrete protective layer. The protective layer consists of 40% fine-grained blast furnace slag (5000 Blaine), 5% silica fume and 55% ordinary Portland cement (CEM I). For the concrete aggregate, high-quality imported granite crushed stone is selected. The water-cement ratio is always guaranteed to be 0.33. The performance of concrete is achieved by means of polycarboxylic acid dispersion admixture and tensid and vinsol resin mixture. Overall, the performance of the concrete is worthy of recognition.
Generally, the air content in freshly mixed concrete is 8%, and a large amount of air will be squeezed out during the pouring of concrete. However, in the actual set concrete samples, the space factor of the air content in the total volume is as low as 3%, about 0.4mm in height, and high as 10%, about 0.15mm in height.
The design compressive strength of concrete is about 70 Mpa, but due to the high air content inside, its actual strength is much lower than this value. When the 28-day compressive strength of the concrete is 45Mpa. One year after the concrete is poured, the core is sampled inside the concrete and the measured strength is 62Mpa. That is to say, one year after pouring, the strength of concrete has changed.
Table 1 shows a set of test results on freeze / thaw tests performed on the protective layer. The test samples are in accordance with the CEN prEn xxxx 1999 standard, referring to the Swedish SS 137244 standard. The test is a zoom test, and the sample is placed in a 3% NaCl solution.
sample
Air,% / spacing factor, mm
28 cycles
56 cycles
Sample 1
10.8 / 0.13
0.02
0.10
Sample 2
7.1 / 0.16
0.02
0.04
Sample 3
2.9 / 0.42
0.07
0.12
Table 1. Scaling test results (kg / m2) on concrete protective layer
The results of this scaling test are very good. The results show that in the freeze / thaw-thaw scaling test, the air content in the concrete has nothing to do with the resistivity results inside the concrete.
On-site monitoring
For different current positions, temperature sensors are installed at three depth positions in the cover layer:
Position 1: The temperature sensor is installed 5 cm below the surface of the protective layer below sea level (submerged area) or 20 cm from the bottom of the concrete protective layer.
Position 2: In the lower part of the intertidal zone, 70cm above the bottom of the concrete protection layer, the two thermocouples will be installed at 5Ccm and 9cm below the surface, respectively.
Position 3: Located in the upper part of the intertidal zone, 70cm below the top of the concrete protection layer, the two thermocouples are installed 5cm and 9cm below the surface.
All five temperature sensors and another temperature sensor installed on the surface of the bridge deck are connected to a data acquisition instrument for recording the temperature of each hour.
The two relative humidity sensors are also installed inside the protective layer. Unfortunately, the test result data shows that the relative humidity inside the protective layer is 100%, which shows that the sensor is filled with concentrated water vapor. There is no doubt that the data of the relative humidity sensor is invalid and wrong. Finally, two sets of corrosion monitors were installed in the protective layer. One set is installed in position two and one set is installed in position three. Each monitor consists of a reference electrode and a corrosion sensor. These corrosion sensors are the main targets of this chapter. The results of temperature testing have been described in other papers (Gudmundsson, 2003).
The concrete protective layer was cast in the late summer of 1999, and the corrosion monitoring started on December 23, 1999. The maximum tidal range of the bridge is 3m.
Results of temperature test:
Figure 2 shows the temperature data collected from December 23, 1999 to February 26, 2001. The data was lost in the summer of 2000 due to a malfunction of the collector. During this period, 66 freezing periods were observed in the collected data. The average temperature was –1.82 ° C and the duration was 36.5 hours.
Corrosion sensor:
The potential test can be used to monitor the start time of concrete structure corrosion. Corrosion sensors are implanted in concrete and are generally installed in place during concrete pouring. This set of corrosion sensors consists of four anodes and a cathode at different heights, and usually a reference electrode is also installed. At present, there are several such corrosion sensors.
Figure 3 CORROWATCH corrosion sensor and reference electrode
Two sets of CORROWATCH corrosion monitors are installed at two different locations in the concrete protective layer, which are the upper and lower parts of the intertidal zone. Each set of sensors consists of a reference electrode and a corrosion sensor. The corrosion sensor has four sacrificial anodes at different heights. As shown in Fig. 3, the positions of the four anodes are respectively 35 (1), 40 (2), 45 (3), and 50 (4) mm below the surface, and the reference electrode is installed 60 mm below the surface.
From the increase in current value, it can be judged that the sharp chloride has developed from a passive state to a non-passive state, that is, corrosion has begun. The data should be read several times in the first year, and the monitoring data can be read once or twice a year. Figure 4 depicts the situation where the sacrificial anode closest to the surface encounters chloride initiation corrosion and is divided into two stages.
Figure 4 shows the beginning of the first sacrificial anode corrosion
When the chloride layer reaches the anode 1, or when the chloride ion content of the concrete around the anode 1 reaches a dangerous level, the anode will begin to corrode. The results show that the test data (potential value) on this anode will change (see Figure 5). The readings of the other anodes will remain at their original values ​​until the surrounding chloride ion content reaches a dangerous level.
Figure 5 Anode 1 reading display in two stages
By installing the CORROWATHC corrosion monitor or other similar monitors, the tendency of any concrete structure to be chlorinated can be tested, and the time to start corrosion can be predicted very accurately.
CORROWATCH corrosion monitor in the concrete cover:
If the corrosion monitors are working well after a few years, and the readings have not changed, it means that chloride has not penetrated into the concrete. Refer to Table 2. Because the performance of concrete is very good, it is not surprising.
date
35mm from the surface
40mm from the surface
45mm from the surface
50 mm from the surface
9-12-1999
11-5-2000
18 mV
23-10-2000
8,5 mV
3,4 mV
11 mV
18 mV
26-01-2001
8,0 mV
4 mV
11,2 mV
20,3 mV
Table 2. CORROWATCH corrosion monitor readings
After 4 years, the concrete core was sampled and tested for chloride ion content. The core sample was ground by a lathe and the powder was dissolved in the HNO3 acid solution. The chloride ion content was measured by titration. The test analysis results are shown in Figure 6.
Figure 6 Chloride content in the structure after 4 years
The analysis data only shows the depth range from 0-12mm, but the first anode is located 35mm below the surface of the structure. In addition, the reinforcement is located 60 mm below the surface of the protective layer. In order to assess the chlorine content at the first anode, a 12-month-old chloride ion diffusivity test is required through surface evaluation current studies. The chloride ion diffusion rate test uses the CTH-test test method. The calculated section is also shown in Figure 6. At 35mm, the chloride content is about 0.03% of the concrete weight. At this time, the steel bar will not rust until the chloride content is greater than 0.1% of the weight of the concrete. In the next 4-5 years, the first anode did not start to corrode as expected, and the chlorine content in the concrete is still very low, insufficient To cause corrosion. Therefore, it is not surprising that the data from the CORROWATCH sensor has not changed significantly. Refer to Figure 2.
The actual problem of using CORROWATCH sensor to obtain test data has appeared, and the signal given by the corrosion sensor needs to be considered together with time. In response to this problem, further tests were carried out in the laboratory, and a corrosion sensor was immersed in saturated Ca (OH) 2. This is to prevent corrosion and simulate the concrete environment. The sensor is connected to the data acquisition instrument and records the voltage value every 15 minutes. The data is shown in Figure 7.
Figure 7 The sensor is placed in a saturated Ca (OH) 2 solution, connected to a data acquisition instrument, and the voltage value is recorded every 15 minutes
In the first three days, the reading has basically stabilized at 10mV. On the third day, the monitoring was stopped and the circuit was disconnected for 6 days. On the ninth day, the reading was re-monitored. The first reading was about 140mV, but the subsequent reading showed that the voltage value dropped rapidly. After one day, the reading returned to 10mV. This is because the ambient temperature fluctuations in the laboratory room caused the readings to fluctuate.
The experience of the test base shows that at least 30 minutes are required to obtain an ideal reading from each anode. The two corrosion sensors have 8 and 8 anodes, a total of 8 readings, which takes a total of 4 hours, which is very Time-wasting is very bad for remote on-site monitoring. The preferred method is to connect these sensors to a data acquisition instrument and collect voltage values ​​at regular intervals.
For a concrete structure with good quality and a low chloride ion permeability coefficient, the corrosion sensor must be installed close to the surface rather than deep inside the structure. In this way, after several years of data collection and analysis, it is possible to assess the time when the concrete reinforcement begins to corrode.
in conclusion:
1. After three years of data collection on CorroWatch, we can see that the first sacrificial anode has no slight corrosion.
2. The corrosion sensor still works as usual after three years, and further tells us that only a very low chloride ion diffusion rate has occurred.
3. Chloride ion diffusion rate data shows that the fastest corrosion may occur after about 8 years.
4. On-site monitoring is extremely time-consuming, so the best way to collect CorroWatch data is to use a data collector.
5. In order to quickly predict the start of corrosion of concrete buildings and locate the location of the corrosion probe, the corrosion probe must be as close to the surface as possible in the research test.
Acknowledgement:
This research was supported by the National Highway Administration's research grants, thanks. Thanks also to Einar Haflidason and Rognvaldur Gunnarss from the National Highway Administration, and Hakon Olafsson and Dr. Olafur Wallevik from the Icelandic Architecture Research Institute for their comments and suggestions during the study.
bibliography:
1. Gudmundsson, G., (1997) Deterioration of concrete bridge piers in Iceland. In: Mechanisms of chemical degradion of cement-based systems. Eds .: KL Scrivener and JF Young. E & FN Spoon, London, 201-208.
2.Andrade, C., (2003) Determination of the chloride threshold in concrete. In: eds .: Cigana, R., Andrade, C., Nürnberger, U., Polder, R., Weydert, R., Seitz, E., Corrosion of steel in reinforced concrete structures, COST Action 521, final report, EUR 20599, 101-111.
3. Gudmundsson, G., (2003) Modified slab tests for testing frost resistance of concrete with regards to both scaling and internal cracking (in Icelandic). IBRI-internal report.
4. Gudmundsson, G., (2003) On site monitoring of high performance concrete during freeze / thaw cycles and relationship to standardized testing. 15. Internationale Baustofftagung, Ibausil- Weimar, 2-0051-2-0062.
5. Gudmundsson, G., Wallevik, O., (1999) Concrete in an aggressive environment – ​​over-crete in Borgarfjordur (in Icelandic). Rb-99-04, 55 pages.
6.Gudmundsson, G., Wallevik, O., (2002) Concrete in an aggressive environment. Proceedings of the Minneapolis Workshop on Frost Damage in Concrete, eds .: Janssen., DJ, Setzer, MJ, Snyder, MB, 87- 102.
7. Gudmundsson, G., Antonsdottir, A. (2003) Chloride diffusion in and out of concrete made with different types of binders. Rilem Pro publication.
8. Gudmundsson, G., Wallevik, O., (2003) Durability of self compacting concrete from standardized test methods. A supplementary paper presented at the 3rd international symposium on Self Compacting Concrete in Reykjavik Iceland, in August 2003.
9. Sørensen, H., Poulsen, E., Mejlbro, L., Frederiksen, JM, (2002) Deterministic model for monitoring of concrete structures using corrosion sensors. In: Cost 521 Workshop, final reports, ed .: Weydert, R ., 97-101.
10. Tang, L., (1996) Electrically accelerated methods for determining chloride diffusivity in concrete – current development. MCR, 48, 173-179.
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