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发 布 者:dt1718  添 加 时 间:2012/10/19  点 击 数:3012 

Thomas Frølund, COWI A/S, Parallelvej 2, DK-2800 Lyngby, TDF@cowi.dk  
Oskar Klinghoffer, FORCE Technology, Park Allé 345, DK-2605 Broendby,  
KEYWORDS: Half cell potential (HCP), Corrosion rate (Icorr), Galvanostatic Pulse Measurements (GPM), concrete structures 
Since 1978 Half Cell Potential (HCP) mapping [1] has been used for detecting corroding areas on concrete structures in Denmark. In the beginning (after balcony gangway has totally collapsed) this method was mainly used on carbonated structures and balconies exposed to de-icing salts. Later the method was used for all kind of structures and the experiences were discussed in Newsletters published by the Danish Corrosion Centre [2]. It was early recognised that the interpretation of the HCP results were difficult or misleading in wet and semi-wet structures where lack of oxygen as well as corrosion would lead to potential gradients.  
A typical potential map of a highway bridge pillar is shown in fig.1. The pillar is exposed to de-icing salts splashed from the passing cars up to a level of 2 meters, but also has a high humidity at the ground level caused by capillary suction. The water filled pore system in the concrete makes the potential drop because the oxygen necessary to maintain the passive film will not be able to diffuse into the concrete fast enough.    
Fig.1. Typical potential map of a high way concrete pillar exposed to de-icing  
The ongoing corrosion process is described by the chemical reaction: 
The Fe++ forms at the high pH in concrete a complex protective film with oxygen. This film can be broken down by the chloride ions from de-icing salts or by neutralising of the high pH e.g. by carbonation.  
The interpretation problem is that the potential will drop either because the loss of passive film due to lack of oxygen (the corrosion process will not be able to proceed without oxygen) or because chlorides break down the passive film and start corrosion. 
In 1994 it was decided to develop an equipment for concrete structures based on well know techniques for determination of corrosion rates to be able to distinguish between active corrosion and the lack of oxygen situation. 
The results presented in this paper are all based on Galvanostatic Pulse Measurements (GPM). This polarisation technique makes it possible within a short time (typically 10 seconds) to calculate the corrosion rate [3,4]. The equipment gives both the corrosion rate and the half-cell potential as well as the resistance between the hand-held electrode placed on the examined concrete surface and the reinforcement. Four different examples from on-site investigations are described below, one where there is a good correlation between the HCP method and the GPM method and 3 where the HCP measurements are misleading. 
2.1 Example no. 1   
2.1 案例一
Two parallel bridges were built in 1965-67 in the Copenhagen area, where the highway crosses over a railway line, a parking lot and two minor roads. The eastern bridge was rehabilitated extensively at a very high price in 1978, after which the western bridge have only received much less rehabilitation, but substantial inspection, test- loadings, probabilistic assessment etc., which essentially have kept the bridge in function at a much less cost that the eastern part. 
Initial inspections, core investigations and chloride profiling in 1999 (fig. 2) pointed out column No.S303 to be suitable for corrosion rate measurements. 
Fig 2. Chloride profiles at level 0.3m and level 1 m 
Electrical continuity in the reinforcement was checked and a permanent connection was welded to the reinforcement. The vertical reinforcement (Ø35 mm) is typically in 60 mm depth and the horizontal (Ø14 mm) in 40 mm depth. Already in 1999 the chloride content in level 0.3 m is so high that active corrosion can be expected. In September 2000 and in April 
2001 corrosion rates were determined together with the half- cell potentials and resistance measurements [5] (fig 3). 
设备其中一端固定连接在钢筋上,并施加连续的脉冲电流。垂直的钢筋(Ø35 mm)在60mm深处,水平钢筋(Ø14mm)在40mm深处。在1999年,0.3米深处的氯离子含量已经非常高了,应该会存在积极的腐蚀情况。2000年的9月份和2001年的4月份,利用半电池电位和电阻率测试方法确定了腐蚀速率。(图3)
In this case there is a rather good correlation between resistance, half-cell potential and corrosion rate mapping. 
To verify the corrosion state a break-up was made at the position 90 degrees south near ground level, see fig. 4. 
Fig. 4 Corroding reinforcement.  Cross section loss: 1-2 mm 
As the constructions have been examined close during the last 20 years it is possible to make a good estimate of the initiation of corrosion. Calculation of the average corrosion rate from the cross section loss of app. 2 mm and assuming the corrosion was initiated after app. 10 years gives an average corrosion rate of 9µA/cm22, which is with in the range of corrosion rates determined at this position by the GPM. The very low half-cell potentials agree with the high corrosion activity. 
2.2 Example 2 
2.2 案例2
The next two examples are from a bridge foundation and a bridge deck in Greenland. The foundation was investigated in the tidal zone as shown in fig. 5 [6]. 
Fig. 5. The investigated area and the location of the chloride profile. 
  图5. 调研区域与氯化物剖面位置
The chloride concentration in the depth of the reinforcement is in the range between 0,3% and 0,7% of the concrete weight. As indicated by the half-cell potential measurements corrosion should therefore be expected. However the measured corrosion rates are low and the verification by visual inspection (fig. 7) shows no damage to the reinforcement. 
Fig. 6 HCP and Icorr of bridge foundation  桥墩的半电池电位与腐蚀电流
Fig. 7 Photo of break-up  
图7 打开结构后的图片
2.3 Example 3
2.3 案例3  
The bridge deck from the same bridge in Greenland showed very different results as shown on fig. 8 and fig. 9. 
Fig.8. The investigated area and the location of the chloride profile. 
The typical dept of the reinforcement is here minimum 40-50 mm and the chloride concentration in this depth is near 0, 3% of the concrete weight. At this high chloride concentrations the half-cell potentials are expected to be low but the most negative values measured are all above -100 mV vs. Ag/AgCl (fig. 9). 
当钢筋处于保护层内至少40-50mm深处,而且氯离子含量接近0.3%时,半电池电位应该是比较低的,但是用Ag/AgCl参比电极测出来的电位值都在-100 mV以上(图9)。
Fig.9. The read circle indicates the location of the chloride profile and the break-up shown at fig. 11. 
  图9.圆形显示了氯化物轮廓面    图11是打开结构的图像
The corrosion rate map fig.10 shows a completely different picture and indicates active corrosion at several locations. 
Fig. 10. Corrosion rate of the bridge deck. The read circle indicates the location of the   
chloride profile and the break-up shown at fig. 11
图10 桥面板的腐蚀速率,圆形指出了氯化物轮廓面 图11则是打开结构后的图片
Fig.11. The first picture shows the corroded reinforcement and some water from cooling 
the diamond-cutting blade. The second picture shows the damage and the mortar repairs. 
General comments to examples 2 and 3 
This bridge is located in a very cold environment. During the measurements described above the temperature was 15 °C at midday and this explains the rather high corrosion rates at the bridge deck. Further there were some damages to the concrete surface due to the traffic directly on the concrete surface and a lot of mortar repairs. 
这座桥处于非常寒冷的环境里,测量过程中,正午的温度是15 °C。在桥面板上会有更高的腐蚀速率,由于交通荷载直接作用在混凝土表面上,所以混凝土表面受到有更多的损害和存在很多的砂浆修补处。
2.4 Example 4 
2.4 案例4
In this example of swimming pool wall the conditions for performing corrosion rate measurements were not ideal. Tiles covered the inside of the swimming pool but preliminary performed GalvaPulse measurements showed that the joint filler was porous. Due to this fact it was possible to conduct the corrosion rate measurements by means of GalvaPulse equipment. The outside reinforcement was corroding at the casting joint between the pool floor and the pool walls and it was found necessary to investigate the inside reinforcement although no rust stains were visible. 
The results were projected to a plane plot where the cast joint is in the centre of the plot fig. 12. 
图12 沿着池底经过结合处再到池壁的腐蚀速率图
Even at these un-ideal measuring conditions the GalvaPulse pointed out the reinforcement corrosion points which was confirmed by visual inspections in breakups fig. 13. 
1.  Two techniques for evaluation of reinforcement corrosion, half-cell potential (HCP) and galvanostatic pulse measurements (GPM) are presented and discussed. 
2.  The evaluation of corrosion by means of the traditional half-cell potential technique using the existing standards may lead to mistakes in cases where the concrete is water saturated, carbonated and also exposed to the very low temperature. 
3.  Complimentary measurements by means of galvanostatic pulse technique determining the corrosion rate contribute to the unambiguous evaluation of reinforcement corrosion also under conditions where the results obtained by the HCP technique could be misleading. 
4.  Four examples from on-site measurements are presented. Three of them show the need of using corrosion rate measurements together with half-cell potential for reliable evaluation of the actual corrosion state. 
5.  Passive areas are defined by galvanostatic pulse measurements as areas where the potential curve on the instruments computer screen has not reached a steady-state after pulsing over 5-10 seconds. In areas with active corrosion, areas where the potential curve exhibit a steady-state potential after 5-10 seconds, the corrosion current is measured as accurate as it can be expected from an on-site measurement taking into account the variation of the area of the reinforcement polarized over, the actual corroding area of the reinforcement and the inherent variations in moisture condition of the concrete and the temperature.
6.  It is not possible to estimate the actual loss of cross sectional area of the reinforcement from a single GPM measurement. If multiple GPM measurements are taken over a period of time, an average value can be estimated. Alternatively the reinforcement must be exposed in the most corrosion active areas as done in these 4 examples. 
[1] American Society of Testing and Materials: “Standard Test Method for Half-Cell Potentials of uncoated Reinforcing Steel in Concrete” ASTM C876, 1987. 
[2] H. Arup: "Potential Mapping of Reinforced Concrete Structures", The Danish Corrosion Centre Report, January 1984 
[3] B. Elsener, O. Klinghoffer, T. Frølund, E. Rislund, Y. Schiegg and H. Böhni: "Assessment of Reinforcement Corrosion by Galvanostatic Pulse Technique, Proc. Int. Conf. on Repair of Concrete Strictures, Svolvaer, Norway, pp 391 - 400,1997.  
[4] J. Mietz and B. Isecke: "Electrochemical Potential Monitoring on Reinforced Concrete Structures using Anodic Pulse Technique", in "Non destructive Testing in Civil Engineering" ed. Bungey, H., The British Institute of NDT, 2, 567,1993. 
[5] T. Frølund.  F.M. Jensen and R. Bässler: "Determination of corrosion rate by means of the galvanostatic pulse technique”, First International Conference on Bridge Maintenance, Safety and Management, IABMAS 2002, Barcelona 2002.  
[6] H. E. Sørensen and T. Frølund: "Monitoring of Reinforcement Corrosion in Marine Concrete Structures by the Galvanostatic Pulse Method", Proceedings of International Conference on Concrete in Marine Environments, Hanoi-Vietnam, October 2002.


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