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Development and prediction strategy of steel corrosion rate in concrete under natural climate
Construction and Building Materials 44 (2013) 287–292
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Development and prediction strategy of steel corrosion rate in concrete under natural climate Jian-hua Jiang a,⇑, Ying-shu Yuan b a b
College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, PR China School of Mechanics & Civil Engineering, China University of Mining & Technology, Xuzhou 221116, PR China
h i g h l i g h t s Steel corrosion rate varies with time and randomly fluctuates in natural climate. Temperature is primary climatic factor that affects steel corrosion rate in concrete. Pore water saturation is a key factor between sheltered and unsheltered conditions. Prediction strategy of steel corrosion rate in concrete is proposed.
a r t i c l e
i n f o
Article history: Received 17 December 2012 Received in revised form 3 March 2013 Accepted 9 March 2013 Available online 9 April 2013 Keywords: Natural climate Concrete Temperature Pore water saturation Corrosion rate Prediction strategy
a b s t r a c t A long-term test on the steel corrosion in concrete was conducted under the sheltered conditions of a natural climate environment. A synchronous test under the sheltered and unsheltered conditions was performed to determine the microenvironment response and steel corrosion in concrete. The effect of environmental climate on the steel corrosion rate in concrete was further determined. The results show that the steel corrosion rate in concrete fluctuates and is time-dependent under natural climatic conditions. The changes of environmental conditions in the concrete microenvironment are less than those in a natural climate environment, particularly relative humidity. The steel corrosion rate in concrete is directly affected by the microenvironment of concrete, which depends on the random fluctuations of natural climate. Temperature is the primary climatic factor that affects the steel corrosion rate in concrete under an atmospheric environment. The pore water saturation of concrete is also a key factor that causes the differences between the steel corrosion rates in concrete under sheltered and unsheltered conditions. A prediction strategy for the steel corrosion rate in concrete under natural climate is proposed based on the effects of environment on the corrosion rate as well as on the responses of the concrete microenvironment. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction A natural climate is the actual environment of a reinforced concrete (RC) structure. The environment is random, unique, and complex because of the effects of the geographic location as well as of meteorological factors, among others. For RC structures located in various regions, their service lives are usually different because of the variations in the natural climate even though these structures are identical. Therefore, studies on the durability of RC structures under a natural climate environment are of high significance. In contrast to the constant artificial climate environment, the natural climate environment shows evident stochastic fluctuations, which cause changes in the microenvironment of concrete ⇑ Corresponding author. Tel.: +86 25 83786633. E-mail address: [email protected] (J.-h. Jiang). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.03.033
[1,2]. Therefore, the effects of the environmental condition must be considered when evaluating the durability of an RC structure. Song et al. [3] presented the equivalent diffusion coefficient model of carbon dioxide in cracked concrete based on the effects of pore water saturation and concrete temperature. By considering the effects of the concrete temperature, relative humidity, and of other factors on the chloride diffusion coefficient, Shi and Luo [4] established a calculation model of the chloride that penetrates the concrete based on the finite difference. Lopez and Gonzalez [5] and Enevoldsen et al. [6] analyzed the effects of the pore water saturation, resistivity, and internal relative humidity of concrete on the steel corrosion rate and suggested that a critical pore water saturation or relative humidity initiate rebar corrosion. Song and Liu [7] established the quantitative relationship between the anode– cathode area ratio of the corroded rebar and the pore water saturation in carbonated concrete. Liu and Weyers [8] developed
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J.-h. Jiang, Y.-s. Yuan / Construction and Building Materials 44 (2013) 287–292
a prediction model for the time-varying steel corrosion rate based on the effects of the chloride-ion concentration, temperature, and resistance of concrete using a long-term exposure test under a natural climate environment. The regularity and mechanism of the effects of environmental climate on deterioration should be determined first to establish an accurate prediction model for the degradation of the RC structure. The response relationships of the temperature and the relative humidity between the natural climate environment and the microenvironment of concrete should be further clarified. This paper focuses on the development of the steel corrosion rate in concrete under a natural climate environment. The responses of the temperature and humidity of the concrete microenvironment and the natural climate environment are clarified. The effects and mechanism of the environment on steel corrosion in concrete are further analyzed. This study is significant for the accurate prediction and assessment of the durability and service life of RC structures under a natural climate.
2. Experiments on the steel corrosion rate in concrete under natural climate 2.1. Specimen design The water–cement ratio (w/c) of the specimen concrete is 0.54, and the concrete mixture proportion is presented in Table 1. The cement was ordinary Portland cement P.O 42.5. River sand was used as the fine aggregate with a fineness modulus of 2.7. Gravel (5–15 mm, primarily consisting of limestone and trachyte) was used as the coarse aggregate. Ordinary tap water was used for mixing. The compressive strength of concrete is 25.4 MPa at 28 days. The specimens were divided into two types. Type I, with specimen dimensions of 63 mm 150 mm 200 mm, was used for the long-term corrosion test under sheltered conditions in a natural climate, and the corresponding serial numbers of the specimens are LCT-1, LCT-2, and LCT-3. Type II (shown in Fig. 1), with specimen dimensions of 100 mm 150 mm 250 mm, was used to determine the effect of the microenvironment of concrete on steel corrosion, and the serial numbers of the specimens are NCE-S1, NCE-S2, NCE-E1 and NCE-E2. A hole was preformed in the Type II specimen using a plastic pipe during concrete pouring to embed the temperature–humidity probe. The diameter of the hole is 20 mm, and the distance from the upper concrete surface to the preformed hole is 25 mm. A hot-rolled ribbed bar with yield strength of 335 MPa and a stainless steel bar were arranged in each specimen to determine the steel corrosion rate. The diameters of bars are 14 mm, and the concrete cover thickness for Types I and II is 15 and 25 mm, respectively. The chloride salt (sodium chloride), which accounted for 5% of the cement, was admixed into the concrete to simulate the chloride ion-induced steel corrosion.
2.2. Test methods The specimens were cured for 28 days under standard conditions after pouring and demolding. The initial relative humidity in the specimens for Type II was then adjusted to approximately 80% corresponding to room temperature, in order to determine the relative humidity response under a natural climate environment. Afterward, the concrete surfaces (except the concrete cover) were sealed with a plastic film and paraffin. The temperature–humidity probe (Fig. 2) was then embedded into the preformed hole using a plastic pipe and sealant. The tests were conducted in an outdoor exposure test site. The NCE-E1 and NCE-E2 specimens were subjected to unsheltered conditions, whereas the others were exposed to sheltered conditions. The corrosion rate of the steel bar in concrete was periodically measured using an electrochemical test system as shown in Fig. 3. The instantaneous corrosion rate of metallic materials can be measured continuously using the potentiodynamic polarization scan and the nonlinear least squares fitting of polarization curve. During the measurement, the saturated CuSO4 electrode acts as reference electrode, the stainless steel bar acts as auxiliary electrode, and the ribbed bar acts as working electrode. Temperature and relative humidity values were simultaneously recorded using the probe.
Table 1 Mixture proportion of concrete (kg/m3). w/c
Water
Cement
Sand
Coarse aggregate
0.54
212
393
630
1165
3. Experimental results and discussion The measurement parameters include the corrosion current density of the steel bars as well as the corresponding internal temperature and relative humidity in the concrete. 3.1. Corrosion rate of steel bars According to the fundamental theory of metal corrosion, the corrosion current density can be used as the representative value of the corrosion rate of steel bars. The measured corrosion current density of the steel bars in Types I and II is shown in Fig. 4a and b, respectively. The test period of Type I is more than three years. As can be seen from Fig. 4a the steel corrosion rate in concrete exhibits an evident time variation under the natural climate environment. The specific details are as follows: (1) The corrosion rate shows a general time-varying trend that can be divided into a descending phase, a steady phase and an ascending phase. This trend is due to the development of a rust layer [9]. However, the variation of the steel corrosion rate in the steady phase is cyclical for one year due to the effect of environmental climate. (2) The corrosion rate shows the detailed fluctuation caused by the changes in the microenvironment of concrete. The timevarying steel corrosion rate in concrete under the natural climate environment differs from that under a constant artificial climate environment. This difference is primarily attributed to the differences in environmental conditions. The differences in the steel corrosion rates in concrete under the two types of conditions are shown in Fig. 4b. The corrosion rates under unsheltered conditions are significantly higher than those under sheltered conditions. Given the same specimen fabrication parameters, the differences in the steel corrosion rates are mainly due to the differences in the micro-environmental conditions of concrete. 3.2. Environmental parameters 3.2.1. Temperature The changes of temperature in the natural climate environment as well as the microenvironment of concrete in the NCE-S1 and NCE-E2 specimens are shown in Fig. 5. The test results in Fig. 5 show that (1) the temperature in concrete fluctuates because of the changes in the ambient temperature, and the fluctuations in the internal temperature of concrete are consistent with those of ambient temperature; and (2) the fluctuations of internal temperature in concrete are smaller than those of the ambient temperature under sheltered conditions. By contrast, the fluctuations of internal temperature are slightly greater under unsheltered conditions due to the effect of sunshine. 3.2.2. Relative humidity The changes of relative humidity in the natural climate environment as well as the microenvironment of concrete in the NCE-S1 and NCE-E2 specimens are shown in Fig. 6. It is indicated that, (1) the relative humidity in concrete fluctuates because of the drastic changes in the ambient humidity; (2) the fluctuations in the internal relative humidity in concrete are significantly lower than those of ambient humidity; and (3) the relative humidity in concrete under sheltered conditions remains at about 80%, whereas that under unsheltered conditions is approximately equal to 100% due to the effect of rainfall.
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Corrosion current density ( µA·cm-2)
Fig. 1. Specimen of steel corrosion in concrete under natural environment (unit: mm).
4.0 LCT-1 LCT-2 LCT-3
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
100 200 300 400 500 600 700 800 900 1000 1100 1200
Measuring time (days)
(a) Results in Type I
Register
Fig. 2. Rotronic temperature–humidity sensor.
Copper-copper sulfate reference electrode Steel bar
Computer
Stainless bar
Corrosion current density (µA·cm-2)
Probe
5.0 4.0
NCE-E1
NCE-E2
NCE-S1
NCE-S2
3.0 2.0 1.0 0.0
0
50
100
150
200
250
300
350
400
Measuring time (days)
(b) Results in Type II
Specimen
Fig. 4. Data for the steel corrosion rate in concrete.
Electrochemical analysis system Fig. 3. Setup for the measurement of the steel corrosion rate.
Thus, the response of temperature is significantly faster than that of relative humidity between the concrete microenvironment and the natural environment. This phenomenon is attributed to the heat transfer and mass transfer involved in the temperature and relative humidity responses, respectively, in which mass transfer of concrete is slower than heat transfer. 4. Analysis of the effects of environmental climate on the steel corrosion rate According to the time variation of the steel corrosion rate in concrete under a constant artificial climate environment, the steel corrosion remains stable before corrosion cracking of concrete cover [10]. However, experimental results indicate that the development of the steel corrosion rate does not exhibit a stable state after the decline; instead, the corrosion rate increases with the fluctuations under the natural climate environment. The reasons are described below.
4.1. Effect of temperature in concrete The changes in the temperature and steel corrosion rate in concrete under the natural climate environment are shown in Fig. 7. As can be seen from Fig. 7 the changes of the steel corrosion rate are relatively consistent with those of the temperature. The corrosion rate of the rebar is affected by the development of a rust layer as well as by the fluctuation of the temperature in concrete. Therefore, the steel corrosion rate fluctuates in all stages without exhibiting a detectable stable state. Steel corrosion in concrete is an electrochemical reaction process. According to the basic law of chemical kinetics, the rate equation of the chemical reaction is as follows [11]:
t ¼ kcaM1 cbN1
ð1Þ
where cM and cN are the concentrations of the reactants; a1 and b1 are the orders of the reaction; k is the reaction rate coefficient. The relationship between k and the temperature can be expressed as the Arrhenius equation:
k ¼ k0 expðEa =RTÞ
ð2Þ
25 20
2.0
15 1.5
10
8-Jun-10
24-Apr-10
14-May-10
9-Mar-10
31-Mar-10
11-Feb-10
(a) NCE-S1
1-Jan-10
-5
23-Jan-10
0
0.0
14-Dec-09
5
0.5
20-Oct-09
1.0
17-Nov-09
27-May-10
27-Apr-10
28-Mar-10
27-Jan-10
26-Feb-10
28-Dec-09
28-Nov-09
29-Oct-09
29-Sep-09
30-Aug-09
1-Jul-09
31-Jul-09
-5
1-Jun-09
5
30
Temperature
2.5
22-Sep-09
15
3.0
Corrosion rate
Temperature (°C)
25
35
3.5
27-Aug-09
In concrete microenvironment
3-Jul-09
35
1-Aug-09
In natural environment
1-Jun-09
45
Corrosion current density (µA·cm-2)
J.-h. Jiang, Y.-s. Yuan / Construction and Building Materials 44 (2013) 287–292
Temperature (°C)
290
Fig. 5. Temperatures in the concrete microenvironment and the natural environment.
8-Jun-10
24-Apr-10
14-May-10
31-Mar-10
9-Mar-10
11-Feb-10
Eqs. (1) and (2) show that the effect of temperature on the steel corrosion rate in concrete can be attributed to the temperature effect on k, which is in accordance with the exponential function.
60 40
27-May-10
27-Apr-10
26-Feb-10
28-Mar-10
27-Jan-10
28-Dec-09
28-Nov-09
29-Oct-09
29-Sep-09
30-Aug-09
1-Jul-09
0
31-Jul-09
20
(a) NCE-S1 120
In natural environment
In concrete microenvironment
100 80 60 40 20
4.2. Effect of pore water saturation in concrete The fluctuation in the relative humidity of the concrete microenvironment, caused by the natural climate, is actually the change in the moisture content or pore water saturation of concrete. Given the temperature variations, the relative humidity cannot reflect the moisture content of concrete alone. Therefore, pore water saturation is used to analyze the effect of moisture content of concrete on the steel corrosion. The pore water saturation of concrete is calculated according to the temperature and relative humidity in the concrete microenvironment. Based on a series of test results, the relationship between the pore water saturation and the relative humidity can be modeled as follows:
27-May-10
27-Apr-10
28-Mar-10
26-Feb-10
27-Jan-10
28-Dec-09
28-Nov-09
29-Oct-09
29-Sep-09
30-Aug-09
31-Jul-09
1-Jul-09
0 1-Jun-09
Relative humidity (%)
Fig. 7. Steel corrosion rate and the temperature in concrete.
In concrete microenvironment
80
1-Jun-09
Relative humidity (%)
In natural environment
23-Jan-10
(b) NCE-E2
120 100
1-Jan-10
0 -5
14-Dec-09
0.5 0.0
17-Nov-09
15 10 5
20-Oct-09
(b) NCE-E2
2.0 1.5 1.0
22-Sep-09
27-May-10
27-Apr-10
28-Mar-10
26-Feb-10
27-Jan-10
28-Dec-09
28-Nov-09
29-Oct-09
29-Sep-09
30-Aug-09
31-Jul-09
1-Jul-09
-5
1-Jun-09
5
35 30 25 20
Corrosion rate Temperature
27-Aug-09
15
4.0 3.5 3.0 2.5
1-Aug-09
25
40
4.5
3-Jul-09
In concrete microenvironment
1-Jun-09
35
Corrosion current density (µA·cm-2)
In natural environment
Temperature (°C)
Temperature (°C)
(a) NCE-S1 45
(b) NCE-E2 Fig. 6. Relative humidity in the concrete microenvironment and the natural environment.
where k0 is the pre-exponential factor or frequency factor; R is the gas constant; T is the absolute temperature; Ea is the activation energy. Ea and k0 can be considered as constants within a certain temperature range.
S¼
k1 H ð1 k2 HÞð1 þ k3 HÞ
k1 ¼ ð2:9142w=c 2:5849Þ 103 t 0:1994w=c þ 0:1647
ð3Þ ð3aÞ
k2 ¼ ð2:907w=c- 1:1446 103 t þ 1:5594 105 t3 þ 4:4465Þ 103 k3 ¼ ð2:158w=c 3:2774Þ 103 t 0:3272w=c þ 0:3154 ð10 C 6 t 6 45 C; 0:48 6 w=c 6 0:62; 0 6 H < 100Þ
ð3bÞ ð3cÞ
291
Corrosion rate
Pore water saturation 0.8
3.0
0.6 2.0 0.4 1.0
8-Jun-10
24-Apr-10
14-May-10
9-Mar-10
31-Mar-10
11-Feb-10
1-Jan-10
23-Jan-10
14-Dec-09
20-Oct-09
17-Nov-09
22-Sep-09
27-Aug-09
3-Jul-09
1-Aug-09
0.2
1-Jun-09
0.0
Meteorological Data in Natural Environment
1.0
4.0
Pore water saturation
Corrosion current density (µA·cm-2)
J.-h. Jiang, Y.-s. Yuan / Construction and Building Materials 44 (2013) 287–292
Prediction model of T /H Response
Predicting of Microenvironment
0.0
Response Spectra of Microenvironment
(a) NCE-S1 5.0
1.0
4.0
0.8
Temperature (T) Relative Humidity (H )
T H T Pore Water Saturation (S)
Prediction Model of Corrosion Rate Corrosion Rate
Corrosion rate 3.0
0.6
Pore water saturation
8-Jun-10
24-Apr-10
14-May-10
31-Mar-10
9-Mar-10
11-Feb-10
23-Jan-10
1-Jan-10
14-Dec-09
17-Nov-09
20-Oct-09
0.0
22-Sep-09
0.0
27-Aug-09
0.2
1-Aug-09
1.0
3-Jul-09
0.4
1-Jun-09
2.0
Pore water saturation
Corrosion current density (µA·cm-2)
Action Spectra of Natural Climate
(b) NCE-E2 Fig. 8. Steel corrosion rate and the pore water saturation in concrete.
where S is the pore water saturation; H is the relative humidity in concrete, %; w/c is the water–cement ratio of concrete; t is the temperature of concrete, °C. However, the detailed study on the pore water saturation of concrete will be described in another paper. The changes in the pore water saturation and the steel corrosion rate in concrete under natural climate are shown in Fig. 8. The effect of the pore water saturation of concrete on the steel corrosion rate is unclear. In fact, the steel corrosion rate should increase when the pore water saturation increases, because a reduction in the resistivity of concrete can strengthen the path of ions between the cathode and anode during steel corrosion. However, the short supply of oxygen in concrete due to water saturation can reduce the steel corrosion rate. In the test, the pore water saturation of concrete did not inhibit the flow of the oxygen supply needed for steel corrosion. Thus, Fig. 8 does not show the effect of pore water saturation but instead demonstrates the comprehensive effects of pore water saturation, temperature, and rust layer on the steel corrosion rate in concrete. The effect of pore water saturation is concealed or counteracted because of the greater effects of temperature and of the rust layer. 5. Prediction strategy for the steel corrosion rate The basic prediction strategy for the steel corrosion rate in concrete under a natural climate environment is presented in a flow chart (Fig. 9) according to the development of the corrosion rate and the effects of the concrete microenvironment. The detailed description of the prediction procedure is as follows: (1) The action spectra of the temperature and the relative humidity under a natural climate are first built based on meteorological data.
of Steel Bars Fig. 9. Prediction procedure of steel corrosion rate in concrete under natural climate.
(2) Based on the action spectra, the response values of the temperature and relative humidity in concrete are calculated using the prediction models of the microenvironment responses. (3) The response spectra of the temperature and of the pore water saturation in concrete are built based on the calculation results of the microenvironment responses. (4) The steel corrosion rate in concrete is predicted using the prediction model for the time-varying steel corrosion rate and based on the response spectra of the concrete microenvironment. 6. Conclusions (1) The corrosion rate of a steel bar in concrete varies with time and significantly fluctuates in a natural climate environment. Therefore, the time variation differs from that under a constant artificial climate environment because of the effects of the concrete microenvironment. (2) The temperature and pore water saturation in the concrete microenvironment significantly affect the steel corrosion rate. Temperature is the primary climatic factor that affects the steel corrosion rate in concrete under an atmospheric environment; their relationship is consistent with the exponential function. The pore water saturation of concrete is also a key factor of the differences between the steel corrosion rate in concrete under sheltered and unsheltered conditions. (3) A prediction strategy for the steel corrosion rate in concrete under a natural climate environment is proposed based on the effects of the environmental climate on the steel corrosion rate as well as on the responses of the concrete microenvironment. Acknowledgements The authors wish to acknowledge the financial supports from the Fundamental Research Funds for the Central Universities (No. 2012B02614). References [1] Andrade C, Sarrı´a J, Alonso C. Relative humidity in the interior of concrete exposed to natural and artificial weathering. Cem Concr Res 1999;29(8): 1249–59. [2] Yuan YS, Jiang JH. Prediction of temperature response in concrete in a natural climate environment. Constr Build Mater 2011;25(8):3159–67.
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[3] Song HW, Kwon SJ, Byun KJ, Park CK. Predicting carbonation in early-aged cracked concrete. Cem Concr Res 2006;36(5):979–89. [4] Shi YH, Luo G. A finite difference calculating model of chloride penetrating into concrete under various factors. Ind Construct 2004;34(5):7–10. [5] Lopez W, Gonzalez JA. Influence of the degree of pore saturation on the resistivity of concrete and the corrosion rate of steel reinforcement. Cem Concr Res 1993;23(2):368–76. [6] Enevoldsen JN, Hansson CM, Hope BB. The influence of internal relative humidity on the rate of corrosion of steel embedded in concrete and mortar. Cem Concr Res 1994;24(7):1373–82.
[7] Song XB, Liu XL. Experiment research on corrosion of reinforcement in concrete through cathode-to-anode area ratio. ACI Mater J 2000;97(2):148–55. [8] Liu T, Weyers RW. Modeling the dynamic corrosion process in chloride contaminated concrete structures. Cem Concr Res 1998;28(3):365–79. [9] Jiang JH, Yuan YS, Peng T. Time-variation full process mode of steel bar corrosion in concrete. J Chongq Univ 2011;34(10):103–8. [10] Yuan YS, Jiang JH, Peng T. Corrosion process of steel bar in concrete in full lifetime. ACI Mater J 2010;107(6):562–7. [11] Logan SR. Fundamentals of chemical kinetics. London: Longman; 1996.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Development and prediction strategy of steel corrosion rate in concrete under natural climate Jian-hua Jiang a,⇑, Ying-shu Yuan b a b
College of Civil and Transportation Engineering, Hohai University, Nanjing 210098, PR China School of Mechanics & Civil Engineering, China University of Mining & Technology, Xuzhou 221116, PR China
h i g h l i g h t s Steel corrosion rate varies with time and randomly fluctuates in natural climate. Temperature is primary climatic factor that affects steel corrosion rate in concrete. Pore water saturation is a key factor between sheltered and unsheltered conditions. Prediction strategy of steel corrosion rate in concrete is proposed.
a r t i c l e
i n f o
Article history: Received 17 December 2012 Received in revised form 3 March 2013 Accepted 9 March 2013 Available online 9 April 2013 Keywords: Natural climate Concrete Temperature Pore water saturation Corrosion rate Prediction strategy
a b s t r a c t A long-term test on the steel corrosion in concrete was conducted under the sheltered conditions of a natural climate environment. A synchronous test under the sheltered and unsheltered conditions was performed to determine the microenvironment response and steel corrosion in concrete. The effect of environmental climate on the steel corrosion rate in concrete was further determined. The results show that the steel corrosion rate in concrete fluctuates and is time-dependent under natural climatic conditions. The changes of environmental conditions in the concrete microenvironment are less than those in a natural climate environment, particularly relative humidity. The steel corrosion rate in concrete is directly affected by the microenvironment of concrete, which depends on the random fluctuations of natural climate. Temperature is the primary climatic factor that affects the steel corrosion rate in concrete under an atmospheric environment. The pore water saturation of concrete is also a key factor that causes the differences between the steel corrosion rates in concrete under sheltered and unsheltered conditions. A prediction strategy for the steel corrosion rate in concrete under natural climate is proposed based on the effects of environment on the corrosion rate as well as on the responses of the concrete microenvironment. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction A natural climate is the actual environment of a reinforced concrete (RC) structure. The environment is random, unique, and complex because of the effects of the geographic location as well as of meteorological factors, among others. For RC structures located in various regions, their service lives are usually different because of the variations in the natural climate even though these structures are identical. Therefore, studies on the durability of RC structures under a natural climate environment are of high significance. In contrast to the constant artificial climate environment, the natural climate environment shows evident stochastic fluctuations, which cause changes in the microenvironment of concrete ⇑ Corresponding author. Tel.: +86 25 83786633. E-mail address: [email protected] (J.-h. Jiang). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.03.033
[1,2]. Therefore, the effects of the environmental condition must be considered when evaluating the durability of an RC structure. Song et al. [3] presented the equivalent diffusion coefficient model of carbon dioxide in cracked concrete based on the effects of pore water saturation and concrete temperature. By considering the effects of the concrete temperature, relative humidity, and of other factors on the chloride diffusion coefficient, Shi and Luo [4] established a calculation model of the chloride that penetrates the concrete based on the finite difference. Lopez and Gonzalez [5] and Enevoldsen et al. [6] analyzed the effects of the pore water saturation, resistivity, and internal relative humidity of concrete on the steel corrosion rate and suggested that a critical pore water saturation or relative humidity initiate rebar corrosion. Song and Liu [7] established the quantitative relationship between the anode– cathode area ratio of the corroded rebar and the pore water saturation in carbonated concrete. Liu and Weyers [8] developed
288
J.-h. Jiang, Y.-s. Yuan / Construction and Building Materials 44 (2013) 287–292
a prediction model for the time-varying steel corrosion rate based on the effects of the chloride-ion concentration, temperature, and resistance of concrete using a long-term exposure test under a natural climate environment. The regularity and mechanism of the effects of environmental climate on deterioration should be determined first to establish an accurate prediction model for the degradation of the RC structure. The response relationships of the temperature and the relative humidity between the natural climate environment and the microenvironment of concrete should be further clarified. This paper focuses on the development of the steel corrosion rate in concrete under a natural climate environment. The responses of the temperature and humidity of the concrete microenvironment and the natural climate environment are clarified. The effects and mechanism of the environment on steel corrosion in concrete are further analyzed. This study is significant for the accurate prediction and assessment of the durability and service life of RC structures under a natural climate.
2. Experiments on the steel corrosion rate in concrete under natural climate 2.1. Specimen design The water–cement ratio (w/c) of the specimen concrete is 0.54, and the concrete mixture proportion is presented in Table 1. The cement was ordinary Portland cement P.O 42.5. River sand was used as the fine aggregate with a fineness modulus of 2.7. Gravel (5–15 mm, primarily consisting of limestone and trachyte) was used as the coarse aggregate. Ordinary tap water was used for mixing. The compressive strength of concrete is 25.4 MPa at 28 days. The specimens were divided into two types. Type I, with specimen dimensions of 63 mm 150 mm 200 mm, was used for the long-term corrosion test under sheltered conditions in a natural climate, and the corresponding serial numbers of the specimens are LCT-1, LCT-2, and LCT-3. Type II (shown in Fig. 1), with specimen dimensions of 100 mm 150 mm 250 mm, was used to determine the effect of the microenvironment of concrete on steel corrosion, and the serial numbers of the specimens are NCE-S1, NCE-S2, NCE-E1 and NCE-E2. A hole was preformed in the Type II specimen using a plastic pipe during concrete pouring to embed the temperature–humidity probe. The diameter of the hole is 20 mm, and the distance from the upper concrete surface to the preformed hole is 25 mm. A hot-rolled ribbed bar with yield strength of 335 MPa and a stainless steel bar were arranged in each specimen to determine the steel corrosion rate. The diameters of bars are 14 mm, and the concrete cover thickness for Types I and II is 15 and 25 mm, respectively. The chloride salt (sodium chloride), which accounted for 5% of the cement, was admixed into the concrete to simulate the chloride ion-induced steel corrosion.
2.2. Test methods The specimens were cured for 28 days under standard conditions after pouring and demolding. The initial relative humidity in the specimens for Type II was then adjusted to approximately 80% corresponding to room temperature, in order to determine the relative humidity response under a natural climate environment. Afterward, the concrete surfaces (except the concrete cover) were sealed with a plastic film and paraffin. The temperature–humidity probe (Fig. 2) was then embedded into the preformed hole using a plastic pipe and sealant. The tests were conducted in an outdoor exposure test site. The NCE-E1 and NCE-E2 specimens were subjected to unsheltered conditions, whereas the others were exposed to sheltered conditions. The corrosion rate of the steel bar in concrete was periodically measured using an electrochemical test system as shown in Fig. 3. The instantaneous corrosion rate of metallic materials can be measured continuously using the potentiodynamic polarization scan and the nonlinear least squares fitting of polarization curve. During the measurement, the saturated CuSO4 electrode acts as reference electrode, the stainless steel bar acts as auxiliary electrode, and the ribbed bar acts as working electrode. Temperature and relative humidity values were simultaneously recorded using the probe.
Table 1 Mixture proportion of concrete (kg/m3). w/c
Water
Cement
Sand
Coarse aggregate
0.54
212
393
630
1165
3. Experimental results and discussion The measurement parameters include the corrosion current density of the steel bars as well as the corresponding internal temperature and relative humidity in the concrete. 3.1. Corrosion rate of steel bars According to the fundamental theory of metal corrosion, the corrosion current density can be used as the representative value of the corrosion rate of steel bars. The measured corrosion current density of the steel bars in Types I and II is shown in Fig. 4a and b, respectively. The test period of Type I is more than three years. As can be seen from Fig. 4a the steel corrosion rate in concrete exhibits an evident time variation under the natural climate environment. The specific details are as follows: (1) The corrosion rate shows a general time-varying trend that can be divided into a descending phase, a steady phase and an ascending phase. This trend is due to the development of a rust layer [9]. However, the variation of the steel corrosion rate in the steady phase is cyclical for one year due to the effect of environmental climate. (2) The corrosion rate shows the detailed fluctuation caused by the changes in the microenvironment of concrete. The timevarying steel corrosion rate in concrete under the natural climate environment differs from that under a constant artificial climate environment. This difference is primarily attributed to the differences in environmental conditions. The differences in the steel corrosion rates in concrete under the two types of conditions are shown in Fig. 4b. The corrosion rates under unsheltered conditions are significantly higher than those under sheltered conditions. Given the same specimen fabrication parameters, the differences in the steel corrosion rates are mainly due to the differences in the micro-environmental conditions of concrete. 3.2. Environmental parameters 3.2.1. Temperature The changes of temperature in the natural climate environment as well as the microenvironment of concrete in the NCE-S1 and NCE-E2 specimens are shown in Fig. 5. The test results in Fig. 5 show that (1) the temperature in concrete fluctuates because of the changes in the ambient temperature, and the fluctuations in the internal temperature of concrete are consistent with those of ambient temperature; and (2) the fluctuations of internal temperature in concrete are smaller than those of the ambient temperature under sheltered conditions. By contrast, the fluctuations of internal temperature are slightly greater under unsheltered conditions due to the effect of sunshine. 3.2.2. Relative humidity The changes of relative humidity in the natural climate environment as well as the microenvironment of concrete in the NCE-S1 and NCE-E2 specimens are shown in Fig. 6. It is indicated that, (1) the relative humidity in concrete fluctuates because of the drastic changes in the ambient humidity; (2) the fluctuations in the internal relative humidity in concrete are significantly lower than those of ambient humidity; and (3) the relative humidity in concrete under sheltered conditions remains at about 80%, whereas that under unsheltered conditions is approximately equal to 100% due to the effect of rainfall.
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Corrosion current density ( µA·cm-2)
Fig. 1. Specimen of steel corrosion in concrete under natural environment (unit: mm).
4.0 LCT-1 LCT-2 LCT-3
3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0
0
100 200 300 400 500 600 700 800 900 1000 1100 1200
Measuring time (days)
(a) Results in Type I
Register
Fig. 2. Rotronic temperature–humidity sensor.
Copper-copper sulfate reference electrode Steel bar
Computer
Stainless bar
Corrosion current density (µA·cm-2)
Probe
5.0 4.0
NCE-E1
NCE-E2
NCE-S1
NCE-S2
3.0 2.0 1.0 0.0
0
50
100
150
200
250
300
350
400
Measuring time (days)
(b) Results in Type II
Specimen
Fig. 4. Data for the steel corrosion rate in concrete.
Electrochemical analysis system Fig. 3. Setup for the measurement of the steel corrosion rate.
Thus, the response of temperature is significantly faster than that of relative humidity between the concrete microenvironment and the natural environment. This phenomenon is attributed to the heat transfer and mass transfer involved in the temperature and relative humidity responses, respectively, in which mass transfer of concrete is slower than heat transfer. 4. Analysis of the effects of environmental climate on the steel corrosion rate According to the time variation of the steel corrosion rate in concrete under a constant artificial climate environment, the steel corrosion remains stable before corrosion cracking of concrete cover [10]. However, experimental results indicate that the development of the steel corrosion rate does not exhibit a stable state after the decline; instead, the corrosion rate increases with the fluctuations under the natural climate environment. The reasons are described below.
4.1. Effect of temperature in concrete The changes in the temperature and steel corrosion rate in concrete under the natural climate environment are shown in Fig. 7. As can be seen from Fig. 7 the changes of the steel corrosion rate are relatively consistent with those of the temperature. The corrosion rate of the rebar is affected by the development of a rust layer as well as by the fluctuation of the temperature in concrete. Therefore, the steel corrosion rate fluctuates in all stages without exhibiting a detectable stable state. Steel corrosion in concrete is an electrochemical reaction process. According to the basic law of chemical kinetics, the rate equation of the chemical reaction is as follows [11]:
t ¼ kcaM1 cbN1
ð1Þ
where cM and cN are the concentrations of the reactants; a1 and b1 are the orders of the reaction; k is the reaction rate coefficient. The relationship between k and the temperature can be expressed as the Arrhenius equation:
k ¼ k0 expðEa =RTÞ
ð2Þ
25 20
2.0
15 1.5
10
8-Jun-10
24-Apr-10
14-May-10
9-Mar-10
31-Mar-10
11-Feb-10
(a) NCE-S1
1-Jan-10
-5
23-Jan-10
0
0.0
14-Dec-09
5
0.5
20-Oct-09
1.0
17-Nov-09
27-May-10
27-Apr-10
28-Mar-10
27-Jan-10
26-Feb-10
28-Dec-09
28-Nov-09
29-Oct-09
29-Sep-09
30-Aug-09
1-Jul-09
31-Jul-09
-5
1-Jun-09
5
30
Temperature
2.5
22-Sep-09
15
3.0
Corrosion rate
Temperature (°C)
25
35
3.5
27-Aug-09
In concrete microenvironment
3-Jul-09
35
1-Aug-09
In natural environment
1-Jun-09
45
Corrosion current density (µA·cm-2)
J.-h. Jiang, Y.-s. Yuan / Construction and Building Materials 44 (2013) 287–292
Temperature (°C)
290
Fig. 5. Temperatures in the concrete microenvironment and the natural environment.
8-Jun-10
24-Apr-10
14-May-10
31-Mar-10
9-Mar-10
11-Feb-10
Eqs. (1) and (2) show that the effect of temperature on the steel corrosion rate in concrete can be attributed to the temperature effect on k, which is in accordance with the exponential function.
60 40
27-May-10
27-Apr-10
26-Feb-10
28-Mar-10
27-Jan-10
28-Dec-09
28-Nov-09
29-Oct-09
29-Sep-09
30-Aug-09
1-Jul-09
0
31-Jul-09
20
(a) NCE-S1 120
In natural environment
In concrete microenvironment
100 80 60 40 20
4.2. Effect of pore water saturation in concrete The fluctuation in the relative humidity of the concrete microenvironment, caused by the natural climate, is actually the change in the moisture content or pore water saturation of concrete. Given the temperature variations, the relative humidity cannot reflect the moisture content of concrete alone. Therefore, pore water saturation is used to analyze the effect of moisture content of concrete on the steel corrosion. The pore water saturation of concrete is calculated according to the temperature and relative humidity in the concrete microenvironment. Based on a series of test results, the relationship between the pore water saturation and the relative humidity can be modeled as follows:
27-May-10
27-Apr-10
28-Mar-10
26-Feb-10
27-Jan-10
28-Dec-09
28-Nov-09
29-Oct-09
29-Sep-09
30-Aug-09
31-Jul-09
1-Jul-09
0 1-Jun-09
Relative humidity (%)
Fig. 7. Steel corrosion rate and the temperature in concrete.
In concrete microenvironment
80
1-Jun-09
Relative humidity (%)
In natural environment
23-Jan-10
(b) NCE-E2
120 100
1-Jan-10
0 -5
14-Dec-09
0.5 0.0
17-Nov-09
15 10 5
20-Oct-09
(b) NCE-E2
2.0 1.5 1.0
22-Sep-09
27-May-10
27-Apr-10
28-Mar-10
26-Feb-10
27-Jan-10
28-Dec-09
28-Nov-09
29-Oct-09
29-Sep-09
30-Aug-09
31-Jul-09
1-Jul-09
-5
1-Jun-09
5
35 30 25 20
Corrosion rate Temperature
27-Aug-09
15
4.0 3.5 3.0 2.5
1-Aug-09
25
40
4.5
3-Jul-09
In concrete microenvironment
1-Jun-09
35
Corrosion current density (µA·cm-2)
In natural environment
Temperature (°C)
Temperature (°C)
(a) NCE-S1 45
(b) NCE-E2 Fig. 6. Relative humidity in the concrete microenvironment and the natural environment.
where k0 is the pre-exponential factor or frequency factor; R is the gas constant; T is the absolute temperature; Ea is the activation energy. Ea and k0 can be considered as constants within a certain temperature range.
S¼
k1 H ð1 k2 HÞð1 þ k3 HÞ
k1 ¼ ð2:9142w=c 2:5849Þ 103 t 0:1994w=c þ 0:1647
ð3Þ ð3aÞ
k2 ¼ ð2:907w=c- 1:1446 103 t þ 1:5594 105 t3 þ 4:4465Þ 103 k3 ¼ ð2:158w=c 3:2774Þ 103 t 0:3272w=c þ 0:3154 ð10 C 6 t 6 45 C; 0:48 6 w=c 6 0:62; 0 6 H < 100Þ
ð3bÞ ð3cÞ
291
Corrosion rate
Pore water saturation 0.8
3.0
0.6 2.0 0.4 1.0
8-Jun-10
24-Apr-10
14-May-10
9-Mar-10
31-Mar-10
11-Feb-10
1-Jan-10
23-Jan-10
14-Dec-09
20-Oct-09
17-Nov-09
22-Sep-09
27-Aug-09
3-Jul-09
1-Aug-09
0.2
1-Jun-09
0.0
Meteorological Data in Natural Environment
1.0
4.0
Pore water saturation
Corrosion current density (µA·cm-2)
J.-h. Jiang, Y.-s. Yuan / Construction and Building Materials 44 (2013) 287–292
Prediction model of T /H Response
Predicting of Microenvironment
0.0
Response Spectra of Microenvironment
(a) NCE-S1 5.0
1.0
4.0
0.8
Temperature (T) Relative Humidity (H )
T H T Pore Water Saturation (S)
Prediction Model of Corrosion Rate Corrosion Rate
Corrosion rate 3.0
0.6
Pore water saturation
8-Jun-10
24-Apr-10
14-May-10
31-Mar-10
9-Mar-10
11-Feb-10
23-Jan-10
1-Jan-10
14-Dec-09
17-Nov-09
20-Oct-09
0.0
22-Sep-09
0.0
27-Aug-09
0.2
1-Aug-09
1.0
3-Jul-09
0.4
1-Jun-09
2.0
Pore water saturation
Corrosion current density (µA·cm-2)
Action Spectra of Natural Climate
(b) NCE-E2 Fig. 8. Steel corrosion rate and the pore water saturation in concrete.
where S is the pore water saturation; H is the relative humidity in concrete, %; w/c is the water–cement ratio of concrete; t is the temperature of concrete, °C. However, the detailed study on the pore water saturation of concrete will be described in another paper. The changes in the pore water saturation and the steel corrosion rate in concrete under natural climate are shown in Fig. 8. The effect of the pore water saturation of concrete on the steel corrosion rate is unclear. In fact, the steel corrosion rate should increase when the pore water saturation increases, because a reduction in the resistivity of concrete can strengthen the path of ions between the cathode and anode during steel corrosion. However, the short supply of oxygen in concrete due to water saturation can reduce the steel corrosion rate. In the test, the pore water saturation of concrete did not inhibit the flow of the oxygen supply needed for steel corrosion. Thus, Fig. 8 does not show the effect of pore water saturation but instead demonstrates the comprehensive effects of pore water saturation, temperature, and rust layer on the steel corrosion rate in concrete. The effect of pore water saturation is concealed or counteracted because of the greater effects of temperature and of the rust layer. 5. Prediction strategy for the steel corrosion rate The basic prediction strategy for the steel corrosion rate in concrete under a natural climate environment is presented in a flow chart (Fig. 9) according to the development of the corrosion rate and the effects of the concrete microenvironment. The detailed description of the prediction procedure is as follows: (1) The action spectra of the temperature and the relative humidity under a natural climate are first built based on meteorological data.
of Steel Bars Fig. 9. Prediction procedure of steel corrosion rate in concrete under natural climate.
(2) Based on the action spectra, the response values of the temperature and relative humidity in concrete are calculated using the prediction models of the microenvironment responses. (3) The response spectra of the temperature and of the pore water saturation in concrete are built based on the calculation results of the microenvironment responses. (4) The steel corrosion rate in concrete is predicted using the prediction model for the time-varying steel corrosion rate and based on the response spectra of the concrete microenvironment. 6. Conclusions (1) The corrosion rate of a steel bar in concrete varies with time and significantly fluctuates in a natural climate environment. Therefore, the time variation differs from that under a constant artificial climate environment because of the effects of the concrete microenvironment. (2) The temperature and pore water saturation in the concrete microenvironment significantly affect the steel corrosion rate. Temperature is the primary climatic factor that affects the steel corrosion rate in concrete under an atmospheric environment; their relationship is consistent with the exponential function. The pore water saturation of concrete is also a key factor of the differences between the steel corrosion rate in concrete under sheltered and unsheltered conditions. (3) A prediction strategy for the steel corrosion rate in concrete under a natural climate environment is proposed based on the effects of the environmental climate on the steel corrosion rate as well as on the responses of the concrete microenvironment. Acknowledgements The authors wish to acknowledge the financial supports from the Fundamental Research Funds for the Central Universities (No. 2012B02614). References [1] Andrade C, Sarrı´a J, Alonso C. Relative humidity in the interior of concrete exposed to natural and artificial weathering. Cem Concr Res 1999;29(8): 1249–59. [2] Yuan YS, Jiang JH. Prediction of temperature response in concrete in a natural climate environment. Constr Build Mater 2011;25(8):3159–67.
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