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Significance of the ambient temperature and the steel material in the process of concrete reinforcement corrosion
Con.stmctim and Build&
Materials, Vol. 1 I, No. 2, pp. 99-103. 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights resewed 0950-0618/97 $17.00 + 0.00
PII:SO950-0618(97)00001-9
Significance of the ambient temperature and the steel material in the process of concrete reinforcement corrosion V. iivica*,
L’. KrajCi, L’. Bagel and M. VargovA
Department of Materials and Rheology, Institute of Construction and Architecture of the Slovak Academy of Sciences, Dtibravsk& cesta 9, 642 20 Bratislava, Slovak Republic Received 6 July 1996; revised 21 January 1997; accepted 5 February 1997 According to the results obtained, the ambient temperature and the steel material of concrete reinforcement represent significant factors of its corrosion rate and corrosion sensitivity under chloride induced corrosion. It has been found that the increase in ambient temperature up to 40°C accelerates the corrosion rate significantly. The increase over the value of 40°C has an opposite effect resulting in the corrosion rate slowing down. The possible causes of the effect are analysed. Three types of steel material, differing in their chemical composition, structure and mechanical properties, showed significantly different corrosion sensitivity. Of the studied steel materials, the steel showing the presence of inclusions and the highest values of yield point and tensile strength and the lowest value of ductility manifested the highest corrosion sensitivity. Therefore, the given characteristics seem to be symptomatic of the level of sensitivity of the material of steel opposite the chloride induced corrosion. 0 1997 Elsevier Science Ltd.
Keywords: corrosion; steel; ambient temperature
factor, E is the activation energy, T is temperature and R is the gas constant. The electrochemical reaction of iron or steel, respectively, consists of two particular processes. The first (anodic reaction) is represented by the formation of iron cations. The increase of temperature usually contributes to the increase of aggressive agent solubility in the pore liquid of concrete and consequently to the development of the anodic reaction. For example the solubility of CaCl, in water is increased by 55% when the ambient temperature from 0°C up to 30°C is increased. The second particular process of iron corrosion (cathodic reaction) is represented by depolarization, which requires the presence of oxygen and water on the cathode. The concentration of these two agents in concrete is very sensitive with regard to the ambient temperature. The oxygen concentration in pore solution is decreased as the ambient temperature is increased. For example when the ambient temperature is increased to 8O”C, the oxygen concentration is decreased by 75%‘. Simultaneously, as the ambient temperature is increased, the pore solution contents are decreased due to the increase in water vapour pressure.
The possibility of initiation of concrete reinforcement corrosion as well.as its corrosion rate depends on the concrete cover quality and on the ambient conditions in which reinforced concrete is placed. The quality of concrete cover is given by the combined effects of technological factors generating concrete properties. The quality of the concrete cover protecting the concrete reinforcement, opposite various unfavourable influences, can be decreased due to effects of carbonation and crack propagation. The ambient conditions are represented by air environment (relative humidity), temperature and present aggressive components CO,, SO,, and NO,. Corrosion of steel represents an electrochemical reaction. Its rate is dependent on the ambient temperature, i.e. the rate is increased as the ambient temperature is increased. This dependence is expressed by the Arrhenius equation [Equation (01 k =A .e(-E/W
(1)
where k is the velocity constant,
*Correspondenceto
V.
A is the frequency
l?ivica 99
Ambient temperature and steel material in concrete reinforcement corrosion: V. kvica et al.
100
Increase in the ambient temperature results in the decrease in concentration of materials needed for the steel corrosion process. Retardation or interruption of the corrosion process can be the final result. The corroding steel reinforcement produces a corrosion electrical current, which is dependent on the difference between anodic and cathodic electrode potentials on the reinforcement. This electrode potential expresses the Nemst equation [Equation (211 For the electrode potential of the anodic reaction Fe + Fe2+ + 2e-
(2)
the equation is valid @A
= @A
I
1
RTln [Fe’+1 TlF Eel
(3)
For the cathodic reaction 4e- + 0, + 2H,O -+ 40H-
(4)
the equation 0c
i?
0c
I I
RTln
nF
[O,].[H,O]
[OH- I4
(5)
where 0; and 0: are so called standard electrode potentials, F is the Faraday constant, n is the ionic valency of the rising ions and data in the square brackets are the molar concentrations of the reactants’-4. The term RT/nF in the given equations expresses the dependence ofthe electrode potentials on the ambient temperature. The difference between the electrode potentials represents the driving force of the electric current produced in the galvanic cell. This current is equal to the rate of chemical reaction and, in the case of iron, is equal to its corrosion rate. It is evident that this rate is also dependent on the ambient temperature. Equations (3) and (5) consider the ambient temperature and confirm its significance for the corrosion rate. The steel corrosion rate is directly dependent on the corrosion electrical current I, according to Equation (6) m -= t
AZ L nF
where m/t is the quantity of corroded metal during the unit of time t, A, is relative atomic weight of the corroding metal, n and F are identical symbols with the ones given in Equations (3) and (5). The significance of the ambient temperature in the process of concrete reinforcement corrosion expresses the fact of its influence on the rate of transfer of ions between anode and cathode. It is known that if there is an increase of the ambient temperature the ionic mobility is increased. Therefore, this effect contributes to the increase of the corrosion rate of reinforcement. The
effect is dependent only on the level of the ambient temperature. Therefore its intensity by the ambient temperature can only be increased. This represents a significant difference from the effect of the ambient temperature on the rate of cathodic reaction. This effect can be decreased when the ambient temperature is increased owing to the resulting oxygen solubility decrease. The significance of the ambient temperature in the process of concrete reinforcement corrosion, as a rule, is not currently considered. Tuutti’ considers the ambient temperature with oxygen diffusion for the significant controlling factors of the corrosion process. According to this author, in the interval of - 20 to + 20°C the dependence of corrosion rate on the temperature increase is linear, and the rate is increased 100 times. However, in the interval + 10 to + 2O”C, the rate is increased only seven times. Schiessl and Raupach6,’ showed that with the ambient temperature increase from + 15 to + 20°C is corrosion electrical current increased by about 50%. According to the authors, the high ambient temperatures and high water content for the concrete reinforcement are the most unfavourable conditions. It can be seen that data about the influence of the ambient temperature are rather poor. This fact was the motivation for our research. The object of this paper is to experimentally determine the influence of the ambient temperature on corrosion rate of concrete reinforcement and the significance of the steel material for its corrosion sensitivity.
Experimental Ordinary Portland cement8 and silica sand’ were used in the study. Three kinds of the steel material were used for the study: steel 11 373; steel 10 216; and steel 10 425*‘-‘*. The increasing quality of mechanical properties of used steel material corresponds to the sequence of the mentioned steels. Chemical composition, structure and mechanical properties of steels used are given in Table 1. High purity calcium chloride was used as a chloride admixture for the study. Mortar mixtures with a cement:sand ratio of 1:3, with a CaCl, portion of 4% (opposite to the cement portion) and w/c value of 0.60 were prepared. From the mortar mixtures the test specimens - cubes with edge length of 20 mm and the prisms 40 x 40 x 160 mm with embedded steel specimens were prepared. The curing procedure of the test specimens was as follows: the first 3 days at relative humidity (RH) of approximately 100% at 20°C; next, 60 days at RH of approximately 90% (over the saturated sodium sulphate-water solution) at 20, 40 and 60°C. After 3 days of hardening, bulk density, compression strength and pore structure of mortars were assessed. A detailed survey of the composition and curing
Ambient temperature and steel material in concrete reinforcement corrosion: V. ,%vica et al. Table 1
101
Chemical composition, structure and mechanical properties of steel material used
Type of steel
Chemical composition (%)
structure
Yield point (MPa)
Tensile strength (MPa)
Ductility (“/of
11313
c Mn P S
0.15 0.12 0.04 0.03
Ferritic with particles of cementite
226
363
26
10 216
c Mn Si P S
0.21 0.48 0.01 0.03 0.07
Ferritic-pearlitic
206
539
24
10 425
C Mn Si P s
0.24 1.14 0.47 0.02 0.03
Ferritic-pearlitic, occurrence of inclusion of oxidizing and sulphidic nature
410
569
14
conditions of the test specimens, as well as their mechanical properties and pore structure parameters, are given in Table 2. During the study the electrode potential and electrical resistance of the embedded steel specimens were assessed. For the study, the following test methods have been used: mercury porosimetry, using the microporosimeter Erba Science mod.2000 with macropore unit 120 for the pore structure analysis within a radius pore range of 3.75 nm to approx. 0.3 mm. The electrode potential method, using saturated calomel electrode WE) and improved electrical resistance method. The improvement of the method is based on a so-called ‘corrosion sensor’ (suitable arranged steel test specimen). This sensor embedded in the mortar test specimen or the concrete structure enables us to check the conditions of concrete reinforcement. The structure of the sensor excludes disturbing effects during the measurement (e.g. influence of temperature changes of the ambient temperature and others) and increases the sensitivity of the method and the reliability of the test results. The weakening of the cross-section, thickness of the corroded layer and the loss of weight of the concrete
reinforcement caused by corrosion can be calculated from the electrical resistance method results. For more detailed information about the methods used see iivica13.
Result and discussion The values of bulk density, compression strength and pore structure of mortars used for the study are given in Table 2. The obtained result showed that the prepared mortar represented the material with usual mechanical properties which, from the point of the given aim of the study, have no deciding significance. The evaluation of the parameters of pore structure of the prepared mortars enables information about the quality of mortar pore structure, which represents the main factor in the corrosion process of concrete reinforcement. Even these results show that the prepared mortar represented the material with its usual quality pore structure. The changes in electrode potential (EP) of the embedded steel test specimens in mortar were very irregu-
Table 2 Composition and curing conditions of the test mortars and their mechanical properties and pore structure parameters after 3 days of hardening Test specimens: mortar prisms 40 Mortar: Steel
cement:sand 1:3,
x 40 x
160 mm with embedded steel test specimens according to STN 72 2121[8]
4% CaCI,, Curing temperature CC)
w/c 0.60
11373 10 216 10 425
20 20 20
40 40 40
60 60 60
Bulk density (kg rne3) 2243
Compression strength (MPa)
Pore volume” (mm3 g- ’ )
Portion of macropores (o/o)
Pore median (nm)
Total porosity (o/o)
19.44
Micro: 76.0 Macro: 8.5 Total: 84.5
10.1
53.6
17.0
’ Macropores, radius > 7500 nm; micropores, radius < 7500 nm.
102
Ambient temperature and steel material in concrete reinforcement corrosion: V. hica
Electrode potential (EP vs. SCE) values of embedded steel test specimens in mortar
Table 3
Temperature of curing CC)
Kind of steel
11373 10 216 10425
20
40
60
- 490” - 3306 - 500” - 270’ -510” - 170b
- 470” - 390b - 480” - 4OOb -480” -4OOh
- 565” - 390b - 480” - 470b ~ 580” - 340b
“Initial value of EP after 3 days of hardening at 20°C. ‘Final value of EP after 60 days of curing at the given temperature.
is
et al.
I
w/c = 0.60
14%
CaC12
2
t7;
v, 2000 LG
2 2
1000
[I:
lar. Therefore it was possible to evaluate only common trends in these changes. The results given in Table 3 indicate the corrosive active state of steel test specimens at the beginning of the measurement (EP values of - 470 to - 580 mV SCE). Later a change from the active corrosive state to the passive one occurred. This occurred in the case of the curing temperature of 20°C (EP values of 170-330 mV on all the studied types of steel) and in the case of steel 10 425 at the curing temperature of 60°C. The results show that the embedded steel test specimens during the certain time period were in a corrosive active state and the tendency of its absolute extinction with the expiration of the curing
CURING
steel
3 000
11 373
z 2 000
TEMPERATURI
-
2ooc 4ooc 60°C
---
... . . .
-2 1000 E z2
3000
X’
steel
.
10 216
; 2000 cr
.
X’
X‘
steel
:
10 425 / f.‘f .** / .+ : f
1000
5
15
25 35 TIME (days)
45
55
Dependence of electrical resistance changes of the embedded steel specimens on the ambient temperature
Figure 1
z
W -I W
40
AM2:ENT
TEMPERATURE
60 1°C)
Dependence of electrical resistance of the embedded steel specimens on the kind of steel material after 25 days of curing Figure 2
Figure I shows a clear influence of the curing temperature level and the steel material on the corrosion rate of the embedded steel test specimens. More practically, Figure 2 shows the influence of both factors. It is evident that the electrical resistance of the steel test specimen or their corrosion rate, respectively, was increased by the increase of the ambient temperature until 40°C. Above this value (40°C) there was a significant decrease in the corrosion rate. This dependence in the case of steel 11 373 and especially 10 425 was expressed. Evidently two mentioned dependences were dominant factors in the corrosion process as shown in Figures 1 and 2, i.e. the dependence solubility of oxygen in water - ambient temperature, and the dependence of the rate of cathodic reaction of the steel corrosion concentration of oxygen or its solubility in pore solution, respectively. Both resulted in continuous attenuation of the corrosion rate over 40°C. Evidently this effect significantly overshadowed the accelerating effects of the ionic mobility increase. According to these results the corrosion sensitivity of the studied steel materials can be evaluated as follows: steel 10 216 low sensitivity; steel 11 373 medium sensitivity; and steel 10 425 high sensitivity. Evaluation of the possible causes of the different corrosion sensitivities of the steels enables the comparison of their properties as shown in Table 1. Steel 10 216 with the relatively low values of yield point and tensile strength and the high value of ductility showed low corrosion sensitivity. The same values of the given properties showed steel 11 373 with medium corrosion sensitivity. It is possible that the decreased Mn content and some differences in structure of both steels were the causes of the increased corrosion sensitivity of steel 11 373. The highly sensitive steel 10 425
Ambient temperature and steel material in concrete reinforcement corrosion: V. 2ivica et al. showed a relatively high value of yield point and tensile strength. However, it also showed a low ductility value. Moreover, it contained inclusions of an oxidizing and sulphidic nature. According to this, the structure and the presence of these inclusions are important factors in corrosion. The related relationships and factors seem to be worthy of the more detailed study. The presence of inclusions in the structure of steel, and its decreased ductility or increased fragility, can be considered the symptoms of its corrosion sensitivity signalling the increased danger of corrosion.
is required for the elimination of reinforced structures.
103
of danger of corrosion
Acknowledgement The authors are grateful to the Slovak Grant Agency for Science (grant No. 999 210/92) for partial supporting of this work.
References Conclusion According to the results, the ambient temperature is the dominant factor in the process of reinforcement corrosion. It has been shown that with increased ambient temperature the steel corrosion is also increased, however, only to a certain limit. The latter is for the given experimental conditions and the studied steels represented by the value of 40°C. In the case of the mentioned phenomenon, the decrease of oxygen content in pore liquid of the mortar can be considered. The oxygen solubility in pore liquid is considerably decreased by the ambient temperature increase. The relationship between the steel corrosion rate, the ambient temperature and the steel material seems to be very complex. The possible causes are chemical and structural composition, presence of inclusions, and probably even the technological procedure, which can significantly influence the quality of concrete steel reinforcement. These factors seem to be very important for the electrochemical behaviour of steel and the possibility of its corrosion. More detailed study
1 Akimov, G.V., Z?reory and Testing Methods Corrosion of MetaIs (in Czech), SNTL, Praha, 1953. Z.P., Physical model for steel corrosion in concrete sea 2 B&t, structures - theory. Joumal of Structuml Divkion, 1979, 105, 1137. 3 B&m, Z.P.. Physical model for steel corrosion in concrete sea structures - application. Journal of Structuml Division, 1979, ;os, 1155. 4 Cemj( M., Corrosive Properties of Metallic Structuml Materials (in Czech). SNTL, Praha, 1984. 5 Tuutti, K., Service life of structures with regard to corrosion of embedded steel. RILEM Quality Control of Concrete Structures. Stockholm, Sweden, 1979, p. 293. 6 Schiessl, P. and Raupach, M., Chloridinduzierte Korrosion von Stahl im Beton. Beton-Infonnationen, 1988, 28, 33. I Schiessl, P. and Raupach, M., Einfluss der Betonzusammensetzung und der Ungebungsbedingungen auf die Chloridinduzierte Korrosion von Stahl im Beton. Beton-Informationen, 1990, 30, 43. 8 STN 72 2121 Portland Cement (in Czech). 9 STN 72 1208 Testing Sands (in Czech). 10 STN 41 1373 Steel il 373 (in Czech). 11 STN 41 0216 Steel 10 216 (in Czech). 12 STN 41 0425 Steel 10 42.5 (in Czech). 13 Zivica, V., Improved method of electrical resistance - a suitable technique for checking the state of concrete reinforcement. Materials and Structures, 1993, 26, 328.
Materials, Vol. 1 I, No. 2, pp. 99-103. 1997 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights resewed 0950-0618/97 $17.00 + 0.00
PII:SO950-0618(97)00001-9
Significance of the ambient temperature and the steel material in the process of concrete reinforcement corrosion V. iivica*,
L’. KrajCi, L’. Bagel and M. VargovA
Department of Materials and Rheology, Institute of Construction and Architecture of the Slovak Academy of Sciences, Dtibravsk& cesta 9, 642 20 Bratislava, Slovak Republic Received 6 July 1996; revised 21 January 1997; accepted 5 February 1997 According to the results obtained, the ambient temperature and the steel material of concrete reinforcement represent significant factors of its corrosion rate and corrosion sensitivity under chloride induced corrosion. It has been found that the increase in ambient temperature up to 40°C accelerates the corrosion rate significantly. The increase over the value of 40°C has an opposite effect resulting in the corrosion rate slowing down. The possible causes of the effect are analysed. Three types of steel material, differing in their chemical composition, structure and mechanical properties, showed significantly different corrosion sensitivity. Of the studied steel materials, the steel showing the presence of inclusions and the highest values of yield point and tensile strength and the lowest value of ductility manifested the highest corrosion sensitivity. Therefore, the given characteristics seem to be symptomatic of the level of sensitivity of the material of steel opposite the chloride induced corrosion. 0 1997 Elsevier Science Ltd.
Keywords: corrosion; steel; ambient temperature
factor, E is the activation energy, T is temperature and R is the gas constant. The electrochemical reaction of iron or steel, respectively, consists of two particular processes. The first (anodic reaction) is represented by the formation of iron cations. The increase of temperature usually contributes to the increase of aggressive agent solubility in the pore liquid of concrete and consequently to the development of the anodic reaction. For example the solubility of CaCl, in water is increased by 55% when the ambient temperature from 0°C up to 30°C is increased. The second particular process of iron corrosion (cathodic reaction) is represented by depolarization, which requires the presence of oxygen and water on the cathode. The concentration of these two agents in concrete is very sensitive with regard to the ambient temperature. The oxygen concentration in pore solution is decreased as the ambient temperature is increased. For example when the ambient temperature is increased to 8O”C, the oxygen concentration is decreased by 75%‘. Simultaneously, as the ambient temperature is increased, the pore solution contents are decreased due to the increase in water vapour pressure.
The possibility of initiation of concrete reinforcement corrosion as well.as its corrosion rate depends on the concrete cover quality and on the ambient conditions in which reinforced concrete is placed. The quality of concrete cover is given by the combined effects of technological factors generating concrete properties. The quality of the concrete cover protecting the concrete reinforcement, opposite various unfavourable influences, can be decreased due to effects of carbonation and crack propagation. The ambient conditions are represented by air environment (relative humidity), temperature and present aggressive components CO,, SO,, and NO,. Corrosion of steel represents an electrochemical reaction. Its rate is dependent on the ambient temperature, i.e. the rate is increased as the ambient temperature is increased. This dependence is expressed by the Arrhenius equation [Equation (01 k =A .e(-E/W
(1)
where k is the velocity constant,
*Correspondenceto
V.
A is the frequency
l?ivica 99
Ambient temperature and steel material in concrete reinforcement corrosion: V. kvica et al.
100
Increase in the ambient temperature results in the decrease in concentration of materials needed for the steel corrosion process. Retardation or interruption of the corrosion process can be the final result. The corroding steel reinforcement produces a corrosion electrical current, which is dependent on the difference between anodic and cathodic electrode potentials on the reinforcement. This electrode potential expresses the Nemst equation [Equation (211 For the electrode potential of the anodic reaction Fe + Fe2+ + 2e-
(2)
the equation is valid @A
= @A
I
1
RTln [Fe’+1 TlF Eel
(3)
For the cathodic reaction 4e- + 0, + 2H,O -+ 40H-
(4)
the equation 0c
i?
0c
I I
RTln
nF
[O,].[H,O]
[OH- I4
(5)
where 0; and 0: are so called standard electrode potentials, F is the Faraday constant, n is the ionic valency of the rising ions and data in the square brackets are the molar concentrations of the reactants’-4. The term RT/nF in the given equations expresses the dependence ofthe electrode potentials on the ambient temperature. The difference between the electrode potentials represents the driving force of the electric current produced in the galvanic cell. This current is equal to the rate of chemical reaction and, in the case of iron, is equal to its corrosion rate. It is evident that this rate is also dependent on the ambient temperature. Equations (3) and (5) consider the ambient temperature and confirm its significance for the corrosion rate. The steel corrosion rate is directly dependent on the corrosion electrical current I, according to Equation (6) m -= t
AZ L nF
where m/t is the quantity of corroded metal during the unit of time t, A, is relative atomic weight of the corroding metal, n and F are identical symbols with the ones given in Equations (3) and (5). The significance of the ambient temperature in the process of concrete reinforcement corrosion expresses the fact of its influence on the rate of transfer of ions between anode and cathode. It is known that if there is an increase of the ambient temperature the ionic mobility is increased. Therefore, this effect contributes to the increase of the corrosion rate of reinforcement. The
effect is dependent only on the level of the ambient temperature. Therefore its intensity by the ambient temperature can only be increased. This represents a significant difference from the effect of the ambient temperature on the rate of cathodic reaction. This effect can be decreased when the ambient temperature is increased owing to the resulting oxygen solubility decrease. The significance of the ambient temperature in the process of concrete reinforcement corrosion, as a rule, is not currently considered. Tuutti’ considers the ambient temperature with oxygen diffusion for the significant controlling factors of the corrosion process. According to this author, in the interval of - 20 to + 20°C the dependence of corrosion rate on the temperature increase is linear, and the rate is increased 100 times. However, in the interval + 10 to + 2O”C, the rate is increased only seven times. Schiessl and Raupach6,’ showed that with the ambient temperature increase from + 15 to + 20°C is corrosion electrical current increased by about 50%. According to the authors, the high ambient temperatures and high water content for the concrete reinforcement are the most unfavourable conditions. It can be seen that data about the influence of the ambient temperature are rather poor. This fact was the motivation for our research. The object of this paper is to experimentally determine the influence of the ambient temperature on corrosion rate of concrete reinforcement and the significance of the steel material for its corrosion sensitivity.
Experimental Ordinary Portland cement8 and silica sand’ were used in the study. Three kinds of the steel material were used for the study: steel 11 373; steel 10 216; and steel 10 425*‘-‘*. The increasing quality of mechanical properties of used steel material corresponds to the sequence of the mentioned steels. Chemical composition, structure and mechanical properties of steels used are given in Table 1. High purity calcium chloride was used as a chloride admixture for the study. Mortar mixtures with a cement:sand ratio of 1:3, with a CaCl, portion of 4% (opposite to the cement portion) and w/c value of 0.60 were prepared. From the mortar mixtures the test specimens - cubes with edge length of 20 mm and the prisms 40 x 40 x 160 mm with embedded steel specimens were prepared. The curing procedure of the test specimens was as follows: the first 3 days at relative humidity (RH) of approximately 100% at 20°C; next, 60 days at RH of approximately 90% (over the saturated sodium sulphate-water solution) at 20, 40 and 60°C. After 3 days of hardening, bulk density, compression strength and pore structure of mortars were assessed. A detailed survey of the composition and curing
Ambient temperature and steel material in concrete reinforcement corrosion: V. ,%vica et al. Table 1
101
Chemical composition, structure and mechanical properties of steel material used
Type of steel
Chemical composition (%)
structure
Yield point (MPa)
Tensile strength (MPa)
Ductility (“/of
11313
c Mn P S
0.15 0.12 0.04 0.03
Ferritic with particles of cementite
226
363
26
10 216
c Mn Si P S
0.21 0.48 0.01 0.03 0.07
Ferritic-pearlitic
206
539
24
10 425
C Mn Si P s
0.24 1.14 0.47 0.02 0.03
Ferritic-pearlitic, occurrence of inclusion of oxidizing and sulphidic nature
410
569
14
conditions of the test specimens, as well as their mechanical properties and pore structure parameters, are given in Table 2. During the study the electrode potential and electrical resistance of the embedded steel specimens were assessed. For the study, the following test methods have been used: mercury porosimetry, using the microporosimeter Erba Science mod.2000 with macropore unit 120 for the pore structure analysis within a radius pore range of 3.75 nm to approx. 0.3 mm. The electrode potential method, using saturated calomel electrode WE) and improved electrical resistance method. The improvement of the method is based on a so-called ‘corrosion sensor’ (suitable arranged steel test specimen). This sensor embedded in the mortar test specimen or the concrete structure enables us to check the conditions of concrete reinforcement. The structure of the sensor excludes disturbing effects during the measurement (e.g. influence of temperature changes of the ambient temperature and others) and increases the sensitivity of the method and the reliability of the test results. The weakening of the cross-section, thickness of the corroded layer and the loss of weight of the concrete
reinforcement caused by corrosion can be calculated from the electrical resistance method results. For more detailed information about the methods used see iivica13.
Result and discussion The values of bulk density, compression strength and pore structure of mortars used for the study are given in Table 2. The obtained result showed that the prepared mortar represented the material with usual mechanical properties which, from the point of the given aim of the study, have no deciding significance. The evaluation of the parameters of pore structure of the prepared mortars enables information about the quality of mortar pore structure, which represents the main factor in the corrosion process of concrete reinforcement. Even these results show that the prepared mortar represented the material with its usual quality pore structure. The changes in electrode potential (EP) of the embedded steel test specimens in mortar were very irregu-
Table 2 Composition and curing conditions of the test mortars and their mechanical properties and pore structure parameters after 3 days of hardening Test specimens: mortar prisms 40 Mortar: Steel
cement:sand 1:3,
x 40 x
160 mm with embedded steel test specimens according to STN 72 2121[8]
4% CaCI,, Curing temperature CC)
w/c 0.60
11373 10 216 10 425
20 20 20
40 40 40
60 60 60
Bulk density (kg rne3) 2243
Compression strength (MPa)
Pore volume” (mm3 g- ’ )
Portion of macropores (o/o)
Pore median (nm)
Total porosity (o/o)
19.44
Micro: 76.0 Macro: 8.5 Total: 84.5
10.1
53.6
17.0
’ Macropores, radius > 7500 nm; micropores, radius < 7500 nm.
102
Ambient temperature and steel material in concrete reinforcement corrosion: V. hica
Electrode potential (EP vs. SCE) values of embedded steel test specimens in mortar
Table 3
Temperature of curing CC)
Kind of steel
11373 10 216 10425
20
40
60
- 490” - 3306 - 500” - 270’ -510” - 170b
- 470” - 390b - 480” - 4OOb -480” -4OOh
- 565” - 390b - 480” - 470b ~ 580” - 340b
“Initial value of EP after 3 days of hardening at 20°C. ‘Final value of EP after 60 days of curing at the given temperature.
is
et al.
I
w/c = 0.60
14%
CaC12
2
t7;
v, 2000 LG
2 2
1000
[I:
lar. Therefore it was possible to evaluate only common trends in these changes. The results given in Table 3 indicate the corrosive active state of steel test specimens at the beginning of the measurement (EP values of - 470 to - 580 mV SCE). Later a change from the active corrosive state to the passive one occurred. This occurred in the case of the curing temperature of 20°C (EP values of 170-330 mV on all the studied types of steel) and in the case of steel 10 425 at the curing temperature of 60°C. The results show that the embedded steel test specimens during the certain time period were in a corrosive active state and the tendency of its absolute extinction with the expiration of the curing
CURING
steel
3 000
11 373
z 2 000
TEMPERATURI
-
2ooc 4ooc 60°C
---
... . . .
-2 1000 E z2
3000
X’
steel
.
10 216
; 2000 cr
.
X’
X‘
steel
:
10 425 / f.‘f .** / .+ : f
1000
5
15
25 35 TIME (days)
45
55
Dependence of electrical resistance changes of the embedded steel specimens on the ambient temperature
Figure 1
z
W -I W
40
AM2:ENT
TEMPERATURE
60 1°C)
Dependence of electrical resistance of the embedded steel specimens on the kind of steel material after 25 days of curing Figure 2
Figure I shows a clear influence of the curing temperature level and the steel material on the corrosion rate of the embedded steel test specimens. More practically, Figure 2 shows the influence of both factors. It is evident that the electrical resistance of the steel test specimen or their corrosion rate, respectively, was increased by the increase of the ambient temperature until 40°C. Above this value (40°C) there was a significant decrease in the corrosion rate. This dependence in the case of steel 11 373 and especially 10 425 was expressed. Evidently two mentioned dependences were dominant factors in the corrosion process as shown in Figures 1 and 2, i.e. the dependence solubility of oxygen in water - ambient temperature, and the dependence of the rate of cathodic reaction of the steel corrosion concentration of oxygen or its solubility in pore solution, respectively. Both resulted in continuous attenuation of the corrosion rate over 40°C. Evidently this effect significantly overshadowed the accelerating effects of the ionic mobility increase. According to these results the corrosion sensitivity of the studied steel materials can be evaluated as follows: steel 10 216 low sensitivity; steel 11 373 medium sensitivity; and steel 10 425 high sensitivity. Evaluation of the possible causes of the different corrosion sensitivities of the steels enables the comparison of their properties as shown in Table 1. Steel 10 216 with the relatively low values of yield point and tensile strength and the high value of ductility showed low corrosion sensitivity. The same values of the given properties showed steel 11 373 with medium corrosion sensitivity. It is possible that the decreased Mn content and some differences in structure of both steels were the causes of the increased corrosion sensitivity of steel 11 373. The highly sensitive steel 10 425
Ambient temperature and steel material in concrete reinforcement corrosion: V. 2ivica et al. showed a relatively high value of yield point and tensile strength. However, it also showed a low ductility value. Moreover, it contained inclusions of an oxidizing and sulphidic nature. According to this, the structure and the presence of these inclusions are important factors in corrosion. The related relationships and factors seem to be worthy of the more detailed study. The presence of inclusions in the structure of steel, and its decreased ductility or increased fragility, can be considered the symptoms of its corrosion sensitivity signalling the increased danger of corrosion.
is required for the elimination of reinforced structures.
103
of danger of corrosion
Acknowledgement The authors are grateful to the Slovak Grant Agency for Science (grant No. 999 210/92) for partial supporting of this work.
References Conclusion According to the results, the ambient temperature is the dominant factor in the process of reinforcement corrosion. It has been shown that with increased ambient temperature the steel corrosion is also increased, however, only to a certain limit. The latter is for the given experimental conditions and the studied steels represented by the value of 40°C. In the case of the mentioned phenomenon, the decrease of oxygen content in pore liquid of the mortar can be considered. The oxygen solubility in pore liquid is considerably decreased by the ambient temperature increase. The relationship between the steel corrosion rate, the ambient temperature and the steel material seems to be very complex. The possible causes are chemical and structural composition, presence of inclusions, and probably even the technological procedure, which can significantly influence the quality of concrete steel reinforcement. These factors seem to be very important for the electrochemical behaviour of steel and the possibility of its corrosion. More detailed study
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