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Modelling of chloride-induced corrosion of reinforcement in cracked high-performance concrete based on laboratory investigations
8 Modelling of chloride-induced corrosion of reinforcement in cracked high-performance concrete based on laboratory investigations M. R A U P A C H and C. D A U B E R S C H M I D T, Aachen University, Germany
8.1
Background
High-strength concrete with a compressive strength above 80 N mm–2 has been used for many years. Besides increasing the load-bearing capacity of the concrete one important aspect for the development of high-performance concrete mixtures is the durability, especially the resistance against chloride diffusion. Numerous investigations have been performed with regard to the abrasion resistance, capillary water suction, permeability to gases and liquids, chloride diffusion resistance or resistance against frost and de-icing salts [1–5]. Nevertheless, questions remain with regard to the behaviour of the reinforcement in high-performance concrete structures when exposed to aggressive environmental conditions. Cracks in reinforced concrete structures allow aggressive agents like chlorides from de-icing salts or sea-water to penetrate into the concrete. As shown in Fig. 8.1, the corrosion rate of a macrocell is a function of the anodic polarisation resistance, RA, the cathodic polarisation resistance, RC, the resistivity of the electrolyte (concrete), Rel, and the difference between the rest potentials at the anode (ER,A) and the cathode (ER,C). It can be deduced from the equation that the corrosion rate is reduced if only one of the three resistances (RA, RC and Rel), especially the most corrosion rate determining resistance, increases. Due to the low permeability and high electrolytic resistivity of highperformance concrete, it might be expected that the corrosion rates of steel in the area of cracks in concrete are far lower than in normal concrete. Another reason for a possible reduction of corrosion rates in high-performance concrete may be the restricted volume expansion of corrosion products at the anode due to the limited space in small cracks causing the anodic polarisation resistance, RA, to increase with time. In order to evaluate the corrosion behaviour of steel in cracked highperformance concrete, laboratory tests have been performed on cracked concrete beams. 89
90
Corrosion of reinforcement in concrete Crack
Ιe =
Concrete
Rel
E R,C – E R,A R A + R C + R el
ER,C RA
∆ E c = Rc I e
RC
ER,A Steel
ER,C
RSt ≈ 0
∆E
∆E
EC,C
∆ Eel = Rel Ie
EC,A
∆EA = RA Ie
Ie Anode
Cathode
ER,A Ie
8.1 Simplified electrical circuit model for the corrosion of steel in cracked concrete [ER,C and ER,A: rest potentials at the cathode and anode; RA, RC and Rel: polarisation resistances at the anode, the cathode and resistivity of the electrolyte/concrete; Ie: macrocell current (~ corrosion rate)] [2]. Section B-B 25 100 25
Section A-A 100
50
Uncoated anodic steel area
Mild steel cathodes
350 Chloride solution 1% Crack
Epoxy coated rebar Activated titanium cathodes
150
100
Titanium cathode 150
700 Top view
䉯B Anode Rebar Ø 12 mm
A
A
50
䉯B 600 700
50
Cathodes: Mild steel Epoxy coated
(Measures in mm)
8.2 Design of test specimens.
8.2
Test programme
The design of the test specimens (Fig. 8.2) allowed the measurement of various corrosion parameters such as macrocell corrosion currents (anodic and cathodic currents), corrosion potentials and electrolytic resistance of the concrete. The main rebar intersecting the crack was designed to be the anode, therefore the active area was restricted by coating the reinforcement in the uncracked area. The cathodes consisted of 24 mild steel rebars, fixed at
Modelling of chloride-induced corrosion of reinforcement
91
defined distances from the crack (50 to 250 mm) and with two concrete covers (23 and 47 mm). After concreting, the specimens were stored for 7 days in a fog-room and afterwards for another 21 days in a climate at 20 °C and 65 % rh. The cracks were formed by fixing the specimens in steel frames and by bending them to the designed crack widths (0.1, 0.25 and 0.5 mm at the surface of the beams). Corrosive conditions were created by wet– dry cycles (1 day immersed with a 1 % chloride solution followed by 6 days dry in a laboratory atmosphere at about 65 % rh). In order to evaluate the corrosion behaviour of reinforcing steel bars in the cracked concrete beams, macrocell corrosion currents, corrosion potentials and electrolytic resistances were measured over a period of 64 cycles (at least 15 months). The concrete mixtures used in the tests are specified in Table 8.1.
8.3
Results
8.3.1
Measurement of the electrolytic resistivity of the concrete, Rel
To measure the resistivity over time at different depths an embedded multiring electrode (MRE) was used [6]. The sensor consisted of several stainlesssteel rings maintained at a defined distance from one another by insulating plastic rings (Fig. 8.3). Cable connections through the sensor enabled the resistivity of the concrete to be determined between each pair of neighbouring stainless-steel rings by means of impedance measurements. The ac resistance values (in Ω) can be converted to resistivity values by a sensor-specific transfer factor determined in aqueous solutions with known conductivity. The standard type of the multi-ring electrode-sensor allowed the measurement of eight resistances between nine rings, down to a distance of 42 mm from the concrete surface. The specimens were stored at 22 °C and 52 % rh (average values). In Fig. 8.4–8.8 the results of the resistivity measurements of the different types of concrete are presented. The highest measurable value is 20 kΩ m. A significant increase in the resistivity with time is noted for all depths for the C 35 concrete mixture (Fig. 8.4). This increase for all depths can be explained because this comparably permeable concrete had fully dried, whereas concrete mixtures C 65 and C 85-0 (Fig. 8.5 and 8.6) had only dried to about 17 mm as a significant rise can be observed only for the curves of 7, 12 and 17 mm. For the concrete mixtures produced with silica fume (Fig. 8.7 and 8.8) drying only occurred to a depth of 12 mm. All curves show a clear profile of decreasing resistivity with increasing depth. There is a tendency for the resistivity of the humid inner area of
Table 8.1 Concrete mix proportions Mixture
–
Cement
Cement
type
content
300
C 65 C 85-0
OPC
Silica fume
kg m3
–
C 35
Water
150
–
Super-plasticiser*
Water-to-binder
Compressive strength†
ratio
7d
%
–
1.7
0.50
28 d N mm–2
36
42
450
160
–
2.7
0.36
–
69
500
135
–
3.1
0.30
–
88
C 85-SF
455
160
30
2.4
0.33
–
92
C 115-SF
550
135
45
6.1
0.23
–
122
*Addiment, FM 93 Determined on cubes 100 × 100 × 100 mm3
†
Modelling of chloride-induced corrosion of reinforcement
93
Ring (noble metal) Cable
Top view
A
A Section A-A Cable
Concrete surface
2.5 2.5 2.5
Electrolytic resistance in Ω
7 12 17 22 27 32 37 42
Distance from surface
(mm)
8.3 Schematic presentation of the multi-ring electrode (MRE). Concrete mixture C 35
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm
17 mm
32 mm
22 mm 27 mm
10000 37 mm
42 mm
1000
100 0
100
200 300 Concrete age (d)
400
500
8.4 Concrete resistivity of specimens C 35 at various distances from surface.
concrete (depth of ~42 mm) to increase with the strength of the concrete. The values show that the resistivity of C 115-SF is up to 10 times higher than that of the normal strength concrete (C 35). On adding silica fume, the resistivity is increased considerably (resistivity of C 85-0 after 1 year: 360 Ω m, C 85 with silica fume: 1080 Ω m) (Fig. 8.9).
94
Corrosion of reinforcement in concrete Concrete mixture C 65
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm
10000 17 mm 22 mm 1000
27 mm
32 mm
37 mm
42 mm
100 0
100
200 300 Concrete age (d)
400
500
8.5 Concrete resistivity of specimens C 65 at various distances from surface. Concrete mixture C85-0
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm 17 mm
10000
22 mm 27 mm 1000 32 mm 37 mm 42 mm 100 0
100
200 300 Concrete age (d)
400
500
8.6 Concrete resistivity of specimens C 85 at various distances from surface.
8.3.2
Results of potential measurements ER,A and ER,C
The average differences in rest potential between the anode and cathodes of the macrocells in each specimen were measured several times. The values differ between 370 mV and 460 mV for all corroding macrocells. No significant influence was observed for the different concrete mixtures or concrete covers (Fig. 8.10).
Modelling of chloride-induced corrosion of reinforcement
95
Concrete mixture C 85-SF
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm 17 mm
10000
22 mm 27 mm 32 mm 37 mm 42 mm
1000
100 0
100
200 300 Concrete age (d)
400
500
8.7 Concrete resistivity of specimens C 85-SF at various distances from surface. Concrete mixture C 115-SF
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm 17 mm
10000
22 mm 27 mm 32 mm 37 mm 42 mm 1000
100
0
100
200 300 Concrete age (d)
400
500
8.8 Concrete resistivity of specimens C 115-SF at various distances from surface.
8.3.3
Results of corrosion current measurements Ie
The currents for specimens made of the five different concrete mixtures with crack widths of 0.10, 0.25 and 0.50 mm were measured for 64 wet-dry cycles. The current has been recorded separately for three macrocells consisting of the four bottom cathodes (concrete cover: 47 mm) with different distances
Corrosion of reinforcement in concrete
Depth under concrete surface (mm)
96
Electrolytic resistivity of concrete (Ω m) 1000 10000
100 0
100000
Concrete age t = 50 d 10 C 65
C 85-S
20
30
C 35 C 85-0
Max. measurable value C 115-S
40
Depth under concrete surface (mm)
50 Electrolytic resistivity of concrete (Ω m) 1000 10000
100 0
100000
Concrete age t = 400 d C 85-S 10 Max. measurable value
C 65
20
C 115-S 30
40
C 35 C 85-0
50
8.9 Profiles of concrete resistivity for all specimens at concrete age of 50 days (upper) and 400 days (lower).
(50, 150 and 250 mm) from the crack and the anode and for three macrocells consisting of the four top cathodes (concrete cover: 23 mm) with different distances from the crack and the anode. To calculate the mass loss of the reinforcement it was necessary to integrate the current versus time. This cumulative charge is shown as a function of the number of wetting periods in Fig. 8.11. By far the highest corrosion charges from 0.5 mm wide cracks were recorded for the reference concrete mixture C 35 (average: 20369 µA d), the lowest for the silica fume containing concrete mixtures C 85-SF (5854 µA d) and C 115-SF (7235 µA d). For the concrete mixtures C 35 and C 65 it is obvious that the corrosion charge is only slightly dependent on crack width (e.g. C 65, crack width 0.10 mm: 6885 µA d, crack width 0.50 mm: 9505 µA d).
Modelling of chloride-induced corrosion of reinforcement Top cathodes c = 23 mm Bottom cathodes c = 47 mm
500
Voltage (mV)
97
450
400
350
300 0.10 0.25 0.50 C 35
0.10 0.25 0.50 0.50 0.50 C 65 C 85-0 C 85-SF
Crack width (mm) 0.50 Concrete mixture C 115-SF
Cumulative corrosion current (µA d)
8.10 Differences in rest potential between anode and cathode in mV (age of specimens: 187 to 383 days).
25000 20000 15000 10000 5000 0 0.10 C 35
0.25
0.50
0.10 0.25 C 65
0.50
0.50
C 85-0
0.50
0.50
C 85-SF
Specimens (average value of two macrocells)
C 115-SF
56 40 24 Number of wetting 8 and drying cycles Crack width (mm) Concrete mixture
8.11 Cumulative macrocell corrosion currents as a function of the number of wetting and drying cycles.
A decrease in the corrosion rate over time due to an increasing anodic polarisation resistance was not observed. However, it can not be excluded that this effect might occur in the course of time, which can only be verified by long-term investigations.
8.3.4
Cathodic current distribution
To evaluate possible changes in the current distribution of macrocells in high-performance concrete mixtures, the local currents for the different anode–
Balance of the current (%)
98
Corrosion of reinforcement in concrete
100% C 35; w = 0.10 mm C 65; w = 0.50 mm C 85-0; w = 0.50 mm C 85-SF; w = 0.50 mm C 115-SF; w = 0.50 mm
75%
50%
25% Top cathodes (c = 23 mm)
250
150
Bottom cathodes (c = 47 mm)
0% 50 50 Distance anode – cathode (mm)
150
250
8.12 Balances of the cathodic currents of the macrocells (w = crack width).
cathode distances have been calculated. Assuming that the total corrosion rate of the anode with the top cathodes is 100 %, the contribution of each anode–cathode cell (three cells with different distances from the crack) was evaluated. Figure 8.12 shows the determined current balances as percentages of the total current for the top cathodes in the left part. The same evaluation was made for the macrocells between anodes and the bottom cathodes (right part of Fig. 8.12). No systematic differences between the corrosion current of the top macrocell and the current of the bottom macrocell could be observed for all tested specimens. Furthermore, no change of the current distribution resulting from the use of high-performance concrete could be evaluated.
8.3.5
Visual examinations
To verify the results of the current measurement the specimens were broken after 64 cycles and the positions of the cracks in the specimens as well as the degree of corrosion were determined. Whereas the cracks in specimens with C 35 and C 65 crossed the anode (main rebar) in the planned uncoated area, in specimens of high-performance concrete with small crack widths (0.10 and 0.25 mm), the cracks were found to cross the anodes in the coated region. It seems that the more brittle the concrete the more the cracks formed at discontinuities in the coating of the reinforcement. The results for specimens where the crack crossed the coated region of the main rebar are neglected in the further evaluation.
Modelling of chloride-induced corrosion of reinforcement
99
However, the degree of corrosion was clearly related to the measured cumulative corrosion current according to Faraday’s law.
8.3.6
Influence of water-to-binder ratio on resistivity and corrosion current
Figure 8.13 shows the water-to-binder ratio of the concrete mixtures (see Table 8.1) versus the resistivity of the concrete measured with the multi-ring electrode at a depth of 42 mm. As expected, a decrease of the water-to-binder ratio leads to an increase in the resistivity of nearby water-saturated concrete. This increase is significantly higher when silica fume is used. In Fig. 8.14 the results of the corrosion current measurement (mean values for each concrete mixture) are related to the water-binder-ratio of the concrete mixtures. The determined average corrosion rate is strongly related to the water-to-binder ratio of the concrete in macrocell corrosion.
8.3.7
Relation between resistivity and corrosion current
Figure 8.15 shows the resistivity against the measured average corrosion rate, as drawn from Fig. 8.13 and 8.14. As can be seen from Fig. 8.15, the decrease of corrosion current is not linearly related to the increase of resistivity. Thus, the corrosion rate is not controlled totally by Rel, but mainly by the polarisation resistances RA and RC. (Multi-ring electrode: depth 42 mm)
Electrolytic resistivity (Ω m)
10000
C 115-S
C 85-S 1000
C 85-0
100 0.2
C 65-0
0.3 0.4 Water-to-binder ratio
C 35-0
0.5
8.13 Water-to-binder ratio of the concrete mixtures versus measured resistivity of the specimens at a depth of 42 mm.
100
Corrosion of reinforcement in concrete 50
Average corrosion current (µA)
C 35-0 40 C 85-0
30
20
C 65-0
C 115-S C 85-S
10
0 0.2
0.3 0.4 Water-to-binder ratio
0.5
8.14 Water-to-binder ratio of the concrete mixtures versus measured corrosion current. 50
Average corrosion current (µA)
C 35-0 40
30
C 65-0 C 85-0
20 C 115-S C 85-S
10
0 100
1000 Electrolytic resistivity (Ωm) (Multi-ring electrode: depth 42 mm)
10000
8.15 Electrolytic resistivity of the concrete (measured with multiringelectrode) versus average corrosion current.
8.4
Numerical simulation
To verify the results of Fig. 8.15 numerical simulations of the corrosion process have been carried out. Actively corroding steel may exhibit microcell action in which anodic and cathodic sites are randomly distributed over the exposure surface of the steel electrode, giving rise to uniform corrosion attack. In this condition, the polarisation behaviour of corroding steel is described by the following equation relating average current density, i, and potential change (overvoltage), ∆U:
Modelling of chloride-induced corrosion of reinforcement
ln(10) ∆U – ln(10) ∆U i = icorr exp – exp b bc a with
ln(10) ba bc icorr
101
(8.1)
= 2.303 = anodic Tafel slope = 90.7 mV dec–1 = cathodic Tafel slope = 176.3 mV dec–1 = self corrosion rate, here 1.0 µA cm –2 [4].
For passive steel it is assumed that anodic reactions can only proceed to a very limited extent. The electrochemical behaviour of passive reinforcing steel under cathodic polarisation is given by:
i = 1 – exp with
–ln(10) ∆U bc
1 – icorr
exp
– ln(10) ∆U bc ilim
(8.2)
ilim = limiting diffusion current density due to oxygen diffusion based on data obtained from experimental investigations [4].
Based on these equations, the corrosion current of the macrocells with coplanar arrangement of local anode and macro cathode as a function of the distance of the cathodes from the anodes can be calculated. Figure 8.16 shows the results for the C 35 and the C 115 concretes with Ue = 400 mV, Ra = 0 Ω and with electrolytic resistivity of 500 Ω m and 5000 Ω m, respectively. Figure 8.17 shows the results of calculations according the equations (8.1) and (8.2) with the potential as a function variable of the distance from the anode. Furthermore, the corrosion current of the specimens can be calculated as a function of the density of reinforcement. In Fig. 8.18, the curves of the
El. current density (µA cm–3)
0.15
I/b/h = 60/15/15 cm Ue = 400 mV 0.12
C 35, Rel = 500 Ω *m
0.09
0.06
0.03
C 115, Rel = 5000 Ω *m
0.00 6
12 18 24 Distance from the anode/crack (cm)
8.16 Calculated corrosion current of cracked specimens with C 35 and C 115-SF concrete as a function of the distance between anode and cathode.
30
102
Corrosion of reinforcement in concrete 0
–50
C 115, Rel = 5000 Ω *m
Potential (mV)
–100 –150 –200 –250 –300 –350 C 35, Rel = 500 Ω *m
–400
I/b/h = 60/15/15 cm Ue = 400 mV
–450 6
12 18 Distance from the anode/crack (cm)
24
30
8.17 Calculated distribution of the potential of cracked specimens with C 35 and C 115-SF concrete as a function of the distance between anode and cathode. 250 C 35, Rel = 500 Ω m
I/b/h = 60/15/15 cm Ue = 400 mV
Macrocell current (µA)
200
150
100 C 115, Rel = 5000 Ω m 50
0 0
5
10
15 20 25 30 35 Reinforcement density (cm2 cm–1)
40
45
50
8.18 Calculated corrosion current of cracked specimens with C 35 and C 115-SF concrete versus density of reinforcement.
calculation with Ue = 400 V and Ra = 0 Ω are shown. With increasing density of reinforcement, the corrosion current increases too. For the high-performance concrete C 115-SF (Rel = 5000 Ω m) the calculated corrosion current is lower than for the concrete C 35 (Rel = 500 Ω m), following a non-linear relationship. Figure 8.19 shows the ratios of calculated corrosion current of C 115-SF to the corrosion current of C 35. For concretes with a 10-fold electrolytic resistivity ratio, the corrosion current decreased only by a factor of two for
Modelling of chloride-induced corrosion of reinforcement
103
Rel. macrocell currents C 115/C 35
1
I/b/h = 60/15/15 cm Ue = 400 mV Rel,C115 = 5000 Ω *m Rel,C35 = 500 Ω *m
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0
10
20 30 Reinforcement density (cm2 cm–1)
40
50
8.19 Ratio of calculated corrosion current of cracked specimens with C 35 and C 115-SF concrete versus content of reinforcement.
low reinforcement densities. For high reinforcement densities this factor is about 4.
8.5
Conclusions
Tests have been carried out to determine the corrosion mechanisms of specimens produced with cracked high-performance concrete beams. As expected, the resistivity of the electrolyte (concrete) increases significantly in high-performance concrete. The resistivity of concrete mixture C 115 in humid conditions is about ten times higher than the resistivity of concrete mixture C 65 in humid conditions. This increase in the electrolytic resistivity leads to a reduction in the corrosion rates in specimens with high-performance concrete. Accordingly, the corrosion currents of macrocells in high-performance concrete mixtures are also reduced, e.g. the average corrosion rate of concrete mixtures C 85SF and C 115-SF is about 1/3 of the current of C 35 (Fig. 8.11) under the conditions investigated. It can be summarised that the mechanisms of corrosion do not change in high-performance concrete: the major type of corrosion is macrocell corrosion, the balances of currents as a function of the distance from the cathode to the anode remain nearly the same for normal and high-strength concrete and there is no sign of a reduction in the current due to insufficient space for corrosion products in specimens of high-performance concrete after 15 months of exposure to aggressive wet and drying cycling. Comparing the ratio of the increasing resistivity and the ratio of reduction of the corrosion rate with regard to the equation of Fig. 8.1, it can be determined
104
Corrosion of reinforcement in concrete
that the corrosion rate is not controlled totally by Rel, but mainly by the polarisation resistances RA and RC. These results have been confirmed by numerical simulations of the corrosion process. These simulations show that the corrosion rate is mainly dependent on the polarisation resistances of the anode and the cathode. The use of high-performance concrete with cracks exposed to severe chloride attack leads to a reduction in the corrosion rate of the anode in comparison to normal strength concrete. So the expected service lifetime of the structure can be prolonged significantly by using high-performance concrete. To investigate long term effects, like a possible anodic self polarisation, additional tests over increased periods are planned.
8.6
References
1. Guse, U. and Hilsdorf, H. K., Durability Aspects of High Strength Concrete. in HighPerformance Concrete. ACI International Conference, Supplementary Papers, Singapore, 1994, Malhotra, V. M., (Ed.), American Concrete Institute, Detroit 1994. 229–250. 2. Raupach, M., ‘Chloride-induced macrocell corrosion of steel in concrete – theoretical background and practical consequences. Constr. Build. Mater. 1996, 10(5), 329–338. 3. Raupach, M., ‘Corrosion of steel in the area of cracks in concrete – laboratory test and calculations using a transmission line model. Corrosion of Reinforcement in Concrete Construction, 4th International Symposium, Cambridge, UK, 1–4 July 1996, Page, C. L.; Bamforth, P. B.; Figg, J. W. (Eds.), The Royal Society of Chemistry, Cambridge, 1996, 13–23. 4. Raupach, M. and Gulikers, J., Electrochemical models for corrosion of steel in concrete – introduction for the planned new EFC-WP11 Task Group, EUROCORR 2000. 5. Raupach, M. and Gulikers, J., ‘A simplified method to estimate corrosion rates – a new approach based on investigations of macrocells, in 8th International Conference on Durability of Building Materials & Components – Service Life and Asset Management, Vancouver, May 30–June 3, 1999, Vol. 1, 376–385. 6. Schießl, P., Breit, W. and Raupach, M., ‘Investigations into the effect of coatings on water distribution in concrete using multi-ring electrodes, in Concrete Bridges in Aggressive Environments, Philip D. Cady International Symposium, Minneapolis, November 9–10, 1993, Weyers, R. E. (Ed.), American Concrete Institute, Detroit ACI SP-151, 1994, 119–133.
8.1
Background
High-strength concrete with a compressive strength above 80 N mm–2 has been used for many years. Besides increasing the load-bearing capacity of the concrete one important aspect for the development of high-performance concrete mixtures is the durability, especially the resistance against chloride diffusion. Numerous investigations have been performed with regard to the abrasion resistance, capillary water suction, permeability to gases and liquids, chloride diffusion resistance or resistance against frost and de-icing salts [1–5]. Nevertheless, questions remain with regard to the behaviour of the reinforcement in high-performance concrete structures when exposed to aggressive environmental conditions. Cracks in reinforced concrete structures allow aggressive agents like chlorides from de-icing salts or sea-water to penetrate into the concrete. As shown in Fig. 8.1, the corrosion rate of a macrocell is a function of the anodic polarisation resistance, RA, the cathodic polarisation resistance, RC, the resistivity of the electrolyte (concrete), Rel, and the difference between the rest potentials at the anode (ER,A) and the cathode (ER,C). It can be deduced from the equation that the corrosion rate is reduced if only one of the three resistances (RA, RC and Rel), especially the most corrosion rate determining resistance, increases. Due to the low permeability and high electrolytic resistivity of highperformance concrete, it might be expected that the corrosion rates of steel in the area of cracks in concrete are far lower than in normal concrete. Another reason for a possible reduction of corrosion rates in high-performance concrete may be the restricted volume expansion of corrosion products at the anode due to the limited space in small cracks causing the anodic polarisation resistance, RA, to increase with time. In order to evaluate the corrosion behaviour of steel in cracked highperformance concrete, laboratory tests have been performed on cracked concrete beams. 89
90
Corrosion of reinforcement in concrete Crack
Ιe =
Concrete
Rel
E R,C – E R,A R A + R C + R el
ER,C RA
∆ E c = Rc I e
RC
ER,A Steel
ER,C
RSt ≈ 0
∆E
∆E
EC,C
∆ Eel = Rel Ie
EC,A
∆EA = RA Ie
Ie Anode
Cathode
ER,A Ie
8.1 Simplified electrical circuit model for the corrosion of steel in cracked concrete [ER,C and ER,A: rest potentials at the cathode and anode; RA, RC and Rel: polarisation resistances at the anode, the cathode and resistivity of the electrolyte/concrete; Ie: macrocell current (~ corrosion rate)] [2]. Section B-B 25 100 25
Section A-A 100
50
Uncoated anodic steel area
Mild steel cathodes
350 Chloride solution 1% Crack
Epoxy coated rebar Activated titanium cathodes
150
100
Titanium cathode 150
700 Top view
䉯B Anode Rebar Ø 12 mm
A
A
50
䉯B 600 700
50
Cathodes: Mild steel Epoxy coated
(Measures in mm)
8.2 Design of test specimens.
8.2
Test programme
The design of the test specimens (Fig. 8.2) allowed the measurement of various corrosion parameters such as macrocell corrosion currents (anodic and cathodic currents), corrosion potentials and electrolytic resistance of the concrete. The main rebar intersecting the crack was designed to be the anode, therefore the active area was restricted by coating the reinforcement in the uncracked area. The cathodes consisted of 24 mild steel rebars, fixed at
Modelling of chloride-induced corrosion of reinforcement
91
defined distances from the crack (50 to 250 mm) and with two concrete covers (23 and 47 mm). After concreting, the specimens were stored for 7 days in a fog-room and afterwards for another 21 days in a climate at 20 °C and 65 % rh. The cracks were formed by fixing the specimens in steel frames and by bending them to the designed crack widths (0.1, 0.25 and 0.5 mm at the surface of the beams). Corrosive conditions were created by wet– dry cycles (1 day immersed with a 1 % chloride solution followed by 6 days dry in a laboratory atmosphere at about 65 % rh). In order to evaluate the corrosion behaviour of reinforcing steel bars in the cracked concrete beams, macrocell corrosion currents, corrosion potentials and electrolytic resistances were measured over a period of 64 cycles (at least 15 months). The concrete mixtures used in the tests are specified in Table 8.1.
8.3
Results
8.3.1
Measurement of the electrolytic resistivity of the concrete, Rel
To measure the resistivity over time at different depths an embedded multiring electrode (MRE) was used [6]. The sensor consisted of several stainlesssteel rings maintained at a defined distance from one another by insulating plastic rings (Fig. 8.3). Cable connections through the sensor enabled the resistivity of the concrete to be determined between each pair of neighbouring stainless-steel rings by means of impedance measurements. The ac resistance values (in Ω) can be converted to resistivity values by a sensor-specific transfer factor determined in aqueous solutions with known conductivity. The standard type of the multi-ring electrode-sensor allowed the measurement of eight resistances between nine rings, down to a distance of 42 mm from the concrete surface. The specimens were stored at 22 °C and 52 % rh (average values). In Fig. 8.4–8.8 the results of the resistivity measurements of the different types of concrete are presented. The highest measurable value is 20 kΩ m. A significant increase in the resistivity with time is noted for all depths for the C 35 concrete mixture (Fig. 8.4). This increase for all depths can be explained because this comparably permeable concrete had fully dried, whereas concrete mixtures C 65 and C 85-0 (Fig. 8.5 and 8.6) had only dried to about 17 mm as a significant rise can be observed only for the curves of 7, 12 and 17 mm. For the concrete mixtures produced with silica fume (Fig. 8.7 and 8.8) drying only occurred to a depth of 12 mm. All curves show a clear profile of decreasing resistivity with increasing depth. There is a tendency for the resistivity of the humid inner area of
Table 8.1 Concrete mix proportions Mixture
–
Cement
Cement
type
content
300
C 65 C 85-0
OPC
Silica fume
kg m3
–
C 35
Water
150
–
Super-plasticiser*
Water-to-binder
Compressive strength†
ratio
7d
%
–
1.7
0.50
28 d N mm–2
36
42
450
160
–
2.7
0.36
–
69
500
135
–
3.1
0.30
–
88
C 85-SF
455
160
30
2.4
0.33
–
92
C 115-SF
550
135
45
6.1
0.23
–
122
*Addiment, FM 93 Determined on cubes 100 × 100 × 100 mm3
†
Modelling of chloride-induced corrosion of reinforcement
93
Ring (noble metal) Cable
Top view
A
A Section A-A Cable
Concrete surface
2.5 2.5 2.5
Electrolytic resistance in Ω
7 12 17 22 27 32 37 42
Distance from surface
(mm)
8.3 Schematic presentation of the multi-ring electrode (MRE). Concrete mixture C 35
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm
17 mm
32 mm
22 mm 27 mm
10000 37 mm
42 mm
1000
100 0
100
200 300 Concrete age (d)
400
500
8.4 Concrete resistivity of specimens C 35 at various distances from surface.
concrete (depth of ~42 mm) to increase with the strength of the concrete. The values show that the resistivity of C 115-SF is up to 10 times higher than that of the normal strength concrete (C 35). On adding silica fume, the resistivity is increased considerably (resistivity of C 85-0 after 1 year: 360 Ω m, C 85 with silica fume: 1080 Ω m) (Fig. 8.9).
94
Corrosion of reinforcement in concrete Concrete mixture C 65
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm
10000 17 mm 22 mm 1000
27 mm
32 mm
37 mm
42 mm
100 0
100
200 300 Concrete age (d)
400
500
8.5 Concrete resistivity of specimens C 65 at various distances from surface. Concrete mixture C85-0
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm 17 mm
10000
22 mm 27 mm 1000 32 mm 37 mm 42 mm 100 0
100
200 300 Concrete age (d)
400
500
8.6 Concrete resistivity of specimens C 85 at various distances from surface.
8.3.2
Results of potential measurements ER,A and ER,C
The average differences in rest potential between the anode and cathodes of the macrocells in each specimen were measured several times. The values differ between 370 mV and 460 mV for all corroding macrocells. No significant influence was observed for the different concrete mixtures or concrete covers (Fig. 8.10).
Modelling of chloride-induced corrosion of reinforcement
95
Concrete mixture C 85-SF
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm 17 mm
10000
22 mm 27 mm 32 mm 37 mm 42 mm
1000
100 0
100
200 300 Concrete age (d)
400
500
8.7 Concrete resistivity of specimens C 85-SF at various distances from surface. Concrete mixture C 115-SF
Resistivity of concrete (Ω m)
100000
Max. measurable value 7 mm
12 mm 17 mm
10000
22 mm 27 mm 32 mm 37 mm 42 mm 1000
100
0
100
200 300 Concrete age (d)
400
500
8.8 Concrete resistivity of specimens C 115-SF at various distances from surface.
8.3.3
Results of corrosion current measurements Ie
The currents for specimens made of the five different concrete mixtures with crack widths of 0.10, 0.25 and 0.50 mm were measured for 64 wet-dry cycles. The current has been recorded separately for three macrocells consisting of the four bottom cathodes (concrete cover: 47 mm) with different distances
Corrosion of reinforcement in concrete
Depth under concrete surface (mm)
96
Electrolytic resistivity of concrete (Ω m) 1000 10000
100 0
100000
Concrete age t = 50 d 10 C 65
C 85-S
20
30
C 35 C 85-0
Max. measurable value C 115-S
40
Depth under concrete surface (mm)
50 Electrolytic resistivity of concrete (Ω m) 1000 10000
100 0
100000
Concrete age t = 400 d C 85-S 10 Max. measurable value
C 65
20
C 115-S 30
40
C 35 C 85-0
50
8.9 Profiles of concrete resistivity for all specimens at concrete age of 50 days (upper) and 400 days (lower).
(50, 150 and 250 mm) from the crack and the anode and for three macrocells consisting of the four top cathodes (concrete cover: 23 mm) with different distances from the crack and the anode. To calculate the mass loss of the reinforcement it was necessary to integrate the current versus time. This cumulative charge is shown as a function of the number of wetting periods in Fig. 8.11. By far the highest corrosion charges from 0.5 mm wide cracks were recorded for the reference concrete mixture C 35 (average: 20369 µA d), the lowest for the silica fume containing concrete mixtures C 85-SF (5854 µA d) and C 115-SF (7235 µA d). For the concrete mixtures C 35 and C 65 it is obvious that the corrosion charge is only slightly dependent on crack width (e.g. C 65, crack width 0.10 mm: 6885 µA d, crack width 0.50 mm: 9505 µA d).
Modelling of chloride-induced corrosion of reinforcement Top cathodes c = 23 mm Bottom cathodes c = 47 mm
500
Voltage (mV)
97
450
400
350
300 0.10 0.25 0.50 C 35
0.10 0.25 0.50 0.50 0.50 C 65 C 85-0 C 85-SF
Crack width (mm) 0.50 Concrete mixture C 115-SF
Cumulative corrosion current (µA d)
8.10 Differences in rest potential between anode and cathode in mV (age of specimens: 187 to 383 days).
25000 20000 15000 10000 5000 0 0.10 C 35
0.25
0.50
0.10 0.25 C 65
0.50
0.50
C 85-0
0.50
0.50
C 85-SF
Specimens (average value of two macrocells)
C 115-SF
56 40 24 Number of wetting 8 and drying cycles Crack width (mm) Concrete mixture
8.11 Cumulative macrocell corrosion currents as a function of the number of wetting and drying cycles.
A decrease in the corrosion rate over time due to an increasing anodic polarisation resistance was not observed. However, it can not be excluded that this effect might occur in the course of time, which can only be verified by long-term investigations.
8.3.4
Cathodic current distribution
To evaluate possible changes in the current distribution of macrocells in high-performance concrete mixtures, the local currents for the different anode–
Balance of the current (%)
98
Corrosion of reinforcement in concrete
100% C 35; w = 0.10 mm C 65; w = 0.50 mm C 85-0; w = 0.50 mm C 85-SF; w = 0.50 mm C 115-SF; w = 0.50 mm
75%
50%
25% Top cathodes (c = 23 mm)
250
150
Bottom cathodes (c = 47 mm)
0% 50 50 Distance anode – cathode (mm)
150
250
8.12 Balances of the cathodic currents of the macrocells (w = crack width).
cathode distances have been calculated. Assuming that the total corrosion rate of the anode with the top cathodes is 100 %, the contribution of each anode–cathode cell (three cells with different distances from the crack) was evaluated. Figure 8.12 shows the determined current balances as percentages of the total current for the top cathodes in the left part. The same evaluation was made for the macrocells between anodes and the bottom cathodes (right part of Fig. 8.12). No systematic differences between the corrosion current of the top macrocell and the current of the bottom macrocell could be observed for all tested specimens. Furthermore, no change of the current distribution resulting from the use of high-performance concrete could be evaluated.
8.3.5
Visual examinations
To verify the results of the current measurement the specimens were broken after 64 cycles and the positions of the cracks in the specimens as well as the degree of corrosion were determined. Whereas the cracks in specimens with C 35 and C 65 crossed the anode (main rebar) in the planned uncoated area, in specimens of high-performance concrete with small crack widths (0.10 and 0.25 mm), the cracks were found to cross the anodes in the coated region. It seems that the more brittle the concrete the more the cracks formed at discontinuities in the coating of the reinforcement. The results for specimens where the crack crossed the coated region of the main rebar are neglected in the further evaluation.
Modelling of chloride-induced corrosion of reinforcement
99
However, the degree of corrosion was clearly related to the measured cumulative corrosion current according to Faraday’s law.
8.3.6
Influence of water-to-binder ratio on resistivity and corrosion current
Figure 8.13 shows the water-to-binder ratio of the concrete mixtures (see Table 8.1) versus the resistivity of the concrete measured with the multi-ring electrode at a depth of 42 mm. As expected, a decrease of the water-to-binder ratio leads to an increase in the resistivity of nearby water-saturated concrete. This increase is significantly higher when silica fume is used. In Fig. 8.14 the results of the corrosion current measurement (mean values for each concrete mixture) are related to the water-binder-ratio of the concrete mixtures. The determined average corrosion rate is strongly related to the water-to-binder ratio of the concrete in macrocell corrosion.
8.3.7
Relation between resistivity and corrosion current
Figure 8.15 shows the resistivity against the measured average corrosion rate, as drawn from Fig. 8.13 and 8.14. As can be seen from Fig. 8.15, the decrease of corrosion current is not linearly related to the increase of resistivity. Thus, the corrosion rate is not controlled totally by Rel, but mainly by the polarisation resistances RA and RC. (Multi-ring electrode: depth 42 mm)
Electrolytic resistivity (Ω m)
10000
C 115-S
C 85-S 1000
C 85-0
100 0.2
C 65-0
0.3 0.4 Water-to-binder ratio
C 35-0
0.5
8.13 Water-to-binder ratio of the concrete mixtures versus measured resistivity of the specimens at a depth of 42 mm.
100
Corrosion of reinforcement in concrete 50
Average corrosion current (µA)
C 35-0 40 C 85-0
30
20
C 65-0
C 115-S C 85-S
10
0 0.2
0.3 0.4 Water-to-binder ratio
0.5
8.14 Water-to-binder ratio of the concrete mixtures versus measured corrosion current. 50
Average corrosion current (µA)
C 35-0 40
30
C 65-0 C 85-0
20 C 115-S C 85-S
10
0 100
1000 Electrolytic resistivity (Ωm) (Multi-ring electrode: depth 42 mm)
10000
8.15 Electrolytic resistivity of the concrete (measured with multiringelectrode) versus average corrosion current.
8.4
Numerical simulation
To verify the results of Fig. 8.15 numerical simulations of the corrosion process have been carried out. Actively corroding steel may exhibit microcell action in which anodic and cathodic sites are randomly distributed over the exposure surface of the steel electrode, giving rise to uniform corrosion attack. In this condition, the polarisation behaviour of corroding steel is described by the following equation relating average current density, i, and potential change (overvoltage), ∆U:
Modelling of chloride-induced corrosion of reinforcement
ln(10) ∆U – ln(10) ∆U i = icorr exp – exp b bc a with
ln(10) ba bc icorr
101
(8.1)
= 2.303 = anodic Tafel slope = 90.7 mV dec–1 = cathodic Tafel slope = 176.3 mV dec–1 = self corrosion rate, here 1.0 µA cm –2 [4].
For passive steel it is assumed that anodic reactions can only proceed to a very limited extent. The electrochemical behaviour of passive reinforcing steel under cathodic polarisation is given by:
i = 1 – exp with
–ln(10) ∆U bc
1 – icorr
exp
– ln(10) ∆U bc ilim
(8.2)
ilim = limiting diffusion current density due to oxygen diffusion based on data obtained from experimental investigations [4].
Based on these equations, the corrosion current of the macrocells with coplanar arrangement of local anode and macro cathode as a function of the distance of the cathodes from the anodes can be calculated. Figure 8.16 shows the results for the C 35 and the C 115 concretes with Ue = 400 mV, Ra = 0 Ω and with electrolytic resistivity of 500 Ω m and 5000 Ω m, respectively. Figure 8.17 shows the results of calculations according the equations (8.1) and (8.2) with the potential as a function variable of the distance from the anode. Furthermore, the corrosion current of the specimens can be calculated as a function of the density of reinforcement. In Fig. 8.18, the curves of the
El. current density (µA cm–3)
0.15
I/b/h = 60/15/15 cm Ue = 400 mV 0.12
C 35, Rel = 500 Ω *m
0.09
0.06
0.03
C 115, Rel = 5000 Ω *m
0.00 6
12 18 24 Distance from the anode/crack (cm)
8.16 Calculated corrosion current of cracked specimens with C 35 and C 115-SF concrete as a function of the distance between anode and cathode.
30
102
Corrosion of reinforcement in concrete 0
–50
C 115, Rel = 5000 Ω *m
Potential (mV)
–100 –150 –200 –250 –300 –350 C 35, Rel = 500 Ω *m
–400
I/b/h = 60/15/15 cm Ue = 400 mV
–450 6
12 18 Distance from the anode/crack (cm)
24
30
8.17 Calculated distribution of the potential of cracked specimens with C 35 and C 115-SF concrete as a function of the distance between anode and cathode. 250 C 35, Rel = 500 Ω m
I/b/h = 60/15/15 cm Ue = 400 mV
Macrocell current (µA)
200
150
100 C 115, Rel = 5000 Ω m 50
0 0
5
10
15 20 25 30 35 Reinforcement density (cm2 cm–1)
40
45
50
8.18 Calculated corrosion current of cracked specimens with C 35 and C 115-SF concrete versus density of reinforcement.
calculation with Ue = 400 V and Ra = 0 Ω are shown. With increasing density of reinforcement, the corrosion current increases too. For the high-performance concrete C 115-SF (Rel = 5000 Ω m) the calculated corrosion current is lower than for the concrete C 35 (Rel = 500 Ω m), following a non-linear relationship. Figure 8.19 shows the ratios of calculated corrosion current of C 115-SF to the corrosion current of C 35. For concretes with a 10-fold electrolytic resistivity ratio, the corrosion current decreased only by a factor of two for
Modelling of chloride-induced corrosion of reinforcement
103
Rel. macrocell currents C 115/C 35
1
I/b/h = 60/15/15 cm Ue = 400 mV Rel,C115 = 5000 Ω *m Rel,C35 = 500 Ω *m
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0
10
20 30 Reinforcement density (cm2 cm–1)
40
50
8.19 Ratio of calculated corrosion current of cracked specimens with C 35 and C 115-SF concrete versus content of reinforcement.
low reinforcement densities. For high reinforcement densities this factor is about 4.
8.5
Conclusions
Tests have been carried out to determine the corrosion mechanisms of specimens produced with cracked high-performance concrete beams. As expected, the resistivity of the electrolyte (concrete) increases significantly in high-performance concrete. The resistivity of concrete mixture C 115 in humid conditions is about ten times higher than the resistivity of concrete mixture C 65 in humid conditions. This increase in the electrolytic resistivity leads to a reduction in the corrosion rates in specimens with high-performance concrete. Accordingly, the corrosion currents of macrocells in high-performance concrete mixtures are also reduced, e.g. the average corrosion rate of concrete mixtures C 85SF and C 115-SF is about 1/3 of the current of C 35 (Fig. 8.11) under the conditions investigated. It can be summarised that the mechanisms of corrosion do not change in high-performance concrete: the major type of corrosion is macrocell corrosion, the balances of currents as a function of the distance from the cathode to the anode remain nearly the same for normal and high-strength concrete and there is no sign of a reduction in the current due to insufficient space for corrosion products in specimens of high-performance concrete after 15 months of exposure to aggressive wet and drying cycling. Comparing the ratio of the increasing resistivity and the ratio of reduction of the corrosion rate with regard to the equation of Fig. 8.1, it can be determined
104
Corrosion of reinforcement in concrete
that the corrosion rate is not controlled totally by Rel, but mainly by the polarisation resistances RA and RC. These results have been confirmed by numerical simulations of the corrosion process. These simulations show that the corrosion rate is mainly dependent on the polarisation resistances of the anode and the cathode. The use of high-performance concrete with cracks exposed to severe chloride attack leads to a reduction in the corrosion rate of the anode in comparison to normal strength concrete. So the expected service lifetime of the structure can be prolonged significantly by using high-performance concrete. To investigate long term effects, like a possible anodic self polarisation, additional tests over increased periods are planned.
8.6
References
1. Guse, U. and Hilsdorf, H. K., Durability Aspects of High Strength Concrete. in HighPerformance Concrete. ACI International Conference, Supplementary Papers, Singapore, 1994, Malhotra, V. M., (Ed.), American Concrete Institute, Detroit 1994. 229–250. 2. Raupach, M., ‘Chloride-induced macrocell corrosion of steel in concrete – theoretical background and practical consequences. Constr. Build. Mater. 1996, 10(5), 329–338. 3. Raupach, M., ‘Corrosion of steel in the area of cracks in concrete – laboratory test and calculations using a transmission line model. Corrosion of Reinforcement in Concrete Construction, 4th International Symposium, Cambridge, UK, 1–4 July 1996, Page, C. L.; Bamforth, P. B.; Figg, J. W. (Eds.), The Royal Society of Chemistry, Cambridge, 1996, 13–23. 4. Raupach, M. and Gulikers, J., Electrochemical models for corrosion of steel in concrete – introduction for the planned new EFC-WP11 Task Group, EUROCORR 2000. 5. Raupach, M. and Gulikers, J., ‘A simplified method to estimate corrosion rates – a new approach based on investigations of macrocells, in 8th International Conference on Durability of Building Materials & Components – Service Life and Asset Management, Vancouver, May 30–June 3, 1999, Vol. 1, 376–385. 6. Schießl, P., Breit, W. and Raupach, M., ‘Investigations into the effect of coatings on water distribution in concrete using multi-ring electrodes, in Concrete Bridges in Aggressive Environments, Philip D. Cady International Symposium, Minneapolis, November 9–10, 1993, Weyers, R. E. (Ed.), American Concrete Institute, Detroit ACI SP-151, 1994, 119–133.