6.1 Case Study 1 – Uncracked Concrete
6.1.6 Interpretation of Results
Data presented in Chapter 2 concerning the electrical resistivity of PC and SFRC (0.5 vol.-% and 1.0 vol.-%) for selected moisture conditions (RH = 45%, RH = 75% and saturated) at T = 20°C is applied to interprete the impact of conductive steel fibres and the moisture content of the concrete on the corrosion rate of embedded reinforce-ment. Data adapted from Chapter 2, viz. the average of the measured electrical resis-tivity and the standard deviation is given in Table 6.4 (data corresponds to Series C in Chapter 2).
Table 6.4 Experimentally obtained values of the electrical resistivity of concrete (PC and SFRC), see Chapter 2.
Material Unit RH = 45 % RH = 75 % Saturated
Avg. St. dev Avg. St. dev Avg. St. dev
PC [Ωm] 5894 397 445 86 172 3
SFRC (0.5 vol.-%) [Ωm] 349 95 64 18 46 8
SFRC (1.0 vol.-%) [Ωm] 106 52 33 17 21 3
It will be seen from a comparison of Figure 6.2, Table 6.1 and Table 6.4 that the aver-age corrosion current densities (and the corrosion rate) corresponding to the measured values of the electrical resistivity are non-negligible (apart from the average corrosion current density corresponding to the measured value of the electrical resistivity of PC at RH = 45%).
The categorization of the corrosion-rate (Table 6.1) is presented in selected figures in the following, for easy identification of the corrosion categories.
6.1 Case Study 1 – Uncracked Concrete Chapter 6 Case Studies
6.1.6.1 Impact of Moisture
The impact of changes in the moisture content of the concrete on the average corro-sion current density of embedded reinforcement is illustrated in Figure 6.3. Results presented in Figure 6.3 are calculated from the average electrical resistivity ± the standard deviation. The input for the numerical simulations presented in the figure, ie the electrical resistivity correspond to the electrical resistivity of PC at T = 20°C.
Figure 6.3 Average corrosion current density, icor,av, as a function of the anode to cathode ratio (input data corresponds to average measured data ± the standard deviation for PC at T = 20°C at various moisture conditions).
It appears from Figure 6.3 that the average corrosion current density is changed by a factor of approx. 6, comparing the average corrosion current density for RH = 45%
and RH = 75%, regardless of the anode to cathode ratio. According to Table 2.7, these relative humidities correspond to a moisture content of 2.3 wt,-% and 3.7 wt.-%, re-spectively.
Comparing the average corrosion current densities corresponding to the electrical re-sistivity of PC at RH = 75% and saturated, it appears from Figure 6.3 that the average corrosion current density is changed by a factor of approx. 2 for all anode to cathode ratios studied. The moisture content of these two conditions correspond to 3.7 wt.-%
and 5.0 wt.-% for RH = 45%, cf. Table 2.7.
The absolute difference in moisture content from RH = 45% to RH = 75% and RH = 75% to saturated is approx. the same, viz. 1.3 wt.-%, cf. Table 2.7. As would be ex-pected, the impact on the electrical resistivity (and thereby the average corrosion cur-rent density) is changed remarkably as the relationship between the moisture content and the electrical resistivity of concrete is non-linear and the largest influence is seen for relatively dry concrete [Hötte, 2003].
For all anode to cathode ratios and conditionings simulated, the calculated corrosion rate is categorized as either low/negligible.
Chapter 6 6.1 Case Study 1 – Uncracked Concrete Case Studies
6.1.6.2 Impact of Conductive Steel Fibres
The average corrosion current density for the average electrical resistivities presented in Table 6.4 are presented in Figure 6.4. For each conditioning, the average corrosion current density is calculated based on the average value of the electrical resistivity measured for each material, ie PC or SFRC (0.5 vol.-% and 1.0 vol.-%). Additionally, for each conditioning the range of the average corrosion current density per material is presented. This range of the average corrosion current density is calculated from the average electrical resistivity ± the standard deviation for each material and represent-ed with errors bars. The same axes have been usrepresent-ed for all figures to ease comparision of graphs in different figures.
(a) (b)
(c)
Figure 6.4 Average corrosion current density, icor,av, as a function of the anode to cathode ratio. Input data corresponds to average measured data ± the standard deviation for PC, SFRC (0.5 vol.-% and 1.0 vol.-%), at T = 20°C and various conditionings. (a): RH = 45%. (b): RH = 75%.
(c): saturated conditions.
The results presented in Figure 6.4 are normalized with the average corrosion current density for PC in Table 6.5, to illustrate the relative impact on the average corrosion
6.1 Case Study 1 – Uncracked Concrete Chapter 6 Case Studies
Table 6.5 Normalized average corrosion current density (for all anode to cathode ratios) for all materials and conditionings investigated.
Unit PC SFRC (0.5 vol.-%) SFRC (1.0 vol.-%)
RH = 45% [-] 1 ~7 ~17
RH = 75% [-] 1 ~4 ~6
Saturated [-] 1 ~2 ~4
The results presented in Table 6.5 indicate that the impact of conductive steel fibres on the average corrosion current density due to their change of the electrical resistivity is reduced when the moisture content is increased.
For a low moisture content (concrete conditioned to RH = 45%, Figure 6.4a) the rela-tive impact of conducrela-tive steel fibres is large. However, the average corrosion current density is still relatively small and for SFRC (1.0 vol.-%) the corrosion rate category is low/moderate (compared to the corrosion rate of PC which can be categorized as negligible). It will be seen from Figure 6.4b that the average corrosion current density at RH = 75%, in PC corresponds to a negligible/low corrosion rate. The average cor-rosion current density for SFRC, 0.5 vol.-% as well as 1.0 vol.-%, is higher, and can be categorized as low/intermediate, depending on the fibre volume content and the anode to cathode ratio. Finally, for saturated concrete the corrosion rate is categorized as low in PC and low/intermediate for SFRC (0.5 vol.-% and 1.0 vol.-%).
6.1.6.3 Impact of Temperature
The impact of the temperature on the average corrosion current density is illustrated in Figure 6.5. The figure presents the average corrosion current density, icor,av, as a func-tion of the anode to cathode ratio (ρ ≈ 450 Ωm).
Figure 6.5 Average corrosion current density, icor,av, as a function of the anode to cathode ratio for ρ ≈ 450 Ωm.
It will be seen from Figure 6.5 that the lower the temperature, the lower the average corrosion current density, icor,av, which is in accordance with the apriori knowledge that increased temperatures increase the corrosion rate as described in eg [Bertolini et al., 2004]. Similar observations have been observed for other values of the electrical
Chapter 6 6.1 Case Study 1 – Uncracked Concrete Case Studies
resistivity investigated as part of this case study, though not reported here. The maxi-mum relative increase in the average corrosion current density within the range of the parameters investigated herein, cf. Table 6.3, is less than 50% when increasing the temperature from T = 0°C to T = 40°C. Thus, compared to the other parameters af-fecting the electrical resistivity, as discussed above, the impact of the temperature on the corrosion rate is minor.
6.1.7 Summary
Based on the results presented in Section 6.1.6 it is summarized:
- For values of the electrical resistivity above approx. 1000 Ωm the corrosion rate can be categorized as negligible.
- For values of the electrical resistivity below 1000 Ωm the average corrosion current density is non-negligible. This range of the electrical resistivity corre-sponds to the values measured for PC and SFRC (0.5 vol.-% and 1.0 vol.-% by the use of AC at 126 Hz.), see Chapter 2.
- Considering the measured values of the electrical resistivity of PC, the corre-sponding corrosion rate can be categorized as negligible/low, depending on the conditioning of the concrete.
- For SFRC (0.5 vol.-%) the corrosion rate can be categorized as low/intermediate depending on the conditioning of the concrete, whereas the corrosion rate of SFRC (1.0 vol.-%) is categorized as low/intermediate. Hence conductive steel fibres (corresponding to depassivated or corroding steel fi-bres) potentially increase the corrosion rate from low to intermediate in satu-rated conditions.
- The largest relative difference in the average corrosion current between PC and SFRC is observed for concrete with a low moisture content. The corrosion rate for low moisture contents, ie conditioned to RH = 45% is categorized as low/moderate.
- An increase in the temperature results in an increasing average corrosion cur-rent density. However, compared to the influence of moisture and/or conduc-tive steel fibres, the impact of the temperature is considered subordinate.
It is stressed that the scenario where all steel fibres conduct current relates to a scenar-io where all fibres are depassivated. As prevscenar-iously presented (Chapter 1), this scenarscenar-io is somewhat hypothetical. In practical applications utilizing combined reinforcement systems this would correspond to a scenario where the traditional reinforcement pre-sumeably has suffered from corrosion for some time, since the fibres are more corro-sion resistant than traditional reinforcement, see descriptions of corrocorro-sion-resistance of traditional reinforcement bars and steel fibres in Chapter 1.