REDUCTION OF 40-YEAR-OLD REINFORCED CONCRETE BEAMS IN NATURAL CORROSION ENVIRONMENTS
3.4.4 Progress of Deterioration and Performance Degradation
The deterioration progress from chloride attack involves the initiation, propagation, acceleration, and deterioration stages. In each stage, the deterioration has different influences on the beam structure. The degree of deterioration progress with performance degradation also varies according to the performance attribute under investigation. Therefore, according to data collected from evaluation after 10 [3.22], 20 [3.23-25], and 40 years of exposure, the
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deterioration degree is determined and presented in Table 3.8. Table 3.8 was constructed based on the criteria for the diagnosis of deterioration degree, as presented in Table 3.7.
Table 3.7 Criteria for diagnosis of deterioration degree Evaluation
Items
Deterioration Degree
0 1 2 3 4 5
Corrosion of reinforcing steel
None Rust spots found on concrete surface
Partial rust stains found on concrete surface
Significant rust staining
Significant floating rust
Dramatic increase in amount of floating rust Cracking None Partial
cracks found on concrete surface
Some cracks
Many cracks, including some of several millimeter s or more in width
Many cracks of several millimeter s in width
-
Spalling covering concrete
None None Partial
floating concrete found
Partial spalling found
Significant spalling
Drastic spalling
Table 3.8 Deterioration degree
Evaluation Items
Deterioration Degree
10 years 20 years 40 years
RC-S-A RC-S-B RC-S-A RC-S-B RC-S-A RC-S-B Corrosion of
reinforcing steel
1 0 2 1 5 2
Cracking 1 1 2 1 4 2
Spalling covering concrete
0 0 1 0 4 0
Total evaluation 2 1 5 2 13 4
The long-term exposure of concrete to marine environments has been studied by PARI since 1975 to evaluate the deterioration progress and performance
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degradation of RC beams. JSCE Standard Chapter 10, Maintenance of Structures Subject to Chloride Attack, Table C10.1.1 [32] was used to identify the deterioration progress. Moreover, the relationships among the deterioration progress, performance degradation, and exposure period are plotted in Fig. 3.25.
In this figure, the degree of deterioration progress is obtained from the total evaluation, according to Table 3. 8. The scale of deterioration, or the total evaluation, ranges from 0 to 15. The relationship of deterioration progress and exposure period can thereby be plotted. Furthermore, the performance degradation can be determined from the ratio of the ultimate flexural moment capacity to the initial (pre-exposure) ultimate flexural moment capacity. The ratio is plotted into the relationship between performance degradation and exposure period.
The concrete cover of the beam is 30 mm thick, which is thinner than that required by the JSCE Standard Specification [3.26] for durability. The minimum concrete cover needed in areas exposed to tidal and splash zones is 60 mm with a maximum water-to-cement ratio of 0.3 without cracking. Moreover, cracks induced by flexural bending moments provide access for chloride to the steel bar surface, thereby reducing the initiation stage [3.33-35]. Therefore, based on the estimation method proposed by Swamy et al. [3.36], the initiation stage is estimated to start after one year of exposure.
The propagation stage lasts from the initiation of steel bar corrosion until the instant of cracking due to corrosion. During the 10-year evaluation, the appearance of RC-S-B as a control beam showed that corrosion-related cracking had occurred in the concrete. It was observed that the longitudinal cracks aligned with the positions of the tensile bars. Therefore, the cracks were generated from corrosion cracking. In addition, after 10 years of exposure, the ultimate flexural moment capacity of RC-S-A was somewhat decreased, while that of RC-S-B was increased with increasing exposure time.
During the 20-year evaluation, the ultimate flexural moment capacity of RC-S-A was observed to increase dramatically. Meanwhile in RC-S-B, the ultimate flexural moment capacity was only slightly increased. In the RC-S-A beam, the cracks were widened and partial rust stains were found on the concrete surface. This indicates that the corrosion of the steel bars proceeded at a high rate,
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aided by corrosion-related concrete cracking. However, the ultimate flexural moment capacity was slightly increased and remained sufficient. Therefore, after 20 years of exposure, the evaluated beam was categorized as being in the acceleration stage for RC-S-A, where deterioration progress was increasing because of the high rate of corrosion but the ultimate flexural moment capacity was still sufficient.
Fig. 3.25 Progress of deterioration progress and performance degradation
After 40 years, the residual ultimate flexural moment capacity in the RC-S-A beam was ~44.8%, while that of the RC-S-B beam was ~12.8% less than the initial ultimate flexural moment capacity. Therefore, indicates that RC-S-B still in acceleration stage. The ultimate flexural moment of the RC beams are affected by the following two phenomena: 1) loss of metal; and 2) degradation of the interfacial bond between steel and concrete [3.29]. From Table 3.6, the
0 5 10 15
10 20 30 40
1
0.5
0 Deterioration Progress (Total Evaluation)Performance Degradation (Mu/Muo)
[Initiation][Propagation]
[Acceleration] [Deterioration]
Exposure period (years) Steel
starts corro ding
Cracking occurs in concrete due to corrosion
steel bar corrodes at a high rate and rust strain
Residual of load-carrying capacity
Deterioration Progress
Performance Degradation Pre-cracked RC Beam
Uncracked RC Beam
Pre-cracked RC Beam Uncracked RC Beam
50 [Acceleration]
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compressive strains of concrete in the RC-S-A and RC-S-B beams are 1590 × 10-6 and 1940 × 10-6, respectively, under ultimate loading. These results indicate that the bond strength of RC-S-A is lower than that of RC-S-B.
Fig. 3.26 Compressive strength and modulus of elasticity
The steel bar corrosion affects the residual bond, generating a slight increase in strength early in corrosion. The increasing of the ultimate flexural moment could be caused by the improved bond strength between the slightly corroded steel bar and concrete, because some corrosion products may fill the voids in the concrete to provide additional interlocking effects [3.7]. However, the loss of bond strength affects the flexural moment due to reinforcement corrosion, which is caused primarily by the breakdown of bond at the steel/concrete interface [3.6].
Therefore, at 10 years, the ultimate flexural moment capacity of RC-S-A is slightly decreased by the loss in bond strength from the pre-cracks. Meanwhile, RC-S-B experienced a slight increase in ultimate flexural moment capacity because of the bond improvement by the corrosion products filling the voids in the concrete. Furthermore, at 20 years, the ultimate flexural moment capacity in RC-S-A is increased. One major contributing factor might be by increasing in concrete compressive strength at 20 years as shown in Fig. 3.26. Increase in concrete compressive strength contribute to increasing ultimate flexural moment capacity of the RC beam at 20 years. Another contributing factor is the improvement of the bond between steel and concrete by corrosion products filling
0 10 20 30 40 50
0 10 20 30 40
Compressive strength (N/mm²)
Exposure Time (years)
0 10 20 30 40 50
0 10 20 30 40
Modulus of elasticity (kN/mm²)
Exposure Time (years)
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gaps and voids [3.7]. However, in RC-S-B, the ultimate flexural moment capacity is slightly increased at 20 years. This is because the corrosion products have reached the maximum volume expansion at 10 years, causing corrosion cracking at this time. At 20 years, the ultimate flexural moment capacity in RC-S-B remained only slightly increased. A summary of the boundary for each deterioration stage observed in this study is shown in Table 3.9.
Table 3.9 Determination of deterioration progress and performance degradation Deterioration
Stage
Time (Years)
Boundary of stage, determined by deterioration progress and performance degradation
Initiation 0-5 Estimated initiation stage was one year [3.37]
Propagation 5-15 Partial cracks found on concrete surface in Fig. 3.6 Acceleration
15-25 (RC-S-A) Steel bar corrodes at a high rate, but the beams show sufficient load-carrying capacity at 20 years 15-45 (RC-S-B) Steel bar corrodes at a low rate and the beams still
show sufficient load-carrying capacity at 20 years Deterioration 25-50 (RC-S-A) Load-carrying capacity is reduced considerably by
the increased corrosion amount 45-50 (RC-S-B)
After 20 years of exposure to the marine tidal environment, the beams were transported into laboratory conditions. Therefore, RC-S-B as the control beam experienced slowed deterioration progress. However, in RC-S-A, the deterioration progress continued. This could be attributed to environmental changes after 20 years; the environmental conditions changed from severe marine conditions to laboratory atmospheric conditions. Therefore, deterioration progress was delayed in RC-S-B to some extent. This result implies that cracks are as important as the environment in the deterioration of an RC beam that has existing cracks before exposure. However, in the RC beam without existing cracks, the environmental condition has greater influence. This implies that even for RC beams transported to a mild environment during the acceleration stage, the deterioration progress is not stopped.
3. 5 CONCLUSIONS
Based on the experimental results, the following conclusions are derived from this comprehensive series of investigations:
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1) Pre-cracks accelerated the deterioration progress and performance degradation in RC beams by up to twice the rate observed in the beam without pre-cracks.
2) The pre-cracks led to changes in the failure mode of the beams. The pre-cracked RC-S-A beam showed, at the ultimate load, large areas of spalled concrete appearing suddenly on the tensile surface in the middle of the beam span. The RC-S-B beam lacking pre-cracks showed, at the ultimate load, crushing of the concrete at the top compressive surface of the beam, with failure conditions identical to those for the normal RC beam.
3) Pre-cracks promoted corrosion crack opening by a factor of almost ten times more than that observed without pre-cracks.
4) Pre-cracks increased the cross-sectional loss of steel reinforcing bars observed in the RC beam compared to that in the beam without pre-cracking, particularly in the tensile bars.
5) A strong correlation was observed between crack width and cross-sectional loss under natural corrosion processes.
6) Corrosion of the steel reinforcing bars significantly affected the mechanical properties of the steel bar by reducing the yield and ultimate capacities, as well as the ductility. Therefore, each 25% point of cross-sectional loss corresponded to a 25% loss in elongation, 15% loss in yielding capacity, and 13% loss in ultimate capacity.
7) For every 10% of local cross-sectional loss in the tensile steel bars, a 10%
reduction in the load-carrying capacity of the RC beams occurs.
8) The effectiveness of pre-crack start to affect from acceleration stage (20 years). Further, after 40 years exposed, RC-S-A (pre-cracked) enter deterioration stage while RC-S-B (no pre-cracked) still in acceleration stage.
ACKNOWLEDGEMENTS
The authors thank PARI (Port and Airport Research Institute) for offering the RC beam specimens tested in this experiment. The first author also wishes to thank the Indonesia Endowment Fund for Education (LPDP) for scholarship during the study.
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