REDUCTION OF 40-YEAR-OLD REINFORCED CONCRETE BEAMS IN NATURAL CORROSION ENVIRONMENTS
VI- 29 References
7.2.2 Impact of Concrete Cracks on Corrosion
VII-5
to cement ratio (w/c) and concrete cover. Therefore, RC beam was estimated for corrosion initiation after 0.96 years exposed in marine environment with w/c of 0.68. This means, after 0.96 years, the chloride content crossed the threshold value, as JSCE standard for breakdown passivity is 1.2 kg /m3. Further, this result was corresponding in Chapter 5, where the chloride content after 1.5 years’ exposure in artificial chloride (i.e., NaCl and sea-water) in laboratory case for un-cracked portion of N-30-0.3-NaCl (0.95 kg/m3) and N-30-0.3-SW (0.86 kg/m3) for OPC.
The result of laboratory case (OPC; w/c of 0.50; cover 30 mm) correlated with field case (OPC; w/c of 0.68; cover 30 mm) on estimation of chloride penetration for crossed threshold value and initiate the corrosion less than 5 years.
VII-6
such marine tidal and splash zone promote reinforcing bar more severe by corrosion.
Fig. 7.4 Corroded area cracked concrete (Chapter 5)
Table 7.1 Limiting values for composition and properties of concrete subject to general structures for 50 years’ design life
Corrosion induced Chlorides
Exposure class XS3 (tidal, splash &
spray zones)
C (tidal, splash & spray zones)
Code EN 206-1 AS 3600
Max w/c 0.45 0.35
Min. cover (mm) 50 50 (f’c ≥ 50MPa)
70 (f’c = 40 MPa)
The result corresponds with the long-term exposure, RC beams with w/c of 68% and concrete cover of 30 mm was predicting corrosion initiation less than five years. N-30-0.3-SW with w/c of 50% and cover 30 mm showed that about 1.5 years then corrosion appeared. This emphasize even with w/c of 50% (OPC) with concrete cover of 30 mm was not enough for concrete subjected into chloride environment particularly tidal and splash zone with initial defect (crack). In addition, the new European Standard (EN 206-1:2000) [7.11] proposed a maximum w/c ratio for a service life of 50 years for chloride-induced corrosion in seawater as presented in Table 7.1. Australian Standard (AS 3600: 2001) [7.12] also gives the minimum concrete cover needed to maintain adequate durability for a design life of 40-60 years (Table 7.1). By considering result of long-term (field) RC beam with
5.81
1.12
0.00 0.00
0 2 4 6 8 10
N-30-0.3-SW N-30-0.3-TW N-50-0.3-TW N-70-0.3-TW
Corroded Area (%)
VII-7
concrete cover 50 mm and short-term (laboratory) N-50-0.3-SW with concrete cover of 30 mm, the minimum required concrete cover for marine structures 50 mm and 70 mm. In addition, in view point of mechanical properties, the compressive strength of RC beam after long-term exposed was around 30 MPa. Therefore, according to AS 3600 in Table 7.1 that minimum required strength of concrete cover 50 mm (f’c ≥ 50MPa) and 70 mm (f’c = 40 MPa) showed that the concrete strength of RC beam was not satisfied.
7. 3 Proposal on Estimation of Long-lasting Performance - Discussions
The relationship between crack width and cross-sectional loss shows a good correlation, as shown in Fig. 3.20 (source: Chapter 3). The R-squared value obtained from the linear regression line shows a relatively high value. Therefore, it is suggested that estimation on cross-sectional loss of steel bar based on crack width more acceptable under natural corrosion process. Moreover, the residual load-carrying capacity of the beam is greatly affected by corrosion of the reinforcements.
The loss of bonding between the steel bar and concrete is caused by cross-sectional loss. Cross-sectional loss greatly affects the load-carrying capacity of the RC beam and the tensile bars in particular are the most severely affected by the flexural bending moment. Therefore, if the local cross-sectional loss is known, it is possible to estimate the residual load-carrying capacity of the beam as shown in Fig. 7.5.
These results are identical to previous reports, thereby confirming the data [7.13,14].
For this reason, the result indicates that each percentage point of local cross-sectional loss in the tensile bars corresponds to a 1% reduction in the residual load-carrying capacity of the beams, which is similar to the previous findings [7-15-17].
Further, by using this estimation proposal then performance degradation can be estimate according to the JSCE Standard Specification [7.18] where each safety factor is set to 1.0.
Service-life of structures depend on deterioration progress. Deterioration due to chloride attack and steel corrosion involved two important parameters; aesthetic appearance and safety. Aesthetic appearance evaluate regarding landscape or actual visual deterioration of the structures. The evaluations such us crack width or
VII-8
concrete spalled. Further, safety consideration evaluates either reduction in load-carrying capacity or stiffness performance. Based on the experimental result summary of estimation on service-life performance as presented in Table 7.2.
Fig. 7.5 Relationship between cross-sectional loss and ultimate capacity loss
Table 7.2 Estimation on service-life performance – Experimental result
Zone Focused On Estimation
Service-life
Aesthetic appearance Safety
1 crack opening ≤ 1mm Capacity loss ≤ 12.8% ≈ 50 years 2 1mm ≤ crack opening ≤ 10mm 12.8% ≤ Capacity loss ≤ 44.8% < 50 years 3 crack opening ≥ 10mm Capacity loss ≥ 44.8% ≤ 40 years 4 1mm ≤ crack opening ≤ 10mm 12.8% ≤ Capacity loss ≤ 44.8% < 50 years
The results of this proposal may guide in the development of longer-lasting reinforcements for concrete structures because it provides a demonstration of the long-term behavior of a commonly used construction material, permitting the prediction of the behaviors of existing structures and the more informed design and study of structures to be built. Based on the evaluation result from the experimental
R² = 1.0
0 10 20 30 40 50
0 10 20 30 40 50
Ultimate Capacity Loss (%)
Cross-section Loss (%)
pre-cracked cover 30 mm
no pre-cracked cover 30 mm
pre-cracked cover 50 mm
3 4
1 2
VII-9
that conducted in field and laboratory case, also long-term and short-term exposed as well, then evaluation comments will have derived. Evaluation comments will have derived necessary information regarding durability and performance points need to be considered.
Table 7.3 An overview of experimental result of long-lasting RC beam
Consideration Evaluation Discussion
Exposure condition
Marine environment has an intense of chloride ions, therefore reinforced concrete severe to corrosion mainly caused by chloride attack. Severely corrosive environment according JSCE Standard [7.19] was environment of marine structures is subjected to tides, splash, or exposed to severe ocean winds.
Here, the beams were subjected to alternating wet and dry conditions because of tidal and splash action. Furthermore, [7.20] reported, that the severest corrosion rate occurred at splash zone.
Crack In presence of initial cracks (initial defects), crack opening almost 10 mm and over allowable crack for severely corrosive environment according JSCE standard as shown in Table 7.4.
Cover Concrete cover of 40 mm is not enough to guarantee long-lasting performance of 50 years except in the atmospheric exposure conditions and for good concrete quality [7.21,22]. Further, in similar condition, long-lasting performance of concrete member of 50 years achieved with concrete cover was 60 mm [7.22].
Concrete cover for the thick 30 mm was not enough for marine structures particularly in tidal and slash zone. Reduced cover is risky even when using high quality concrete since initial defects such as cracks become more significant than they are with normal cover and may provide a low resistance path to the reinforcing bar.
Chloride content
Concrete with w/c of 0.30 and 50 mm concrete cover, the chloride may exceed 0.4% weight of cement (commonly assumed corrosion threshold) [7.23] within 30 years in splash zone [7.22].
Based on the JSCE standard for chloride threshold 1.2 kg/m3 [7.3] showed that the amount of chloride over the threshold value already. However, it is suggested that over time, chloride ions will eventually penetrate even un-cracked concrete, initiating more widespread corrosion. Thus, after a long-lasting exposed,
VII-10
the difference between the amount of chloride ions in cracked and un-cracked concrete will be minor.
Cross-sectional loss
The cross-sectional loss differs significantly in tensile bars. The local cross-sectional losses of the tensile bars in beam with and without initial cracks are 36.1% and 13.4%, respectively. It is suggested that each percentage point of local cross-sectional loss in the tensile bars corresponds to a 1% reduction in the residual load-carrying capacity of the beams
Load-carrying capacity
The losses in ultimate capacity for beam with and without initial cracks are 44.8% and 12.8 %, respectively. Further, the losses in ultimate capacity had strong correclation with cross-sectional loss of reinforcing bar.
Service life 1. In presence of initial cracks, estimation of service life maximum 40 years.
2. In absence of initial cracks, estimation of service life maximum 50 years.
Table 7.4 Limit value of crack width [7.19]
Type of reinforcement
Environmental conditions for reinforcement corrosion Normal Corrosive Severely corrosive Deformed bars
and plain bars 0.005c 0.004c 0.0035c
Prestressing steel 0.004c --- ---
One parameters to evaluate deterioration and performance of prestressed concrete (PC) beam by determine prestress loss. The remaining tendon forces in the beam were determined using the so called crack re-opening method. The beams loaded until flexural cracks appeared in the bottom. The initial crack was marked and the beam unloaded, then the beam prepared for reloaded. The compressive stress in concrete at the level of prestressing steel to continually decrease until it becomes zero at a load level termed the decompression load. The decompression load was determined by intersecting the two slopes. Plot of applied load vs.
displacement to determine the decompression load.
The total prestress loss obtained by determined of decompression load and a simple elastic analysis. The effect of pre-cracked for both PC-O and PC-R clearly
VII-11
seen in affected to prestress loss. The total prestress loss for PC-O and PC-R due to cracked about 23.72% and 38.21% respectively. Post-tensioning type, pre-cracked also contributed of prestress loss due to produce deteriorated a part of anchorage zone. Perhaps during loading for pre-cracked (i.e., 0.65 moment ultimate) anchorage zone have an increasing local stress and produced numerous of crack. Numerous of crack on anchorage zone lead to anchorage slip and produce prestress loss. The prestress loss likelihood may be produced by accumulative of deterioration of the beams (i.g., spalling, longitudinal cracks, anchorage zone cracks) as the result of the corrosion products.
Table 7.5 An overview of experimental result of long-lasting PC beam
Consideration Evaluation Comments
Exposure condition
Marine environment has an intense of chloride ions, therefore reinforced concrete severe to corrosion mainly caused by chloride attack. Severely corrosive environment according JSCE Standard [7.19] was environment of marine structures is subjected to tides, splash, or exposed to severe ocean winds.
Here, the beams were subjected to alternating wet and dry conditions because of tidal and splash action. Furthermore, [7.20] reported, that the severest corrosion rate occurred at splash zone.
Crack High strength prestressing steels show a far more sensitive reaction to corrosion attack than reinforcing bars, Therefore, JSCE standard [7.19] in Table 7.2 recommend no cracks are allowed for severe corrosive environments such as marine exposures subjected to tidal and splash zone.
Cover Concrete cover for 30 mm was not enough. Even in un-cracked area, PC wires still corroded. Particularly for PC tendons, in spite of cement paste was grouted between tendons and sheath after post-tensioning, corrosion still occurred.
Prestressed Loss
Allowable prestress loss for pre-tensioning and post-tensioning were 25% and 20%, respectively [7.24,25]. The total prestress loss for pre-tensioning type for un-cracked and pre-cracked were 22.18% and 38.21%, respectively. Whereas, post-tensioning type for un-cracked and pre-cracked were 23.73% and 48.86%, respectively. Both type PC beam, pre-tensioning and post-tensioning type for pre-cracked condition, over allowable
VII-12
prestress loss already. For un-cracked condition, only post-tensioning type over the allowable prestress loss.
Service life 1. In presence of initial cracks, estimation of service life maximum 40 years for pre-tensioning and less than 40 years post-tensioning type.
2. In absence of initial cracks, estimation of service life maximum 50 years for pre-tensioning and maximum 40 years post-tensioning type
7. 4 Summary
This study completes an extended evaluation of the corrosion-related deterioration of RC and PC beams exposed to naturally corrosive environments. In particular, we examine the deterioration and properties of RC and PC beams after 40 years of exposure to maritime tidal and splash-zone environments. The resulting data set provides a clear demonstration of the effects of corrosion on pre-cracked and un-cracked RC and PC beams over the course of nearly half a century. Such valuable information is necessary for the optimal durability design and construction of long-lasting buildings and facilities in harsh environments. Based on the evaluations and consideration of the current codes and standards then provides an appropriate assessment for long-lasting performance of RC and PC member with initial defects under environmental action, the evaluation overviews as follows:
1. Cracks (initial defects) accelerate the onset of corrosion and reduce the service life of RC and PC member.
2. It is suggested that over time, chloride ions will eventually penetrate even un-cracked concrete, initiating more widespread corrosion. Thus, after a long-lasting exposed, the difference between the amount of chloride ions in cracked and un-cracked concrete will be minor.
3. In marine environmental condition, particularly tidal and splash zone, where mostly affected by the chloride-induced corrosion of reinforcing bar, concrete cover of 30 mm, with w/c of 0.68 and Normal Portland Cement (OPC) was not sufficient for expected long-lasting performance of 50 years.
4. Concrete cover for 30 mm and 40 mm with w/c of 0.37 was not sufficient to protect PC tendons (with sheath and grout cement paste) and PC wires from corrosion, respectively. Therefore, was not satisfactory for expected long-lasting performance of 50 years.
VII-13
5. A strong correlation was observed between crack width and cross-sectional loss and cross-sectional loss with load-carrying capacity as well under natural chloride environment. Therefore, it is possible to estimate deterioration progress and performance degradation.
6. It is suggested that Supplementary Cementitious Materials (SCMs) by Blast Furnace Slag (BFS) increased resistance against chloride particularly in marine environmental condition.
7. The environment has less influence on the RC and PC member with existing cracks, compared to that without existing cracks.
VII-14
References
[7.1] K. Okada, Reinforcement corrosion in concrete structures, Report of the Corrosion and Corrosion Inhibition Committee JSMS 1976;14(74).
[7.2] 6349-1: 2000 “Maritime structures, Part I: Code of Practice for General Criteria, London: BSI, British Standard.
[7.3] JSCE, Standard Specification for Concrete Structures (Part :Maintenance), Japan Society of Civil Engineers, Japan, 2007.
[7.4] R. F. Bakker, Permeability of blended cement concretes, Special Publication 1983;79:589-606.
[7.5] B. Lothenbach, G. Le Saout, M. B. Haha, R. Figi, E. Wieland, Hydration of a low-alkali CEM III/B–SiO 2 cement (LAC), Cement and Concrete Research 2012;42(2):410-23.
[7.6] M. Sahmaran, G. Yildirim, T. K. Erdem, Self-healing capability of cementitious composites incorporating different supplementary cementitious materials, Cement and Concrete Composites 2013;35(1): 89-101.
[7.7] S. Qian, J. Zhou, E. Schlangen, Influence of curing condition and precracking time on the self-healing behavior of engineered cementitious composites, Cement and concrete composites 2010;32(9):686-93.
[7.8] S. Qian, J. Zhou, M. De Rooij, E. Schlangen, G. Ye, K. Van Breugel, Self-healing behavior of strain hardening cementitious composites incorporating local waste materials, Cement and Concrete Composites 2009;31(9):613-21.
[7.9] K. Van Tittelboom, E. Gruyaert, H. Rahier, N. De Belie, Influence of mix composition on the extent of autogenous crack healing by continued hydration or calcium carbonate formation, Construction and Building Materials 2012;37:349-59.
[7.10] R. Swamy, H. Hamada, T. Fukute, S. Tanikawa, J. Laiw, Chloride penetration into concrete incorporating mineral admixtures or protected with surface coating material under chloride environments, Proc. of CONSEC 1995;95.
[7.11] EN 206-1, Concrete-Part 1 : Specification, Performance, Production and Conformity, European Standard, 2000.
VII-15
[7.12] Guidelines for the Design Maritime Structures AS 4997, Australian Standard, 2005.
[7.13] J. Rodriguez, L. Ortega, J. Casal, Load carrying capacity of concrete structures with corroded reinforcement, Constr. Build. Mater.
1997;11(4):239-48.
[7.14] R. Zhang, A. Castel, R. François, Serviceability limit state criteria based on steel–concrete bond loss for corroded reinforced concrete in chloride environment, Mater. Struct. 2009;42(10):1407-21.
[7.15] G. Malumbela, M. Alexander, P. Moyo, Variation of steel loss and its effect on the ultimate flexural capacity of RC beams corroded and repaired under load, Constr. Build. Mater. 2010;24(6):1051-59.
[7.16] R. Zhang, A. Castel, R. François, Concrete cover cracking with reinforcement corrosion of RC beam during chloride-induced corrosion process, Cem. Concr. Res. 2010;40(3):415-25.
[7.17] A. A. Torres-Acosta, S. Navarro-Gutierrez, J. Terán-Guillén, Residual flexure capacity of corroded reinforced concrete beams, Eng. Struct.
2007;29(6):1145-52.
[7.18] H. Watanabe, H. Hamada, H. Yokota, T. Yamaji, Long-term Performance of Concrete and Reinforced Concrete under Marine Environment, in: N.
Banthia, K. Sakai, O.E. Gjørv (Eds.) Proceedings 3rd International Conference on Concrete under Severe Conditions, The University of British Columbia, Vancouver, Canada, 2001.
[7.19] JSCE, Standar Specification for Concrete Structures (Part : Design), Japan Society of Civil Engineers, Japan, 2007.
[7.20] R. W. Revie, H. H. Uhlig, Uhlig's corrosion handbook, John Wiley &
Sons2011.
[7.21] A. Costa, J. Appleton, Chloride penetration into concrete in marine environment—Part I: Main parameters affecting chloride penetration, Materials and Structures 1999;32(4):252-59.
[7.22] A. Costa, J. Appleton, Chloride penetration into concrete in marine environment-Part II: Prediction of long term chloride penetration, Materials and Structures 1999;32(5):354-59.
VII-16
[7.23] G. Glass, N. Buenfeld, The presentation of the chloride threshold level for corrosion of steel in concrete, Corrosion Science 1997;39(5):1001-13.
[7.24] T. Y. Lin, N. H. Burns, Design of prestressed concrete structures, 1981.
[7.25] E. G. Nawy, Prestressed concrete, Pearson Education2011.
VIII-1
Chapter 8
CONCLUSIONS AND FUTURE RESEARCH WORK
8. 1 Summary
The durability of steel reinforced concrete in chloride environments is of great interest to design engineers, infrastructure owners and maintainers, and researchers.
The problem is how to achieve durability through careful design of the cement matrix and its microstructures, and determine the minimum concrete cover for reinforcement that meets the requirements of environmental actions. Optimal design necessary advances in the knowledge base relevant to the durability evaluation of steel reinforced concrete in chloride environments, including the role of supplementary cementitious materials (SCMs) for durability, the methods of measuring the chloride penetration into concrete, the challenges in assessing concrete durability from its chloride diffusivity, and design for long-lasting of reinforced concrete in chloride environments.
The aim of this study is to provide an overview related durability of RC and PC member due to initial defects (crack and chloride) under environmental action.
This study will be contributing for necessary information because it provides a demonstration of the long-term behavior of a commonly used construction material, permitting the prediction of the behaviors of existing structures and the more informed design and study of structures to be built. This study completes an extended evaluation of the corrosion-related deterioration of RC and PC beams exposed to naturally corrosive environments. The results may guide engineers in the development of longer-lasting reinforcements for concrete structures.
VIII-2
8. 2 Overview of Findings and Conclusions
In Chapter 1, the back ground of this study, research objective, research contribution and standing point. The aim of this study is to provides an overview related durability of RC and PC member due to initial defects (crack and chloride) under environmental action. Further, long-term exposure by field case (i.e., natural corrosion environments) and laboratory case (i.e., artificial corrosion environments) regarding of corrosion behavior and deterioration progress of RC and PC member is of the advantage from this study. The expected result of this study is to propose an overview assessment regarding durability evaluation of concrete member embedded steel due to corrosion based on the long-term exposure by field case and laboratory case. Therefore, handling and right countermeasure could be planned for the future design of long-lasting performance of RC and PC member.
In Chapter 2, previous study is presented to illustrate the existing body of knowledge that has been established by previous researches. Provide literature regarding role chlorides for the corrosion of reinforcements. The amount chloride for breakdown or initiate corrosion. Further, concrete cracking and the effect to reinforcing bar corrosion. Evaluation report obtained from the long-term performance of RC and PC member. These such literature or evaluation became necessary information in order to provide an appropriate assessment regarding durability of concrete member. The evaluation result will provide necessary information and reference for the future design and construction of long-lasting reinforcements for concrete structures particularly in marine environments.
Chapter 3 describes evaluation of deterioration progress and performance reduction of 40-year-old corroded reinforced concrete (RC) beams in natural corrosion environments. The corrosion process was natural, without acceleration by current application, admixture inclusion, or exposure to an artificial chloride environment. In this study, two 40-year-old RC beams, exposed to real marine environments (i.e., tidal and splash zones). The beams are categorized into two