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Structural effects of simultaneous loading and reinforcement corrosion on performance of concrete beams
Construction and Building Materials 39 (2013) 148–152
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Structural effects of simultaneous loading and reinforcement corrosion on performance of concrete beams Yingang Du a,b,⇑, Martin Cullen a, Cankang Li b a b
Glasgow Caledonian University, Glasgow G4 0BA, UK The University of Birmingham, Birmingham B15 2TT, UK1
h i g h l i g h t s " Tests done on concrete beams under simultaneous loading and corrosion (SLC). " SLC causes a beam to exceed its limiting deflection for service prematurely. " SLC decrease beam strength and especially, its ductility much more rapidly. " SLC causes a beam to fail less ductile and even in a brittle way.
a r t i c l e
i n f o
Article history: Available online 13 June 2012 Keywords: Concrete beam Reinforcement corrosion Applied loads Simultaneous loading Ultimate strength Ductility
a b s t r a c t The paper presents an experimental investigation into the structural performance of concrete beams under simultaneous loading and reinforcement corrosion. Four out of five concrete beams were subjected to an accelerated corrosion of their tensile bars, while they sustained a constant point load and their selfweight, until they failed in their load-bearing capacity. On the basis of the experimental results, it has been found that, under the same service loads, the time-dependant deflection of a corroded beam increases more rapidly than that of a non-corroded beam, and is likely to reach the limiting deflection for its serviceability requirements prematurely. Under simultaneous loading and reinforcement corrosion, both ultimate strength and maximum deflection of a concrete beam decrease more than those of the beams tested under a separate loading and corrosion condition. Either a further development of corrosion or an occasional over-loading or both are likely to cause a corroded beam under service loads to fail and even collapse suddenly without significant warning in term of its deflection. Hence, attention should be paid to the inspection and maintenance of corroded structures that are being used in our society to avoid any loss of life and property. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction Corrosion of reinforcement is one of the major causes for structural deterioration of reinforced concrete buildings and bridges. For the purpose of cost-effective decision-making in respect of their use and maintenance, mechanical performance, residual strength and service life of these structures with corroding reinforcements should be fully understood. Over the past few decades, many experimental investigations have been carried out to address the structural effects of reinforcement corrosion on the performance of concrete beams. On basis of ⇑ Corresponding author. Address: School of Engineering and Built Environment (EBE), Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 0BA, UK. Tel.: +44 0141 3313560. E-mail address: [email protected] (Y. Du). 1 Formerly.
the experimental results, it has been reported that, in contrast with the ductile failure of a non-corroded beam, a corroded beam would possibly fail either in shear, or in shear–compression, or by bond ineffectiveness with a less ductile mode [1–5]. With regards to the magnitude of the reduction of beam strength due to corrosion, however, completely consistent results have not been achieved from the previous tests. In particular, all the above experimental investigations were carried out in such a way that concrete beams were first corroded to an expected extent, before they were loaded to failure to assess the variation of their mechanical behaviour due to corrosion [1–5]. This clearly does not reflect the real world of corroded structures in which corrosion of reinforcement occurs simultaneously with service loads that are being applied on the structures. Recently, Yoon et al., Ballim et al., Vidal et al. and Malumbela et al. conducted the tests on concrete beams which were subjected to simultaneous loading and reinforcement corrosion [6–10]. It has
0950-0618/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.05.006
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reported that, under simultaneous loading and reinforcement corrosion, the longitudinal tensile strain, surfacing cracks and flexural deflection of a corroded beam increased more than those of a non-corroded one [9,10]. Due to a simultaneous loading, there were faster initiation of the corrosion of reinforcements and a shift of failure mode of a concrete beam [6]. However, these investigations are mainly focused on beam behaviour at its service stage on aspects of corrosion imitation, corrosion propagation, surface cracks and the time-dependant deflection, etc. In addition, the level of sustained loads that were applied on the beams with corroding reinforcements was only about 8% and 12% of beam ultimate strength [9,10], which seems too small when compared with the level of service loads that are commonly applied on most concrete beams in actual structures. Hence, this paper presents an experimental investigation with an aim of studying the structural behaviour of reinforced concrete beam subjected to simultaneous loading and reinforcement corrosion. A particular attention will be paid to their ultimate strength, maximum deflection and failure modes at its ultimate limit state. 2. Experimental work 2.1. Beam specimen and materials Five reinforced concrete beams with the dimensions of 100 mm wide by 150 mm deep by 1300 mm long were made for an anticipated flexural failure, prior to corrosion of their reinforcements, as shown in Fig. 1. The measured yield and ultimate strengths of the H8 rib bars were 478 N/mm2 and 557 N/mm2, while those of the R6 plain bars were 286 N/mm2 and 335 N/mm2. All concrete beams were cast using the same batch of the concrete mix and had the same thickness of concrete cover of 20 mm. The measured cubic strengths of the concrete were 25.6 N/mm2 after 28 days cure in the water tank, and 39.2 N/mm2 when beams failed under simultaneous loading and corrosion.
2.2. Test programme and techniques Following both 28 days cure in the water tank and further 29 days natural exposure, all concrete beams were subjected to three points bending over a 1000 mm span, as illustrated in Fig. 1. In addition to beam self-weight, a point load was applied at the mid-span of each beam and progressively increased up to 15 kN, 60% of their design ultimate load of 25 kN, and then kept constant. The loading was implemented by using two screwed rods that stood on each side of a beam. Each rod had its one end fixed to the floor of the laboratory and the other end projected above the test beam, as shown in Fig. 2. The point load was measured and monitored using a load cell that was positioned between a loading distribution steel beam linking the two rods and the top surface of each beam. The deflection of the beam was measured using a dial gauge that was located in the middle span of each beam adjacent to the point load. While keeping the above point load at the constant level of 15 kN, four out of five beams, except for the control beam CB5, were then subjected to accelerated corrosion on the parts of their H8 bottom tensile bars simultaneously for the remaining 50–60 days, as shown in Fig. 3. To ensure uniform moisture condition of different beams and to improve the electrical conductivity of the concrete, 3.5% sodium solution was sprayed for 24 h to the part of a beam to be corroded. After this initial ‘wet’ phase, the power suppliers were immediately switched onto impress the currents of 0.25 mA/cm2 and 0.50 mA/cm2 onto the H8 bars during the first two weeks and the remaining six weeks, respectively, when the beams were under a cycle of ‘wet’ and ‘dry’ conditioning. This cycle of wet and dry conditions was established by having 2 h
Fig. 1. Specimen of reinforced concrete beam.
Screw Distribution beam Load cell Beam specimen Water tank for wet/dry cycling Screwed rod
Fig. 2. Test rig for beam loading.
of sodium–chloride solution spraying and the remaining 22 h of indoor natural exposure of the beam every day. A timer was used to control the turn on and off of water-spraying while the current was kept at a constant level. At the end of the above process, both the control beam CB5, which was only subjected to the applied loads without corrosion, and the corroded beam CB4 were further loaded to failure using Mand Testing Rig under displacement control to measure their ultimate strength and maximum deflections. A beam was deemed to have failed in its load-bearing capacity, if either its tensile steel bar fractured or its post-peak load decreased by 50% of its ultimate strength. At the end of the test, both corroded bars and non-corroded bars were removed from the tested specimens. The amount of corrosion of reinforcing bars was calculated on the basis of its weight loss, which represents an average loss of steel crosssectional area and is about 1–5 times less that is determined using its minimum residual cross-section area [12].
3. Results and discussion The main experimental results of concrete beams subjected to simultaneous loadings and reinforcement corrosion are shown in Figs. 4–6 in terms of their load-dependant and time-dependant deflections, respectively. Here, the deflections of the beams under the initial loading of up to 15 kN and under simultaneous loading of 15 kN and reinforcement corrosion were shown in Figs. 4 and 5. The increased deflections of the beams CB4 and CB5 under the further loading of up to 20 kN and 25.6 kN, respectively, were added to Fig. 5, while those under the simultaneous loading and reinforcements only in Fig. 6. 3.1. Structural performance of concrete beam under loading and corrosion As shown in Figs. 4 and 5, the structural performance of reinforced concrete beams subjected to simultaneous loading and reinforcement corrosion can be divided into three different stages, i.e., stage A for initial loading, stage B for time-dependant deflection and stage C for final failure. Initially, when the point load was gradually increased to between 5.0 and 7.0 kN, the deflections of all concrete beam almost linearly increased to between 0.28 mm and 0.51 mm and the first set of flexural cracks occurred in the middle span of the concrete beams. When the point load was increased to the designated level of 15 kN, the development of the flexural cracks on beam surface tended to stabilise and the deflections of the five beams increased to between 1.46 and 2.03 mm with an average value of 1.70 mm, as shown in Figs. 4 and 5. During the 24 h of the initial ‘wet’ phase, the average deflection of the five beams increased from 1.70 mm to 2.72 mm with an average value of 2.7 mm. This was still well below the limit deflection of 4.0 mm in term of L/250 specified in the BS EN 1992-1-1 for a concrete beam at its serviceability limit state [11], as shown in Fig. 5. Following the initiation of the corrosion of reinforcements in the beams CB1–CB4, some rust stains and corrosion cracks appeared on beam surfaces along the length of the longitudinal
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Power supply -
+
Lc
Stainless steel plate Conductive foam
CB1 (Lc=150mm); CB2 (Lc=500mm)
Water pipe
+
Lc
CB3 (Lc=300mm); CB4 (Lc=650mm); CB5 (Lc=0mm) Fig. 3. Test rig for reinforcement corrosion.
B
2.50
C
Increase of Deflection (mm)
A 30
Applied Load (kN)
25 20 15 10 5
2.00 1.50 1.00 0.50 0.00 0
10
20
0 0
10
20
30
40
50
60
70 CB5
Midspan Deflection (mm) CB5
CB1
CB2
CB3
CB4
C
B
10.0 9.0
Deflection (mm)
8.0 7.0 6.0 5.0 4.0
3.0 Limit deflection for serviceability
2.0 1.0 0.0 0
10
20
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40
50
60
70
Time (Days) CB5
CB1
CB2
CB3
40
50
CB3
CB4
60
CB1
CB2
Fig. 6. Increased time-dependent deflection of beams under loading and corrosion.
Fig. 4. Load-dependent deflections of beams under simultaneous loading and corrosion.
A
30
Time (Days)
CB4
Fig. 5. Time-dependent deflections of beams under loading and corrosion.
reinforcements, crossed with the previously formed flexural cracks. The defections of the beam continued to increase, as shown in Fig. 5. In addition, the deflections of corroded beams CB1–CB4 increased much more than those of the control beam CB5, as shown in Fig. 6, and had exceeded their limit deflection of 4.0 mm much earlier than the control beam CB5 prematurely, as shown in Fig. 5. Hence, in addition to the rusts stains and corrosion cracks, which stay on beam surfaces and damage structural aesthetics, the deflection of a corroded beam is likely to exceed its limiting deflection and therefore may cause the beam no longer to satisfy the requirements for its serviceability limit state, as indicated in Fig. 5. After about 50 days of simultaneous loading and reinforcement corrosion, the beams CB1, CB2 and CB3 with 14%, 19% and 16% corrosions, respectively, first became unstable suddenly with their deflections out of control, as shown in Figs. 4 and 5. The beam CB4 with 27% of corrosion and the control beam CB5 reached their ultimate loads of 20 kN and 25.6 kN, respectively, following an extra 12 days of the testing and a further loading using Mand Testing Rig. A careful examination on all tested beams shows that one out of two tensile bars of the beams had fractured either under the simultaneous loading and reinforcement corrosion (CB1, CB2 and CB3) or under the further loading (CB4 and CB5). Therefore they were deemed to fail in their load-bearing capacity at their ultimate limit state.
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3.2. Failure modes of beam under simultaneous loading and reinforcement corrosion Experimental results also indicate that, in additional to an earlier failure at serviceability limit state due to an excessive deflection that exceeds the limiting deflection of a beam prematurely, the simultaneous loading and reinforcement corrosion also cause the beams CB1–CB4 to fail in a less ductile and even brittle ways at their ultimate limit state with a fracture of their tensile bars. In contrast to the control beam CB5 that failed in very ductile manner with wider flexural cracks and substantial deflection, for the corroded beams CB1–CB4, once their ultimate strengths decreased to below the value of the sustained loads of 15 kN, they suddenly collapsed with a fracture of their corroded tensile bars, as typically shown in Figs. 4 and 7. The anticipated ductile flexure failure of an under-reinforced beam was replaced by a brittle failure. This is due to the reduction of ductility of corroded reinforcement and non-uniform corrosion along the length of a corroded bar [13–15]. 3.3. Effect of simultaneous actions of loading and corrosion on beam behaviour Du et al. carried out an experimental investigation on several types of under-reinforced concrete beams that had the same type of the concrete mix as those used for the beams CB1–CB5, but with different dimensions of 150 200 2100 mm and different reinforcements [15]. Similar to those for the beams CB1, CB2 and CB3, only 300 mm long bottom tensile bars within the mid-span of these concrete beams were artificially corroded. Instead of the simultaneous loading and reinforcement corrosion, as described above for the beams CB1, CB2 and CB3, however, Du et al’s. previously tested beams were first corroded to the expected levels without loading, except for their self-weight, and then subjected to an increasing loading until their failure to check the variation of their structural performance due to reinforcement corrosion. By dividing the differences of both the measured strengths and deflections, respectively, between the control and corroded beams with those of the control beam, the percentage reduction of beam strengths and maximum deflection under the two sets of the tests were determined and shown in Fig. 8. It is clear that both ultimate strength and maximum deflection of a concrete beam decrease as a result of the corrosion of its tensile bars. In addition, for the same amount of corrosion, the
Structural Strength Residual Strength of Corroded beams Sustained loading P=15kN
T
0
Service Time
Reduction of beam strength and deflection
Y. Du et al. / Construction and Building Materials 39 (2013) 148–152
120% 100% 80% 60% 40% 20% 0% 0
5
10
15
20
Amount of corrosion (%) Strength-Simultanous Deflection-Simultanous Strength-Simultanous Deflection-Simultanous
Strength-Separate Deflection-Sepatate Strength-Separate Deflection-Separate
Fig. 8. Simultaneous effects of loading and corrosion on beam behaviour.
maximum deflection of a concrete beam, namely, beam ductility decreases more rapidly than its ultimate strength. In particular, both ultimate strength and maximum deflection of the beams CB1, CB2 and CB3 under simultaneous loading and reinforcement corrosion reduce much more rapidly than those of the previously tested beams under separate loading and reinforcement corrosion. In other words, the simultaneous action of both loading and corrosion impairs both residual strength and ductile behaviours of corroded structures more significantly than the case where corrosion and loading occur separately. This may be due to a localised corrosion that is much more easily developed along the length of reinforcement under tension stress. Hence, care should be taken when interpreting the results of structural elements that were corroded without applied loads into the real structures. 4. Conclusions From the above experimental results, the following conclusions can be drawn: (1) Due to the corrosion of tensile bars, the time-dependant deflection of a corroded beam under service loads increase more rapidly than those of a non-corroded beam. (2) In addition to rusts stains and surface cracks, corrosion also likely causes a concrete beam under service loads to reach at its limiting deflection prematurely, which damages structural aesthetes and cause the beam to no longer satisfy the requirements for its serviceability limit state. (3) Under simultaneous loading and reinforcement corrosion, a concrete beam would fail and collapse without significant warning of signs, except for the rusts stains and surface cracks. An anticipated ductile failure can be replaced by a less ductile or even brittle failure. (4) The ultimate strength and maximum deflection of a beam tested under simultaneous loading and corrosion decrease much rapidly than those tested under a separate loading and corrosion.
Acknowledgements
Service Life of Structure
Fig. 7. Failure of a beam under simultaneous loading and reinforcement corrosion.
The tests were carried out at The University of Birmingham, and were partially sponsored by the Institution of Structural Engineer (IStructE) under the Institution 2008 MSc Research Grant.
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References [1] Tachibana Y, Kajikawa Y, Kawamura K. The mechanical behaviour of RC beams damaged by corrosion of reinforcement. Concr Libr JSCE 1990;14:177–88. [2] Almusallam AA, Al-Gahtani AS, Aziz AR, Dakhil FH, Rasheeduzzafar. Effect of reinforcement corrosion on flexural behaviour of concrete slab. J Mater Civ Eng 1996;8:123–7. [3] Rodriguez J, Ortega LM, Casal J, Diez JM. Assessing structural condition of concrete structures with corroded reinforcement. In: Dhir RK, Jones MR, editors. Concrete repair, rehabilitation and protection. E & FN Spon; 1996. p. 65–78. [4] Mangat PS, Elgarf MS. Flexural strength of concrete beams with corroding reinforcement. ACI Struct J 1999:149–58. [5] Torres-Acosta AA, Navarro-Gutierrez S, Teran-Guillen J. Residual flexural capacity of corroded reinforced concrete beams. Eng Struct 2007;29:1145–52. [6] Yoon S, Wang K, Weiss WJ, Shah SP. Interaction between loading corrosion and serviceability of reinforced concrete. ACI Mater J 2000;97(6):637–44. [7] Ballim Y, Reid JC, Kemp AR. Deflection of RC beams under simultaneous load and steel corrosion. Mag Concr Res 2001;53(3):171–81.
[8] Ballim Y, Reid JC. Reinforcement corrosion and the deflection of RC beam – an experimental critique of current test methods. Cem Concr Res 2003;25:625–32. [9] Malumbela G, Moyo P, Alexander M. Behaviour of RC beams under sustained service loads. Constr Build Mater 2009;23:3346–51. [10] Malumbela G, Alexander M, Moyo P. Steel corrosion on RC structures under sustained service loads – a critical review. Eng Struct 2009;31:2518–25. [11] EN 1992-1-1:2004. Eurocode 2, Design of concrete structures, Part 1–1: General rules and rules for buildings. British Standard Institution (BSI), London. [12] Gonzalez JA, Andrade C, Alonso C, Feliu S. Comparison of rates of generation corrosion and maximum pitting penetration on concrete embedded steel reinforcements. Cem Concr Res 1995;25(2):257–64. [13] Du YG, Clark LA, Chan AHC. Residual capacity of corroded reinforcing bars. Mag Concr Res 2005;57(3):135–47. [14] Du YG, Clark LA, Chan AHC. Effect of corrosion on ductility of reinforcing bars. Mag Concr Res 2005;57(7):407–19. [15] Du YG, Clark LA, Chan AHC. Impact of reinforcement corrosion on ductile behaviour of reinforced concrete beams. ACI Struct J 2007;104(3):285–93.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Structural effects of simultaneous loading and reinforcement corrosion on performance of concrete beams Yingang Du a,b,⇑, Martin Cullen a, Cankang Li b a b
Glasgow Caledonian University, Glasgow G4 0BA, UK The University of Birmingham, Birmingham B15 2TT, UK1
h i g h l i g h t s " Tests done on concrete beams under simultaneous loading and corrosion (SLC). " SLC causes a beam to exceed its limiting deflection for service prematurely. " SLC decrease beam strength and especially, its ductility much more rapidly. " SLC causes a beam to fail less ductile and even in a brittle way.
a r t i c l e
i n f o
Article history: Available online 13 June 2012 Keywords: Concrete beam Reinforcement corrosion Applied loads Simultaneous loading Ultimate strength Ductility
a b s t r a c t The paper presents an experimental investigation into the structural performance of concrete beams under simultaneous loading and reinforcement corrosion. Four out of five concrete beams were subjected to an accelerated corrosion of their tensile bars, while they sustained a constant point load and their selfweight, until they failed in their load-bearing capacity. On the basis of the experimental results, it has been found that, under the same service loads, the time-dependant deflection of a corroded beam increases more rapidly than that of a non-corroded beam, and is likely to reach the limiting deflection for its serviceability requirements prematurely. Under simultaneous loading and reinforcement corrosion, both ultimate strength and maximum deflection of a concrete beam decrease more than those of the beams tested under a separate loading and corrosion condition. Either a further development of corrosion or an occasional over-loading or both are likely to cause a corroded beam under service loads to fail and even collapse suddenly without significant warning in term of its deflection. Hence, attention should be paid to the inspection and maintenance of corroded structures that are being used in our society to avoid any loss of life and property. Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
1. Introduction Corrosion of reinforcement is one of the major causes for structural deterioration of reinforced concrete buildings and bridges. For the purpose of cost-effective decision-making in respect of their use and maintenance, mechanical performance, residual strength and service life of these structures with corroding reinforcements should be fully understood. Over the past few decades, many experimental investigations have been carried out to address the structural effects of reinforcement corrosion on the performance of concrete beams. On basis of ⇑ Corresponding author. Address: School of Engineering and Built Environment (EBE), Glasgow Caledonian University, Cowcaddens Road, Glasgow G4 0BA, UK. Tel.: +44 0141 3313560. E-mail address: [email protected] (Y. Du). 1 Formerly.
the experimental results, it has been reported that, in contrast with the ductile failure of a non-corroded beam, a corroded beam would possibly fail either in shear, or in shear–compression, or by bond ineffectiveness with a less ductile mode [1–5]. With regards to the magnitude of the reduction of beam strength due to corrosion, however, completely consistent results have not been achieved from the previous tests. In particular, all the above experimental investigations were carried out in such a way that concrete beams were first corroded to an expected extent, before they were loaded to failure to assess the variation of their mechanical behaviour due to corrosion [1–5]. This clearly does not reflect the real world of corroded structures in which corrosion of reinforcement occurs simultaneously with service loads that are being applied on the structures. Recently, Yoon et al., Ballim et al., Vidal et al. and Malumbela et al. conducted the tests on concrete beams which were subjected to simultaneous loading and reinforcement corrosion [6–10]. It has
0950-0618/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.05.006
149
Y. Du et al. / Construction and Building Materials 39 (2013) 148–152
reported that, under simultaneous loading and reinforcement corrosion, the longitudinal tensile strain, surfacing cracks and flexural deflection of a corroded beam increased more than those of a non-corroded one [9,10]. Due to a simultaneous loading, there were faster initiation of the corrosion of reinforcements and a shift of failure mode of a concrete beam [6]. However, these investigations are mainly focused on beam behaviour at its service stage on aspects of corrosion imitation, corrosion propagation, surface cracks and the time-dependant deflection, etc. In addition, the level of sustained loads that were applied on the beams with corroding reinforcements was only about 8% and 12% of beam ultimate strength [9,10], which seems too small when compared with the level of service loads that are commonly applied on most concrete beams in actual structures. Hence, this paper presents an experimental investigation with an aim of studying the structural behaviour of reinforced concrete beam subjected to simultaneous loading and reinforcement corrosion. A particular attention will be paid to their ultimate strength, maximum deflection and failure modes at its ultimate limit state. 2. Experimental work 2.1. Beam specimen and materials Five reinforced concrete beams with the dimensions of 100 mm wide by 150 mm deep by 1300 mm long were made for an anticipated flexural failure, prior to corrosion of their reinforcements, as shown in Fig. 1. The measured yield and ultimate strengths of the H8 rib bars were 478 N/mm2 and 557 N/mm2, while those of the R6 plain bars were 286 N/mm2 and 335 N/mm2. All concrete beams were cast using the same batch of the concrete mix and had the same thickness of concrete cover of 20 mm. The measured cubic strengths of the concrete were 25.6 N/mm2 after 28 days cure in the water tank, and 39.2 N/mm2 when beams failed under simultaneous loading and corrosion.
2.2. Test programme and techniques Following both 28 days cure in the water tank and further 29 days natural exposure, all concrete beams were subjected to three points bending over a 1000 mm span, as illustrated in Fig. 1. In addition to beam self-weight, a point load was applied at the mid-span of each beam and progressively increased up to 15 kN, 60% of their design ultimate load of 25 kN, and then kept constant. The loading was implemented by using two screwed rods that stood on each side of a beam. Each rod had its one end fixed to the floor of the laboratory and the other end projected above the test beam, as shown in Fig. 2. The point load was measured and monitored using a load cell that was positioned between a loading distribution steel beam linking the two rods and the top surface of each beam. The deflection of the beam was measured using a dial gauge that was located in the middle span of each beam adjacent to the point load. While keeping the above point load at the constant level of 15 kN, four out of five beams, except for the control beam CB5, were then subjected to accelerated corrosion on the parts of their H8 bottom tensile bars simultaneously for the remaining 50–60 days, as shown in Fig. 3. To ensure uniform moisture condition of different beams and to improve the electrical conductivity of the concrete, 3.5% sodium solution was sprayed for 24 h to the part of a beam to be corroded. After this initial ‘wet’ phase, the power suppliers were immediately switched onto impress the currents of 0.25 mA/cm2 and 0.50 mA/cm2 onto the H8 bars during the first two weeks and the remaining six weeks, respectively, when the beams were under a cycle of ‘wet’ and ‘dry’ conditioning. This cycle of wet and dry conditions was established by having 2 h
Fig. 1. Specimen of reinforced concrete beam.
Screw Distribution beam Load cell Beam specimen Water tank for wet/dry cycling Screwed rod
Fig. 2. Test rig for beam loading.
of sodium–chloride solution spraying and the remaining 22 h of indoor natural exposure of the beam every day. A timer was used to control the turn on and off of water-spraying while the current was kept at a constant level. At the end of the above process, both the control beam CB5, which was only subjected to the applied loads without corrosion, and the corroded beam CB4 were further loaded to failure using Mand Testing Rig under displacement control to measure their ultimate strength and maximum deflections. A beam was deemed to have failed in its load-bearing capacity, if either its tensile steel bar fractured or its post-peak load decreased by 50% of its ultimate strength. At the end of the test, both corroded bars and non-corroded bars were removed from the tested specimens. The amount of corrosion of reinforcing bars was calculated on the basis of its weight loss, which represents an average loss of steel crosssectional area and is about 1–5 times less that is determined using its minimum residual cross-section area [12].
3. Results and discussion The main experimental results of concrete beams subjected to simultaneous loadings and reinforcement corrosion are shown in Figs. 4–6 in terms of their load-dependant and time-dependant deflections, respectively. Here, the deflections of the beams under the initial loading of up to 15 kN and under simultaneous loading of 15 kN and reinforcement corrosion were shown in Figs. 4 and 5. The increased deflections of the beams CB4 and CB5 under the further loading of up to 20 kN and 25.6 kN, respectively, were added to Fig. 5, while those under the simultaneous loading and reinforcements only in Fig. 6. 3.1. Structural performance of concrete beam under loading and corrosion As shown in Figs. 4 and 5, the structural performance of reinforced concrete beams subjected to simultaneous loading and reinforcement corrosion can be divided into three different stages, i.e., stage A for initial loading, stage B for time-dependant deflection and stage C for final failure. Initially, when the point load was gradually increased to between 5.0 and 7.0 kN, the deflections of all concrete beam almost linearly increased to between 0.28 mm and 0.51 mm and the first set of flexural cracks occurred in the middle span of the concrete beams. When the point load was increased to the designated level of 15 kN, the development of the flexural cracks on beam surface tended to stabilise and the deflections of the five beams increased to between 1.46 and 2.03 mm with an average value of 1.70 mm, as shown in Figs. 4 and 5. During the 24 h of the initial ‘wet’ phase, the average deflection of the five beams increased from 1.70 mm to 2.72 mm with an average value of 2.7 mm. This was still well below the limit deflection of 4.0 mm in term of L/250 specified in the BS EN 1992-1-1 for a concrete beam at its serviceability limit state [11], as shown in Fig. 5. Following the initiation of the corrosion of reinforcements in the beams CB1–CB4, some rust stains and corrosion cracks appeared on beam surfaces along the length of the longitudinal
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Y. Du et al. / Construction and Building Materials 39 (2013) 148–152
Power supply -
+
Lc
Stainless steel plate Conductive foam
CB1 (Lc=150mm); CB2 (Lc=500mm)
Water pipe
+
Lc
CB3 (Lc=300mm); CB4 (Lc=650mm); CB5 (Lc=0mm) Fig. 3. Test rig for reinforcement corrosion.
B
2.50
C
Increase of Deflection (mm)
A 30
Applied Load (kN)
25 20 15 10 5
2.00 1.50 1.00 0.50 0.00 0
10
20
0 0
10
20
30
40
50
60
70 CB5
Midspan Deflection (mm) CB5
CB1
CB2
CB3
CB4
C
B
10.0 9.0
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8.0 7.0 6.0 5.0 4.0
3.0 Limit deflection for serviceability
2.0 1.0 0.0 0
10
20
30
40
50
60
70
Time (Days) CB5
CB1
CB2
CB3
40
50
CB3
CB4
60
CB1
CB2
Fig. 6. Increased time-dependent deflection of beams under loading and corrosion.
Fig. 4. Load-dependent deflections of beams under simultaneous loading and corrosion.
A
30
Time (Days)
CB4
Fig. 5. Time-dependent deflections of beams under loading and corrosion.
reinforcements, crossed with the previously formed flexural cracks. The defections of the beam continued to increase, as shown in Fig. 5. In addition, the deflections of corroded beams CB1–CB4 increased much more than those of the control beam CB5, as shown in Fig. 6, and had exceeded their limit deflection of 4.0 mm much earlier than the control beam CB5 prematurely, as shown in Fig. 5. Hence, in addition to the rusts stains and corrosion cracks, which stay on beam surfaces and damage structural aesthetics, the deflection of a corroded beam is likely to exceed its limiting deflection and therefore may cause the beam no longer to satisfy the requirements for its serviceability limit state, as indicated in Fig. 5. After about 50 days of simultaneous loading and reinforcement corrosion, the beams CB1, CB2 and CB3 with 14%, 19% and 16% corrosions, respectively, first became unstable suddenly with their deflections out of control, as shown in Figs. 4 and 5. The beam CB4 with 27% of corrosion and the control beam CB5 reached their ultimate loads of 20 kN and 25.6 kN, respectively, following an extra 12 days of the testing and a further loading using Mand Testing Rig. A careful examination on all tested beams shows that one out of two tensile bars of the beams had fractured either under the simultaneous loading and reinforcement corrosion (CB1, CB2 and CB3) or under the further loading (CB4 and CB5). Therefore they were deemed to fail in their load-bearing capacity at their ultimate limit state.
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3.2. Failure modes of beam under simultaneous loading and reinforcement corrosion Experimental results also indicate that, in additional to an earlier failure at serviceability limit state due to an excessive deflection that exceeds the limiting deflection of a beam prematurely, the simultaneous loading and reinforcement corrosion also cause the beams CB1–CB4 to fail in a less ductile and even brittle ways at their ultimate limit state with a fracture of their tensile bars. In contrast to the control beam CB5 that failed in very ductile manner with wider flexural cracks and substantial deflection, for the corroded beams CB1–CB4, once their ultimate strengths decreased to below the value of the sustained loads of 15 kN, they suddenly collapsed with a fracture of their corroded tensile bars, as typically shown in Figs. 4 and 7. The anticipated ductile flexure failure of an under-reinforced beam was replaced by a brittle failure. This is due to the reduction of ductility of corroded reinforcement and non-uniform corrosion along the length of a corroded bar [13–15]. 3.3. Effect of simultaneous actions of loading and corrosion on beam behaviour Du et al. carried out an experimental investigation on several types of under-reinforced concrete beams that had the same type of the concrete mix as those used for the beams CB1–CB5, but with different dimensions of 150 200 2100 mm and different reinforcements [15]. Similar to those for the beams CB1, CB2 and CB3, only 300 mm long bottom tensile bars within the mid-span of these concrete beams were artificially corroded. Instead of the simultaneous loading and reinforcement corrosion, as described above for the beams CB1, CB2 and CB3, however, Du et al’s. previously tested beams were first corroded to the expected levels without loading, except for their self-weight, and then subjected to an increasing loading until their failure to check the variation of their structural performance due to reinforcement corrosion. By dividing the differences of both the measured strengths and deflections, respectively, between the control and corroded beams with those of the control beam, the percentage reduction of beam strengths and maximum deflection under the two sets of the tests were determined and shown in Fig. 8. It is clear that both ultimate strength and maximum deflection of a concrete beam decrease as a result of the corrosion of its tensile bars. In addition, for the same amount of corrosion, the
Structural Strength Residual Strength of Corroded beams Sustained loading P=15kN
T
0
Service Time
Reduction of beam strength and deflection
Y. Du et al. / Construction and Building Materials 39 (2013) 148–152
120% 100% 80% 60% 40% 20% 0% 0
5
10
15
20
Amount of corrosion (%) Strength-Simultanous Deflection-Simultanous Strength-Simultanous Deflection-Simultanous
Strength-Separate Deflection-Sepatate Strength-Separate Deflection-Separate
Fig. 8. Simultaneous effects of loading and corrosion on beam behaviour.
maximum deflection of a concrete beam, namely, beam ductility decreases more rapidly than its ultimate strength. In particular, both ultimate strength and maximum deflection of the beams CB1, CB2 and CB3 under simultaneous loading and reinforcement corrosion reduce much more rapidly than those of the previously tested beams under separate loading and reinforcement corrosion. In other words, the simultaneous action of both loading and corrosion impairs both residual strength and ductile behaviours of corroded structures more significantly than the case where corrosion and loading occur separately. This may be due to a localised corrosion that is much more easily developed along the length of reinforcement under tension stress. Hence, care should be taken when interpreting the results of structural elements that were corroded without applied loads into the real structures. 4. Conclusions From the above experimental results, the following conclusions can be drawn: (1) Due to the corrosion of tensile bars, the time-dependant deflection of a corroded beam under service loads increase more rapidly than those of a non-corroded beam. (2) In addition to rusts stains and surface cracks, corrosion also likely causes a concrete beam under service loads to reach at its limiting deflection prematurely, which damages structural aesthetes and cause the beam to no longer satisfy the requirements for its serviceability limit state. (3) Under simultaneous loading and reinforcement corrosion, a concrete beam would fail and collapse without significant warning of signs, except for the rusts stains and surface cracks. An anticipated ductile failure can be replaced by a less ductile or even brittle failure. (4) The ultimate strength and maximum deflection of a beam tested under simultaneous loading and corrosion decrease much rapidly than those tested under a separate loading and corrosion.
Acknowledgements
Service Life of Structure
Fig. 7. Failure of a beam under simultaneous loading and reinforcement corrosion.
The tests were carried out at The University of Birmingham, and were partially sponsored by the Institution of Structural Engineer (IStructE) under the Institution 2008 MSc Research Grant.
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