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Laboratory investigations of steel bar corrosion in concrete Background document SB3.10

PRIORITY 6

SUSTAINABLE DEVELOPMENT

GLOBAL CHANGE & ECOSYSTEMS

INTEGRATED PROJECT

Sustainable Bridges SB-3.10 2007-11-30 2 (137)

This report is one of the deliverables from the Integrated Research Project “Sustainable Bridges - Assessment for Future Traffic Demands and Longer Lives” funded by the European Commission within 6th Framework Programme. The Project aims to help European railways to meet increasing transportation demands, which can only be accommo-dated on the existing railway network by allowing the passage of heavier freight trains and faster passenger trains. This requires that the existing bridges within the network have to be upgraded without causing unnecessary disruption to the carriage of goods and passengers, and without compromising the safety and economy of the railways.

A consortium, consisting of 32 partners drawn from railway bridge owners, consultants, contractors, research insti-tutes and universities, has carried out the Project, which has a gross budget of more than 10 million Euros. The Euro-pean Commission has provided substantial funding, with the balancing funding has been coming from the Project partners. Skanska Sverige AB has provided the overall co-ordination of the Project, whilst Luleå Technical University has undertaken the scientific leadership.

The Project has developed improved procedures and methods for inspection, testing, monitoring and condition as-sessment, of railway bridges. Furthermore, it has developed advanced methodologies for assessing the safe carrying capacity of bridges and better engineering solutions for repair and strengthening of bridges that are found to be in need of attention.

The authors of this report have used their best endeavours to ensure that the information presented here is of the highest quality. However, no liability can be accepted by the authors for any loss caused by its use.

Copyright © Authors 2007.

Figure on the front page: Photo of the Ullasund bridge in Norway, reinforced concrete bridge, corroding under envi-ronmental conditions (sea water).

Project acronym: Sustainable Bridges Project full title: Sustainable Bridges – Assessment for Future Traffic Demands and Longer Lives Contract number: TIP3-CT-2003-001653 Project start and end date: 2003-12-01 -- 2007-11-30 Duration 48 months Document number: Deliverable D3.10 Abbreviation SB3.10 Author/s: G. Horrigmoe, I. Sæther, Roy Antonsen and Bård Arntsen, Norut Technology Date of original release: 2007-11-30 Revision date:

Project co-funded by the European Commission within the Sixth Framework Programme (2002-2006)

Dissemination Level

PU Public X PP Restricted to other programme participants (including the Commission Services) RE Restricted to a group specified by the consortium (including the Commission Services) CO Confidential, only for members of the consortium (including the Commission Services)


Summary

Sustainable Bridges is an integrated project in FP6 which develops procedures for better assessment of railway bridges to allow higher axle loads and increased speeds on exist-ing railway bridges. Data obtained during inspection and condition assessment are cru-cial to estimate the current state of railway bridges. The European project Sustainable Bridges was funded by the European Commission in the 6th Framework Programme, contract no. TIP3-CT-2003-001653. The new economic structure of Europe requires an increase of interoperability, mobility and barrier-free transport of passengers and goods. The transport of goods by rail is expected to double or even treble within the next 20 years. To be prepared to accom-modate these future rail traffic demands, the objectives of the project Sustainable Bridges are:

- Increase the transport capacity of existing bridges by allowing axle loads up to 33 t for freight traffic with moderate speeds or for speeds up to 350 km/h for passen-ger traffic with low axle loads.

- Increase the mean residual service life of existing bridges with up to 25%. - Enhance management, strengthening, and repair systems.

The present report deals with corrosion of steel reinforcement which is the principal cause of deterioration of reinforced concrete infrastructure. Previous research on steel bar corrosion has mainly been devoted to the causes and mechanisms of corrosion while relatively little attention has been given to the problem of assessing the residual strength of corroded structures. The objective of the present report is to investigate the effects of steel bar corrosion on structural performance. In the first phase of this re-search, a comprehensive evaluation was carried out based on published laboratory test data on corrosion of steel bars in concrete, see Appendix D. In the second phase, ex-perimental research was conducted to investigate the effects of steel bar corrosion, us-ing both specimens collected from the field (30 years exposure) and standard speci-mens cast in the laboratory. The experimental data from the present research is used to develop methods for assessment of the structural consequences of reinforcement corro-sion (deliverable D3.11). Corrosion of reinforcing steel bars reduces performance and service life of European railway bridges. The major threat is posed by the reduction in bond caused by steel bar corrosion. Bond, or the composite action between steel reinforcing bars and concrete, is the key ingredient of reinforced concrete. The reduction of bond in vital structural com-ponents may therefore lead to impaired stiffness and strength. The need to assess the structural consequences of reduced bond is made more intricate by the fact that loss of bond is a hidden effect which does not lend itself to direct observation or measurement in the field. Further, laboratory tests have revealed that reduced bond may change the structural behaviour from that of ductile, flexural failure to brittle, shear or anchorage failure. All of this emphasizes the need for improved methods of assessing the bond conditions of corroding steel bars in the field, and of reliable methods for estimating the effects of bond loss on structural performance and safety.


The effect of steel bar corrosion on bond behaviour needs experimental verification. The present investigation does not only consider the influence of corrosion on the maximum value of bond stress (i.e., the bond strength), but includes the entire bond behaviour of corroded bars. The chief motivation behind this is that realistic bond stress-slip relation-ships are a prerequisite for developing reliable methods for assessing performance and capacity of reinforced concrete structures with corroded reinforcement. Any attempt to estimate the structural response of corroding RC structures, without realistic bond stress-slip models, will be of limited value. The work described herein deals with laboratory analyses of bond behaviour of corroded steel bars determined by standard pullout tests. The lack of field data on bond is one of the major shortcomings for the development of reliable methods for assessment of the effects of corrosion on bond behaviour. The present study comprised samples subjected to approximately 30 years of field exposure before they were subjected to pullout testing in the laboratory. These specimens were also used to study how conventional “patch” repair affects bond. A specific difficulty with standard laboratory experiments on steel bar corrosion is due to the fact that such experiments are generally highly accelerated by an induced electric current. The applied current densities are often orders of magnitude greater that the highest values measured in the field. Hence, the correlation between laboratory results and field experience is not always very good. In the present investiga-tion it was attempted to relieve some of these shortcomings by using relatively low cur-rent densities to accelerate the corrosion process. The bulk of the specimens subjected to 30 years of exposure under field conditions had corrosion levels between 0 % and 3 % weight loss. Within this interval bond strength was almost unaffected by corrosion. An increase in initial stiffness of the corroded specimens was recorded and the post peak behaviour remained relatively ductile. For the standard laboratory specimens corrosion resulted in reduced bond strength for all three current densities employed in the present study (0.05 mA/cm2, 0.1 mA/ cm2 and 0.25 mA/ cm2). The largest scatter was found for the specimens subjected to the highest current density. Bond stress-slip diagrams were recorded for each of the three test series. A careful evaluation of these results was presented together with a detailed comparison with results from similar investigations published in the literature. Quantita-tive information from the present and other experimental studies can be used to estab-lish realistic bond stress-slip relationships for corroded reinforcement. Further experi-mental research is needed in order to improve our understanding of bond behaviour of corroded steel bars.


Table of Contents Summary .................................................................................................................... 3

1. Introduction............................................................................................................. 6

1.1 Corrosion of steel reinforcement .................................................................... 6

1.2 Experimental studies of reinforcement corrosion ........................................... 6

1.3 Scope of the present investigation ................................................................. 7

2. Experimental program ............................................................................................ 9

2.1 Field specimens............................................................................................. 9

2.2 “Repaired” specimens.................................................................................. 12

2.3 Laboratory specimens.................................................................................. 13

3. Bond strength ....................................................................................................... 14

3.1 Field specimens........................................................................................... 14



4. Bond stress-slip behaviour ................................................................................... 26

4.1 Uncorroded bars .......................................................................................... 26

4.2 Previous studies........................................................................................... 32

4.3 Field specimens........................................................................................... 42



5. Concluding remarks ............................................................................................. 54

6. References........................................................................................................... 56

7. APPENDICES ...................................................................................................... 57


1. Introduction

1.1 Corrosion of steel reinforcement The European railway bridge stock comprises bridges made of common engineering materials such as steel, concrete, and masonry (brick, stone). Reinforced concrete bridges constitute a significant portion of these bridges, although there are variations between the individual countries in Europe regarding construction materials. Reinforced concrete bridges, like other types of concrete structures, suffer from various forms of deterioration, e.g., steel bar corrosion, cycles of freezing and thawing, alkali aggregate reactions, etc. Among these deterioration mechanisms, corrosion of steel re-inforcement is the principal cause of degradation of concrete structures, in general. Re-inforcement corrosion is commonly classified according to the type of attack. The term general corrosion is used to characterize the case where corrosion is spread over the entire surface of the bar. If, on the other hand, the attack is localized, it is called local corrosion or pitting. General corrosion may be attributed to either carbonation or chloride contamination, whereas local corrosion is invariably associated with chlorides penetrat-ing the concrete cover and not with carbonation. Reinforced concrete road bridges lo-cated in or close to the sea are susceptible to attack by chlorides originating from sea water. Even more severe chloride induced corrosion of road bridges may be caused by the use of deicing salt during the winter season. It is reasonable to assume that steel bar corrosion due to chloride contamination is a lesser problem for reinforced concrete rail-way bridges than it is for road bridges and that carbonation plays a relatively more im-portant role for railway bridges. Steel bar corrosion affects the reinforcement itself (loss of bar cross section), the sur-rounding concrete (cracking and spalling of concrete cover) and the composite action between steel and concrete (reduced bond). Previous research has been mainly con-cerned with the causes and mechanisms of reinforcement corrosion, and, in particular, with corrosion caused by chloride contamination. The effects of steel bar corrosion on structural performance and safety, however, have received relatively little attention. Loss of bar cross section, cracking of concrete cover and reduced bond all contribute to the structural consequences of reinforcement corrosion. Based on the review of published results from laboratory investigations on steel bar corrosion (Appendix D) it can be con-cluded that the development of longitudinal cracks is associated with significant reduc-tions in bond strength and that this usually occurs before the bar cross sectional area is noticeably reduced. Moreover, the reduction in bond strength is greater than the loss of bar cross section. From a structural point of view, bond loss due to corrosion is particu-larly intriguing since it may lead to sudden, brittle shear or anchorage failure.

1.2 Experimental studies of reinforcement corrosion The major portion of published experimental data on the influence of corrosion on bond has been obtained from pullout tests. These laboratory investigations have used a vari-ety of specimens and bar types, and it is therefore not surprising that the bond strength values reported in the literature differ widely. Further scatter is caused by the variations in the adopted procedures for conditioning of specimens for corrosion studies. In the majority of bond tests on corroded bars, the corrosion process has been activated by


chloride salts. In order to accelerate the corrosion process to achieve reasonable dura-tions of the tests, relatively high current densities have in many cases been adopted. These current densities may be orders of magnitude greater than the highest values re-corded in the field. Steel bar corrosion is a slow process, which under field conditions takes decades, whereas laboratory experiments on corrosion are designed to take months (or weeks). Hence, results from laboratory tests do not always show very good correlation with experiences from the field. Continued experimental research into the influence of corrosion on bond is of vital im-portance to improve our understanding of the structural consequences of reinforcement corrosion. To achieve better correspondence with field conditions, future laboratory tests should be less accelerated than the majority of previous laboratory investigations of steel bar corrosion. Moreover, corrosion influences the entire bond stress-slip behaviour of affected bars, and not only the maximum bond stress, i.e., the bond strength. Whereas bond failure of uncorroded bars in well confined concrete is by ductile, pullout failure, corroded bars may exhibit brittle, splitting failure due to loss of confinement caused by longitudinal cracking. Systematic field investigations on steel bar corrosion are complicated to perform and the associated costs are in most cases prohibitive. As a result, reliable field data are very scarce. Information on reinforcement corrosion under field conditions may be obtained, however, when reinforced concrete structures attacked by corrosion are being demol-ished. Large pieces of concrete can be cut from the structure and transported to a labo-ratory where suitable specimens can be prepared and tested. By this approach, it is possible to obtain long time field data for steel bar corrosion.

1.3 Scope of the present investigation Corrosion of steel reinforcement in concrete affects the reinforcement itself, the sur-rounding concrete and the composite action between reinforcement and concrete. Of these, the latter is the most important and, at the same time, the most intriguing. Bond, or the composite action, between steel bars and concrete is the essence of reinforced concrete. The reduction of bond due to steel bar corrosion may therefore pose threats to the performance and safety of reinforced concrete structures. Loss of bond is a hidden effect which does not lend itself to direct observation or measurement in concrete struc-tures in the field. Since we can only measure (or more precisely, estimate) the degree of steel bar corrosion, it is important to investigate how reinforcement corrosion influences bond behaviour. This line of research will enable us to develop more reliable methods for condition assessment of concrete structures with corroding reinforcement, with re-gard to loss of bond and its possible effects on structural performance and safety. The major portion of previous experimental research on the effects of steel bar corrosion on bond has been concerned with bond strength (i.e., the maximum bond stress) and how varying levels of corrosion influence bond strength. A review of published test data on bond strength of corroded reinforcing bars is compiled in Appendix D. It should be borne in mind; however, that corrosion affects the entire bond stress-slip behaviour and not only the maximum value of bond stress. Accordingly, we need to establish bond stress-slip diagrams for corroded steel bars. This type of data is a prerequisite for devel-oping reliable mathematical models and associated, numerical simulation methods for structural assessment of deteriorated concrete structures.


The present report focuses attention on bond stress-slip behaviour of corroded steel re-inforcement. Test data from a comprehensive laboratory study are presented. Bond stress-slip diagrams determined from pullout tests are examined and compared with re-search results published by other investigators. A unique feature of the present study is that it made use of standard laboratory specimens as well as concrete with corroded steel bars collected from the Ullasund bridge (Figure 1.1), which was demolished in 1998. The latter specimens had been subjected to approximately 30 years of field expo-sure under harsh environmental conditions.

Figure 1.1: The Ullasund bridge.


2. Experimental program

2.1 Field specimens From the pieces of concrete collected from Ullasund bridge cubic specimens with di-mensions 150 mm x 150 mm x 150 mm were prepared. These specimens contained a single steel bar of 25 mm diameter and no transverse reinforcement, as illustrated in Figure 2.1. The specimens were cast in concrete cylinders with diameter 320 mm and height 160 mm to fit the requirements of the testing machine, see Figure 2.2.

Figure 2.1: Specimen from Ullasund bridge.

Figure 2.2: Specimen cast in concrete cylinder for pullout testing.


A total of 22 cylindrical specimens were stored in the laboratory and subjected to pullout testing after 28 days. Of these specimens, two (no. 0 and no. 7) were destroyed during the mechanical testing. For four specimens (numbers 3, 6, 9 and 12) errors occurred with the data registration. Accordingly, the effective number of field specimens from the Ullasund bridge was reduced to 16. The steel bars were ribbed of type Ks40 (cf. Norwegian Code NS 481) with a nominal diameter of 25 mm. The guaranteed yield strength was 400 MPa. The Ullasund bridge was constructed during 1968-1969. Records of the exact concrete mix could not be retrieved. In order to obtain strength data, three cylinders were cored and tested. The average uniaxial compressive strength obtained from these drilled cores was 40.3 MPa. Specimen data are compiled in Table 2.1. Table 2.1 Compressive strength of Ullasund field concrete (drilled cores). Specimen no.

Height [mm]

Weight [kg]

Volume [mm3]

Failure load [kN]

Compressivestrength [MPa]

1 200 3.628 1561 345 43.9

2 200 3.660 1560 282 35.9

3 200 3.614 1546 325 41.4

Average 40.3

After pullout testing of the Ullasund field specimens had been completed, the corrosion products accumulated on the surface of the steel bars were removed by sandblasting. The corrosion level was defined as the relative weight loss of the bar. The 22 corroded steel bars are shown in Figure 2.3 which illustrates that the degree of corrosion not only varies from specimen to specimen, but there were also significant variations in the cor-rosion attack along the length of individual bars. The corrosion level was determined by a procedure in which each corroded steel bar was gradually lowered into a cylinder partially filled with water. The lowering process was carried out in small steps so that the water level was raised by 10 mm in each step. A cleaning liquid was added to the water in order to reduce surface stresses and thereby facilitate the readings of the water level in the cylinder. Afterwards, the bars were cleaned by sandblasting and then weighted to check the results obtained from the sub-merging process. By this procedure, not only the total weight loss but also the variations in cross sectional area along the axis of the corroded bar, could be established. How-ever, these measurements were unsuccessful in the sense that the production toler-ances of the steel bars were too large to obtain meaningful measurements of the varia-tions in cross sectional area along the bar axis. Hence, only the total weight loss could be established.


Figure 2.3: Corrosion of steel bars in Ullasund field specimens.


2.2 “Repaired” specimens The most widely used repair procedure for corrosion damaged structures continues to be that of removal of carbonated or chloride contaminated concrete, followed by sand-blasting of corroded steel bars and application of a cement-based mortar to replace the removed concrete. This procedure is commonly referred to as “patch” repair. However, the term may be somewhat misleading since in some cases the total area of a single “patch” may be quite large, both in absolute terms and compared to the dimensions of the structural element under consideration. Unless repair work is carefully executed and all chloride contaminated concrete is removed, patch repair has in many cases proved to be inefficient to prevent subsequent corrosion. Another inherent problem with patch repair is that the cleaned steel bars will have re-duced cross sectional area, and, more importantly, the bond strength of these bars may be impaired. Although these shortcomings can be relieved by adding extra reinforcing bars before the repair mortar is applied, this may not be feasible from a practical point of view, and, in most cases, extra steel bars are not employed. One of the objectives of the present investigation was to study the bond characteristics of corroded steel bars after proper repair had been conducted. With this in mind, a total of 11 of the steel bars from the Ullasund field specimens were selected for a separate study in which they were subjected to a procedure similar to conventional patch repair. After sandblasting, the corroded steel bars were cast in new concrete specimens. These specimens were of cylindrical shape with diameter 320 mm and height 160 mm, see Figure 2.2. It was attempted to use a concrete mix resembling the concrete used for construction of Ullasund bridge during 1968-69. The mix proportions of the concrete used for the “repaired” specimens are listed in Table 2.2. This mix resulted in a uniaxial compressive strength of 30 MPa (28 days). It should be noted that no chlorides were added to the concrete mix. The corroded steel bars employed in this study had varying levels of corrosion, ranging from 0.6 % to 15.8 % weight loss. The “repaired” specimens were stored in the laboratory and subjected to pullout testing at the age of 28 days. Table 2.2 Mix proportions of concrete used in Ullasund “repaired” specimens.

Constituent Weight, kg/m3

Cement 280

Sand 0-8 mm 958

Coarse aggregate 8-22 mm 958

Water 190

Water-cement ratio 0.68


2.3 Laboratory specimens Standard laboratory specimens were cast with the objective of studying the effects of the accelerated test conditions on steel bar corrosion and bond behaviour. These specimens consisted of a single, uncorroded steel bar embedded in a concrete cylinder with diame-ter 320 mm and height 160 mm, see Figure 2.2. Ribbed steel bars, B500B, were used (cf. Norwegian Code NS 3576). The bars had 25 mm nominal diameter and guaranteed yield strength of 500 MPa. The concrete mix was identical to that used in the Ullasund “repaired” specimens (Table 2.2), the only excep-tion being that 3 % NaCl was added to initiate corrosion. The average 28 days compres-sive strength was 30 MPa. The corrosion process was accelerated by applying an electrical current. Three different current densities were used, 0.05, 0.1 and 0.25 mA/cm2, respectively. The specimens were split into there different categories depending on the applied current density. To identify the different test series, the terminology Lab 0.05, Lab 0.1 and Lab 0.25 will be used throughout this report. Detailed information about each of the three series of labo-ratory tests is compiled in Table 2.3. It is to be noted that in the test conducted in the present study, the slip (i.e., the relative displacement between the steel bar and concrete) was measured at the loaded end dur-ing pullout testing. This is commonly referred to as loaded end slip. Table 2.3 Laboratory specimens. Series

Lab 0.05 Series Lab 0.1

Series Lab 0.25

No. of specimens 9 9 9

No. of reference specimens 3 3 3

Steel bar diameter, mm 25 25 25

Embedded length, mm 160 160 160

Constant current density, mA/cm2

0.05 0.1 0.25

Length of corrosion process, days

127 52 14


3. Bond strength

3.1 Field specimens The 22 specimens prepared from concrete taken from Ullasund bridge were effectively reduced to 16, for reasons already explained in Section 2.1. These specimens covered a wide range of corrosion levels, from 0.6 % to 15.8 % weight loss. The associated reduc-tions in “equivalent” bar diameter were 0.1 mm and 2.2 mm, respectively. The bond strength was obtained from pullout tests assuming that the bond stress was uniformly distributed over the portion of the surface of the bar embedded in concrete. Denoting the maximum pullout load by Pmax, the bond strength maxτ is defined as the shear stress corresponding to Pmax, i.e.,

L

P⋅⋅

=φπ

τ maxmax (3.1)

where φ is the nominal bar diameter and L the embedded length.

A compilation of data from the pullout tests is provided in Table 3.1, which contains, for each individual specimen, the embedded length L, calculated bond strength maxτ accord-ing to Eq. (3.1), measured slip (i.e., relative displacement) at failure, relative corrosion weight loss, and relative reduction in equivalent diameter. For the sake of completeness, data for all 22 specimens are listed in Table 3.1. It is seen that five of the specimens contained visible cracks which were recorded at the time the specimens were being pre-pared in the laboratory. Three of these specimens (no. 3, 6 and 9) were discarded due to data registration problems during testing. Hence, only two of the remaining 16 speci-mens (no. 15 and 21) contained visible cracks. Test data from the Ullasund field speci-mens are compiled in Appendix A. Table 3.1 reveals that the specimens covered corrosion levels ranging from 0.0 % (ap-proximately) to 15.8 % weight loss. The average bond strength for the 16 effective specimens listed in Table 3.1 was 8.3 MPa. The minimum recorded value of the bond strength was 7.0 MPa and the maximum 11.2 MPa. Note that for these field specimens, no reliable reference bond strength for uncorroded bars was available. Figure 3.1 shows a plot of recorded bond strengths versus weight loss. It is observed that the bulk of the data covers corrosion levels between 0 % and 3 % weight loss and that only four specimens had corrosion levels in excess of 3 % weight loss. The general trend in Figure 3.1 seems to be that for these field specimens, bond strength is almost unaffected of increasing levels of corrosion up to 3 % weight loss. Another observation is that the scatter in the results is relatively low.


Table 3.1 Ullasund field specimens; pullout tests.

Specimen no.

Embed-ded length (mm)

Bond strength (MPa)

Slip at failure (mm)

Corrosion weight loss (%)

Corrosion diameter loss (%)

Re-marks

0 Specimen destroyed during testing

1 138 8.17 0.37 2.08 1.04

2 139 8.57 1.17 1.82 0.91

3 139,5 ? ? 6.60 3.35 cracked

4 140 8.00 1.09 1.61 0.81

5 138 7.02 0.47 2.62 1.32

6 130 ? ? 6.32 3.21 cracked

7 Specimen destroyed during testing

8 143 8.11 1.26 15.80 8.24

9 136 ? ? 1.42 0.71

10 137.5 8.19 0.97 0.56 0.28

11 137.5 11.16 1.27 3.97 2.00

12 145 ? ? 11.44 5,90 cracked

13 135 7.88 1.21 0.88 0.44

14 140 7.59 2.26 1.50 0.75

15 145 9.09 1.58 8.80 4.50 cracked

16 138.5 8.47 2.84 2.05 1.03

17 137.5 8.16 3.14 -0.57 -0.28

18 140 8.32 1.78 2.57 1.30

19 140 7.85 1.14 1.94 0.97

20 135 8.45 3.61 1.43 0.72

21 135 8.36 1.73 6.12 3.11 cracked


0

2

4

6

8

10

12

0 2 4 6 8 10 12 14 16 18

corrosion, weight loss [%]

bond

str

engt

h, [M

Pa]

Ullasund, field

Figure 3.1: Ullasund field specimens; bond strength.

3.2 “Repaired” specimens Complete test data from the Ullasund “repaired” specimens are available in Appendix B. Some of the results obtained with these 11 specimens are compiled in Table 3.2.The preparation of these specimens has been previously described in Section 2.2. The cor-rosion level for the steel bars, after field exposure and before casting in new concrete specimens, varied from 0.6 % to 15.8 % weight loss. The lowest and highest value of bond strength recorded was 5.6 MPa and 7.9 MPa, respectively, with the mean value being 6.9 MPa. Comparing these values to the corresponding bond strengths recorded for the original Ullasund field specimens, it is observed that all three characteristic val-ues are lower for the “repaired” specimens. Bond strength versus weight loss is plotted in Figure 3.2. It is seen that bond strength remains almost unaffected by increased corrosion. A direct comparison between bond strengths obtained from the field specimens (16) and from the “repaired” specimens (11) is provided in Figure 3.3. At first, it may appear as if the bond strengths of the “repaired” specimens are very close to bond strengths of the original field specimens. Since the same steel bars were used both in the Ullasund field and in the Ullasund “repaired” specimens, it is possible to trace the performance specimen by specimen. To do this, we first note that reinforcing bars from some of the field specimens for which data errors occurred during testing, where reused in the Ullasund “repaired” series. More specifi-cally, this was the case with specimens no. 3, 6 and 12 in Table 3.2 which are also in-cluded in the illustrations in Figures 3.2 and 3.3. By removing these three specimens from the Ullasund “repaired” series, the remaining 9 specimens will have steel bars for which bond strength was also measured during testing of the Ullasund field series. Bond strengths for these specimens are compiled in Table 3.3. For all 9 steel bars, the bond


strength after “repair” was lower than the corresponding bond strength measured for the field specimens. The reductions in bond strength varied between 7.2 % and 31.7 % with a mean value of 17.2 %. Table 3.2 Ullasund “repaired” specimens; pullout tests.

Specimen

Embedded length

Bond strength

Slip at failure

Corrosion,weight loss

Corrosion, diameter loss

no. (mm) (MPa) (mm) (%) (%)

u 01b 138 6.72 1.04 2.08 1.04

u 03 b 139 6.40 4.13 6.60 3.35

u 06b 130 7.30 2.88 6.32 3.21

u 08b 143 6.53 4.72 15.80 8.24

u 10b 137 7.12 2.05 0.56 0.28

u 11b 137 7.62 3.07 3.97 2.00

u 12b 145 5.56 2.45 11.44 5.90

u 14b 140 6.30 1.48 1.50 0.75

u 15b 145 7.20 3.70 8.80 4.50

u 16b 138 7.86 3.00 2.05 1.03

u 21b 135 7.51 2.85 6.12 3.11


0

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10 12 14 16 18


bond

str

engt

h, [M

Pa]

Ullasund, repaired

Figure 3.2: Ullasund “repaired” specimens; bond strength.

0

2

4

6

8

10

12

0 2 4 6 8 10 12 14 16 18


bond

str

engt

h, [M

Pa]

Ullasund, field

Ullasund, repaired

Figure 3.3: Ullasund field and Ullasund “repaired” specimens; bond strength.


Table 3.3 Ullasund field and “repaired” specimens; comparison of bond strength. Bond strength MPa

Difference in bond strength Specimen

no.

Corrosion, weight loss %

Ullasund, field

Ullasund repaired MPa %

0

1 2.08 8.17 6.72 -14.89 -17.70

2 1.82 8.57

3

4 1.61 8.00

5 2.62 7.02

6

7

8 15.80 8.11 6.53 -1.58 -19.45

9

10 0.56 8.20 7.12 -1.07 -13.09

11 3.97 11.16 7.62 -3.53 -31.68

12

13 0.88 7.88

14 1.50 7.59 6.30 -1.29 -16.97

15 8.80 9.09 7.20 -1.89 -20.81

16 2.05 8.47 7.86 -0.61 -7.21

17 -0.57 8.16

18 2.57 8.32

19 1.94 7.85

20 1.43 8.45

21 6.12 8.37 7.51 -0.86 -10.25

3.3 Laboratory specimens As described in Section 2.3, standard laboratory specimens were cast and subjected to an imposed electrical current to accelerate the corrosion process. Three different current densities were used, 0.05, 0.1 and 0.25 mA/cm2. Accordingly, the test series were de-noted by Lab 0.05, Lab 0.1 and Lab 0.25, respectively. To get an indication of the de-gree of acceleration of the corrosion conditions, it is noted that the associated durations of the tests were 127 days, 52 days and 14 days, respectively. Test data for the three


series of laboratory specimens are contained in Appendix C. Lowest current density, i = 0.05 mA/cm2 Laboratory series Lab 0.05 was subjected to the lowest current density, 0.05 mA/cm2. A total of 12 specimens were employed. Of these, three specimens were reference speci-mens which were not subjected to corrosion. These specimens were used to establish a reference value for the bond strength, which was 10.7 MPa. In Table 3.4, the results from pullout tests of series Lab 0.05 are listed. It is observed that the corrosion weight loss for these specimens varied from 3.1 % to 6.8 %, whereas the bond strengths ranged between 5.2 MPa and 9.7 MPa. The relative bond strengths are illustrated in Figure 3.4 as a function of percentage weight loss. These data all lie between 3 % and 7 % weight loss. Within this interval, the average bond strength was 7.5 MPa, which is 70 % of the reference value for the uncor-roded specimens. Bond strength seems to be little affected when corrosion increases from 3 % to 7 % in these tests. Since there are no data available for weight losses be-tween 0 % and 3 %, it is not possible to make any statements whether the low induced current density used in this series would have produced an increase in bond strength at low levels of corrosion. Table 3.4 Laboratory specimens, series Lab 0.05; pullout tests.

Specimen Embedded length

Bond strength

Slip at failure

Corrosionweight loss

no (mm) (MPa) (mm) (%)

ref 1 160 9.84 1.77

ref 2 160 10.48 2.15

ref 3 160 11.91 1.60

1 160 7.68 2.09 5.62

2 160 8.09 2.19 5.84

3 160 9.68 2.64 3.40

4 160 7.67 2.35 3.05

5 160 5.17 2.24 5.58

6 160 7.17 2.52 5.19

7 160 6.10 0.95 4.15

8 160 9.03 3.59 5.19

9 160 6.48 2.51 6.82


0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

0 1 2 3 4 5 6 7 8


Rela

tive

bond

str

engt

h

0.05 mA/cm2

Figure 3.4: Laboratory specimens, series Lab 0.05; bond strength. Intermediate current density, i = 0.1 mA/cm2 In laboratory series Lab 0.1, 9 specimens were subjected to a current density of 0.1 mA/cm2. Of these, two specimens were discarded, so that effectively 7 corroded speci-mens remained in this series. In addition, three reference specimens (no corrosion) were included for the purpose of establishing a reference value for the bond strength. How-ever, the specimen ref. 3 was rejected since its weight was outside the tolerance limits. The two remaining specimens produced a reference bond strength of 9.6 MPa. Data from pullout tests of series Lab 0.1 are compiled in Table 3.5. The corrosion level for these specimens all lie in a relatively narrow band between 3 % and 5 % weight loss and the associated bond strengths varied from 7.3 MPa to 11.3 MPa. The average bond strength for these specimens was 8.9 MPa, which corresponds to 86 % of the bond strength for an uncorroded bar. Relative bond strengths versus percentage weight losses are portrayed in Figure 3.5. From this diagram it is seen that there is an almost 20 % increase in bond strength be-tween 3 % and 4 % weight loss, although this is based on the results from two speci-mens only. Due to lack of data for corrosion levels lower than 3 %, it is not possible to make any judgements regarding possible increases in bond strength for low levels of corrosion (0-3 %) using an induced current density of 0.1 mA/cm2. In Figure 3.5 there are five specimens with almost the same degree of corrosion (from 4.4 % to 4.7 % weight loss). These specimens produced an average bond strength of 8.0 MPa, or 77 % of the reference value.


Table 3.5 Laboratory specimens, series Lab 0.1; pullout tests.

Specimens Embedded length

Bond strength

Slip at fracture ( loaded)

Corrosion,weight loss

nr (mm) (MPa) (mm) (%)

ref 1 160 9.13 1.9 0

ref 2 160 10.00 2.28 0

ref 3

U 23 160 11.12 2.24 3.20

U 24 160 9.33 2.89 -0.89

U 25 160 8.17 1.90 -0.26

U 26 160 7.89 2.42 4.69

U 27 160 8.08 2.58 4.35

U 28 160 11.32 2.42 3.88

U 29 160 9.04 2.40 4.45

U 30 160 7.64 1.74 4.39

U 31 160 7.28 2.40 4.52

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

0 1 2 3 4 5 6 7 8


Rela

tive

bond

str

engt

h

0.1mA/cm2

Figure 3.5: Laboratory specimens, series Lab 0.1; bond strength.


Highest current density, i = 0.25 mA/ cm2

In series Lab 0.25, a total of 12 specimens were subjected to accelerated corrosion us-ing a current density of 0.25 mA/ cm2 . For two of the specimens (no. U8 and no. U15) data failure occurred, so that this series contained effectively 10 specimens. In addition, there were three references specimens. The calculated reference value of the bond strength was 10.4 MPa. Results from pullout tests of this series are given in Table 3.6. It is seen that the weight losses ranged from 0.8 % to 5.3 % whereas the bond strengths varied between 3.9 MPa and 13.6 %. The average bond strength for the 10 specimens subjected to corrosion was 8.7 MPa, or 84 % of the average value for the uncorroded, reference specimens. The relative bond strengths are plotted in Figure 3.6 as a function of relative weight loss. First, it can be observed that the data cover the entire range from 0 % to 5.3 % weight loss. The scatter in the results is greater for this series than for the two other series based on lower induced current densities. Both the highest (13.6 MPa) recorded value of bond strength and the lowest (3.9 MPa) value belong to this series. It is seen from Fig-ure 3.6 that some specimens produced increased bond strengths for low levels of corro-sion, but there is a large scatter in the results. The five specimens with weight loss greater than 3 % had an average bond strength of 7.7 MPa (75 % of the reference value).

Table 3.6 Laboratory specimens, series Lab 0.25; pullout tests.

Specimens Embedded length

Bond strength

Slip at failure

Corrosion weight loss

no (mm) (MPa) (mm) (%)

ref.1 160 10.43 2.28

ref.2 160 10.92 1.94

ref.3 160 9.69 2.73

U 3 160 8.70 1.79 2.09

U 4 160 13.63 3.45 1.78

U 5 160 4.93 3.23 3.09

U 6 160 10.89 2.72 1.31

U 7 160 5.91 2.356 2.22

U 8 Fault

U 9 160 13.02 3.12 3.16

U 10 160 9.36 1.97 0.79

U 12 160 7.93 2.35 4.10

U 13 160 3.89 3.41 5.33

U 15 Fault

U 20 160 8.70 2.14 4.13


0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

0 1 2 3 4 5 6 7 8


Rel

ativ

e bo

nd s

tren

gth

0.25 mA/cm2

Figure 3.6: Laboratory specimens, series Lab 0.25; bond strength. Comparison A compilation of characteristic values of bond strength for the three series is provided in Table 3.7. The relative bond strengths for all laboratory specimens are plotted versus weight loss in Figure 3.7. Although the absolute value of the bond strength has been scaled by the reference bond strength for each series, this diagram is also representa-tive for the absolute values since the reference bond strengths for the three series are very close. The following conclusions can be drawn from the analysis of laboratory specimens:

1. Corrosion leads to reduced bond strength for all three applied current densities, 0.05, 0.1 and 0.25 mA/cm2.

2. By considering all specimens within each of the three series, the mean values of the bond strength were 70 % (0.05 mA/ cm2), 86 % (0.1 mA/ cm2) and 84 % (0.25 mA/ cm2) of the respective reference values.

3. Limiting the scope to corrosion levels corresponding to weight losses between 3 % and 5 %, the reductions in bond strength were 30 % (0.05 mA/ cm2), 16 % (0.1 mA/ cm2) and 16 % (0.25 mA/ cm2).

4. Other investigations have revealed significant increases in bond strength for low levels of corrosion, especially when high current densities were applied (strongly accelerated tests). In the present investigation, the number of specimens with


corrosion levels in the area 0 % to 3 % was too few to draw any define conclu-sions.

5. The largest scatter in the measured bond strengths was obtained for the highest current density, 0.25 mA/ cm2. The absolute maximum and minimum values of the bond strength occurred in this series, which also produced the largest stan-dard deviation.

Table 3.7 Laboratory specimens; characteristic values of bond strength. Bond strength [MPa]

Lab 0.05 i=0.05 mA/cm2

Lab 0.1 i=0.1 mA/cm2

Lab 0.25 i=0.25 mA/cm2

Reference (uncorroded) 10.74 9.57 10.35

Maximum 9.68 11.32 13.63

Minimum 5.17 7.28 3.89

Mean 7.50 8.91 8.70

Standard deviation 1.415 1.667 3.231

Maximum – Minimum 4.51 4.04 9.74

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

0 1 2 3 4 5 6 7 8


Rel

ativ

e bo

nd s

tren

gth

0.05 mA/cm2

0.1mA/cm2

0.25 mA/cm2

Figure 3.7: Laboratory specimens; relative bond strength.


4. Bond stress-slip behaviour

4.1 Uncorroded bars We first consider the bond behaviour of uncorroded steel bars in concrete. In mathe-matical terms, bond behaviour is uniquely defined by the functional relationship between bond stress τ and slip (i. e., relative displacement) s. It is important to note that we con-sider the relationship ττ = (s) as a constitutive relation. This means that this relation is valid for an arbitrary point on the surface of the reinforcing bar, and this is the way it is used in numerical simulations based on the finite element method. By contrast, the ex-perimental determination of the relation ττ = (s) is based on pullout tests, and a stan-dard pullout test (Figure 2.2) represents a structure (and not a point) where both bond stress and slip varies along the embedded reinforcing bar. This should be borne in mind when the accuracy of the constitutive relation between bond stress τ and slip s is con-sidered. The bond stress-slip behaviour of reinforcing bars is closely related to the type of failure that occurs, and the type of bond failure is governed by the degree of confinement pro-vided to the steel bar. There are three common sources of confinement: transverse rein-forcement (e.g. stirrups), external compressive stresses (e.g. caused by reactive forces at supports) and sufficient concrete material surrounding the reinforcing bar. In the latter case, the ratio between concrete cover c and bar diameter φ is used to characterize the degree of confinement. If the c/φ -ratio is large or the concrete is well confined by transverse reinforcement, or both, bond failure will occur as pure pullout failure in which the steel bar is pulled out from the concrete. The associated behaviour is ductile where the bond stress increases monotonically towards a maximum value which is maintained as the slip increases. Eventually, the bond stress drops to a final, residual value. This residual bond strength is caused by the confinement which remains partially intact and offers resistance against pullout. If the c/φ -ratio is small, and the concrete is either unconfined or moderately confined (as is the case in most structural applications), a longitudinal crack will occur and penetrate the concrete cover from the steel bar to the concrete surface. The corresponding failure is called splitting failure. Since there is no transverse reinforcement to provide confine-ment of the steel bar, the behaviour will be brittle, that is, the bond capacity drops rapidly after the maximum value has been reached. Pullout failure For steel bars embedded in well confined concrete the mode of failure is by pullout. There is solid experimental evidence that the bond strength maxτ associated with pullout failure is proportional to the square root of the concrete compressive strength ckf , where

ckf is the characteristic uniaxial compressive strength measured on cylindrical speci-mens. The corresponding bond stress-slip relationship is well established and adopted by Comité Euro-International du Beton (1993), subsequently referred to as MC 90. This


relationship is illustrated in Figure 4.1. The various branches of the bond stress-slip dia-gram in Figure 4.1. is defined by the following mathematical formulas,

( )α

ττ ⎟⎟⎠

⎞⎜⎜⎝

⎛=

1max s

ss for 10 ss ≤≤ (4.1a)

( ) maxττ =s for 21 sss ≤< (4.1b)

( ) ( ) ⎟⎟⎠

⎞⎜⎜⎝

⎛−−

−=23

2max ss

sss fτττ for 32 sss ≤< (4.1c)

( ) fs ττ = for 3ss > (4.1d)

Figure 4.1: Bond stress-slip diagram for well confined concrete (pullout failure).

The bond stress-slip relationship is completely defined by the bond strength maxτ , the residual bond capacity fτ , the exponent α , and the three slip values 1s , 2s and 3s .

The ascending portion of the curve in Figure 4.1 is given by Eq. (4.1a), i. e.,

( )α

ττ ⎟⎟⎠

⎞⎜⎜⎝

⎛=

1max s

ss for 10 ss ≤≤ (4.2)


The initial stiffness is governed by the exponent ( )10 ≤≤ αα . The value 0=α corre-sponds to an instantaneous increase in bond stress up to the maximum value maxτ whereas 1=α yields a linear relation between bond stress τ and slip s. In MC 90, it is suggested to use the value 4.0=α for all bond conditions, but it is pointed out that the stiffness (i.e., the slope of the τ -s diagram) depends on the confinement. Values for the parameters occurring in Eqs. (4.1a) – (4.1d) are compiled in Table 4.1.

It is seen from Table 4.1 that MC 90 suggests the following values for bond strength maxτ and residual bond capacity fτ for confined concrete and good bond conditions,

ckf5.2max =τ (4.3)

max40.0 ττ =f (4.4)

For all other bond conditions, maxτ is reduced by 50 % and the same relative reduction applies to maxτ . Table 4.1 Parameters defining the bond stress-slip relationship, Eqs. (4.1a) – (4.1d), according to MC 90.

Unconfined concrete Confined concrete Parameter Good bond

conditions All other bondconditions

Good bondconditions

All other bond conditions

s1 0.6 mm 0.6 mm 1.0 mm 1.0 mm

s2 0.6 mm 0.6 mm 3.0 mm 3.0 mm

s3 1.0 mm 2.5 mm Clear rib spacing

Clear ribspacing

α 0.4 0.4 0.4 0.4

maxτ ckf0.2 ckf0.1 ckf5.2 ckf25.1

fτ max15.0 τ max15.0 τ max40.0 τ max40.0 τ Splitting failure For unconfined concrete, bond failure occurs by splitting of the concrete around the rein-forcing bar. This type of failure is also dealt with in MC 90, where the bond stress-slip relationship for splitting failure is obtained by modifying the corresponding relationship for pullout failure.


The suggested modifications for good bond conditions are as follows:

1. The bond strength ,maxτ is reduced by 20 %, cf. Eq. (4.3),

ckf0.2'

max=τ (4.5) The prime ( ' ) is introduced to distinguish bond behaviour corresponding to split-ting failure from that associated with pullout failure.

2. The residual bond capacity is reduced significantly, cf. Eq. (4.4), '

max' 15.0 ττ =f (4.6)

where '

maxτ is given by Eq. (4.5).

3. The slip values ,1s and ,

2s are made to coincide, i. e., '

2'1 ss = (4.7)

4. The numerical values for the ,1s , ,

2s and ,3s are reduced,

see Table 4.1, ='

1s 0.6 mm (reduced from 1.0 mm) (4.8) ='

2s 0.6 mm (reduced from 3.0 mm) ='

3s 1.0 mm (reduced from clear rib spacing)

The bond stress-slip curve corresponding to splitting failure is illustrated in Figure 4.2 (dotted line), together with the curve representing pullout failure (solid line). The differ-ences in behaviour are evident from this diagram.


Figure 4.2: Bond stress-slip diagram; pullout failure (solid line), splitting failure (dotted line). Harajli (2004) reported results from laboratory studies of bond behaviour during pullout failure as well as during splitting failure using varying concrete strengths, bar diameters and c/φ -ratios. It was concluded in Ref. 3 that the experimental results for bond strength corresponding to splitting failure was best fitted by the formula

max

3/2'max τ

φδτ ≤⎟⎟

⎠

⎞⎜⎜⎝

⎛=

cfck (4.9)

where the coefficient δ = 0.75 for normal strength concrete ( )MPa48<ckf .

In MC 90 a ratio of concrete cover to bar diameter equal to 1.0 represents unconfined concrete. Substituting c/φ = 1.0 in Eq. (4.9) yields

ckf75.0'max =τ (4.10)

which is only 37.5 % of the bond strength specified in MC 90 for good bond conditions, c.f. eq. (4.5). The bond stress-slip diagram for splitting failure proposed by Harajli (2004) uses the cor-responding diagram for pullout failure in MC 90 as a basis, see Figure 4.3. The shape of the diagram is slightly modified since it is suggested that the coefficient α in the expres-sion for the ascending branch of the curve, Eq. (4.2), should be assigned the value 0.3, whereas α = 0.4 is recommended in MC 90. This will produce a somewhat stiffer behav-iour. Also, note that the slip parameters s1, s2 and s3 are assigned values as follows:


ocs 15.01 = (1.5 mm)

ocs 35.02 = (3.5 mm) (4.11)

ocs =3 (10 mm)

where oc is the clear rib spacing of the reinforcing bar. The values given in parentheses can be used in cases where information about the bar properties are lacking. The latter set of values are generally larger than those recommended in MC 90, see Table 4.1.

Figure 4.3: Bond stress-slip diagram for splitting failure; Harajli (2004). The bond stress-slip curve suggested by Harajli (2004) for splitting failure is shown by the dotted line in Figure 4.3. Compared to MC 90 (Figure 4.2) this model is more refined. The curve for splitting failure breaks off from the envelope (pullout failure) at the bond stress '

maxτβτ ⋅= where the coefficient β is assigned a value of 0.7. Splitting occurs at 'maxττ = as defined by Eq. (4.9). Since it is difficult to quantify the loss in bond resistance

immediately after splitting, it is suggested to take the coefficient δ as anywhere between 0.6 and 0.7 for normal strength concrete. Beyond splitting, Harajli (2004) found that the test data were fitted with reasonably accuracy by the formula

2/1

'max

'max)(

−

⎟⎟⎠

⎞⎜⎜⎝

⎛=

sss τδτ (4.12)


where 'maxs is the slip at splitting, see Ref. 3.

4.2 Previous studies Previous laboratory studies on steel bar corrosion have to a large extent focused atten-tion on the effect of corrosion on bond strength. Several investigators have reported re-sults showing an increase in bond strength for low levels of corrosion, but as corrosion continues and longitudinal cracking develops bond strength rapidly deteriorates to only a small fraction of its initial value. It has been demonstrated that bond strength of corroded bars is influenced by a number of different parameters such as bar roughness, concrete strength, concrete cover, transverse reinforcement, etc. Last but not least, conditioning of specimens and accelerated testing have a strong influence on measured values of bond strength as discussed in the review of previous work (Appendix D). The influence of corrosion on bond strength has been dealt with in Chapter 3. As already pointed out, corrosion not only influences the peak bond stress (i.e., the bond strength) but it has a profound impact on the entire bond stress-slip relationship. Thus, in order to gain improved understanding of how corrosion affects bond, it is necessary to study the complete bond stress versus slip behaviour of corroded bars. Moreover, a proper bond stress-slip model is a key ingredient in numerical simulations of the structural perform-ance of reinforced concrete structures with corroding reinforcement. Any attempt to cal-culate performance and strength of corroding RC structures, without realistic models for the bond stress-slip behaviour, will be of limited value. Experimental investigations on bond stress-slip relations for corroded reinforcing bars are limited in number. Al-Sulaimani et al. (1990) reported results from pullout tests on 150 mm cubic concrete specimens using 10, 14 and 20 mm bars embedded centrally, giving cover-to-bar diameter ratios of 7.50, 5.36 and 3.75, respectively. The concrete specimens were cast from a 0.55 w/c ratio concrete having an average compressive strength of 30 MPa. A relatively high current density (2 mA/cm2) was adopted in these experiments. Increased bond stress was recorded in the precracking range (up to about 1 % corrosion). With further corrosion, the bond stress declined consistently until it be-came negligible for 6.5-8.5 % corrosion (post cracking). Bond stress versus slip curves for different levels of corrosion for a 10 mm bar is depicted in Figure 4.4. According to Al-Sulaimani et al. (1990), similar behaviour was recorded for bars with 14 mm and 20 mm diameter.


Figure 4.4: Bond stress versus slip for 10 mm bar; Al-Sulaimani et al. (1990). A series of pullout tests on corroded reinforcing bars was conducted by Tachibana et al. (1990). The prismatic specimens had two eccentrically embedded steel bars of 16 mm diameter. Minimum cover-to-bar diameter ratio was estimated to 1.38 based on the data provided by Tachibana et al. (1990). The average compressive strength was 35.6 MPa. Specimens were subjected to an electrical current density of 0.5 mA/cm2 for 3 days, 6 days, 10 days and 15 days, respectively. No explicit measurements of the corresponding levels of corrosion are given by Tachibana et al. (1990). The obtained bond stress-slip diagram shows increased initial stiffness (i.e. slope) for all corrosion periods. For 3 days of corrosion, the bond strength was approximately the same as for uncorroded speci-mens, whereas longer durations led to a consistent reduction in bond strength. Cabrera and Ghoddoussi (1992, 1996) conducted pullout tests on 150 mm concrete cubes with a 12 mm diameter reinforcing bar centrally embedded in the cube, corre-sponding to a cover-to-bar diameter ratio of 5.75. Concrete made of both ordinary Port-land cement (opc) and of a mixture of Portland cement and fly ash (pfa) was used in the experiments. For both mixes, the w/c ratio was 0.55 and the uniaxial compressive strength 56 MPa. The applied current density is not explicitly given by Cabrera and Ghoddoussi (1992, 1996). Figure 4.5 shows bond stress versus slip diagrams for opc specimens where the degree of corrosion ranged from 0 % to 12 % weight loss.


Figure 4.5: Bond stress versus slip for 12 mm bar; Cabrera and Ghoddoussi (1996). Results from pullout tests of corroded steel bars were reported by Auyeung et al. (2000) and by Auyeung and Balaguru (2001). The specimens consisted of concrete prisms (cross section 177.8 x 177.8 mm) in which two steel bars were centrally embedded. Long and short bars were separated by 50.8 mm. When the two bars were pulled in op-posite directions, the shorter bar pulled out of the concrete. The bar diameter was 19 mm, corresponding to a cover-to-bar diameter ratio of 8.35. The concrete had a water-cement ratio of 0.6. The concrete compressive strength, measured on cylindrical speci-mens, was specified to be 30 MPa (Aeyeung and Balaguru 2001). An average current density of 3 mA/cm2 was maintained for each corrosion process. Bond stress-slip be-haviour for an uncorroded bar is illustrated in Figure 4.6, whereas the effect of corrosion is depicted in Figure 4.7. Note that in this diagram, a normalized bond stress is used. In the initial stages of corrosion, the bond stress-slip curve is steeper due to increased fric-tion. Increased bond was observed up to a mass loss of approximately 2 %. Beyond this value, cracks started to form along the corroded bar and the stiffness reduced consis-tently. Even though both stiffness and bond strength increased at low levels of corrosion, the slip at failure decreased substantially. This is a clear indication that ductility is re-duced even at low levels of corrosion. Experimental investigations of bond properties of corroded reinforcement were reported by Lee et al. (1996). In the pullout tests which were conducted on 81.3 mm cubic specimens, material and geometric parameters were subject to variations in order to study the effects of individual parameters. The investigation comprised ribbed as well as plain reinforcing bars. Specimens without or with transverse reinforcement were used, and cover-to-bar diameter ratios were 1.5, 2.5 and 3.5, respectively. The concrete mixes employed had w/c ratios of 0.45, 0.55 and 0.65, and the associated compressive strengths were 42.1 MPa, 33.0 MPa and 24.7 MPa, respectively. The level of corrosion was controlled by impressing a direct current of 1 A on the specimens for specified peri-ods of time using a constant electrical current supply. Bond stress-slip diagrams re-ported by Lee et al. (1996) for ribbed bars were based on concrete specimens with w/c = 0.65 and cover-to-bar diameter ratio equal to 3.5. In Figure 4.8 results for specimens without transverse reinforcement are portrayed. The curves correspond to three different levels of corrosion, 0 %, 3.2 % and 16.8 % weight loss. The bond strength of laterally


unreinforced specimens was abruptly lost by splitting of the concrete along the crack formed by corrosion. Similar curves for specimens with transverse reinforcement are depicted in Figure 4.9. A more ductile behaviour was observed for these specimens, de-spite the fact that they contained a longitudinal crack due to corrosion. The explanation for this behaviour is that the lateral reinforcement prevented splitting of the concrete. For specimens with or without lateral reinforcement, the maximum bond stress was consis-tently reduced for increasing levels of corrosion.

Figure 4.6: Bond stress versus slip for 19 mm uncorroded bar; Auyeung et al. (2000).

Figure 4.7: Normalised bond stress versus slip for 19 mm bar; Auyeung et al. (2000).


Figure 4.8: Bond stress versus slip for specimens without lateral reinforcement; Lee et al. (1996). Similar findings are available in the paper by Fang et al. (2004). Here, bars (ribbed or plain) of 20 mm diameter were cast in prismatic concrete specimens with 140 mm square cross section. The study included specimens without or with stirrups. The water-cement ratio of the concrete was 0.44, corresponding to a compressive strength of 52.1 MPa. To accelerate reinforcement corrosion, a direct electric current (0-2A) was im-pressed on the steel bar embedded in the specimen. All the corroded specimens had longitudinal corrosion-induced cracks. Load versus slip curves for specimens without stirrups are shown in Figure 4.10 for 0 %, 4 % and 9 % weight loss, respectively. It is seen from this diagram that corrosion caused a significant reduction in the maximum load (45 % reduction for 4 % weight loss). Bond behaviour of specimens with stirrups is illustrated in Figure 4.11. The associated levels of corrosion were 0 %, 3.8 % and 6.0 %, respectively. It is observed that as the degree of corrosion increased, bond strength was to a large extent maintained for these corrosion levels. For 6 % weight loss the bond strength was only reduced by 12 % compared to that of the uncorroded specimen. Also for the specimens with stirrups, bond failure was caused by splitting of the specimen along the corrosion-induced cracks. The high ductility recorded for these specimens was attributed to the confinement provided by the stirrups.


Figure 4.9: Bond stress versus slip for specimens with lateral reinforcement; Lee et al. (1996).

Figure 4.10: Load versus slip for specimens without stirrups; Fang et al. (2004).


Figure 4.11: Load versus slip for specimens with stirrups; Fang et al. (2004). Comparisons The literature survey of pullout test data for corroded reinforcing bars has demonstrated that, although the investigations are relatively few in number, they do not lend them-selves easily to comparison. The experimental investigations cited in this section have made use of varying specimen geometries, bar diameters, cover-to-bar ratios, concrete qualities, and induced current densities (where specified). Comparisons of absolute val-ues of measured bond stresses versus associated, absolute values of slip will be highly scattered and therefore create some confusion. In an attempt to establish a rational ba-sis for direct comparison of the available test data, the measured bond stress in a test series was scaled by the associated concrete compressive strength. Similarly, the slip values recorded in the test were divided by the steel bar diameter. The bond stress-slip diagrams reported by other investigators can now be recast using the dimensionless quantities introduced in the present report. Figure 4.12 shows results for bond stress/compressive strength versus slip/bar diameter for uncorroded bars calculated from previous investigations. As could be expected, there is a significant variation in the values of the relative, maximum bond stress. The slip at maximum load divided by bar diameter seems to lie in the interval from 0.01 to 0.02.


0

0,1

0,2

0,3

0,4

0,5

0,6

0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10

free end slip/bar diameter

bond

str

ess/

com

pres

sive

str

engh

t

Tachibana (1990)Al-Sulaimani (1990)Auyeung (2000)Cabrera (1992, 1996), opcCabrera(1992,1996), pfaFang (2004)Lee(2002)

Figure 4.12: Dimensionless bond stress versus dimensionless slip for uncorroded bars.

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

0,000 0,010 0,020 0,030 0,040 0,050 0,060 0,070

free end slip/bar diameter

Bon

d st

ress

/com

pres

sive

str

engh

t

Auyeung (2000), 5.19%

Tachibana(1990), 6 days

Fang (2004), 4.0%

Lee(2002), 3.2%

Cabrera(1992, 1996), 2.40%, pfa

Cabrera( 1992,1996), 2.42%, opc

Al-Sulaimani (1990), 4.27%

Figure 4.13: Dimensionless bond stress versus slip for corroded bars (2.4 – 5.2 % weight loss).


Similar curves for corroded bars are depicted in Figure 4.13. Again, the scatter in peak values is considerable. The dimensionless values of slip corresponding to peak bond stresses exhibit less scatter. This quantity is approximately 0.001 – 0.01 for cover-to-bar diameter ratios less than 4.5 and somewhat larger, 0.02 – 0.05, when the cover-to-bar diameter ratio exceeds 4.5. This relationship between bond stress τ and slip s for pullout failure of uncorroded bars is specified by MC 90 and illustrated in Figure 4.1. The ascending branch of this curve is defined by

α

ττ ⎟⎟⎠

⎞⎜⎜⎝

⎛=

1max s

s for 1ss ≤ (4.13)

There are three variables in this formula, the bond strength maxτ , the slip 1s at failure and the exponent α . Numerical values for these parameters differ considerably between investigators, even for uncorroded reinforcing bars. As could be expected, the values of

maxτ , 1s and α differ widely between these tests. Limiting the scope to the coefficient α , it is observed that α varies between 0.10 and 0.65 for uncorroded bars, whereas data for corroded bars produced values of α in the interval from 0.01 to 0.82.

In the state-of-the-art report (fib Task Group Bond Models 2000), Eq. (4.13) was fitted to published data from pullout tests comprising uncorroded as well as corroded bars.This approach was also adopted in the present study. The references considered are listed are listed in Table 4.2 together with the computed values of maxτ , 1s and α . The coeffi-cient α was calculated by fitting a curve through the three points with bond stress 0,

maxτ /2 and maxτ .

It is of interest to investigate the effect of corrosion on slip 1s at failure. Published test data are compiled in Figure 4.14. As could be expected, the results exhibit considerable scatter.


0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

0 2 4 6 8 10 12 14 16 18

corrosion, [%], weight loss

free

end

slip

, [m

m]

Cabrera (1992), pfaCabrera (1992), opcAugueng (2000)Al-Sulaimani (1990)Fang( 2004)Lee(2002)Augueng (2001), φ=10 mmAugueng (2001), φ=13 mmAugueng (2001), φ=16 mmAugueng (2001), φ=19 mmAugueng (2001), φ=25 mm

Figure 4.14: Slip 1s at failure versus degree of corrosion.


Table 4.2 Fitting of test data to MC 90. Author(s) Weight loss ,

% α s1 ,

mm τmax , MPa

0 0.35 0.16 15.7 Al-Sulaimani et al., (1990)

0.78 0.4 0.27 4.0

0 0.35 0.27 12 Tachibana et al., (1990)

6 days 0.3 0.08 9

0 0.25 0.43 20 Cabrera & Ghoddouissi, ( 1992) 6.9 0.35 0.52 15

0 0.1 0,7 16 Almussallam et al. (1996)

5.1 0.01 0,1 13.5

0 0.41 0,254 4.71 Auyeung et al., ( 2000)

5.19 0.82 0,038 1.46

0 0.44 0.315 21.95 Fang et al. ( 2004) No stirrups 4 0.14 0.071 12.06

0 0.30 0.5625 21.95 Fang et al. ( 2004) Stirrups 3.8 0.21 0.65 20.87

0 0.65 0.276 6.15 Lee et al., (2002) No stirrups 0.32 0.15 0.0095 3.95

0 0.27 1.34 7.85 Lee et al., (2002) Stirrups 13 0.15 1.34 6.95

4.3 Field specimens As explained in Chapter 2, pullout tests were conducted on Ullasund field specimens, Ullasund “repaired” specimens and on the three series of laboratory specimens. During the pullout tests, the applied force and corresponding displacement was registered at small intervals. Figure 4.14 shows a typical bond stress-slip diagram produced from one of these tests. As can be seen, the curve has an irregular shape, which makes direct comparisons of curves representing different tests difficult. This is particularly true for the descending branch in Figure 4.14. To overcome this difficulty, it was decided to simplify the bond stress-slip curves by replacing each curve by three straight line segments passing through the points A, B, C and D in Figure 4.14. These four points are defined as follows: Point A: Origin of the τ versus s diagram, i. e., 0,0 == sτ .

Point B: Point corresponding to maximum value of τ , i. e. 1max , ss =τ .


Point C: Point representing the turning point of the descending branch.

Point D: The final point of the test, i. e., maxmin , ss == ττ .

Of these points, A, B and D, are well defined, while in some cases the identification of point C is less than obvious.

0

2

4

6

8

10

12

0 2 4 6 8 10 12

loaded end slip, [mm]

bond

str

ess,

[MPa

]

ExperimentSimplified

A

B

C

D

Figure 4.15: Simplified representation of experimentally determined bond stress-slip diagram. In total, there were 16 field specimens from the Ullasund bridge. These specimens were split into three groups depending on the degree of corrosion. 0.0 – 2.5 % weight loss (9 specimens) 2.6 – 5.0 % weight loss (3 specimens) 5.1 – 8.8 % weight loss (2 specimens) Two specimens had corrosion levels outside the selected intervals. Specimen no. 8 had a recorded weight loss of 15.8 % and was left out of consideration. For specimen no. 17, the measured weight loss after approximately 30 years of field exposure was 0.0 %. For each specimen the four points A, B, C and D on the bond stress-slip diagram (Figure 4.15) were identified with the corresponding s,τ - values. Mean values for each of the three groups were calculated, resulting in the average simplified bond stress-slip dia-grams shown in Figure 4.16. For comparison, the diagram for specimen no. 17 is also included in Figure 4.16. The computed values of bond stress and slip corresponding to points A – D are listed in Table 4.3. The slip 1s corresponding to the maximum bond stress maxτ (point B) seems to be almost unaffected by varying corrosion levels (0.0 – 8.8


%). Moreover, the measured residual bond stress for maxss = (point D) was greater than zero (0.3 – 2.2 MPa) in these tests. The curve for each individual specimen is available in Appendix A.

0

2

4

6

8

10

0 2 4 6 8 10 12loaded end slip, [mm]

bond

str

ess,

[MPa

]

mean 0-2.5%

mean 2.6 - 5.0%

mean 5.1- 8.8%

0% (specimen no.17)

A

B

C

DD

Figure 4.16: Ullasund field specimens; simplified bond stress-slip diagrams. Table 4.3 Ullasund field specimens; points A – D.

A B C D Corrosion %, weight loss

slip mm

bond stress MPa

slip mm

bond stressMPa

slip mm

bond stress MPa

slip mm

bond stressMPa

ref. 17 0 0 3.14 8.16 3.64 1.04 10.00 0.45

0 - 2.5% 0 0 1.47 8.13 3.73 2.39 9.57 0.93

2.6 - 5.0% 0 0 1.17 8.83 2.58 4.61 9.81 2.21

5.1 - 8.8% 0 0 1.65 8.73 2.91 1.99 9.94 0.34

4.4 “Repaired” specimens The 9 “repaired” specimens from the Ullasund bridge were divided into three categories according to the same principles as adopted for the Ullasund field specimens. For the repaired specimens, only one reinforcing bar had weight loss between 2.6 % and 5.0 % (specimen no. u11b, w = 3.97 %). Test results for the individual specimens can be found in Appendix B. Simplified bond stress-slip diagrams for each of the three groups are shown in Figure 4.17 and the associated values of bond stress and slip are tabulated in


Table 4.4. It is observed that for these specimens, the residual bond strength (point D) was practically zero. By comparing the slip at maximum bond stress (point B), the aver-age value for the field specimens (Table 4.3) is 1.86 mm while the corresponding value for the “repaired” specimens is 2.75 mm. Hence, the stiffness is reduced after repair. Table 4.4 Ullasund “repaired” specimens; points A – D.

A B C D Corrosion, %, weight loss

slip mm

bond stress MPa

slip mm

bond stressMPa

slip mm

bond stress MPa

slip mm

bond stressMPa

0 - 2.5% 0 0 1.89 7.00 4.09 0.21 9.03 0.13

2.6 - 5.0% 0 0 3.07 7.62 4.13 0.13 10.00 0.11

5.1 - 8.8% 0 0 3.28 7.35 4.64 0.59 10.00 0.26

0

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

mean 0 - 2.5%

3.97%

mean 5.1-8.8%

DD

C

B

A

Figure 4.17: Ullasund “repaired” specimens; simplified bond stress-slip diagrams.


4.5 Laboratory specimens

Reference specimens (no corrosion) The laboratory specimens were grouped into three categories depending on the applied current density (0.05 mA/cm2, 0.1 mA/ cm2 and 0.25 A/ cm2, respectively), as already described in Section 2.3. Accordingly, the series were denoted Lab 0.05, Lab 0.1 and Lab 0.25. Each series included three reference specimens which were not subjected to any corrosion prior to the mechanical testing. The mean values of bond stress and slip at points A – D for each group of reference specimens were calculated from the data given in Appendix C. The corresponding bond stress versus slip curves are depicted in Figure 4.18. Also shown in this diagram is the curve representing the mean of all 9 ref-erence specimens. Further details are available in Table 4.5. Figure 4.17 reveals that the results from the reference specimens exhibits very little scatter and that the bond stress-slip curves are very close. The residual bond stress (point D) was very low (be-tween 0.2 MPa and 0.9 MPa).

0

2

4

6

8

10

12

0 2 4 6 8 10


bond

str

ess,

[MPa

]

mean ref. Lab. 0.05

mean ref. Lab. 0.1

mean ref. Lab. 0.25

mean, ref. w = 0

A

B

DC

Figure 4.18: Laboratory specimens, reference (no corrosion); simplified bond stress-slip diagrams.


Table 4.5 Laboratory specimens, reference (no corrosion); points A – D. A B C D

Series Corrosion, %, weight loss

slip mm

bond stress MPa

slip mm

bond stress MPa

slip mm

bond stress MPa

slip mm

bond stress MPa

Lab. 0.05 0 0 0 1.84 10.74 5.67 0.79 7.75 0.87

Lab. 0.1 0 0 0 2.09 9.56 5.28 0.52 9.05 0.18

Lab. 0.25 0 0 0 2.32 10.34 5.28 0.4 7.85 0.21

mean ref. 0 0 0 2.08 10.21 5.41 0.57 8.22 0.42

Lowest current density (i = 0.05 mA/cm2) The intention was to split laboratory specimens into three groups with weight loss 0 – 2.5 %, 2.6 – 5.0 % and ≥ 5.1 %, respectively. However, for the 9 specimens of series Lab 0.05, three specimens had weight losses in the range 2.6 – 5.0 %, the remaining 6 specimens had corrosion levels from 5.1 % to 7.5 %, see Appendix C. Simplified bond stress-slip diagrams for these two groups are shown in Figure 4.19 together with the reference curve (uncorroded bars). It is observed that increased corrosion leads to a consistent reduction in bond strength and that the descending branch beyond the maxi-mum value is steeper for corroded bars than for uncorroded bars.

0

2

4

6

8

10

12

0 2 4 6 8 10


bond

str

ess,

[MPa

]

mean 2.6 - 5%

mean 5.1 - 7.5%mean, ref. w = 0

A

B

C D

Figure 4.19: Laboratory specimens; series Lab 0.05; simplified bond stress-slip diagrams.


Intermediate current density (i = 0.1 mA/cm2) Laboratory series Lab 0.1 contained 9 corroded specimens plus two reference speci-mens (Appendix C). Measured weight losses for the corroded specimens were all in the range 2.6 – 5.0 %. The simplified bond stress-slip diagrams for these specimens are shown in Figure 4.20 which also contains the corresponding diagram for the reference specimens. The average bond strength of the corroded bars was 14 % lower than that of the refer-ence bars, but beyond this corrosion seems to have little effect on the shape of the bond stress-slip curve. Highest current density (i = 0.25 mA/cm2) Weight losses for the 10 corroded steel bars in series Lab 0.25 were as follows (see Appendix C): 0.0 – 2.5 % 5 specimens 2.6 – 5.0 % 4 specimens 5.3 % 1 specimen

0

2

4

6

8

10

12

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

mean 2.6 - 5%

mean, ref. w = 0

A

B

CD

Figure 4.20: Laboratory specimens; series Lab 0.1; simplified bond stress-slip diagrams. In Figure 4.21 the associated simplified bond stress-slip diagrams are shown together with the curve representing the mean of the three reference specimens. It is observed that the bond strength is consistently reduced for increasing levels of corrosion. The re-


duction is dramatic for the highest corrosion level (5.3 %), however, this curve is based on one specimen only. An increase in the slip value s1 at maximum bond stress (point B) for increasing levels of corrosion, is visible in Figure 4.20. Comparison, 2.6 – 5.0 % corrosion All three series of laboratory specimens contained specimens with corrosion levels be-tween 2.6 % and 5.0 % weight loss. This corrosion interval was therefore taken as a ba-sis for an attempt to study the effect of the applied current density on bond stress-slip behaviour. An illustration is provided in Figure 4.22 which shows simplified bond stress-slip diagrams representing mean values of the reference specimens (no corrosion) and curves representing each of the series Lab 0.05, Lab 0.1 and Lab 0.25. The three latter curves correspond to corrosion levels in the range 2.6 – 5.0 % weight loss. It is seen from Figure 4.22 that corrosion leads to reduced bond strength for all three current den-sities applied in the present investigation. The reduction in bond strength varies between 13 % and 23 %. However, there is no clear relation between bond strength and applied current density. The slip value s1 corresponding to maximum bond strength (point B) ex-hibits relatively little variation for the data portrayed in Figure 4.22.

0

2

4

6

8

10

12

0 2 4 6 8 10


bond

stre

ss, [

MPa

]

mean 0 -2.5%mean 2.6 - 5%5.3%mean, ref. w = 0

A

B

C D

Figure 4.21: Laboratory specimens, series Lab 0.25; simplified bond stress-slip diagrams.


0

2

4

6

8

10

12

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

mean Lab.0.05mean Lab.0.1

mean Lab.0.25mean, ref. w = 0

A

B

CD

Figure 4.22: Laboratory specimens with corrosion levels 2.6 % to 5.0 % weight loss; simplified bond stress-slip diagrams. Comparisons with test data from Cabrera and Ghoddoussi (1992, 1996) The two most important parameters for characterizing the bond stress-slip relation are the bond strength (i. e, the maximum value of bond stress) and the slip s1 corresponding to maximum bond stress. In Figure 4.23 the slip s1 is plotted versus weight loss for the three series of laboratory specimens. The scatter in these data is considerable, even among the reference specimens (no corrosion). A slight increase in the slip value s1 for increasing levels of corrosion may be viewed from this diagram. The experimental investigation conducted by Cabrera and Ghoddoussi (1992, 1996) and the present laboratory tests are in many respects closely related, as seen from the data listed in Table 4.6. The main deviations between these two investigations are the differ-ences in bar diameter and concrete compressive strength. Also, note that Cabrera and Ghoddoussi (1992, 1996) employed concretes made of ordinary Portland cement (opc) and of a mixture of Portland cement and fly ash (pfa). The applied current density was not specified by Cabrera and Ghoddoussi (1992, 1996). A comparison is provided in Figure 4.24 which shows a plot of the slip value s1, corre-sponding to maximum bond stress, versus weight loss. The values of the slip s1 meas-ured in the present tests are generally greater than those reported by Cabrera and Ghoddoussi (1992, 1996). The latter data seem to exhibit less scatter. The increase in slip with increasing corrosion is more pronounced for the specimens containing fly ash (pfa) than for those made of ordinary Portland cement. In Figure 4.25 dimensionless


bond strength (i. e., cf/maxτ ) is plotted against dimensionless slip s1 (i.e., ϕ/1s ). As al-ready pointed out, the tests reported by Cabrera and Ghoddoussi (1992, 1996) em-ployed specimens with relatively high compressive strength, fc = 56 MPa, and steel bars with relatively small diameters ϕ = 12 mm. The corresponding parameters in the present investigations were fc = 30 MPa and ϕ = 25 mm. Notwithstanding, the dimensionless quantities seem to be in relatively good agreement.

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

0 1 2 3 4 5 6 7 8


load

ed e

nd s

lip, [

mm

]

Lab. 0.05

Lab. 0.1

Lab. 0.25

Figure 4.23: Laboratory specimens; slip s1 versus weight loss.


Table 4.6 Characteristic test parameters. Cabrera and

Ghoddoussi 1992,1996)

Present laboratory studies

Specimen geometry cube cylinder

Bar diameter, φ [mm] 12 25

Cover-to-bar diameter ratio, c/ φ 5.75 5.9

compressive strength, fc [Mpa] 56 opc and pfa

30

Yield strength of reinforcement [MPa]

460 500

Embedded length 4 · φ 6.4 · φ

Chloride content 2% NaCl 3% NaCl

Water- cement ratio 0.55 0.68

Mix proportion 1 : 2.3 : 3.5 1 : 3.4 :3.4

Current density [mA/cm2] ? 0.05, 0.1, 0.25

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

0 2 4 6 8 10 12 14


slip

s1,

[mm

]

Cabrera (1992, 1996),opcCabrera (1992, 1996),pfaPresent

Figure 4.24: Slip s1 versus weight loss.


0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,50

0,00 0,02 0,04 0,06 0,08 0,10 0,12 0,14 0,16

end slip/diam (s1 /φ)

bond

stre

ngth

/com

pres

sive

stre

nght

Cabrera (1992, 1996), opc

Cabrera (1992, 1996), pfa

Present

Figure 4.25: Bond strength versus slip s1 (dimensionless quantities).


5. Concluding remarks

Steel reinforcement corrosion continues to pose threats to the performance and integrity of reinforced concrete structures. Although steel bar corrosion affects the reinforcement itself, the surrounding concrete and the composite action between steel and concrete, the latter effect is the most important. Bond, or the composite action between steel rein-forcing bars and concrete, is the key ingredient of reinforced concrete. The reduction of bond due to steel bar corrosion in vital structural components may therefore lead to im-paired stiffness and strength. Loss of bond is a hidden effect which does not lend itself to direct observation or meas-urement in field concrete structures. Moreover, bond reductions may change the mode of structural behaviour, from that of ductile, flexural failure to brittle, shear or anchorage failure. All of this points to the need for improved methods of assessing the bond condi-tions of corroding steel bars in the field, and of reliable methods for estimating the ef-fects of bond loss on structural performance and safety. While the major portion of earlier investigations has restricted attention to the influence of steel bar corrosion on bond strength, the scope of the present study was broadened to include the entire bond behaviour and not only the maximum value of bond stress. This was motivated by the fact that realistic bond stress-slip relations are a prerequisite for the development of methods for assessing the performance and capacity of rein-forced concrete structures attacked by corrosion. Any attempt to estimate the structural response of corroding RC structures, without realistic models for the bond stress-slip behaviour, will be of limited value. Laboratory experiments on steel bar corrosion have a major shortcoming due to the fact that such experiments are generally highly accelerated in order to achieve reasonable durations of the tests. Steel bar corrosion is a slow process which under field conditions takes decades, whereas laboratory experiments are designed to take weeks or months. Hence, results from laboratory testing often deviate from experiences from the field. Fur-thermore, systematic field data on reinforcement corrosion are rare due to the complexi-ties and high costs associated with such field investigations. In the present study, it was attempted to overcome these difficulties. First, the corrosion process was only moder-ately accelerated using applied current densities which were lower than those adopted in similar laboratory studies reported in the literature. Second, concrete samples subjected to approximately 30 years of field exposure were collected from the Ullasund bridge in Norway, transported to the laboratory where specimens were prepared, and then sub-jected to pullout testing. These specimens were also subjected to a procedure similar to that applied during conventional repair of concrete structures with corroded reinforce-ment, commonly referred to as “patch” repair. The first part of the present study was concerned with the effects of steel bar corrosion on bond strength. The bulk of the Ullasund field specimens had corrosion levels be-tween 0 % and 3 % weight loss. Within this interval, bond strength remained almost un-affected by corrosion. For the Ullasund “repaired” specimens, bond strengths were gen-erally reduced as compared to the original field specimens. Standard laboratory speci-mens were cast with the objective of studying the effects of accelerated test conditions on steel bar corrosion and bond behaviour. Three different applied current densities were used, 0.05, 0.1 and 0.25 mA/cm2, respectively. Corrosion resulted in reduced


bond strength for all three applied current densities. For corrosion levels between 3 % and 5 % weight loss, the reductions in bond strength were 30 % (0.05 mA/cm2), 16 % (0.25 mA/cm2) and 16 % (0.25 mA/cm2). The largest scatter in the measured bond strengths was obtained for the strongest accelerated specimens (0.25 mA/cm2). Other investigators have reported significant increases in bond strength for low levels of corro-sion. In the present study, the number of specimens with corrosion level in the range 0-3 % were too few to draw any definite conclusions. The second part of this investigation has dealt with the effect of corrosion on the entire bond behaviour of reinforcing bars. Bond stress-slip diagrams depend on whether failure occurs as pullout failure (confined concrete) or as splitting failure (unconfined concrete). A discussion on bond stress-slip diagrams for uncorroded steel bars was presented in Section 4.1. Previous experimental investigations on bond stress-slip relations for cor-roded reinforcing bars are limited in number. Available published test data were evalu-ated in Section 4.2. Bond stress-slip behaviour of corroded bars was studied extensively in the laboratory experiments reported herein. Although the Ullasund field and Ullasund “repaired” specimens were included in the present investigations, the emphasis regard-ing bond stress-slip behaviour was on the standard laboratory specimens. In order to facilitate direct comparisons of bond stress-slip diagrams from different pullout tests, a simplified procedure was introduced whereby each test diagram was replaced by straight lines through four selected points. In the Ullasund field specimens, corrosion led to an increase in the initial stiffness. The post peak behaviour was relatively ductile with a significant residual bond capacity. The Ullasund “repaired” specimens displayed less ductility and the residual bond capacity was practically zero. For laboratory specimens with uncorroded reinforcement the slip s1 at maximum bond stress was approximately 2 mm. This slip value increased with increasing levels of cor-rosion for all of the three induced current densities adopted in the present investigation. For the reference specimens (no corrosion) a certain, small residual bond capacity was recorded. However, the residual bond capacity decreased to practically zero for increas-ing levels of corrosion. The present results for bond strength and slip s1 at maximum bond stress were compared to results from similar experimental investigations con-ducted by Cabrera and Ghoddoussi (1992, 1996). In the present laboratory study the uniaxial compressive strength was kept constant at 30 MPa and the cover to bar diameter ratio was kept at a constant value of 5.9, with the bar diameter being 25 mm. Despite these limitations and the natural scatter in the re-sults, data from the present study together with other published test data discussed in the present report yield improved understanding of bond behaviour of corroded steel bars. Quantitative information from these experimental investigations can be used to establish realistic bond stress-slip relationships for corroded reinforcement.


6. References

Al-Sulaimani, G.J., Kaleemullah, M., Basanbul, I.A. and Rasheedduzzafar, 1990. In-fluence of corrosion and cracking on bond behaviour and strength of reinforced con-crete members. ACI Structural Journal, 87 (2), pp.220-231. Auyeung, Y., Balaguru, P. and Chung, L., 2000. Bond behaviour of corroded rein-forcement bars. ACI Materials Journal, 97 (2), pp.214-220. Auyeung, Y. and Balaguru, P., 2001. Effects of corrosion on the bond properties of reinforcing bars. In N. Banthia, K. Sakai and O.E. Gjørv, eds. Proceedings of the third international conference on concrete under severe conditions. Vancouver, Canada: The University of British Columbia, pp.112-119. Cabrera, J.G., 1996. Deterioration of concrete due to reinforcement steel corrosion. Cement & Concrete Composites, 18, pp.47-59. Cabrera, J.G. and Ghoddoussi, P., 1992. The effect of reinforcement corrosion on the strength of the steel/concrete “bond”. International conference on bond in con-crete, CEB, Riga, Latvia, pp. 10/11-10/4. Comité Euro-Internatinal du Beton, 1993. CEB-FIP model code 1990, London: Tho-mas Telford. Fang, C., Lundgren, K., Chen, L. and Zhu, C., 2004. Corrosion influence on bond in reinforced concrete. Cement and Concrete Research, 34 (11), pp.2159-2167. Fib Task Group Bond Models, 2000. Bond in reinforced concrete, fib Bulletin 10, Lausanne, Switzerland. Harajli, M.H., 2004. Comparison of bond strength of steel bars in normal- and high-strength concrete. ASCE Journal of Materials in Civil Engineering, 1 , pp.365-374. Lee, H.S., Tomosawa, F. and Noguchi, T., 1996. Effects of rebar corrosion on the structural performance of singly reinforced beams, In C. Sjöström, ed. Durability of building materials & components 7, 1. London: E.F. Spon, pp.571-580. Tachibana, Y., Maeda, K.-I., Kajikawa, Y. and Kawamura, M., 1990. Mechanical be-haviour of RC beams damaged by corrosion of reinforcement. In K. Treadaway and P. Bamford, eds. Corrosion of reinforcement in concrete, Elsevier, pp.178-187.


7. APPENDICES

A. Field specimens B. “Repaired” specimens C. Laboratory specimens D. Review of laboratory test results on the ef-

fects of steel bar corrosion


7.1 APPENDIX A Field specimens


0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

2.1% 1.8% 1.6% 0.6% 0.9%1.5% 2.1%1.9% 1.4%

Fig. A1. Bond stress- slip curves. Ullasund, field . 0 - 2.5 % weight loss

0

2

4

6

8

10

12

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

2.6%

4.0%

2.6%

Fig A2. Bond stress- slip curves. Ullasund, field . 2.6 – 5.0 % weight loss


0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

8.8%

6.1%

Fig.A3. Bond stress- slip curves. Ullasund, field 5.1 – 15.8 % weight loss

0

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10 12


bond

stre

ss, [

MP

a]

2.1%1.8%1.6%0.6%0.9%1.5%2.1%1.9%1.4%mean 0-2.5%

A

B

C

DD

Fig. A4. “Simplified” bond stress- slip curves. Ullasund, field . 0 - 2.5 % weight loss.


0

2

4

6

8

10

12

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

2.6%4.0%2.6%mean 2.6 - 5.0%

A

B

CDD

Fig. A5. “Simplified” bond stress- slip curves. Ullasund, field . 2.6 – 5.0 % weight loss

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

8.8%

6.1%

mean 5.1- 8.8%

A

B

C

DD

Fig. A6. “Simplified” bond stress- slip curves. Ullasund, field . 5.1-8.8 % weight loss


Table A1. “Simplified” bond stress- slip curves. Ullasund, field . 0 - 2.5 % weight loss.

Specimen Corrosion, A B C D

slip bond stress slip

bond stress slip

bond stress slip

bond stress

nr %, weight

loss mm MPa mm MPa mm MPa mm MPa

1 2.1 0 0 0.37 8.17 1.97 3.52 9.64 1.52

2 1.8 0 0 1.17 8.57 4.19 2.08 9.51 0.95

4 1.6 0 0 1.09 8.00 2.40 2.55 8.51 1.24

10 0.6 0 0 0.97 8.20 5.01 1.68 9.89 0.86

13 0.9 0 0 1.21 7.88 2.50 3.17 10.00 0.88

14 1.5 0 0 2.26 7.59 6.77 0.37 10.00 0.13

16 2.1 0 0 2.84 8.47 5.22 1.25 10.00 0.38

19 1.9 0 0 1.14 7.85 2.00 2.14 9.88 0.37

20 1.4 0 0 2.19 8.45 3.53 4.76 8.70 2.04

mean 0 - 2.5 % 0 0 1.47 8.13 3.73 2.39 9.57 0.93

Table A2. “Simplified” bond stress- slip curves. Ullasund, field . 2.6 – 5.0 % weight loss.



bond stress slip

bond stress slip

bond stress

nr %, weight loss mm MPa mm MPa mm MPa mm MPa

5 2.6 0 0 0.7 7.02 2.00 3.07 10.00 1.07

11 3.97 0 0 1.27 11.16 2.43 5.31 10.00 1.43

18 2.6 0 0 1.78 8.32 3.31 5.45 9.43 4.14

mean 2.6 – 5.0% 0 0 1.17 8.83 2.58 4.61 9.81 2.21


Table A3. “Simplified” bond stress- slip curves. Ullasund, field . 5.1-8.8 % weight loss



bond stress slip

bond stress slip

bond stress

nr %, weight


15 8.8 0 0 1.58 9.09 2.36 1.66 10.00 0,20

21 6.1 0 0 1.73 8.37 3.46 2.32 9.89 0,47

mean 5.1-8.8% 0 0 1.65 8.73 2.91 1.99 9.94 0,34

Table A4. Mean values of the “Simplified” bond stress- slip curves, Ullasund field

Corrosion, A B C D


bond stress slip

bond stress slip

bond stress %, weight


ref. 17 0 0 3.14 8.16 3.64 1.04 10.00 0.45

0 - 2.5% 0 0 1.47 8.13 3.73 2.39 9.57 0.93

2.6 - 5.0% 0 0 1.17 8.83 2.58 4.61 9.81 2.21

5.1 - 8.8% 0 0 1.65 8.73 2.91 1.99 9.94 0.34


7.2 APPENDIX B “Repaired” specimens


0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

2.1%

0.6%

1.5%

2.1%

Fig. B1. Bond stress- slip curves. “Ullasund, repaired” 0 - 2.5 % weight loss.

0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

3.97%

Fig. B2. Bond stress- slip curves. “Ullasund, repaired” 2.6- 5.0 % weight loss


0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

6.1%

8.8%

Fig.B3. Bond stress- slip curves. “Ullasund, repaired” 5.1 – 8.8 % weight loss.

0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

2.1%0.6%1.5%2.1%mean 0 - 2.5%

A

B

CDD

Fig. B4. “Simplified” bond stress- slip curves. Ullasund, “repaired”. 0 - 2.5 % weight loss


0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

3.97%

A

B

C DD

Fig. B5. “Simplified” bond stress- slip curve. Ullasund, “repaired”. 2.6 – 5.0% weight loss

0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

6.1%

8.8%

mean 5.1-8.8%

A

B

C

DD

Fig. B6. “Simplified” bond stress- slip curves. Ullasund, “repaired”. 5.1 -8.8 % weight loss


0

1

2

3

4

5

6

7

8

9

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

mean 0 - 2.5%

mean 3.97%

mean 5.1-8.8%

DD

C

B

A

Fig. B7. Comparison of “simplified” bond stress- slip curves. Ullasund, “repaired”. Table B1. “Simplified” bond stress- slip curves. Ullasund, “repaired”. 0 - 2.5 % weight loss



bond stress slip

bond stress slip

bond stress

nr %, weight


u 01b 2.1 0 0 1.04 6.72 4.87 0.11 9.67 0.09

u 10b 0.6 0 0 2.05 7.12 3.81 0.04 9.66 0.06

u 14b 1.5 0 0 1.48 6.30 2.61 0.55 6.80 0.31

u 16b 2.1 0 0 3.00 7.86 5.07 0.13 10.00 0.06

mean 0-2.5% 0 0 1.89 7.00 4.09 0.21 9.03 0.13


Table B2. “Simplified” bond stress- slip curve. Ullasund, “repaired”. 2.6 – 5.0% weight loss



bond stress slip

Bond stress slip

bond stress


u 11b 3.97 0 0 3.07 7.62 4.13 0.13 10.00 0.11

mean 3.97% 3.97 0 0 3.07 7.62 4.13 0.13 10.00 0.11

Table B3. “Simplified” bond stress- slip curves. Ullasund, “repaired”. 5.1 -8.8 % weight loss



bond stress slip

bond stress slip

bond stress


u 15b 8.8 0 0 3.70 7.20 3.77 0.33 10.00 0.20

u 21b 6.1 0 0 2.85 7.51 5.50 0.86 10.01 0.32

mean 5.1- 8.8% 0 0 3.28 7.35 4.64 0.59 10,00 0.26

Table B4. Mean values of the “Simplified” bond stress- slip curves, Ullasund , “repaired”

Corrosion, A B C D


bond stress slip

bond stress slip

bond stress %, weight


0 - 2.5% 0 0 1.89 7.00 4.09 0.21 9.03 0.13

2.6 - 5.0% 0 0 3.07 7.62 4.13 0.13 10.00 0.11

5.1 - 8.8% 0 0 3.28 7.35 4.64 0.59 10.00 0.26


7.3 APPENDIX C Laboratory specimens


0

2

4

6

8

10

12


bond

str

ess,

[MPa

]

3.4%

3.1%

4.2%

Fig. C1. Bond stress- slip curves. Lab. 0.05. 2.6 - 5.0 % weight loss.

0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

5.6%5.8%5.6%5.2%5.2%6.8%

Fig. C2.Bond stress- slip curves. Lab. 0.05. 5.1 – 7.5% weight loss


0

2

4

6

8

10

12

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

3.4%

3.1%4.15%

mean 2.6 - 5%

A

B

C D

Fig. C3. “Simplified” bond stress- slip curves. Lab. 0.05. 2.6 - 5.0 % weight loss

0

2

4

6

8

10

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

5.6%5.8%5.6%5.2%5.2%6.8%mean 5.1 - 7.5%

A

B

CD

Fig. C4. “Simplified” bond stress- slip curves. Lab. 0.05. 5.1-7.5 % weight loss


Table C1. “Simplified” bond stress- slip curves. Lab. 0.05. 2.6 – 5.0 % weight loss.



bond stress slip

bond stress slip

bond stress


3 3.4 0 0 2.64 9.68 2.84 1.96 7.06 0.00

4 3.0 0 0 2.35 7.67 4.52 0.20 10.00 0.16

7 4.2 0 0 0.95 6.10 2.13 0.02 7.50 0.05

mean 2.6 – 5.0% 0 0 1.98 7.82 3.16 0.73 8.19 0.07

Table C2. “Simplified” bond stress- slip curves. Lab. 0.05. 5.1- 7.5 % weight loss



bond stress slip

bond stress slip

bond stress


1 5.6 0 0 2.09 7.68 3.85 0.84 7.30 0.46

2 5.8 0 0 2.19 8.09 7.71 0.21 10.00 0.13

5 5.6 0 0 2.24 5.17 4.36 0.18 6.70 0.11

6 5.2 0 0 2.52 7.17 5.95 0.23 10.00 0.19

8 5.2 0 0 3.59 9.03 5.03 0.15 6.96 0.21

9 6.8 0 0 2.51 6.48 4.16 0.18 8.14 0.13

mean 5.1- 7.5% 0 0 2.52 7.27 5.18 0.30 8.18 0.20


0

2

4

6

8

10

12

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

4.4%3.2%4.7%4.6%3.9%4.5%4.4%4.5%

Fig. C5. Bond stress- slip curves. Lab. 0.1 2.6 - 5.0 % weight loss.

0

2

4

6

8

10

12

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

3.2%4.7%4.4%3.9%4.5%4.4%4.5%mean 2.6 - 5%

A

B

CD

Fig. C6 “Simplified” bond stress- slip curves. Lab. 0.1. 2.6- 5.0 % weight loss.





bond stress slip

bond stress slip

bond stress


U 23 3.2 0 0 2.24 11.12 3.82 0.78 10.00 0.19

U 26 4.7 0 0 2.42 7.89 6.66 0.14 9.67 0.11

U 27 4.4 0 0 2.58 8.08 4.47 0.83 10.00 0.39

U 28 3.9 0 0 2.42 11.32 5.19 0.19 8.81 0.14

U 29 4.4 0 0 2.40 9.04 5.17 0.12 10.00 0.24

U 30 4.4 0 0 1.74 7.64 5.00 0.79 9.59 0.17

U 31 4.5 0 0 2.40 7.28 5.20 0.31 8.61 0.25

mean 2.6 –5.0 % 0 0 2.32 8.91 5.07 0.45 9.53 0.21

Lab. Series 0.25

0

2

4

6

8

10

12

14

16


bond

str

ess,

[MPa

]

2.1%

1.8%

1.3%

2.2%

0.8%

Fig. C7. Bond stress- slip curves. Lab. 0.25. 0 - 2.5 % weight loss.


0

2

4

6

8

10

12

14

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

3.1%

4.1%

4.1%

3.2%

Fig. C8. Bond stress- slip curves. Lab. 0.25. 2.6 - 5.0 % weight loss.

0

2

4

6

8

10

0 2 4 6 8 10


bond

str

ess,

[MPa

]

5.3%

Fig. C9. Bond stress- slip curves. Lab. 0.25. 5.1 – 7.5 % weight loss.


0

2

4

6

8

10

12

14

16

0 2 4 6 8 10 12


bond

stre

ss, [

MPa

]

2.1%1.8%1.3%2.2%0.8%mean 0 -2.5%

A

B

CD

Fig.C10. “Simplified” bond stress- slip curves. Lab.0.25. 0 -2.5 % weight loss.

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

3.1%4.1%4.1%3.2%mean 2.6 - 5%

A

B

C D

Fig. C11. “Simplified” bond stress- slip curves. Lab.0.25. 2.6- 5.0% weight loss.


0

2

4

6

8

10

0 2 4 6 8 10


bond

str

ess,

[MPa

]

5.3%

A

B

C D

Fig. C12. “Simplified” bond stress- slip curves. Lab.0.25. 5.1 – 7.5% weight loss. Table C4. “Simplified” bond stress- slip curves. Lab. 0.25. 0 - 2.5% weight loss.



bond stress slip

bond stress slip

bond stress


U 3 2.1 0 0 1.79 8.70 5.68 0.92 8.93 0.07

U 4 1.8 0 0 3.45 13.63 7.13 0.29 9.57 0.24

U 6 1.3 0 0 2.72 10.89 3.68 0.83 8.85 0.09

U 7 2.2 0 0 2.36 5.91 7.24 0.86 10.00 0.09

U 10 0.8 0 0 1.97 9.36 3.56 0.96 9.01 0.07

mean 0 - 2.5% 0 0 2.46 9.70 5.46 0.77 9.27 0.11


Table C5. “Simplified” bond stress- slip curves. Lab. 0.25. 2.6 – 5.0 % weight loss



bond stress slip

bond stress slip

bond stress


U 5 3.1 0 0 3.23 4.93 7.41 0.11 9.57 0.05

U 9 3.2 0 0 3.12 13.02 6.35 0.09 9.09 0.08

U 12 4.1 0 0 2.35 7.93 5.00 0.06 8.33 0.06

U 20 4.1 0 0 2.14 8.90 3.37 0.73 9.53 0.19

mean 2.6 - 5.0% 0 0 2.71 8.69 5.53 0.25 9.13 0.10




bond stress slip

bond stress slip

bond stress


U 13 5,3 0 0 3.41 3.89 5.21 0.05 9.31 0.05


Reference specimens

0

2

4

6

8

10

12

14

0 2 4 6 8 10loaded end slip, [mm]

bond

str

ess,

[MPa

]

ref.1

ref.2

ref.3

Fig. C13. Reference bond stress- slip curves. Lab. 0.05.

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

ref.1

ref.2

ref.3

Fig.C14. Reference bond stress- slip curves. Lab. 0.1.


0

2

4

6

8

10

12


bond

str

ess,

[MPa

]

ref.1

ref.2

ref.3

Fig. C15. Reference bond stress- slip curves. Lab. 0.25.

0

2

4

6

8

10

12

14

0 2 4 6 8 10loaded end slip, [mm]

bond

str

ess,

[MPa

]

ref.1ref.2ref.3mean ref. Lab. 0.05

A

B

C

D

Fig. C16. “Simplified” reference bond stress- slip curves. Lab. 0.05.


0

2

4

6

8

10

12

0 2 4 6 8 10 12


bond

str

ess,

[MPa

]

ref.1ref.2mean ref. Lab. 0.1

AC

B

D

Fig. C17.“Simplified” reference bond stress- slip curves. Lab. 0.1.

0

2

4

6

8

10

12

0 2 4 6 8 10


bond

str

ess,

[MPa

]

ref.1ref.2ref.3mean ref. Lab.0.25

DCA

B

Fig. C18. “Simplified” reference bond stress- slip curves. Lab. 0.25.


Table C7. “Simplified” reference bond stress- slip curves. Lab. 0.05.



bond stress slip

bond stress slip

bond stress


ref.1 0 0 0 1.77 9.84 5.17 0.17 7.61 1.17

ref.2 0 0 0 2.15 10.48 7.50 1.36 8.69 1.27

ref.3 0 0 0 1.60 11.91 4.32 0.85 6.94 0.17

mean Lab.0.05 0 0 0 1.84 10.74 5.67 0.79 7.75 0.87

Table C8. “Simplified” reference bond stress- slip curves. Lab. 0.1.



bond stress slip

bond stress slip

bond stress


ref.1 0 0 0 1.90 9.13 4.45 0.27 8.26 0.10

ref.2 0 0 0 2.28 10.00 6.11 0.77 9.83 0.26

mean Lab.0.1 0 0 2.09 9.56 5.28 0.52 9.05 0.18

Table C9. “Simplified” reference bond stress- slip curves. Lab. 0.25



bond stress slip

bond stress slip

bond stress


ref.1 0 0 0 2.28 10.43 5.50 0.98 6.92 0.41

ref.2 0 0 0 1.94 10.92 5.86 0.11 7.09 0.13

ref.3 0 0 0 2.73 9.69 4.47 0.12 9.52 0.10

mean Lab.0.25 0 0 0 2.32 10.34 5.28 0.40 7.85 0.21


0

2

4

6

8

10

12

0 2 4 6 8 10


bond

str

ess,

[MPa

]

mean ref. Lab. 0.05

mean ref. Lab. 0.1

mean ref. Lab. 0.25

mean, ref.

Fig. C19. “Simplified” reference bond stress- slip curves Table C10. Mean values of the “Simplified” reference bond stress- slip curves

Serie Corrosion, A B C D


bond stress slip

bond stress slip

bond stress

%, weight loss mm MPa mm MPa mm MPa mm MPa

Lab. 0.05 0 0 0 1.84 10.74 5.67 0.79 7.75 0.87

Lab. 0.1 0 0 0 2.09 9.56 5.28 0.52 9.05 0.18

Lab. 0.25 0 0 0 2.32 10.34 5.28 0.4 7.85 0.21

mean ref. 0 0 2.08 10.21 5.41 0.57 8.22 0.42


7.4 APPENDIX D Review of laboratory test results on the effects of steel bar corrosion


SUMMARY Corrosion of embedded steel bars continues to be the principal cause of deterioration of reinforced concrete structures. Previous research has been chiefly concerned with the causes and mechanisms of deterioration while relatively little attention has been given to the problem of assessing the residual strength of corroded structures. Cur-rent service life prediction models are only concerned with the corrosion initiation pe-riod, i. e., the time needed to initiate the corrosion attack. By contrast, a growing number of reinforced concrete structures is reaching a stage where deterioration has already progressed beyond the initiation period and corrosion is propagating. All of this has exposed the need for improved understanding of the influence of corrosion damage upon structural integrity. In particular, there is a growing need for reliable methods for estimating stiffness and strength of deteriorated structures, since any attempt to optimize maintenance and repair requires the capability to predict the re-maining service of corroding structures. Experimental research represents the foundation for developing methods for predict-ing the performance of deteriorated concrete structures. Since the assumption that the end of service life occurs at the onset of corrosion is far too conservative, it is im-portant to conduct research designed to clarify to which extent steel bar corrosion will impair structural performance and safety. The present study is devoted to reviewing published results from experimental inves-tigations on the structural consequences of reinforcement corrosion. Steel bar corro-sion is known to affect the reinforcement itself, the concrete and the composite action between reinforcement and concrete. In laboratory tests, an induced electrical cur-rent is used to accelerate corrosion. Typically, durations of laboratory tests are of the order of weeks or months, while the corrosion of reinforced concrete structures under field conditions may continue for decades. Hence, quantitative results from laboratory tests cannot be directly applied to concrete structures in the field. This report is primarily concerned with the effect of steel bar corrosion on bond strength, since this is the property strongest influenced by corrosion. As could be ex-pected, there is considerable scatter in the published results. Test data reveal that loss of bond strength is more severe than loss of bar cross section. Bond strength has been shown to be reduced by 50 % when loss of bar section was only 10 – 15 %. Initially, bond strength increases with corrosion. Bond strength seems to be main-tained as long as visible, longitudinal cracks do not occur. In the presence of such cracking, however, bond strength is rapidly impaired to only a fraction of its initial value. The reduction in bond strength was more severe for specimens without trans-verse reinforcement (stirrups) than for specimens with such reinforcement. Hence, intact stirrups play an important role in maintaining bond strength of corroded bars.


Table of Contents 1 Introduction........................................................................................................... 88

2 Effects of steel bar corrosion ................................................................................ 89

3 Laboratory studies of reinforcement corrosion ..................................................... 91

4 Pullout tests, general information ......................................................................... 94

4.1 Specimens without transverse reinforcement .............................................. 94

4.2 Specimens with transverse reinforcement ................................................... 95

5 Pullout tests; evaluation of results ........................................................................ 96

5.1 Longitudinal cracking ................................................................................... 96

5.2 Loss of bond ................................................................................................ 96

5.3 Increased bond strength for low levels of corrosion ..................................... 97

5.4 Effect of bar diameter................................................................................... 99

5.5 Effect of cover to bar diameter ratio ........................................................... 100

5.6 Effect of bar position .................................................................................. 100

5.7 Effect of impressed current density............................................................ 100

6 Pullout tests on field concrete............................................................................. 103

6.1 Specimens from the Ullasund bridge ......................................................... 103

6.2 Pullout tests on field specimens................................................................. 104

6.3 Pullout tests on “repaired specimens”........................................................ 104

6.4 Pullout tests on laboratory specimens ....................................................... 106

7 Beam tests ......................................................................................................... 108

7.1 Ultimate load capacity................................................................................ 108

7.2 Bond strength............................................................................................. 110

7.3 Simultaneous loading and corrosion .......................................................... 112

8 Bond stress-slip relationship............................................................................... 113

8.1 Uncorroded bars ........................................................................................ 113

8.2 Corroded bars............................................................................................ 115

9 Concluding remarks ........................................................................................... 116

10 References......................................................................................................... 117

Appendices: A Pullout tests B Beam tests


Introduction

Corrosion of steel reinforcement continues to be the principal cause of deterioration of reinforced concrete structures. The degree to which structural performance is im-paired as a result of steel bar corrosion is a matter of great concern to those respon-sible for assessment and maintenance of affected structures. While much research effort has been devoted to investigating the causes and mechanisms of reinforce-ment corrosion and to questions of protection systems and repair materials, relatively little attention has been devoted to the problem of assessing the residual strength of corroded structures. This is somewhat surprising since information on the effect of corrosion on structural integrity is essential for the development of effective tools for prediction of residual service life and for development of cost effective repair strate-gies. Design Codes and Standards are intended primarily for new construction, and gener-ally do not contain the information required for assessment of deteriorated structures. Detailed guidelines on assessment of residual strength of corrosion damaged struc-tures are scarce and generally provide very limited guidance on quantifying loss of structural stiffness and strength due to corrosion. Nonlinear finite element analysis can be applied to simulate the behaviour of rein-forced concrete structures suffering from steel bar corrosion. In order to achieve suf-ficient accuracy and reliability of finite element simulations, it is essential that the un-derlying constitutive and kinematical models represent realistic approximations to the true, physical behaviour of concrete structures with corroding reinforcement. Hence, these models must be based on existing experimental evidence on the structural consequences of reinforcement corrosion. The aim of this report is to review avail-able information on the effects of steel bar corrosion on the structural performance and integrity of reinforced concrete structures. More precisely, the main focus of the report is to examine current experimental data on the changes in bond characteristics of steel reinforcement caused by corrosion and the associated consequences of that reduction on residual structural capacity of reinforced concrete structures or structural elements. The review is conducted in such a way that the extracted experimental re-sults can be employed in suitable constitutive and kinematical models which are key ingredients in the nonlinear finite element simulations.


2. Effects of steel bar corrosion

The alkaline environment surrounding reinforcement in concrete results in the forma-tion of a passivating layer on the surface of the steel bar. If this passivating layer is broken by carbonation of cover concrete or by penetration of chlorides through the cover, active corrosion may develop. It is common to classify corrosion of reinforcement according to the type of attack. If corrosion is spread over the entire surface of the bar, general corrosion is said to oc-cur. If, on the other hand, the attack is localized, it is called local corrosion or pitting. General corrosion may occur due to either carbonation or due to chloride contamina-tion. Local corrosion is invariably associated with chloride contamination and not with carbonation. Corrosion of reinforcing steel bars affects the reinforcement, the concrete and the composite action between steel and concrete. It is convenient to distinguish between the effects of corrosion on the local level and on the structural level. The local effects of steel bar corrosion are briefly summarized below. Reduction in bar cross sectional area The most obvious consequence of reinforcement corrosion is the reduction in cross sectional area of the affected steel bars. The maximum force carried by a single steel bar is reduced proportionally. For local corrosion there is an additional effect attrib-uted to the large localized strains at the pit, which lead to impaired ductility. Cracking and spalling of concrete cover General corrosion is associated with formation iron oxides, commonly referred to as “brown rust”. The volume of these oxides is several times greater than that of the par-ent steel. The volumetric expansion of a corroding bar generates hoop strains in the surrounding concrete, leading to longitudinal cracking. If corrosion is allowed to con-tinue, spalling of the concrete cover may result. This also serves as a warning of more significant loss of steel bar section. The corrosion products formed during local corrosion do not exhibit the same degree of volumetric expansion as brown rust. Consequently, the tendency of splitting the concrete cover along the corroding bar is less with local corrosion. It is important to note that pitting corrosion may cause extreme loss of steel bar cross section without visible signs of deterioration at the concrete surface. Reduction in bond strength and changes in bond stress-slip behaviour Bond between steel reinforcement and concrete is a prerequisite for ensuring com-posite action between the two materials; in essence, bond is the main feature of rein-forced concrete. Bond stress is commonly regarded as pure shear stress acting on the surface of a bar. It is important to note that this represents a considerable simpli-fication of the real physical behaviour since bond of ribbed bars involves a number of different effects such as chemical bonds between steel and hardened cement, fric-tion, and direct bearing stresses, or mechanical interlock. For a more comprehensive


treatment on bond in concrete including recent research results it is recommended to consult the state-of the-art report (Fib Task Group Bond Models, 2000). The relative displacement between the steel bar and the surrounding concrete, in the direction of the bar, is denoted slip. When slip occurs, frictional forces develop be-tween bar and concrete. Corrosion affects bond between a steel bar and concrete in several ways. In the ini-tial stage, the corrosion products accumulate on the surface of the bar and the asso-ciated increase in bar diameter causes an increase in radial stresses between the bar and concrete, thereby increasing the frictional component of bond. At a later stage, longitudinal cracking reduces the effective confinement of the bar and also the bond strength. Corrosion products are mechanically weaker than the original steel and this contributes to reduced bond strength. Further reduction in bond strength can be anticipated at more advanced stages of corrosion where the height of the ribs of the deformed bar is being reduced. Most of the available experimental data on the effect of corrosion on bond behaviour only considers bond strength. However, the corrosion process is likely to have an impact on the entire bond stress-slip behaviour. Systematic field measurements on corrosion of steel reinforcement in concrete struc-tures are expensive and difficult to perform, which explains the lack of reliable field data. With new developments in sensors, instrumentation and monitoring it is realistic to anticipate that more comprehensive and useful field data on reinforcement corro-sion will be available in the near future. The collection of specimens from real struc-tures, with long time field exposure, and subsequent investigation of these speci-mens in the laboratory, represents an alternative way of obtaining field data. The op-portunity of obtaining specimens from concrete structures in situ may materialize in cases when reinforced concrete structures are being demolished. By careful planning and execution, such specimens can be collected without major additional costs. New results from laboratory testing of this type of field specimens are presented in the cur-rent report. The available experimental data on steel bar corrosion almost exclusively have been obtained on small scale specimens corroded and tested in the laboratory. Such tests are generally conducted under accelerated conditions to be practically feasible. The basic features of laboratory investigations of steel bar corrosion are described in Chapter 3.


3. Laboratory studies of reinforcement corrosion

Investigations into the influence of corrosion on bond have used a wide variety of specimens and bar types, and it is therefore not surprising that the bond strength values reported in the literature differ widely. Furthermore, there have been consid-erable variations in the adopted procedures for conditioning of specimens for corro-sion studies, and this has been proven to influence residual bond strength. The major portion of published experimental data on bond strength has been ob-tained from pullout tests of various types. The concentric pullout test (ASTM C234) illustrated in Fig. 1 is the most widely used pullout test. A single steel bar is embed-ded in a rectangular or cylindrical concrete specimen and the applied force required to pull out the bar or to make it slip excessively is measured. In calculating the asso-ciated bond strength, it is assumed that the bond stress (i. e., the shear stress) is uni-formly distributed over the embedded surface of the steel bar.

Figure 1 Concentric pullout test, specimen without transverse reinforcement. A version of an eccentric pullout test is shown in Fig. 2. Specimens belonging to this category typically contain several bars that are pulled individually. For the assessment of results on bond strength obtained from pullout tests, it is common to distinguish between specimens with no transverse reinforcement and specimens having


Figure 2 Eccentric pullout test; specimen with transverse reinforcement. transverse reinforcement. In the vast majority of the research studies considered herein, bond strengths were measured for varying degrees of corrosion. Only a lim-ited number of publications contained information of the general bond stress-slip be-haviour of corroded bars. Research into the influence of corrosion on bond has also made use of structural elements such as beams and slabs. Most commonly, small scale beams with rectan-gular cross section and subjected to four point loading have been employed, see Fig. 3. The amount of reinforcement may vary considerably in such beams. This holds for both the longitudinal reinforcement (tensile/compressive) and the transverse rein-forcement (cross sectional area and spacing of stirrups).

Figure 3 Simply supported beam used in laboratory experiments.


Probably the most important factor in laboratory investigations on the influence on steel reinforcement corrosion is the specimen conditioning. In the majority of bond tests on corroded bars, the corrosion process has been activated by chloride salts. Acceleration of the corrosion process is provided by electrical polarization of the rein-forcement. A positive electrical potential is applied to the steel bars to make the rein-forcement anodic and facilitate dissolution of Fe2+ ions. The process may be consid-ered as the opposite of cathodic protection, where a negative electrical potential is applied to the reinforcement to make it cathodic and retain Fe2+ ions. Sometimes the conditioning is performed with the specimen submerged, wholly or partially, in a salt solution. In other cases the solution is sprayed over the specimen. Continuous submerging produces different conditions than those associated with cy-cles of wetting and drying. The applied current density, measured in mA/cm2, is an important parameter in the conditioning of laboratory specimens. In order to accelerate the corrosion process to achieve reasonable durations of the tests, relatively high current densities have in many cases been adopted. These current densities may be orders of magnitude greater than the highest values recorded in service. Steel bar corrosion is a slow process, which under field conditions takes decades. On the other hand, in laboratory studies, specimen corrosion is designed to take months (or weeks). Not surprisingly, results from laboratory tests do not always show very good correlation with field ex-periences. The most widely adopted measure of corrosion is the weight of metal lost after com-pletion of the test. Hence, corroded bars must be cleaned and weighted carefully. Throughout the present report, percentage weight loss, i.e., weight loss divided by original weight, expressed as a percentage, is adopted as a measure of corrosion. This is equivalent to section loss. Some investigators prefer to use average corrosion penetration as a measure of corrosion, which may be beneficial in the assessment of residual bond strength. The use of different measures of corrosion damage may sometimes cause confusion in interpreting the experimental results.


4. Pullout tests, general information

4.1 Specimens without transverse reinforcement The major portion of the published test data on the effects of corrosion on bond is concerned with pullout tests conducted on specimens without transverse reinforce-ment (stirrups, links). The results discussed in the present report have been ex-tracted from selected references. These tests comprise a variety of specimen ge-ometries, conditioning procedures, test configurations, bar diameters etc. The num-ber of individual tests carried out during each series of experiments also varies con-siderably. More detailed information about the tests considered herein , and others of the same category, can be found in Appendix A, Table A1.

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ativ

e bo

nd s

tren

gth

Al-Sulaimani (1990),φ=10mmCabrera (1992, 1996),φ=12mmClark (1993),bottom cast,φ=8mmRodriguez (1994), type3,top cast,φ=16mmRodriguez (1994),type3,bottom cast,φ=16mmLee (1996),φ=10mmAuyeung (2000), φ=19mmGhandehari (2000),φ=19mmShima (2001), type1,φ=22mmAuyeung ( 2001),φ=16mmFang (2004),φ=20mm

Figure 4 Results from pullout tests; specimens without transverse reinforcement. Experimental results collected from these references are depicted in Figure 4, which shows the relative bond strength, i.e. the bond strength of the corroded bar divided by the bond strength of the uncorroded, reference bar, as a function of the measured, relative weight loss in percent.


As could be expected, the results portrayed in Fig. 4 exhibit a large amount of scatter. The data cover weight losses from 0 up to 28.9 % with the bulk of the results in the range 0 – 10 %. It is seen that corrosion has a strong influence on bond strength. In some of the tests series shown in Fig. 4, the bond strength increased considerably in the initial stage of corrosion (for weight losses less than 1.0 – 1.5 %). This point is further discussed in section 5.3. As corrosion continues, bond strength rapidly dete-riorates, and in all test data reported here bond strength of corroded bars varied be-tween 20 % and 80 % of the reference value when the weight loss exceeded 5 %.

4.2 Specimens with transverse reinforcement Published results from pullout tests on specimens with transverse reinforcement (stir-rups) are considerably fewer in number than for specimens without transverse rein-forcement. The data investigated in the present study are based on the work reported by Rodriguez et al (1994a), Almusallam et al.(1996a), Shima (2001) and Fang et al. (2004). Details from these tests are compiled in Appendix A, Table A2, which also contains additional references. The extracted results on bond strength are depicted in Fig. 5. The scatter appears to be less for specimens with transverse reinforcement than for specimens without such reinforcement (Fig. 4). Notwithstanding, the measured bond strengths in this diagram vary between 20 % and 120 % of the reference value for uncorroded bars for weight losses up to 20 %. It is seen that there is very little data available for weight loss greater than 8 %. Note also that the increase in bond strength at low levels of corro-sion for specimens with transverse reinforcement seems to be less pronounced than depicted in Fig. 4 for specimens without transverse reinforcement.

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Rodriguez (1994),type1,bottom cast,φ=16mmRodriguez (1994),type2,bottom cast,φ=16mmAlmusallam (1996),φ=12mmShima (2001),series2,φ=22mmFang (2004),φ=20mm

Figure 5 Pullout test results; specimens with transverse reinforcement (stirrups).


5. Pullout tests; evaluation of results

5.1 Longitudinal cracking Development of longitudinal cracks is a visible sign of corroding steel bars, and it is important to investigate under which conditions such cracks are being formed. More precisely, it is of interest to establish the amount of corrosion (expressed as attack penetration or percentage weight loss) corresponding to crack development. Accelerated corrosion tests conducted by Rodriguez et al. (1994a) revealed that cracking first developed at attack penetrations (i.e., reduction in bar radius) of 0.015 mm to 0.040 mm. For bars with relatively small diameter (say, 12 mm), this corre-sponds to less than 2 % reduction in cross sectional area, and less for bars of larger diameter. Al-Sulaimani et al. (1990) reported research results that showed that cracks were initiated when corrosion penetrations reached approximately 0.1 mm. For the diameters used, this corresponds to reductions in cross sectional area in the range 2.0 % to 4.5 %. Similar findings were reported by Carbrera and Ghoddoussi (1992) and Clark and Saifullah (1993) . Although that the relationship between sec-tion loss and formation of longitudinal cracks depends on a number of parameters such as specimen conditioning, concrete cover,etc., it can be concluded that longitu-dinal cracking develops before the reduction in bar cross sectional area becomes significant.

5.2 Loss of bond The significant scatter in bond strengths reported from pullout tests, as evidenced in Fig. 4 and Fig. 5, cannot hide the fact that all studies seem to have confirmed the same general pattern of changes in bond strength as corrosion develops. First bond strength is increased at low levels of corrosion, but further increases in corrosion lead to reductions in bond strength. In the pullout tests without transverse reinforcement reported in Fig. 4, bond strength was reduced by 20 to 80 % for corrosion levels greater than 5 % weight loss. Lesser reductions in bond strength, but still of consid-erable magnitude, were recorded in the pullout tests with transverse reinforcement (Fig. 5) for weight losses between 5 and 10 %. Despite the scatter and inconsistencies between the quantitative reductions in bond strength, there seems to be clear evidence that the reduction in bond strength ex-ceeds the reduction in bar cross sectional area. In other words, the reduction in bond strength of a corroded steel bar is greater than the associated reduction in force ca-pacity of the affected bar. The significant loss of bond due to corrosion has implica-tions for the stiffness and strength of structural elements attacked by reinforcement corrosion. Moreover, this also explains why simple section analysis is no longer valid in situations with partial or complete loss of composite action between reinforcement and concrete. The fundamental assumption in bending of beams and plates that “plane sections remain plane” is no longer valid and the distribution of strains over the cross section will no longer be linear.


5.3 Increased bond strength for low levels of corrosion The volumetric expansion of a corroding bar subjected to general corrosion causes increase in bar diameter accompanied by an increase in radial stresses between the bar and the surrounding concrete. This leads to increased friction and increased bond. Hence, it is natural to assume that in the early stage of the corrosion process there should be an increase in bond strength. This effect was briefly mentioned in Chapter 4 in connection with the overall results from pullout tests and is examined in more detail in the following. Figure 6 shows bond strengths for selected pullout tests conducted on specimens with no transverse reinforcement. Since the objective is to study variations in bond strength during the early stage of the corrosion process, only data for weight losses up to 5 % are included in this diagram.

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Al-Sulaimani (1990),φ=10mmCabrera (1992, 1996),φ=12mmClark (1993),bottom cast,φ=8mmRodriguez (1994), type3,top cast,φ=16mmRodriguez (1994),type3,bottom cast,φ=16mmLee (1996),φ=10mmAuyeung (2000), φ=19mmGhandehari (2000),φ=19mmShima (2001), type1,φ=22mmAuyeung ( 2001),φ=16mmFang (2004),φ=20mm

Figure 6 Pullout tests with no transverse reinforcement; increase in bond strength for low levels of corrosion. Al-Sulaimani et al (1990), Clark and Saifulla (1993) and Auyeung et al. (2000, 2001) reported increases in bond strength up to 40 %, and even beyond that. In the work carried out by Cabrera and Ghoddoussi (1992) the bond strength in the early stage of corrosion increased approximately by 20 %. It can be observed from Fig. 6 that the


increase in bond strength in these investigations seems to peak for weight losses in the area of 1.0 – 1.5 %. Figure 7 shows similar results obtained on specimens with transverse reinforcement. In this diagram, only data published by Rodriguez and et al. (1994a), Almusallam et al. (1996a), Shima (2001) and Fang et al. (2004) are included. The results reported by Almussallam et al. (1996a) showed an increase in bond strength up to approxi-mately 4 % weight loss, at which the bond strength had increased by 16 %. Beyond this point there is a drop in bond strength. The same tendency is not observed in the data published by Fang et al. (2004). Here, the data points are more scattered and the bond strength is almost unaffected by the increased radial stresses. The results published by Shima (2001) indicate a monotone reduction in bond strength with in-creased weight loss. However, in this reference there are no measurements for weight losses between 0 % and 2.7 %, so the data are insufficient in this respect. Another important observation is that the increase in bond strength is much more modest for specimens with transverse reinforcement (Fig. 7) than for specimens with no transverse reinforcement.

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0 1 2 3 4 5Corrosion, weight loss [%]

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Rodriguez (1994),type1,top cast,φ=16mmRodriguez (1994),type2,top cast,φ=16 mmAlmusallam (1996),φ=12mmShima (2001),series2,φ=22mmFang (2004),φ=20mm

Figure 7 Pullout tests with transverse reinforcement; increase in bond strength for low levels of corrosion. As already mentioned, it is logical to anticipate a certain increase in bond strength in the early stage of the corrosion process, due to increased radial stresses caused by expansion in bar diameter. The relatively large increases in bond strength obtained in laboratory studies may, in part, be explained by the accelerated tests. At lower rates of corrosion, it may be easier for the corrosion products to dissolve through the con-


crete cover which will tend to reduce radial stresses. The same quantitative increases in bond strength may therefore not be attainable under field conditions. The deterioration in bond strength beyond a certain level of corrosion seems to be associated with the development of longitudinal cracking. As long as visible cracks are not present, the bond strength seems, at least, to be maintained at the initial value (intact bar). The abrupt drop in bond strength in some tests may in part be at-tributed to the accelerated testing conditions.

5.4 Effect of bar diameter A systematic study of possible influences of bar diameter on measured bond strength values for corroded bars is not readily conducted from the available bulk of test data, since there are large variations in specimen geometry, conditioning procedures, etc. Therefore, a single series of consistent tests was selected in order to illustrate the effect of bar diameter. Auyeung and Balaguru (2001) conducted pullout tests on specimens with no transverse reinforcement. The bar diameter was varied from 10 to 25 mm. These data are portrayed in Fig. 8 which shows relative bond strength as a function of weight loss for different bar diameters.

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φ=10mmφ=13mmφ=16mmφ=19mmφ=25mm

Figure 8 Influence of bar diameter on bond behaviour; from Auyeung and Balaguru (2001). A relatively high current density (3 m A/cm2) was applied in this investigation, so some caution should be exercised in interpreting the results. Figure 8 indicates that


bars with smaller diameters (10 mm, 13 mm) seem to have a more ductile behaviour than larger bars (19 mm, 25 mm), that is, the reduction in bond strength with increas-ing weight loss is less abrupt in the smaller bars. Furthermore, bars with large diame-ter (25 mm) seem to suffer complete loss of bond strength whereas the small diame-ter bars (10 mm) maintain a significant portion of their initial bond strength during cor-rosion.

5.5 Effect of cover to bar diameter ratio The ratio of concrete cover to bar diameter has strong influence on bond strength and bond stress-slip behaviour of uncorroded steel bars. Increasing the cover to bar diameter ratio leads to increased bond strength. Systematic studies of the influence of cover to bar diameter ratio for corroded bars are relatively few in number. Rodriguez, Ortega and Garcia (1994b) performed tests with cubic concrete specimens reinforced with four bars (one in each corner), with and without stirrups. The specimens were subjected to a constant shear force. It was concluded that the influence of cover to bar diameter ratio on bond strength was neg-ligible. The rationale behind this is that once the cover is cracked due to corrosion, cover to bar diameter ratio is no longer important. Similar findings were reported by Andrade, Alonso and Molina (1993).

5.6 Effect of bar position Bond strength of uncorroded bars has been shown to be influenced by bar position, evidenced by the fact that bottom cast bars exhibited 10 – 15 % higher bond values than top cast bars. Similar tests on corroded bars are very few, but it appears that this effect is less pronounced for corroded bars. Clark and Saifulla (1993) found that bottom cast bars showed marginally greater bond strength than top cast bars, but the difference was not significant. Rodriguez et al. (1994a) concluded that the influence of bar position was negligible.

5.7 Effect of impressed current density The impressed electrical current density applied to accelerate corrosion in laboratory experiments exerts a significant influence on measured values of bond strength. The high current densities used in laboratory studies is a major obstacle in achieving good correlation between laboratory tests and field measurements. According to Rodriguez et al. (1994a), the highest current densities recorded in ser-vice for chloride contaminated concrete are of the order of 10 – 25 μ A/cm2. Current densities in structures affected by carbonation tend to be lower. On the other hand, laboratory studies reported in the literature have used current densities up to 1 – 5 mA/cm2. Even the lower current densities (0.05 – 0.5 mA/cm2) applied in the labora-tory are appreciably higher than typical field values. Several authors have investigated the effect of current density on bond strength. Clark and Saifulla (1993) studied the amount of corrosion (expressed as percentage weight loss) to cause cracking over a bar with a cover/bar diameter ratio of 1.0 under different corrosion rates, measured by the impressed current density. For high rates of corrosion (4m A/cm2 and a few hours to cracking) the weight loss was 0.2 %. At


low rates (0.04 mA/cm2 and 8 – 12 days to cracking) cracking occurred at a weight loss of 1.2 %. The use of high corrosion current densities for conditioning of labora-tory specimens was criticized by several authors and it has been suggested that cur-rent densities should not exceed 0.05 mA/cm2. Results from pullout tests corresponding to relatively low current densities (0.1 – 0.7 mA/cm2) are compiled in Fig. 10. The bar diameter varied in these investigations, from 8 mm (Clark and Saifulla 1993) to 16 mm (Rodriguez et al. 1994a) and 19 mm (Auyeung et al. 2000; Ghandehari et al. 2000). Considerable scatter in the measured bond strengths is observed from Fig. 10.

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Clark (1993),bottom cast,φ=8mm,i=0.5mA/cm2

Rodriguez (1994),bottom cast,φ=16 mm, i=0.1mA/cm2

Auyeung (2000),φ=19mm, i=0.1 mA/cm2

Ghandehari (2000),φ=19mm, i=0.62 mA/cm2

Figure 10 Comparisons of bond strength for relatively low current densities (0.1 – 0.7 mA/cm2). Figure 11 shows bond strengths measured in pullout tests corresponding to relatively high current densities (2 – 3 mA/cm2). The data portrayed in Fig. 11 are taken from the papers by Auyeung and Balaguru (2001), using bars with diameters 10 mm and 16 mm, and Al-Sulaimani et al. (1990) who employed bars with 10 and 14 mm di-ameter. The two current densities used in these tests are too close to produce any significant differences in the values of bond strength. A closer examination of the effect of current density on bond strength is attempted in Fig. 12. This diagram contains test data published by Al-Sulaimani et al. (1990), Clark and Saifullah (1993), Rodriguez et al.(1994a), Ghandehari et al. (2000), Shima (2001), and Auyeung and Balaguru (2001). These test data were obtained using im-pressed current densities in the range 0.1 – 3.0 mA/cm2. A significant amount of scat-ter in the results is observed from Fig. 12, where an exponential fit of the data points has been attempted.


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thAuyeung (2001),φ=10mm, i=3mA/cm2

Al-Sulaimani (1990), φ=10mm, i=2 mA/cm2

Auyeung (2001),φ=16mm,i=3mA/cm2

Al-Sulaimani (1990),φ=14mm, i=2 mA/cm2

Figure 11 Comparisons of bond strength for relatively high current densities (2 – 3 mA/cm2).

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Al-Sulaimani (1990)Clark (1993)Rodriquez (1994)Ghandehari (2000)Shima (2001)Auyeng (2001)y=exp(-0,4103x)

Figure 12 Relationship between bond strength and current density (0.1 – 3.0 mA/cm2).


6. Pullout tests on field concrete

6.1 Specimens from the Ullasund bridge The Ullasund highway bridge (Fig. 13) on the coast of Norway was completed in 1969. The degradation due to chloride induced corrosion was so severe and the as-sociated cost of repair so high, that in 1998 the bridge was demolished after only 29 years in service. Figure 13 Ullasund bridge, Norway; completed 1969, demolished 1998. Large pieces of concrete were sawed from one of the bridge columns in areas with varying corrosion levels and transported to Norut Technology’s laboratory for further preparation and testing. These specimens are unique in the sense that they provide information on corrosion of steel bars in concrete subjected to approximately 30 years of field exposure in a chloride contaminated environment. After testing some of the specimens were subjected to a procedure similar to that during conventional re-pair of corrosion damaged concrete structures, i.e., the corroded bar was cleaned by sandblasting, then cast in a new concrete specimen and subsequently subjected to pullout testing. Standard laboratory specimens were also cast with uncorroded rein-forcement of the same diameter as the field specimens. These specimens were split in three different series, each subjected to a different impressed current density, and then subjected to pullout testing.


6.2 Pullout tests on field specimens The specimens prepared from the Ullasund bridge contained a single bar of 25 mm diameter with no transverse reinforcement. Standard pullout testing was performed. Calculated bond strengths obtained from the failure loads are shown in Fig. 14 as a function of measured weight loss. Again, the results show a certain degree of scatter, but some general trends can be observed. First, there seems to be only a minor in-crease in bond strength in the initial stages of corrosion, and, second, further corro-sion up to a weight loss of approximately 16 % does not appreciably impair bond strength.

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Ullasund, field

Figure 14 Pullout tests results; field specimens from the Ullasund bridge.

6.3 Pullout tests on “repaired specimens” After the pullout testing of the field specimens from the Ullasund bridge had been completed, a portion of these specimens were subjected to further investigations in the laboratory. Each corroded bar was cleaned by sandblasting and cast in a new concrete specimen. The average concrete compressive strength of these “repaired” specimens was 30 MPa, while the field specimens collected from the Ullasund bridge had an average compressive strength of 40.3 MPa. The “repaired” specimens were stored in the laboratory and subjected to pullout testing at the age of 28 days.


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Figure 15 Pullout tests results; “repaired” specimens. Bond strength values are given in Fig. 15 as a function of weight loss. It is seen that the bond strength seems to be almost unaffected by increased levels of corrosion up to about 16 % weight loss. An interesting comparison can be made from Fig. 16, which shows bond strengths for field specimens and for “repaired” specimens in the same diagram. The “repaired” specimens had bond strengths which were only slightly less than those obtained from the field specimens.


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Ullasund, repaired

Figure 16 Comparison of bond strengths from field and “repaired” specimens.

6.4 Pullout tests on laboratory specimens Standard laboratory specimens, each containing a single bar of 25 mm diameter with no transverse reinforcement, were also cast. The average compressive strength was 30 MPa. The corrosion process was accelerated by applying an electrical current. Three different current densities were used, 0.05, 0.1 and 0.25 mA/cm2, respectively. The results from pullout tests are depicted in Fig. 17. The scatter in measured bond strengths is greatest for the highest current density (0.25 mA/cm2). Apart from that, there seems to be no clear correlation between bond strength and applied current density in these tests. In Fig. 18, bond strengths from field specimens (Ullasund bridge), “repaired” speci-mens and laboratory specimens are compiled. Measured bond strengths for labora-tory specimens are somewhat higher than for the two other categories in the initial stage of corrosion (weight loss up to 4 %). Note that data for the laboratory speci-mens cover weight losses up to 7 % only.


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0.05 mA/cm2

0.1mA/cm2

0.25 mA/cm2

Figure 17 Pullout test results; laboratory specimens.

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Ullasund, repaired

Laboratory, 0.05

Laboratory, 0.1

Laboratory, 0.25

Figure 18 Comparison of field, “repaired” and laboratory specimens.


7. Beam tests

7.1 Ultimate load capacity Beams without stirrup reinforcement A typical beam test configuration is shown in Fig. 3. In some investigations beams with tensile reinforcement but without stirrups were employed. The chief motivation for leaving out the stirrups is the ability to produce uniform corrosion of the longitudi-nal bars over the entire beam length. On the other hand, this increases the risk of shear or anchorage failure. To eliminate the possibility of shear failure, some authors, like Mangat and Elgarf (1999a), [have supplied steel collars during the mechanical testing. The tests on corroded beams reviewed here include the studies conducted by Tachi-bana et al. (1990), Almusallam et al. (1996a), Lee et al. (1996), Uomoto and Misra (1998) and Mangat and Elgarf (1999a). All these investigations employed relatively small scale beams or slabs with spans ranging from 600 mm to 1500 mm and span to height ratios in the range 7.0 – 9.6. The longitudinal tensile reinforcement con-sisted of two bars (diameter 10 – 16 mm), the only exception being the slabs tested by Almusallam et al. (1996a) where 5 φ 6 mm bars were used. Detailed information of these tests are compiled in Appendix B, where additional references are also in-cluded.

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Uomoto (1988)Tachibana (1990)Lee (1996), without hooksLee (1996), with hooksAlmusallam (1996)Mangat (1999), groups 6

Figure 19 Ultimate load capacity of beams without stirrups.


The ultimate load capacity of beams without stirrups is illustrated in Fig. 19 as a func-tion of degree of corrosion measured as percentage weight loss of the tensile rein-forcement. For low levels of corrosion (weight loss less than 5 %) these tests re-vealed reductions in ultimate capacity of 10 – 20 %. As corrosion continues the strength is further impaired to about 50 % of the reference value (beam with intact reinforcement). Beams with stirrup reinforcement Studies of the influence of corrosion on beams with stirrups were reported by Takagi-shi et al (1998), Al-Sulaimani et al (1990), Cabrera and Ghoddoussi (1992), Rodriguez et al. (1994a) and Mangat and Elgarf (1999a). All these investigations employed relatively small beams (L = 1000 – 2000 mm,h = 100 – 200 mm) cast in the laboratory, except for the research conducted by Takagishi et al. (1998), where the concrete was taken from an access bridge in Tokyo Bay which had been exposed to 15 years of service. The slabs sawed from this concrete had dimensions (b x h x L) 1000 x 300 x 1500 mm. The amount of longitudinal (tensile and compressive) reinforcement varied con-siderably in these investigations.

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Takigishi (1988)Al-Sulaimani (1990), seriesIIIAl-Sulaimani (1990), seriesIVCabrera (1992), series1Rodriguez (1997), 5 types of beamsMangat (1999), groups4

Figure 20 Ultimate load capacity of beams with stirrups.


Measured values of ultimate strength are shown in Fig. 20 for different levels of cor-rosion. It is observed from this diagram that in the tests reported by Al-Sulaimani et al. (1990) increased strength was recorded in the initial stage of corrosion. The other studies compiled in Fig. 20 did not reveal the same behaviour. The values of ultimate capacity obtained for beams with stirrups seem to predict a more consistent behav-iour (i. e., less scatter) than the corresponding results depicted in Fig. 19 for beams without stirrups, despite the fact that the impressed current density varied widely for the beams with stirrups. In Fig. 20 the load-carrying capacity decreases continuously with increasing weight loss. It is important to observe that the loss of strength is con-siderably less for beams with stirrups as compared to beams without transverse rein-forcement. Hence, it can be concluded that presence of transverse reinforcement is important for maintaining ultimate capacity.

7.2 Bond strength Bond strength can be measured using beam specimens. This is most commonly achieved using specimens comprised of two half lengths of the beam, interconnected at the bottom by two steel bars. A hinge is provided near the top to allow for rotation of the concrete blocks, according to RILEM recommendations (1973). Beams without stirrups Bond strengths for corroded bars determined by bending tests have been reported by Mangat and Elgarf (1999b) and Stanish et al. (1999). In the latter study, bond strength was calculated using moment and curvature measurements. The results of these investigations are illustrated in Fig. 21. It is to be noted that Mangat and Elgarf (1999b) used applied current densities of 0.8 mA/cm2 and 2.4 mA/cm2, whereas the current was approximately 100 mA in the work of Stanish et al. (1999).


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Mangat (1999)Stanish (1999)

Figure 21 Bond strength obtained from tests of beams without stirrups. Beams with stirrups Berra et al. (1997) carried out an experimental program where bond strength of cor-roded bars was determined by testing beams with transverse reinforcement. The span of the beams was 650 mm and the height 180 mm. The testing was conducted in accordance with ASTM prescriptions.


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th

Al-Sulaimani, 1990/seriesIIIAl-Sulaimani (1990),seriesIVCabrera (1992), series1Cabrera (1992), series2Berra (1997), series1Berra (1997), series4

Figure 22 Bond strength obtained from beams with stirrups. In the work reported by Berra et al. (1997), results from similar investigations by Al-Sulaimani et al. (1990) and Cabrera and Ghoddoussi (1992) were also given. A com-pilation of these test data is provided in Fig. 22. The obtained bond strengths are fairly consistent without excessive scatter. Moreover, bond strength did not drop ap-preciably. The tests conducted by Al-Sulaimani et al. (1990) showed an increase in bond strength in the initial stage of corrosion, but this was not confirmed in the other investigations.

7.3 Simultaneous loading and corrosion In laboratory investigations of steel bar corrosion, the specimens are first subjected to corrosion and then subjected to loading in a mechanical test (pullout strength, beam ultimate load, etc.). Concrete structures in the field, however, are attacked by corrosion under sustained and variable loading. The interaction between mechanical load and corrosion is likely to accelerate corrosion, which was confirmed in a study conducted by Yoon et al. (2000).


8. Bond stress-slip relationship

8.1 Uncorroded bars The bond stress-slip relationship for intact or uncorroded bars is known to depend on a number of factors, of which the most important are:

- bar roughness (related rib area) - concrete strength - position and orientation of the bar during casting - state of stress - boundary conditions - concrete cover

The shape of the bond stress-slip diagram is influenced by the type of failure, that is, whether failure occurs by pullout of the bar or by splitting of the concrete. This is illus-trated in Fig. 23, where the decending branch corresponding to splitting failure has been drawn as a straight line, to simplify matters.

Figure 23 Bond stress-slip relationships for pullout and splitting failure. An analytical model for the bond stress-slip relationship for pullout failure is provided in the CEB-FIP Model Code ( Comité Euro-International du Beton 1993). Accordingly,


the bond stress τ between the concrete and the reinforcing bar can be calculated as a function of the relative displacement (i.e., slip) s as follows:

( )αττ 1max / ss= for 10 ss ≤≤ (1)

maxττ = for 21 ss ≤ (2)

⎟⎟⎠

⎞⎜⎜⎝

⎛−−

−−=23

2maxmax )(

ssss

fττττ for 32 sss ≤< (3)

fττ = for ss <3 (4)

In the above equations, maxτ is the ultimate bond strength and fτ denotes the resid-ual bond strength, see Fig. 23. The selection of slip values 1s , 2s and 3s , as well as the exponent α , for different bond conditions can be made with the aid of Table 1. Here, the concrete compressive strength is denoted by ckf .

The parameters involved in these equations are defined in Fig. 23. The selection of numerical values for bond strength and slip can be made on the basis of Table 1. Table 1 Parameters for defining the bond stress-slip relationship according to eqs. (1) – (4).

Unconfined concrete Confined concrete

Good bond conditions


Good bond conditions


s1 0.6 mm 0.6 mm 1.0 mm 1.0 mm

s2 0.6 mm 0.6 mm 3.0 mm 3.0 mm

s3 1.0 mm 2.5 mm Clear rib spac-ing

Clear rib spac-ing

α 0.4 0.4 0.4 0.4

τmax ckf0.2 ckf0.1 ckf5.2 ckf25.1

τf max15.0 τ max15.0 τ max40.0 τ max40.0 τ


The first curved part in Fig. 23 refers to the stage where the ribs penetrate into the surrounding concrete material and is characterized by local crushing and micro cracking. The horizontal portion of the curve only occurs for confined concrete, corre-sponding to progressive crushing and shearing off the concrete between the ribs. (For unconfined concrete, s1 = s2, see Table 1). The descending branch in Fig. 23 is associated with the reduction in bond strength due to the occurrence of splitting cracks along the bars. The horizontal section represents the residual bond capacity, which is maintained by a minimum of transverse reinforcement, keeping a certain degree of integrity intact.

8.2 Corroded bars The bond stress-slip relationship for corroded bars is less well established. Experi-ments to determine bond stress-slip behaviour of corroded steel bars have been re-ported by Al-Sulaimani et al. (1990), Tachibana et. al. (1990), Cabrera and Ghod-doussi (1992), Almussallam et al. (1996a), Auyeung, Balaguru and Chung (2000), Lee et al. (1996) and Fang et al. (2004) and do not facilitate simple, straightforward conclusions. Attempts have been made to establish bond stress-slip relationships for corroded reinforcement on the basis of existing experimental documentation. Such efforts in-clude the models suggested by Tørlen andHorrigmoe (1998), Horrigmoe and Sand (2002), Castellani and Coronelli (1999) and Coronelli and Gambarova (2004).


9. Concluding remarks

The present report has been devoted to reviewing published laboratory data on the structural consequences of steel bar corrosion in concrete. Corrosion experiments conducted in the laboratory are accelerated. The published test results exhibit con-siderable scatter. Furthermore, accelerated conditions during laboratory testing devi-ate significantly from field conditions. Quantitative results from laboratory investiga-tions are therefore not directly applicable to concrete structures in the field. Steel bar corrosion affects the reinforcement itself, the surrounding concrete and the composite action between steel and concrete. The major portion of the test data is concerned with measurements of bond strength of corroded bars. In the present report, data from pullout tests (bond strength) and small scale beam tests (ultimate load-carrying capacity) were evaluated. The effects on bond strength of a number of parameters such as concrete quality, bar diameter, concrete cover, bar position, transverse reinforcement, impressed electrical current, etc., were inves-tigated. The examined data revealed that loss of bond strength due to corrosion can be more severe than loss of bar cross section. As long as visible, longitudinal cracks do not appear at the concrete surface, bond strength seems to be maintained at its initial value (uncorroded bar). After the formation of longitudinal cracks bond strength is rapidly reduced. The presence of intact, transverse reinforcement seems to be an important factor in maintaining bond strength even in the more advanced stages of corrosion. Lack of systematic field measurements of steel bar corrosion is a serious shortcom-ing in the development of reliable models for the structural effects of corrosion. Re-cent test results obtained on specimens subjected to approximately 30 years of field exposure prior to testing in the laboratory were included in the present investigation. It is believed that more reliable field data can be obtained by this avenue of research, which should be encouraged.


10. References

Al-Sulaimani, G.J., Kaleemullah, M., Basanbul, I.A. and Rasheeduzzafar, 1990. Influence of corrosion and cracking on bond behaviour and strength of reinforced concrete members. ACI Structural Journal, 87 (2), pp.220-231. Almussallam, A.A., Al-Gahtani, A.S., Aziz, A.A. and Rasheeduzzafar, 1996a. Ef-fect of reinforcement corrosion on bond strength. Construction and Building Ma-terials, 10 (2), pp.123-129. Almussallam, A.A., Al-Gahtani, A.S., Aziz, A.R., Dakhil, F.H. and Rasheeduz-zafar, 1996b. Effect of reinforcement corrosion on flexural behaviour of concrete slabs. ASCE Journal of Materials in Civil Engineering, 8 (3), pp.123-127. Andrade, C., Alonso, C. and Molina, F.J., 1993. Cover cracking as a function of rebar corrosion: Part1 – Experimental test. Materials and Structures, 26, pp.453-464. Auyeung, Y., Balaguru, P. and Chung, L., 2000. Bond behaviour of corroded re-inforcement bars. ACI Materials Journal, 97 (2), pp.214-220. Auyeung, Y. and Balaguru, P., 2001. Effects of corrosion on the bond properties of reinforcing bars. In N. Banthia, K. Sakai and O.E. Gjørv, eds. Proceedings of the third international conference on concrete under severe conditions. Vancou-ver, Canada: The University of British Columbia, pp.112-119. Berra, M.A., Castellani, A. And Coronelli, D., 1997. Bond in reinforced concrete and corrosion of bars. Structural faults and repair – 97. Edinburgh: Engineering Technics Press,2, pp. 349-356. Cabrera, J.G. and Ghoddoussi, P., 1992. The effect of reinforcement corrosion on the strength of the steel/concrete “bond”. International conference on bond in concrete. Riga, Latvia, pp. 10/11-10/4. Cabrera, J.G., 1996. Deterioration of concrete due to reinforcement steel corro-sion. Cement & Concrete Composites, 18, pp.47-59. Castellani, A. and Coronelli, D., 1999. Beams with corroded reinforcement: Evaluation of the effects of cross-section losses and bond deterioration by finite element analysis. Structural faults and repair - 99. London. Clark, L.A. and Saifullah, M., 1993. Effect of corrosion on reinforcement bond strength, Proceedings of the 5th international conference on structural faults and repair. Edinburgh: Engineering Technics Press, pp. 113-119. Comité Euro-International du Beton, 1993. CEB-FIP model code 1990. London : Thomas Telford. Coronelli, D. and Gambarova, P., 2004. Structural assessment of corroded rein-forced concrete beams: Modelling guidelines. ASCE Structural Journal of Struc-tural Engineering, 130 (8), pp.1214-1224. Fang, C., Lundgren, K., Chen, L. and Zhu, C. 2004. Corrosion influence on bond in reinforced concrete. Cement and Concrete Research, 34 (11), pp.2159-2167.


fib Task Group Bond Models, 2000. Bond of reinforcement in concrete. fib Bulle-tin 10, Lasusanne, Switzerland. Ghandehari, M., Zulli, M. and Shah, S.P., 2000. Influence of corrosion on bond degradation in reinforced concrete. Proceedings of the ASCE mechanical engi-neering conference, Austin, Texas. Horrigmoe, G. and Sand, B., 2002. Residual strength of deteriorated and retrofit-ted concrete structures: A numerical approach. IABSE symposium towards a bet-ter built environment – Innovation, sustainability, information technology. Mel-bourne, Australia. Lee, H.S., Tomosawa, F. and Noguchi, T., 1996. Effects of rebar corrosion on the structural performance of singly reinforced beams. In C. Sjöström, ed. Durability of building materials & components 7, London: E.F. Spon, pp.571-580. Mangat, P.S. and Elgarf, M.S., 1999a. Flexural strength of concrete beams with corroding reinforcement. ACI Structural Journal, 96 (1), pp.149-158. Mangat, P.S. and Elgarf, M.S., 1999b. The bond characteristics of corroding rein-forcement in concrete beams. Materials and Structures, 32, pp.89-97. RILEM/CEB/FIP Recommendation, 1973. Bond test for reinforcing steel. Materi-als and Structures, 6 (32). Rodriguez, J., Ortega, L.M. and Casal, J., 1994a. Corrosion of reinforcing bars and service life of reinforced concrete structures: Corrosion and bond deteriora-tion. International conference on bond loss across borders. Odense, Denmark, 2, pp.315-326. Rodriguez, J., Ortega, L.M. and Garcia, A.M., 1994b. Assessment of structural elements with corroded reinforcement. In R.N. Swamy, ed. Corrosion protection of steel in concrete. Sheffield: Sheffield Academic Press, pp.171-185. Rodriguez, J., Ortega, L.M.,Casal, J. and Diez, J.M., 1996. Assessing structural conditions of concrete structures with corroded reinforcement. 4th international congress on concrete in service of mankind, Dundee. Shima, H., 2001. Local bond stress-slip relationship of corroded steel bars em-bedded in concrete. In N. Banthia, K. Sakai and O.E. Gjørv, eds. Proceedings of the third international conference on concrete under severe conditions. Vancou-ver, Canada: The University of British Columbia, 1, pp.454-462. Stanish, K., Hooton, R.D. and Pantazopoulou, S.J., 1999. Corrosion effects on bond strength in reinforced concrete. ACI Materials Journal 96 (6), pp.915-921. Tachibana, Y. Maeda, K.-I., Kajikawa, Y. and Kawamura, M., 1990. Mechanical behaviour of RC beams damaged by corrosion of reinforcement. In K. Treadaway and P. Bamford, eds. Corrosion of reinforcement in concrete, El-sevier, pp.178-187. Takagishi, Y., Ichikawa, H., Tabuci, H. and Moriwake, A., 1998. An experimental study on deterioration and repair of a marine concrete structure, ACI Special Publication SP-109, Detroit, pp.353-276.


Tørlen, A. and Horrigmoe, G., 1998. Modelling of bond between reinforcement and concrete for deteriorated and repaired beams (in Norwegian). Norut Tech-nology. Narvik, Norway: Report No. NTAS A99034. Uomoto, T. and Misra, S., 1998. Behaviour of concrete beams and columns in marine environment when corrosion of reinforcing bars takes place. ACI Special Publication SP-109, Detroit, pp.127-146. Yoon, S., Wang, K., Weiss, J. and Shah, S.P., 2000. Interaction between loading, corrosion and serviceability of reinforced concrete. ACI Materials Journal 97 (6), pp.637-644.


APPENDIX A

Laboratory studies of bond strength of corroded steel bars determined from pullout tests


A

Table A1. Pullout tests; specimens without stirrups

Ref. no.

Author

Type of bar

Main rein-

force-ment

Number/

diam.

Cover/

diam.

ratio

Embedded

length

Compr.

strength

(cylinder/

cube)

Dimension

of specimen

Current

density

Duration

Conditions

procedure

Corrosion

(weight loss)

Residual

bond stress

Calculation of reference

value

Remarks

ribbed/

plain

mm

-

mm

MPa

mm×mm×mm

mA/cm2

%

%

Cabrera (1992)

precorr. – 32 days

Series 1

Opc mix

under corr

2-40 days

0 – 12.6 25

7

Series 2

Pfa mix

ribbed

12

48

56

(?)

150×150×150

?

Voltage:

3 Volts

under corr

10-80 days

Central embed-ded.

2% of Na as NaCl

was added

5% NaCl solu-tion

0- 6.90 85

table

0 – 9.1

10 125×125×200 5 56

13 125×125×250 30

16 125×125×350 29

19 175×175×350 24

23

Auyeung

(2001)

ribbed

(two bars)

25

30

(cylinder)

175×175×450

3.0

3 % by cement weight of CaCl2were added

7

curve


24

Rodriguez (1994)

Type 3

ribbed

4 × 16

24/16

210

(13 · diam)

40

(cube)

300×300×300

0.1

Allowed the bars to cor-rode in sev-eral month

Adding 3 % CaCl2 .

Top and

bottom cast

Corrosion level

0

1(2.9 – 3.7 )

2 (5.4 – 9.1)

18 - 23

table

47

Tachibana

(1990)

ribbed

2 × 16

64

35.6

(?)

100×150×200

0.5

0, 3, 6, 10, 15 days

Accelerated galvanostatic method

-

-

curve Bond stress-

slip curves

only

48

Auyeung

(2000)

mild steel

2 × 19

127

178

28

(cylinder)

355×178×178

Current density

0.14 –

0.15 A

Placed in 3% salt solution.

Current induced

corrosion was started after the specimens were soaked in salt water for a minimum of 3 days.

0 – 5.19

24

table

50

Shima

(2001)

Series 1

ribbed

22

24.4 – 28.4

(cylinder)

400×300×500

2.8

Surface of the bar was re-moved by

10 % ammonium citrate solution. Specimens were soaked in artifi-cial salt solution of 3, 3 % con-centration.

0 – 28.9

≥ 37

Curves τmax/τnon

cor


54

Clark

(1993)

ribbed

plain

4 × 8

0.5

1.0

2.0

110

30

(?)

150×175

0.5

first crack formed 9, 21,

49 hours

( for c/φ=0.5, 1.0, 2.0) (ribbed bar) and 14, 27, 56 hours ( for c/ φ = 0.5, 1.0, 2.0) (plain bar)

3.5 % sodium chloride solution in a tank.

0 – 25

5

12.5

106 -100

75

Curves τmax/τnon

cor

0-10

19

40

(?)

Cylinder φ= 102 mm, L =

203 mm

0.616

5

7.5

37

0

19

40


304 mm

0.616

5

7.5

16

0

9.5

40


203 mm

0.329

7.5

10

23

0

56

Ghandehari (2000)

ribbed

9.5

40


304 mm

0.329

4 weeks

Specimen sub-merged in 5 % by weight NaCl solution

6 hours intervals for 4 weeks

10

110

curves τmax/τnon

cor

curve

60

Al-Sulaimani (1990)

ribbed 10

14

22

7.50

5.36

3.75

30

( cube (?))

150×150×150

2.0 Central embed-ded.

Placed underwa-ter soaked in water for several days

5

7

10

35

15

0

table


82

Williamson

(1999)

ribbed

plain

2 × 8

2 × 8

Two oppsite

face

0.5

1.0

2.0

?

150×150×150

0.25

2 month corrosion

Is not central.

Specimens were placed in a tank of 3,5 % sodium chloride electro-lyte solution

0 and 20

20

36

-

curve

Pre –load

No pre-load

114

Lee

(2002)

ribbed

D13

1.5 · D

2.5 · D

3.5 · D

6 · D

42.1

33.0

24.7

8·D×8·D×8·D

1A

Central embed-ded

0

3

15

20

30

- - Bond stress/

slip

115

Lee

(1996)

10

1.0

110

70.1

100×100×150

1 A

Is not central

( corner)

0 - 12 - curve Bond stress/

slip

deformed 0 - 9.2

9

45

curve

113

Fang

(2004) smooth

20

80

52.1

(cube)

140×140×180

0-2 A

92,

95- 324,

32 times 3 - 10

125

Saifullah

(1994)

ribbed

4 × 8

1.0

110

Grade 30

150×150×175

4.0

2.0

1.0

0.5

0.25

0.15

0.09

0.04

4 days

9

15

30

60

100

150

400

Top and bottom cast

3.5 % sodium chloride solution

12.5

12.5

12.5

50

105

60


Table A2. Pullout tests; specimens with stirrups

Ref.

no.

Author

Type of bar

Main rein-

forcement

Number/

diam.

Cover/

diam.

ratio

Embedded

length

Stirrups

Diam/ spacing

Compr.

strength

Dimension

of specimen

Current

density

Duration

Conditions

procedure

Corrosion

(weight loss)

Residual bond stress

Remarks

mm

mm

mm/mm

MPa

mm×mm×mm

mA/cm2

%

%

19

Cairns

(2002)

plain

16

20/16

40

3 × 6mm /?

58.1

(cube)

200×300×380

0.01-0.05

140 days under corro-

sion

0 – 3.95

(bottom cast)

72 - 120

Corrosion penetration

0 – 0.20 mm

Fang

(2004)

deformed

97

113

smooth

20

80

2 × 6 mm/40

140×140×180

Current

0 - 2 A

0 -7.6

Rodriguez (1994)

Type 1 ribbed

4 × 16

24/16

210

(13 · D)

8/70

40

( cube)

300×300×300

0.1


Top and

bottom cast

Corrosion

level

0

1 (2,9 – 3,7)

2 (5,4 – 9,1)

70 - 89

24

Type 2 ribbed

4 × 16

24/16

210

(13 · D)

6/100

40

300×300×300

0.1


Top and

bottom cast

Corrosion level

0

1 (2.9 – 3.7 )

2 (5.4 – 9.1)

62 - 69


value - table.


Type 4 ribbed

4 × 16

40/16

210

(13 · D)

6/100

40

300×300×300

0.1


Top and

bottom cast

Corrosion level

0

2 (5.4 – 9.1)

51 - 60

Type 5 ribbed

4 × 10

15/10

180

(18 · D)

6/100

40

300×300×300

0.1


Top and

bottom cast

Corrosion level

0

2 (5,4 – 9,1)

71 - 84

Shima

(2001)

0 – 10.9

Series 2 40/22 19.5-25.2

(cylinder)

0 – 10.8 35

Series 3 25/22 25.3-27.6 0 - 9.9

47

50

Series 4

ribbed

22

10/22

50 mm from bar

10 mm/100

20.5-23.3

400×300×500

2.8

Central embedded

0 – 10.9

82


value - curve,

Conclusion in ref.

Bond stress at large

degree of corrosion are about 50% with-

out longitu-dinal crack

59

Almusallam

(1996)

ribbed 12

4 #3 – comp.

63.5/12

102

#2 – shear /76

U- sharped

30

(cylinder)

152×254×279

0.4 A

Is not cen-tral. Speci-

men

was placed in water

0 - 80 15 Curve

Residual strength/cor

rosion


APPENDIX B

Laboratory studies of the influence on steel bar corrosion using small scale beam specimens


B Table B1. Beam tests

Tensile Ref.

no.

Author Type of

bars Compr.

reinfor-cement

Transverse

/stirrups

Compr.

strength

Dimensions

L b h

Concrete

cover

Duration Current

density

Conditioning procedure

Details

of

end

ancho-

rage

Corrosion Long. crack width

Pcor /Pnon

Type

of failure

mm mm/mm MPa mm×mm×mm mm mA/cm2 % mm

Berra

(1997)

Series1 6/100

50 0 – 2.14 107

110

Series 4

ribbed

14

6/50

?

100×180×650

30

1 mA/m2

Concrete – 2% chlorides

Corrosion – 3% NaCl in a

tank

0 – 4.29 114

Stanish

(1999)

(slab)

Nonsilica fume mixture

35

0 - 22

6.25 47

22

Silica fume mixture

3 × 10 M

No

43

1300×150×350

20

100 mA

Tank with 3% NaCl

straight

21

14


?

Stanish

(1997)

( from CEB- FIB)

? No 0.21 10 60

?

Coronelli

(1997)

( from CEB- FIB)

? Yes

c/φ=2.5

0.05

110


Table B1. Beam tests (contd.) Tensile Ref.

no.

Author Type of

bars Compr.

reinforce-ment

Transverse

/stirrups

Compr.

strength

Dimensions

L b h

Concrete

cover

Duration Current

density

Condit. procedu-

re

Details

of

end

ancho-

rage

Corrosion Long. crack width

Pcor /Pno

Type

of failure

mm MPa mm×mm×mm mm mA/cm2 % mm

58

Almusallam (1996)

(Slab)

ribbed

5 × 6/57

No

711×65.5×305

2 A

8.5 · φ straight

Up to 75

100-15

Precracking –flexure

Postcrack.

- bond/shear

≥87

Non-corr. Flexure

3- days 100 Flexure

6- days 90 Shear-compr.

10- days 85 Bond-shear

47

Tachibana (1990)

ribbed

2 × 16

No

35.6

(?)

2000×150×200

30

15- days

0.5

Straight

up to 5

( 5 % ≈ 15 days)

88 Bond shear

Kawamura (1995)

117

Test 1 – small beams

ribbed

1× D13

No

31.1-37.6

(?)

1200×100×100

Direct current

(?)

Chloride

solution in a tank

Hooks

Up to 1 mm

85

Flexure


Mangat (1999)

ribbed

2 × 8 Series 3,7

2 × 6

2 ×10

17

Series 2,4,5,6,8,9

2× 6

No

40

(cube)

910×100×150

25

10 days-1 year

(series 9) under

corrosion

3.0

U-sharped hooks

5

10

69

23

4

Mangat (1999)

ribbed

2 × 10 mm

No

45

20

Pre corr –

15 days, corr

duration 14, 4 - 96

hours

0.8

2.4

(for 5 % corr.)

0 - 5

-

-

Bond stress/ slip

6 days to cracking

( 0.05)

94 - 84

74

Lin

(1980)

ribbed

#5

No

?

1016×76×152

0.05-0.1

Sea water for

several days

Hooks

2 days to cracking

( 0.1)

44 - 56

0-7.9

0 – 10.4

5 92.2 Hooks

25

-

60

5 71.6

115

Lee

(1996)

ribbed 2 × 10 mm

No 70.1 800×100×100 10 Current of 1 A

NaCl 3 %

No hooks

10.4

-

38


1 96

1.2 92

124

Uomoto

(1988)

Type A

ribbed

2 × D6

2 × D10

2 × D16

2 × D19

No

28

37

48

700×100×100

10-20

7 - 14 days

167mA

- 1A

NaCl, 1.25

kg/m3

NaCl solution

?

2.4

(for φ= 16 mm)

83

( cube)

2350×130×200

1 × 16

2 × 6

34.9

Debonded length

250

100

1 × 16

2 × 6

36.7

850

94

42

Nokhasteh

(1992)

smooth mild “black” steel

2 × 16

2 × 6

No

( stirrups), shear links

29.9

850

25

hooks

68.3

15

Yoon

(2000)

ribbed

1 × 19

No

42

1170×100×150

30 Effect of loading


Table B1. Beam tests (contd.)

Tensile Ref. no.

Author Type of

bars Compr.

reinforce-ment

Transverse

/stirrups

Compr.

strength

Dimensions

L b h

Concrete

cover

Duration Current

density

Conditioning procedure

Details

of

end

ancho-rage

Corrosion Long.

crack

width

Pcor /Pnon

Type

of failure

mm mm/mm MPa mm×mm×mm mm mA/cm2 % ( or mm) mm %

1 × 12

Al-Sulaimani

(1990)

series III

6/50

2.5·12 Bond failure

60

series IV

ribbed 2 × 10

6/50

30 (cube (?))

1000×150×150

5·12

2.0

12 φ straight

up to 4

up to

1.3

≥90

Flexure

Kawamura (1995)

hooks

Test 1 – small beams

1× D13 ? /100 31.1-36.6

(?)

1200×100×100 Direct current

(?)

Chloride solution in a

tank

≥91 Flexure

117

Test 2 – large beams

Non splice

(H2)

ribbed

2 × 19

10/200(100, 600)

29.4

(?)

2800×200×300

40

0 - 14 days

?

100


2 × 12 Cabrera (1992)

2 ×10

Up to

2.90 93 Series I-opc

9.2

78.6

Flexure

2.86 94 Series I-pfa

6.10

90

4.3 104 SeriesII-opc

7.8

100

Bond failure

3.6 104

7

Series II-pfa

ribbed

8/40

56

(?)

1000×125×160

25

Voltage

of 3 volts

None (ref.

CEB-FIB)

6.3

97

2 × 10 Mangat (1999)

2 × 6

Up to 0.3-0.6

5 91

17

Series 1,4

ribbed

6/70

40

(cube)

910×100×150

25

15-18 days

precorr.

32-128 hrs – corr.

3.0 Hooks, U - sharped

10

54

Okada

(1988)

Serie C9-10 6/180(200) 100×200×1200

Serie C7-8 6/180(140) 100×200×1400

76

Serie C1-6

ribbed

2 × 10

2 ×10

Counter D13

6/100

?

100×200×1600

20 Current 0.1 mA

End hooks

0.02-

0.15

≥ 91 ?


Rodriquez

(1995, 1996)

100-200 days

Type 1

ribbed

2 ×12

2 × 8

6/150 42

2300×150×200

149

0.1

3 % Ca Cl2

Straight

0.54 mm - - Bending (tensile

reinforce-ment)

2 × 10 Type 11

2 × 8

6/170

6/170

50

34

101-190 0.36-0.71mm

0.2-0.6

55 - 74 Bending

(tensile reinforc.)

4 ×12 Type 12

2 × 8

6/170

6/170

48

35

104-175 0.32-0.41

mm

0.2-0.3

57 - 80 Bending concrete/

shear

2×12 + 2×12 (-)

Type 13

(-) 2×12 cut-off bars

2 × 8

6/170

6/170

52

37

108-175 0.32-0.40 mm

0.2-0.5

58 - 70 Shear/ bond shear

4 ×12 Type 21

4 × 8

6/170

6/170

50

35

108-181 0.31-0.53

mm

0.2-0.4


shear

4 × 12

16,

18

Type 31

4 × 8

6/85

6/85

49

37

111-190

0.30-0.51

mm

0.2-0.3


shear

?

Daly

(1995)

from CEB-FIB (unpublished)

?

?

Yes

?

?

?

?

?

?

12 φ straight

up to

17

-

≥ 70

1 96

124

Uomoto

(1988)

Type B

ribbed

2 × D6

2 × D10

2 × D16

2 × D19

6/170

28

37

48

2100×100×200

10-20

7-14 days

167mA-1A

NaCl, 1.25 kg/m3

NaCl solution in a tank

? 1.2

2.4

92

83


123

Takagishi (1988)

Slabs

Sample D without repair

A costal struc-ture in the

Tokyo Bay, serious dam-age after 15

years

16 mm

Yes

27.4

2200×200×500

15 years

-

-

12.4

(table)

-

80 - 90

1×16

14

Ballim

(2003)

Deflection ratio/

corrosion %

ribbed

2×8

8/60 47 1500×100×160 32 30 days 0.4 straight - - -

0

0.0

Max load

42.3

Bond slip

2 × 10

8.6

0.45

51.8

Flexure

2 × 10

6/125

1000×150×200

25

0.06

11.5

0.58

53.1

Flexure

0

0

74.9

Bond slip

3 ×16 7.1 0.55 78.3 Bond slip

23

Clairns

(2003)

Type sbf

Type sbs

Left configura-tion

plain mild steel

2 × 8

6/125

1000×150×200

25

7.6 0.89 79.4 Flexure/

slip


0 0 94.3 Bond slip 3 ×16

7.1 0.55 104.7 Flexure

Right

configuration

2 × 8

7.6 0.89 104.7 Flexure

2 ×16 6/150 92 Flexure

2 × 20 6/100 142 Flexure

2 × 25 6/50 167 Flexure

2 × 20 6/100 256 Flexure

46

Cairns

(1993)

2 × 25 6/100

35 - 42

166 Flexure

laboratory investigations of steel bar corrosion in concrete ...1337406/fulltext01.pdfcorrosion of...

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