Effect of corrosion of reinforcement on the mechanical behaviour of highly corroded RC beams

Engineering Structures 56 (2013) 544–554

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Effect of corrosion of reinforcement on the mechanical behaviour of highly corroded RC beams Wenjun Zhu a, Raoul François a,⇑, Dario Coronelli b, David Cleland c a

Université de Toulouse, UPS, INSA, LMDC (Laboratoire Matériaux et Durabilité des Constructions), Toulouse, France Politecnico di Milano, Dipartimento di Ingegneria Strutturale, Milano, Italy c School of Planning, Architecture & Civil Engineering, Queen’s University, Belfast, UK b

a r t i c l e

i n f o

Article history: Received 25 September 2012 Revised 15 February 2013 Accepted 22 April 2013

Keywords: Corrosion Concrete Bending tests Tensile tests Residual performance

a b s t r a c t This paper describes an experimental investigation of the behaviour of corroded reinforced concrete beams. These have been stored in a chloride environment for a period of 26 years under service loading so as to be representative of real structural and environmental conditions. The configuration and the widths of the cracks in the two seriously corroded short-span beams were depicted carefully, and then the beams were tested until failure by a three-point loading system. Another two beams of the same age but without corrosion were also tested as control specimens. A short span arrangement was chosen to investigate any effect of a reduction in the area and bond strength of the reinforcement on shear capacity. The relationship of load and deflection was recorded so as to better understand the mechanical behaviour of the corroded beams, together with the slip of the tensile bars. The corrosion maps and the loss of area of the tensile bars were also described after having extracted the corroded bars from the concrete beams. Tensile tests of the main longitudinal bars were also carried out. The residual mechanical behaviour of the beams is discussed in terms of the experimental results and the cracking maps. The results show that the corrosion of the reinforcement in the beams induced by chloride has a very important effect on the mechanical behaviour of the short-span beams, as loss of cross-sectional area and bond strength have a very significant effect on the bending capacity. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The problem of the deterioration of reinforced concrete structures due to chloride-induced corrosion of the steel reinforcement is considered one of the most important factors for the durability of reinforced concrete structures and has drawn great attention all over the world [1–4]. The major effect of the corrosion process is the formation of rust, whose volume is greater than steel (from two to six times depending on environmental conditions) [5] and this results in cracking and spalling of the concrete, which can influence various characteristics such as the mechanical performance and load capacity of the concrete structures. Most experimental research deals mainly with the effect of steel corrosion on the flexural behaviour of reinforced concrete elements [6–9], in contrast little research has been done about the influence of corrosion on the shear behaviour of corroded reinforced concrete beams. Xia et al. [10] have studied the effect of the corroded stirrups on the shear performance of reinforced concrete beams. Wang et al. [1,11] have carried out some research on the impact of partial length corrosion on shear behaviour of rein⇑ Corresponding author. Tel.: +33 561559901. E-mail address: [email protected] (R. François). 0141-0296/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.engstruct.2013.04.017

forced concrete beams, but all of these tests were carried out on beams subjected to accelerated corrosion due to an impressed electrical current and therefore the results may not be representative of concrete structures in service. Also it is difficult to find the relationship between the current-induced corrosion and the natural corrosion, which reduces the applicability of the research results. Differently from most of the research on the corroded structures resulting from the application of an impressed current or adding an admixture of CaCl2 in concrete, François and Arliguie [12] have carried out a long-term program about corrosion of reinforced concrete beams stored in a chloride environment under service load since 1984. The corrosion was much closer to the one actually observed in real structure conditions, including the distribution of corrosion, the nature of the corrosion and the oxide products. A series of results have been published ranging from the serviceability limit state [13] to the ultimate load capacity in a bending test [14]. As part of this program, this paper reports on the mechanical behaviour of corroded short-span reinforced concrete beams. These beams were extracted from one highly corroded longer span beam (labelled B2Cl2), which was tested to destruction at the age of 26 years, by cutting into two parts to produce beams of length 1.15 m. The same tests were carried out on

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two control beams without corrosion obtained through the same process, i.e. by cutting a 26 years old control beam (B2T) into two parts. It should be pointed out that for the experimental test on the longer span beams (span equal to 2850 mm), the ultimate capacity was 37.6 kN and 49.2 kN for B2Cl2 and B2T respectively. The mechanical experimental tests on the short-span beams were undertaken immediately after the short-span beams were formed which meant that there was no subsequent corrosion tests. In other words, the corrosion damage in the short beams was the same as that obtained in the long span configuration during 26 years. A first bending test was done on the long span configuration then the two small span beams were created by sawing and tested immediately in three point bending. During the long-term chloride exposure, the level of sustained load was higher than the ULS design load, therefore a stabilized cracking phase was reached and the additional load to failure in long span configuration did not modify the cracking pattern. Furthermore, the plastic hinge was concentrated at mid-span of the long span beams and in the part not included within the short beams. Neither the control nor the corroded beams were virgin beams in regards to load and cracks, mainly because the sustained loading was applied for all beams. therefore, we can expect that the comparison between control beams and corroded beams highlights the effect of long term corrosion. 2. Experimental context This program of long-term experiments has been carried out since 1984 at Laboratoire Matériaux et Durabilité des Constructions (L.M.D.C.) in Toulouse, southwest of France. The goal was to better understand the steel corrosion process in reinforced concrete elements and its influence on the mechanical performance. Thirty-six beams were cast, with dimensions of 3000  280  150 mm which was a typical size in the construction industry during that time, and then stored in a chloride environment under service load so as to model the structures in service, designated as Group A and B. The beams in both groups were loaded in level 1 (Mser1 = 13.5 kN m) and level 2 (Mser2 = 21.2 kN m) respectively. Another 36 beams of the same composition were cast but stored in the laboratory condition to be treated as the control beams. At different stages throughout the 26 years, experimental studies were executed on some beams to determine the relationship between structural and corrosion cracking [15,16], the chloride content [15] and the mechanical behaviour [17]. This paper presents experiments on Beam B2Cl2, where the first letter B refers to the group of beams, the first 2 means the loading level 2, the Cl means the beam were stored in the chloride environment, and the last digit (2) is the beam number within the group.

content was adjusted in order to obtain a constant workability of 70 mm in the slump test. The average compressive stress was found to be 45 MPa and the elastic modulus was found to be 32 GPa by the test of cylinder specimens (£110  220 mm) at 28 days. All of the reinforcing bars including the stirrups were made of high yield reinforcement steel with nominal yield strength of 500 MPa. The compressive strength of the concrete was about 60 MPa after 26 years, and the properties of the corroded tensile bars and non-corroded tensile bars were investigated by mechanical tests after 26 years and are discussed in Section 3.3. 2.2. Reinforced concrete specimens The layout of the reinforced concrete beam B2Cl2 is shown in Fig. 1. The concrete cover was 10 mm, according to French regulation [18] at the time of casting the beam, which corresponded to the minimum depth in non-aggressive environment. For the bars, the diameter of the bottom bars was 12 mm, while the diameter of the top bars and the stirrups was 6 mm. 2.3. Ageing regime A three-point loading system was applied to the beams by coupling a Group A beam with a Group B beam (Fig. 2), and for beam B2Cl2, the moment was Mser2 = 21 kN m, which corresponded to 80% of the predicted failure load. The moment was applied by controlling the strain of the loading device [19]. Therefore the maximum stress in the tensile reinforcement was calculated to be 380 MPa. According to French Standard, this was about twice the serviceability limit state design loading for exposure to a chloride environment. The beams were exposed in a saline environment with a salt fog of 35 g/l which corresponded to the salt concentration of sea water. The fog was generated by four spray nozzles located in each upper corner of a sealed room (Fig. 2). Having been exposed in the fog environment for 6 years, the beams were then subjected to wetting–drying cycles (Table 2) to accelerate the corrosion process. 3. Description of beams This research program was mainly based on the two short-span beams obtained from beam B2Cl2. A bending test had been carried out on Beam B2Cl2 and B2T at a 2850 mm span. Bending failure occurred in the middle of the span. Subsequently it was divided into two short-span beams B2Cl2-1 and B2Cl2-2 (Fig. 1) by sawing to remove the centre cracked portion. The load tests were carried out on these two corroded beams directly once they were formed after removing the middle sections of the long-span beam, together with the two control beams.

2.1. Material and composition 3.1. Cracking map For the beams of Group B, the concrete composition and the cement chemical composition are shown in Table 1. The concrete mixes were fabricated with a water/cement of 0.5 but the water

Before the load test on the 2850 mm span beam, crack maps of the two short-span corroded beams B2Cl2-1 and B2Cl2-2 were

Table 1 Concrete composition. Mix composition Rolled gravel (silica + limestone) Sand Portland cement: OPC HP (high perform) Water

1220 kg/m3 820 kg/m3 400 kg/m3 200 kg/m3

5/15 mm 0/5 mm

Cement composition

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

Na2O

Weight (%)

21.4

6.0

2.3

63.0

1.4

3.0

0.5

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W. Zhu et al. / Engineering Structures 56 (2013) 544–554

280

3000

220

220

220

220

220

220

220

220

220

220

220

220

220

220

220

B2Cl2 1150

1150

280

10

Ribbed 6

10

14 stirrups 6 220

220

220

100

220 950

220

Ribbed 12

220

150

100

100

220 950

100

Short span beam B2Cl2-2

Short span beam B2Cl2-1

Fig. 1. Layout of the reinforcement for the target beams (mm).

2m

Spray nozzles

gs Loadin

ystem

B1 A1

Strain measurement

B2

2m

A2

3.5m

Fig. 2. Loading system and chloride environment.

Table 2 Wetting–drying cycles of the corroded beams. Periods (years)

Spraying state

Loading conditions

Conservation conditions

Temperature (°C)

0–6

Continuous spraying

Loaded

About 20 °C

6–9

Wetting–drying cycles (1 week each) Wetting–drying cycles (1 week each) No spraying

Loaded

Salt fog chamber in the Lab Salt fog chamber in the Lab Salt fog chamber outside Outside

9–19

19–26

Loaded

Loaded

About 20 °C

SWFC

SWFC

SWFC: the south-west of France climate, ranging from 5 to 21 °C average value per month.

produced. All the cracks of the corroded beams were described carefully, including the locations and the configurations of both the transverse cracks and the longitudinal cracks after 26 years of exposure in a chloride-rich environment. The widths of the cracks were also measured by video-microscope with an accuracy of 0.02 mm and the magnification ranging from 25 times to 175 times.

The cracking maps of the corroded beams at the age of 26 years are drawn in Fig. 3. In order to better analyse the relationship between corrosion of the bars and the cracking behaviour, the locations of bars in the beams are also outlined by dashed lines in the cracking maps. It is necessary to point out that the cracking before the mechanical experiment could better show the influence of the long-time chloride corrosion on the RC beam. Moreover, the cracking during the test of the 2850 mm span beam was mainly concentrated in the middle portion which was discarded in creating the short span beams. Other cracking was slight and closed on unloading. In these maps, the ordinary lines stand for the cracks whose widths are smaller than 1 mm. The cracks appeared in all four surfaces. In the bottom surface and the lower parts of the side, the longitudinal cracks were dominant; however, in the higher parts of the sides there were more transverse cracks, appearing mainly parallel to the stirrups. Some transverse cracks in the four faces (sides, top and bottom) had interconnected, such as cracks No. 9 – No. 2 – No. 10 – No. 5 in beam B2CL2-2. It should be pointed out that all the crack maps were depicted before the mechanical test on the longer span beam. Therefore all the cracks are due to bending during the first 19 years and the corrosion during 26 years. For example, the transverse crack 7 in B2Cl2-2 on the back side developed from the compressive side to the tensile section due to the corrosion of the stirrup. The wider lines in the cracking maps stand for the cracks whose widths were larger than 1 mm, and the dashed zones represent the spalling of the corroded beams. According to DuraCrete Final Technical Report [20], the spalling can be observed when cracks reach 1.0 mm in width. In the bottom surface, the wider cracks and the spalling area had connected all along one tensile bar (zone A in Beam B2Cl2-2 in Fig. 3); along the other tensile bar of the same beam, the maximum crack widths were either in the middle of the span or at the end of the beams. 3.1.1. Longitudinal cracks Compared with the transverse cracks of the corroded beams (Fig. 3), many longitudinal cracks are visible on all faces. Especially on the bottom surface, the longitudinal cracks developed faster and wider than any of the transversal cracks. Along the tensile bars, the corrosion cracks had stretched the whole span in the bottom areas. Some cracks wider than 1 mm were present around the tensile bars in the two adjacent faces (i.e. side and bottom respectively), of which the biggest one reached 5.78 mm near the support, much wider than the value proposed by Duracrete [20] for spalling. This suggests that the force supplied by the support confined the concrete cover and prevented

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W. Zhu et al. / Engineering Structures 56 (2013) 544–554

0,02

0,02

0,09 0,02

0,07

0,04

0,01

0,09

0,07

Top side

0,40 0,144

0,25

9

0,06

0,06

0,06

0,25

0,03 0,02

0,22

0,02

0,07

2,12

0,33

1,90

5,78 0,07 0,74

0,03

0,10

0,74

0,03

0,12

0,09

0,88

1,09

0.1

0,59

0,02 0,24

0,04

0,34

1,97

0,02

Down side

0,31

0,07

0.3

0.4

0.5

B2CL2-1

0.6

0.7

0.8

0.9

1.0

0,12

0,65

10

0,09

0,05

0,34 0,01

0,05

0,03

SUPPORT D 3,25

2,92 0,05

0,09

1,71

0,07

0,07

0,41

0,10

1,05

ZONE C

1.2 m 0

0.1

0,02

0,32

0,08 5,83

2,59

0.2

7

0.3

0.4

0,04 0,12

0.5

0,07

0.6

0.7

0.8

0,01

8 0,30

0,07

B2CL2-2

26 years

0,04 0,20

0,07

0,15

1.15

6

0,08

5 0,52

0,94

2,77

0,02

Back side

1.1

4

0,03

0,22

1,93 0,02

2,14

1,15

0,02

3

SUPPORT C

0,02 0,02 1,29

0.2

0,02

0,17

2,21

0,04

0,02

0,03 0,06

0,06

0,08

2

0,04

0,07

ZONE B

0,02

0

0,03

0,03

SUPPORT B

ZONE F 0,02

Front side 0,01

0,18

0,50 0,28

0,16

0,04

0,03

ZONE A

SUPPORT A

0,08

0,15

0,08

1 0,02 0,26 0,05

0,03

2,01

ZONE E 0,55

0,02

0,15

0,02

ZONE D 0,05

0,36

0,03

0,06 0,02

0,89

0.9

1.0

1.15

1.1

1.2m

26 years

Fig. 3. Cracking maps of beams B2Cl2-1 and B2Cl2-2 after 26 years of storage in corroded environment (crack widths in mm).

spalling. Nevertheless, the tensile bars are highly corroded as were shown in Fig. 4. In the compressive area, the general cracks appeared but they had not interconnected throughout the surface along the bars, the distribution was random, either near the support or in the middle.

3.1.2. Spalling of the concrete cover According to [20], when the corrosion crack reaches 1.0 mm, the spalling of the concrete cover begins to happen. For both corroded beams, many spalling zones appeared in both the tensile surface and the compressive surface, which exposed the steel to the aggressive environment directly. This contributed to the accelera-

tion of corrosion including pitting which appeared in large area of the bottom bars (Fig. 4).

3.2. Corroded tensile bars 3.2.1. Corrosion map Following the mechanical tests, the bottom steel bars of the corroded beams were extracted by breaking the concrete. The corrosion distributions of the tensile bars were drawn carefully in upward view and downward view respectively, including the local corrosion and general corrosion. The corrosion maps of the tensile bars were drawn in the bottom (downward view) and top (upward view) faces of the beams

Fig. 4. Corrosion maps for the tensile bars (mm).

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(a) The failure points

(b) The cross-section with pitting corrosion

Fig. 5. The bottom bars in the short-span tests at the location of failure.

Fig. 6. Loss of the cross-sectional area of the longitudinal tension bars in B2Cl2-1.

(Fig. 4). In the bottom face, due to the low concrete cover (10 mm), general corrosion had progressed almost throughout the whole length of the bars. Pitting corrosion was also visible, but distributed randomly. Especially in the spalling areas, pitting corrosion was in evidence all over the exposed surface. In the upper part of the circumference, both the generalised and pitting corrosion were less serious than those at the lower part of the circumference due to the longer path for chloride ingress. However, at the failure locations in the load tests, both of the tensile bars at the bottom face were affected by serious pitting corrosion (Fig. 5). In fact, by comparison with the cracking maps (Fig. 3), the spalling in B2Cl2-1 was more serious than B2Cl2-2, which led to the tensile bars being more corroded. 3.2.2. Loss of cross-section of the tensile bars The corroded bars were cleaned carefully with Clarke solution to separate the corrosion products from the sound steel. The residual diameter of the tensile bars was measured in two ways: by vernier calliper and by residual mass. The vernier calliper with an accuracy of 0.02 mm gave the depth of pitting corrosion which corresponded to the difference between the initial diameter and the minimal residual diameter. However, the residual cross-section with pitting corrosion was rather irregular as shown in Fig. 6b. If the residual diameter obtained the vernier calliper was used to calculate the residual cross-section, only the part of the steel hatched in Fig. 3 was considered. As a result, the loss of cross-section was significantly over-estimated. Therefore another measurement by the residual mass of the corroded bars was carried out. The tensile bars were cut into coupons and all the coupons were short enough so as to make sure that the loss of cross-section was approximately the same and equably distributed throughout the length of the coupons. The length of the coupons was dependent on the corro-

sion distribution and varied from 130 mm to as small as 5 mm; the smaller value corresponding to positions of deep pitting corrosion. therefore corrosion measurement by loss of mass took account of pitting corrosion, so there was no need to use a pitting factor, which would be necessary if an average measurement was done on a whole length of bar. Compared with the results made from the vernier calliper, the cross-section calculated by residual mass of small coupons was closer to the true residual cross-section than the one calculated from the loss of diameter measured by the vernier. Therefore the results from the residual mass were adopted for the longitudinal bars in the following discussion. The original mass of all the sections of the steel bars can be calculated by Eq. (1). And then the average loss of cross-section for the small sections of the corroded bars could be calculated by the following formulation:

m0 ¼ q  As  L m0  m DAs ¼  As m0

ð1Þ ð2Þ

where q (g/cm3) is the density of the steel bars, it is considered to be 7.85 g/cm3. L (mm) is the length of each small part of the steel bars, measured by Vernier Calliper. DAs (mm2) is the average loss of the cross-section of the corroded bar over the small part length. As (mm2) is the nominal cross-section of the steel bars. m (g) is the residual mass of the small parts of the corroded bars. m0 (g) is the nominal mass of the steel bars. The loss of cross-section of the corroded longitudinal bottom bars in the two short corroded beams was calculated using formulae (1) and (2) over the length of the bars (Figs. 6 and 7).

Loss of cross-section (mm2)

W. Zhu et al. / Engineering Structures 56 (2013) 544–554

549

Stirrups

Location (mm) Fig. 7. Loss of the cross-sectional area of the longitudinal tension bars in B2Cl2-2.

The main trend of the loss of the diameter was similar between the front bottom bar and the back bottom bar. The three failure points occurred at the location of peak area loss as shown in the graphs, rather than at the location of the stirrups. It should be noticed that high levels of corrosion in the longitudinal bars were not generally found at the location of stirrups.

3.2.3. Loss of diameter of the stirrups The residual diameter of the stirrups was measured by Vernier Calliper. By comparing with the nominal diameter of 6 mm, the percentage of diameter loss is showed in Fig. 8. The stirrups also exhibited corrosion. However, the distribution was variable. For B2Cl2-2, the corrosion was concentrated in the top horizontal legs of the stirrups. However, for B2Cl2-1, most of the corrosion appeared in the bottom leg. There was one failure of a stirrup though the failure point was not in the vertical leg as might be expected. It took place in B2Cl2-2 but in the middle of the bottom leg for the third stirrup, with a 67% loss of diameter. This is thought to have been a secondary failure effect due to the slip of the two tensile bars being significant. The stirrups and longitudinal bars had been welded together before the beams were casted so that with differential slip between the two tensile bars the stirrup was tensioned by the distortion in the middle of the bottom leg at the location with significant pitting corrosion.

3.3. Tensile tests of the steel bars Mechanical tests on tensile bars were also carried out, including the bars from the corroded beams and the control beams, so as to assess the influence of corrosion on the mechanical behaviour. The force–displacement and stress–strain curves of the tensile experiment are shown in Fig. 9, including the results of two noncorroded bars and three corroded bars. In calculating the stress, the effective cross-section of the tensile bar obtained from the residual mass was used. The corrosion had played an important influence in the mechanical properties of the tensile bars, especially in the ultimate capacity and the ductility. By comparing with the non-corroded bars, the ultimate strength of all the corroded bars was improved by about 30%, and reached 760 MPa. The ductility of the corroded bars was reduced more than 50%. Moreover, there was no plateau in the post-yielding stage, as in the case of the non-corroded bars. However, for the corroded tensile bars, the yield value was about 600 MPa, while 530 MPa for the non-corroded bars. It is important to note that although the ultimate strength of the corroded bars was higher than that of the non-corroded bar the ultimate force which the corroded bars could resist was still far inferior to the non-corroded bars. The reason for this was the large loss in cross-section of the corroded bars. The pitting corrosion concentrates the stress and strain sharply in the corroded bars. Therefore the ultimate ductility was more or

Fig. 8. Diameter loss of the stirrups by percentage in B2Cl2-1 and B2Cl2-2.

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W. Zhu et al. / Engineering Structures 56 (2013) 544–554

70

Force (kN)

60 50 40 30

Corroded bar 1 Corroded bar 3 Non-corroded bar 2

20 10

Corroded bar 2 Non-corroded bar 1

0 0

2

4

6

8

10

12

14

16

Displacement (mm)

(a) The force and displacement curves of the tensile tests

(Fig. 10). The load and deflection at the mid-span of the beams were recorded throughout the loading process, the latter by a linear variable differential transformer (LVDT) with an accuracy of 0.01 mm. During the load test, four other LVDT’s were fixed at the ends of the concrete beam so that the sensor touched the ends of the bottom bars in Fig. 10b, where support A and support D correspond to the two ends showed in Fig. 3, in order to obtain detailed information of the slip behaviour of the bars. For these sensors, the accuracy was 1  104 mm. At the end which was not sawn a small hole was drilled in the concrete so that the LVDT’s could be located on the end of the reinforcing bar.

800

Stress (MPa)

700

4.2. Crack propagation and failure

600 500 400 300

Corroded bar 1

Corroded bar 2

200

Corroded bar 3

Non-corroded bar 1

100

Non-corroded bar 2

0

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Strain

(b) The stress-strain curves of the tensile tests Fig. 9. The results of the tensile tests for the tensile bars.

less dependent on the shape and the intensity of the pitting corrosion, and the ultimate strain in the corroded bars varied from each other and from the bars in the control beams. The results for the tensile test (Fig. 9 and Table 3) showed that the corrosion had a detrimental influence on the mechanical behaviour. The yield strength and ultimate strength of the corroded bars varied from the non-corroded bars. And all the corroded bars had almost the same ultimate strength, but the ultimate strength was improved about 30% by comparison with the non-corroded bars, which was quite a surprising result. The strain of the tensile bars varied from 0.025 to 0.035, much smaller than that of the non-corroded bars with a value over 0.07, which meant that the ductility of the tensile bar was greatly influenced by the corrosion. The second feature of the tensile tests was that the curves of the post-yielding behaviour for the corroded bars showed more hardening than the non-corroded bars. The post-yielding stiffness of the corroded bars was definitely superior to that of the control bars, though the values varied from each other.

4. Load testing of beams 4.1. Test arrangement The load tests on the short-span beams were performed by increasing the mid-span load monotonically up to failure

The behaviour under load and the failure modes of the corroded beams were different from those of the control beams. For the two control beams without any previous damage, the first cracks appeared on the side faces of the beam. These were either vertical or inclined at a slight angle and started at the bottom close to mid-span and progressed upwards toward the location of the applied load (Fig. 11). All the cracks propagated in both width and length, and then one main crack was formed, extending from the bottom of the beam to the loading point. With the crack widening progressively, failure occurred quite suddenly with the inclined crack opening wide. Surprisingly all of the main cracks appeared in the half of the beam with more stirrups. In Beam B2T-2, simultaneously there was rupture in one of the main tension bars and two vertical legs of the stirrup, the latter close to the tension bar (Fig. 12). For the corroded beam B2Cl2-1, the main crack was almost vertical along a stirrup nearest to the loading point, i.e. at mid-span of the short beam. The crack developed gradually from the bottom towards the top of the beam. As failure approached, the rate of deflection with load increased due to yielding of the tensile bars. The beam failed due to the rupture of the two tension bars: one near the support and the other at the crack at mid-span of the beam. For the corroded beam B2Cl2-2, the main cracks divided into two branches on the sides of the beam. One crack inclined gradually in the diagonal direction at first, while with the load increasing, another transverse crack appeared gradually, parallel to the stirrup in the middle of the span. As the load increased, the concrete cover to the bottom reinforcement became delaminated from the parent concrete as illustrated in Fig. 11. At last, the beam failed when the front side tension bar ruptured at the mid-span. The rupture of tension bars near the mid-span showed that the post-yielding behaviour played a dominant role in the mechanical test of the corroded beams, and they finally ruptured after the beam had deflected considerably. The detailed information about the mid-span deflections of the four short-span beams was recorded together with load; the load–deflection curves are shown in Fig. 13.

Table 3 The mechanical properties of the corroded tensile bars and the non-corroded bars. Corroded bars

Yield strength (MPa) Ultimate strength (MPa) Length of the tensile bar, L0 (mm) Ultimate elongation, dL (mm) Ultimate strain, e Residual diameter (mm) Residual cross-section (mm2) Loss of cross-section %

Non-corroded bars

Corroded bar-1

Corroded bar-2

Corroded bar-3

Non-corroded bar-1

Non-corroded bar-2

625 770 222 7.76 0.033 8.56 78.34 30.69

630 760 201 6.32 0.031 9.26 74.29 34.28

628 772 201 4.86 0.024 10.70 89.87 20.50

532 592 200 14 0.07 12 113.04 –

539 601 201 16.5 0.08 12 113.04 –

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W. Zhu et al. / Engineering Structures 56 (2013) 544–554

load

slip test

slip test 100

475

475

100

1150

(a) The loading system (dimensions in mm)

(b) The support A/D

Fig. 10. Three-point loading system and slip test.

Fig. 11. The failure points and the main cracks of the tests (dimensions in mm).

Shear failure mode with one tensile bar and srrup broken

c

Load (kN)

Pure shear failure mode 160 140 120 100 80 60 40 20 0

Pure bending failure mode with large spalling

0

5

10

15

20

Bending failure mode with shear effect

B2T-1

B2T-2

B2Cl2-1

B2Cl2-2

25

30

35

Deflection (mm) Fig. 13. Load–deflection curves of short-span bending tests.

Fig. 12. The failure point of the stirrup of the control beam.

Surprisingly, the control beams exhibited less ductility than the corroded beams. This is illustrated in Fig. 13 where the onset of yielding is clearly visible but the development of strain hardening

of the reinforcement was shortened for both control beams. A possible explanation for this is that shear failure took place just after yielding of the longitudinal reinforcement. Another aspect which applied in the case of B2T-1 was that the steel knife-edge load was acting on the beam directly. As a result, the load was more concentrated and may have made a difference between the responses of the two control beams. In the other control beam and

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160 B2Cl2-1

Force(kN)

140

FS support A FS support B BS support A BS support B

160 140

B2Cl2-2

120

120

100

100

80

FS support C

80

60

FS support D

60

40

BS support C

40

20

0 -0.25 -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15

Force(kN)

BS support D

20 0

-0.25 -0.2 -0.15 -0.1 -0.05

Slip (mm)

0

0.05 0.1 0.15

Slip (mm)

Fig. 14. Load–slip behaviour for mechanical tests of the corroded beams.

the two corroded beams a steel plate of 150 mm length and a rubber strip were used to spread the load. For the corroded beams, there was a considerable degradation of concrete cross-section (spalling, cracking), a loss of cross-section for longitudinal bars (tension and compression) and for stirrups. Usually the corrosion is assumed to lead to a more brittle behaviour of corroded RC elements. But as is shown in Fig. 11, the degradation due to corrosion led to an increase of ductility but with a small reduction in the ultimate capacity. When the corroded beams reached the yield point, the capacity still increased gradually with the development of large deflections until failure, which showed that the corroded beams retained good post-yielding characteristics while the increase of the capacity for the control beams was quite limited once the yield point was achieved. These surprising results could be explained by a change in failure mode. Indeed, if a nonlinear model was used to compute the ultimate deflection of a short span beam in a virtual bending failure mode [21], using a steel elongation of 7% corresponding to ultimate strain of steel bars (Fig. 9), a ultimate deflection of 50 mm with would be found. Then the same model applied to the corroded beam taking into account the loss of cross-section and the reduction of ultimate strain at 3.5% would lead to an ultimate deflection of 25 mm which was closed to the experimental one. Thus the loss of steel ductility led to a loss of beam ductility for the same failure mode. However, the difference of failure mode between the corroded and control beams, i.e., bending versus shear, meant a more brittle behaviour of the control beam even though the steel bars were more ductile. Nevertheless, for the control beams the failure mode involved interaction between flexure and shear, not untypical of short-span beams. For example yielding of the reinforcement began to take place but before strain hardening could develop shear failure occurred in the reduced compression zone. However with advanced corrosion the flexural capacity is reduced more than the shear capacity so that the failure mode changes to pure flexure.

4.3. Slip of the tensile bars Slip of the longitudinal bars was monitored by measuring movement at the ends of the bars. Neither of the control beams exhibited slip of the bars before failure although just at failure slip of the bars increased sharply, which was not considered to be the cause of the failure of the beam. For example, for the B2T-2, during the failure process, the concrete cover near the support D became loose (Figs. 11 and 15), the bond between the bars and the concrete decreased sharply, and slip of the tension bars followed rapidly.

Corroded beam B2Cl2-2 showed the same behaviour as the control beams. However, B2Cl2-1 which had a large area of spalling near the support B, as shown in Fig. 3, exhibited slip of reinforcing bars from an applied load on the beam of 80 kN. The slip of tension bars in beam B2Cl2-1 was quite different from other beams. When the load reached 80 kN, slip began to occur, as evidenced by the load–slip curve in Fig. 14, and this coincided with a change in slope in the load–deflection relationship of B2Cl2-1 (Fig. 13). The slip increased rapidly which could also have contributed to the ultimate deflection of the beam. In spite of the slip, the load which the beam was able to support increased slightly in a manner similar to Beam B2Cl2-2. Therefore even with extensive spalling near support B there appears to have been sufficient anchorage to allow the beam to behave in a ductile manner. 4.4. Result analysis In order to further investigate the failure mechanisms of the short span beams, the flexural capacity (yield and ultimate) and shear capacity of the beams were calculated and compared with the experimental results. The bending value and the shear value were determined with the help of Eurocode 2 [22]. In all cases partial safety factors were taken as 1.0. 4.4.1. The yield capacity of the beams With the help of Fig. 9, the yield of the corroded and non-corroded tensile bars was assumed to be 600 MPa and 530 MPa respectively. The strength of the stirrups was considered the same as the tension bars for the corroded beams and control beams respectively. The cross-section loss of the corroded stirrups was considered as 10% for shear calculation (Fig. 8) while 31% and 25% loss of cross-section of the tension bars was assumed for B2Cl2-1 and B2Cl2-2 respectively (Figs. 6 and 7). The shear capacity was calculated using the capacity of the concrete, Vdr,c, and the capacity of the stirrups, Vrd,s. Although the spacing of the links was approximately equal to the effective depth, the inclusion of the latter was justified on the grounds that the experimental evidence was that the cracks crossed one link and in the case of Beam B2T-2 stirrups were actually fractured at failure. In Table 4 the experimental results are compared with theoretical predictions for flexural and shear capacity based on the steel yield strength. For the control beams, both the theoretical flexural capacity and shear capacity match the experimental value closely, with the shear capacity only a little higher than the flexural capacity. For the corroded beams, the theoretical flexural capacity is significantly lower than the theoretical shear capacity indicating that flexural failure will govern. The flexural capacity also agrees well

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Force (kN)

160

140

140

B2T-1

B2T-2

120

120

100

100

FS support A

80

FS support C 80

FS support B

60

FS support D 60

BS support A

40

BS support C 40

BS support B

20

BS support D 20

0

0 -0.25 -0.2 -0.15 -0.1 -0.05

Force (kN)

160

0

0.05 0.1 0.15

-0.25 -0.2 -0.15 -0.1 -0.05

Slip (mm)

0

0.05

0.1 0.15

0.2

Slip (mm)

Fig. 15. Load–slip behaviour for mechanical tests of the control beams.

Table 4 Comparison of the results of the yield for the beams.

B2T1 B2T2 B2Cl2-1 B2Cl2-2

Theoretical shear capacity, PPV (kN)

Theoretical bending capacity, PPB (kN)

Net Experimental PT/PP theoretical capacity, capacity, PT (kN) PP (kN)

144.4 149.0 150.2 150.2

126.8 136.9 101.6 110.9

126.8 136.9 101.6 110.9

137.0 145.0 88.0 110.0

1.08 1.06 0.87 1.00

Table 5 Comparison of the results of the ultimate capacity for the beams.

B2T1 B2T2 B2Cl2-1 B2Cl2-2

Theoretical shear capacity, PPV

Theoretical bending capacity, PPB

Net theoretical capacity, PP

Experimental capacity, PT

PT/PP

143.7 156.2 167.4 167.4

140.7 153.7 128.0 139.6

140.7 153.7 128.0 139.6

138.0 159.0 123.0 148.0

0.98 1.03 0.96 1.06

with the experimental results. This tends to confirm the earlier observation in Section 4.2, i.e. that the control beams reached the yield moment in bending but because the shear capacity was only slightly greater a shear failure developed soon after yielding and before significant ductile behaviour had developed. With corrosion the corroded beams had a significantly reduced flexural capacity because of the reduced area of the tension reinforcement. The shear capacity of the stirrups is reduced but the shear capacity of the concrete is barely affected so the overall reduction in shear capacity due to corrosion is quite modest, provided there is no significant loss of anchorage. 4.4.2. The ultimate capacity of the beams The same calculation was carried out but based on the ultimate strength of the steel obtained from the tensile tests (Fig. 9). According to Fig. 9, the values for the control beam were fsu = 590 MPa. The stirrups were not actually tested and so the same value was assumed. For the corroded beams, the ultimate stress of fsu = 760 MPa was used. The same loss of the cross-section of the corroded bars was also considered as was referred above. The theoretical flexural capacity and the shear capacity have also been calculated as before and are presented in Table 5. According to Table 5, the theoretical bending predictions agree well with the experimental results. The calculated shear capacity is stronger than the experimental results in all cases although the calculated shear capacity of control beams is very close.

In determining the shear capacity after the main reinforcement has yielded no account has been taken of any reduction in the concrete shear component due to widening of cracks. This is likely to reduce the shear capacity and account for shear failure occurring after yielding but before full ductility can develop. 5. Conclusion An experimental investigation of the behaviour of corroded short span reinforced concrete beams has been reported. The long term corrosion process used is representative of the real structural conditions, concerning both environmental and mechanical aspects. Two seriously corroded short-span beams were tested until failure by three-point loading system. Another two beams of the same age but without corrosion were also tested as the control ones. 1. The corroded steel bar exhibit a less ductile failure in tensile tests due to corrosion pitting and the ultimate elongation is reduced by more than 50% in comparison with uncorroded reinforcement bars. 2. The failure modes of the corroded beams were different from those of the control beams. The non-corroded specimens exhibited a brittle shear failure. In contrast, both corroded beams failed in flexure with a ductile response despite the fact than corroded steel is less ductile than non-corroded steel. 3. The corrosion of reinforcement can lead to a change in the failure mode of short beams from shear to flexure for the corroded beam because the reduction in cross-section of the reinforcement has a larger effect on the flexural capacity than on the shear capacity. This consequence of corrosion increases the complexity for modelling strategy to evaluate the load-bearing capacity. 4. Despite large corrosion cracks and spalling near the support, the anchorage of tensile bars was still sufficient to allow bending failure in the corroded beams. This shows that natural corrosion, which is not constant all along the bar and around the perimeter, is less detrimental to bond properties than might be assumed from accelerated corrosion by impressed current. 5. For the beams tested, the prediction of both shear and flexural bending capacity with simple analytical models was in a good agreement with experimental results. The assessment of a deteriorated RC structure required knowledge of the geometry and cross section of steel, the material properties and the possible failure modes. This paper had studied the flexural and shear failure modes. The measurement of the steel cross section had been carried out in the laboratory after testing the beams to failure. For practical applications, it was not easy to measure

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the loss of cross section due to corrosion on-site. Nevertheless, there are some possible approaches to evaluate the loss of cross-section. As an example, it is possible to predict the loss of cross-section through the measurement of the width of corrosion cracks on the concrete surface [23–26]. Alternatively, the modelling of the loss of steel cross section due to corrosion could be based on corrosion kinetics [25].

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