BENDING PERFORMANCE OF CORRODED RC BEAMSUSED FOR MANY YEARSMichiaki OYADO1, Toshiyuki KANAKUBO2 Yasuhiko YAMAMOTO3,Toshiyuki KUROIWA4
ABSTRACT : It is essential to check the safety performance of existing reinforced
concrete (RC) structures because the number of existing RC structures that have been inservice for many years is still increasing. However, an appropriate method for evaluating
the effect of severely corroded reinforcing bars on the safety performance is notestablished yet. This is because only insufficient knowledge is available with regard tothe effect of the corrosion on the mechanical properties. This paper presents the resultsof an experimental investigation conducted to examine the effect of corrosion ofreinforcing bars on the mechanical performance and behavior of a RC beam under static
loading. The beam was taken from a real railway frame structure that had been used as aviaduct for approximately 70 years. Close examinations of the beam before and after theloading test revealed that the reinforcing bars in the beam had been corroded to a large
extent due to the carbonation of concrete, and the corrosion had induced cracking andspalling of the cover concrete. When a bending force was applied to the beam, the maincrack at the mid-span shows about 50 mm width at ultimate because of the debonding of
the longitudinal bar due to corrosion and failure of cover concrete. The corroded
reinforcing bar is modeled by tensile test results and measured values of distribution ofcross section investigated by caliper and 3D scanner. Using this analytical model of thecorroded bar, fiber analysis for the beam specimen is carried out. It was indicated thatthe bending strength of the corroded RC beam could be estimated to a satisfactory
accuracy if the distribution of net cross section of reinforcing bar excluding the corroded
portions was taken into account in its calculation.
KEYWORDS: existing structure, corrosion, net cross section, bending strength, carbonation
1. INTRODUCTION
To utilize existing infrastructures effectively, it is important to check the effect of the deterioration.
Therefore it is essential to check the performance of existing reinforced concrete (RC) structures. In
many cases the effect of the corrosion of the reinforcing bar is checked under the viewpoint of theprevention from stain the appearance, but it is also important to evaluate the safety performance when
the reinforcing bars is seriously damaged by the corrosion. On the other hand, it should be consideredthat existing structures are designed in accordance with the old design standard and constructed using
old materials and production methods different from those of today. This paper was aimed to present
an typical example of the evaluating test for old structural members based on the old design code
deteriorated after its long-term service.
2. TEST PROGRAM
Test program was conducted as follows: For the first step, inspection survey of the appearance and themeasurement of the position of the reinforcing bars were carried out for the specimen using the nondestructive bar detector. In the next step, static loading test was carried out to the beam specimen after

1

Concrete Structures, Railway Technical Research Institute, Japan.
Graduate School of Systems and Inf. Engineering, University of Tsukuba, Japan.
3
Graduate School of Systems and Inf. Engineering, University of Tsukuba, Japan.
4
Technical Research Institute, Tokyu-Construction, Japan.
2

providing sensors to measure the strain or the displacement. Finally, the loading test result wasverified by the analysis after required checks on the deterioration of concrete and reinforcing bars.
3. TEST SPECIMEN

Fig.3 shows the bottom view of the beamspecimen. The side and bottom surface of thespecimen was stained with the sand from therailway girder upheld. Thus it was conceivablethat the water was provided continuously to thisbeam and induced the corrosion of the tensile
reinforcing bar at the mid-span. The cover mortar
without gravel was spalled and stripped off thebeam, while the ordinary concrete is arranged inanother region of the cover concrete. In addition,

Shoe bearing

Utility pole
6500

Cut off line

G.L
Numbers areapproximative value（unit：mm）

Fig.1 Outline of objective structure

ＣＬ 5772
2286
2014
φ 13
φ960 0 Haunch
Lo a d i ng a n d
su p p or t i ng p oi n t s

φ 224001067

400

610
6 0 490 60

10 7Re t r o fi t t e d
co n c r e t e
Si d e A: O b se r b e d
Si d e B: M e as u r e dwh e n lo a d i ng t e s t

Side BSide A60Fig.2 shows the details of the specimen and thenames of the sides and reinforcing bars referred in
the following chapters. The reinforcing bar
arrangements shown in this figure was verified atseveral major positions with the use of anelectromagnetic wave radar and an electromagnetic induction meter. The followings can bepointed out comparing with the today’s conventional railway viaduct:(1)The ratio of tensile reinforcement is 0.37
percent, which is less than that of the ordinary
railway structures today.
(2)The lap splices with the semicircular hook intensile reinforcements are provided at the samecross section at the quarter span of the beam. At
present, the lap splices are not provided in generalat such positions subjected to high stresses
because the lap splices in bar reinforcement tend
to become weak spots.
(3)The tensile reinforcing bars are not thedeformed bar but the round bar, and they are bent
up at the beam-column joints. Bending momentat the ends of the beam caused by the seismic
load is transmitted by the reinforcement at thehaunches.
(4)Four longitudinal reinforcing bars are respectively arranged and encircled by stirrups separately.

10000

106 7900

Fig.1 shows the outline of the objective structure.
This is a RC rigid-framed pier used as a railway
viaduct in a warm urban area away from coast.
The specimen is beam portion of this structure cut
off by a wire saw at the line of the column sidesurface as shown in Fig.1. Shoe bearing provided
on the top of the columns instead of the beam
segment sustained the weight of the girder. The
structure has been in service for nearly 70 years
since its construction.

un i t ： m m
６ ５ ４ ３ ２ １ No . o f b a r s

Fig.2 ofDetails
Fig.2 Details
beamof beam specimen
specimena very large gravel (approximately
300mm or larger in diameter) existed atthe particular position. It is supposedthat these regions had existed since thecompletion considering the existence ofthe continuous vestige of the mold joint
recognized at the bottom surface of thespecimen. These situations suppose thepossibility that the structure had been
constructed in the following procedure;
large gravels were used as the bar
spacer to keep bars at the arranged
position, the aperture was filled withpoor mixed mortar and the ordinary
concrete was cast to the remaining
major part of the beam section above
the longitudinal reinforcing bars.

Side A

Mortar(without gravel)

Concrete*** Vestigeof mold joints
Between haunches
Fig.3 Bottom view of the beam specimen

4. LOADING TEST
4.1 Loading method
The beam specimen was tested by four-point
static bending to determine their load-deflection
curves. Fig.2 shows the test setup and theloading / supporting points. The length of thepure bending region is 800 mm, and shear spandepth ratio is approximately two. Because
supporting points are located at the haunch, newreinforcing bars are arranged and new concreteis cast in this haunch portion to get stable
reaction force. The concrete strength of newly
cast is approximately 29.4 N/mm2 at the age of
16 days. To improve insufficient anchorage ofthe longitudinal reinforcement, small steel plates
were welded at the end of the reinforcing bars.
The beam specimen was loaded under theloading program shown in Fig.4, where Pycal
indicates the calculated value of the yieldP, δ failure3δy2δyP/2(a)Pycal(b)1.5δy(c)2δyP/2

δ

1.5δyPycal(3/4) Pycal(1/2) Pycal(1/4) Pycal(d)after failure
Fig.4 Loading program

Fig.5 Crack pattern caused by loading teststrength and δy indicates the deflection at the yield.
100Carbonation depth (mm)

4.2 Test results

60
40200Thickness of alkaline
section (mm)After yielding of the longitudinal reinforcement,
the deflection at the mid-span has been proceeded.
Though compression failure at the top side wasobserved, sudden failure was not observed. The
number of the cracks is only two. The main crack
at the mid-span shows approximately 50 mm
width at the last loading step. Fig.5 shows thecrack pattern observed in the loading test. The
reason of a small number of the cracks is assumed
to be that the bond strength between thelongitudinal reinforcement and the concrete wasreduced by the followings. (1)The longitudinal
reinforcement is not the deformed bar but theround bar. (2)The cover concrete at the mid-spanalmost falls because of the corrosion. The lap
splice of the longitudinal reinforcing bar does not
fail in this loading test.

MortarConcrete80

123
4Bar No.

561234Bar No.

5660
40
200-20-40

Fig.6 Results of carbonation test

5. DETERIORATION CHECKS
5772

268

1795

(2)

(3)

268

435

(1)
Side A (4)

(5)

(6)

st-3

st-2

1300
1300
Test hammer

st-1
core Φ 75
core Φ 100

Fig.7 Test position of concrete strength (unit:mm)

30.0
20.0

st-3
ave.

st-2

st-3
ave.

(4)
(5)
(6)
ave.

st-1
st-2

10.0
0.0
(1)
(2)
(3)

(N/mm )50.0
40.0
2Fig.7 shows the test position of theconcrete strength. It was measured
by the compression test of thecylindrical core samples extracted
from the beam specimen whose
diameters were about 75mm or
100mm. Fig.8 shows the test resultsof the concrete strength comparedwith the estimated value by theSchmidt Test Hammer. Concrete
strength measured by the compress-Stirrup 1350

Compressive strength

Fig.6 indicates the investigation
results of carbonation of the concrete,
which was tested by the solution of
the phenolphthalein splayed at thefresh surface exposed by the partial
demolition removal of the coverconcrete.
Thickness of alkaline
section (which was calculated by thedifference between the thickness ofthe cover concrete and thecarbonation depth) at the place of thecover mortar is smaller than 10 mm,
which leads the reinforcing bar tocorrode. It is evident that the beamspecimen had been corroded bycarbonation of the cover concrete ormortar.

Fig.8 Test results of the concrete strength

ion test is larger than the estimated value ofthe test hammer. The averaged test result
for the elastic modulus is approximately
18.4N/mm2, which is smaller than thecalculated value by the design code.

1.2
1.1

Net area ratio

1.0

Fig.9 shows the net area ratio of the crosssection of the corroded longitudinal
reinforcing bar excluding the corroded
portions, which was measured by a caliper
after the loading test and removing thecover concrete. The net area ratio iscalculated by the cross sectional area of thecorroded reinforcing bar divided by that of
the non-corroded bar (diameter is 22 mm).
The cross sectional area is assumed to beequal to the area of the circle whose
diameter is to be the minimum value
measured by the caliper in every direction.

-2000

600

800

Measuring position (mm)(a)example of a test piece

Measured by 3D scanner

Net area ratio

caliper
3D scanner400

246-1000

01000

2000

Fig.9 Net area ratio (measured by caliper)

measured by

200

135

Measured position referring the center of the beam (mm)

0.8

00.7

0.5

1.0

0.2
0.0

0.8

0.6

1.2

0.6
0.4

0.9

1.2
1.0

y = 0.99 x

0.8
0.6
0.4
0.2

Net area
ratio

0.00.0 0.2 0.4 0.6 0.8 1.0 1.2

Measured by caliper(b)Averaged in each test piece

Fig.10 Comparison of values measured by caliper and 3D scannerFig.10 shows the comparison of the measured values between caliper and 3D scanner. The 3D
scanner can digitize the shape of the objective measuring article set on the turn-table by tracing theposition of the laser beam spot. According to the Fig.10(a), the net area ratio measured by caliper has
sufficient accuracy. However, the measured area by caliper is totally a little larger than that by the 3D
scanner according to the Fig.10(b).
According to Fig.9, the net area ratio of the bar No.1 located at the corner of the cross section (see inFig.2) is small. It is the effect of the water provided from the railway girder upheld. The net area ratioof the cross section averaged in the 400 mm span at the beam center is approximately 0.85.
6. ANALYSYS TO VERIFY LOADING TEST
In order to verify the load-deflection relationship of the loading test of the beam specimen, ananalytical program is conducted considering the material characteristics of the deteriorated concrete
and reinforcing bar. The test results of the concrete strength for the sampled core and the reduced
strength of the reinforcing bar considering the effect of the corrosion have been adopted.

400

0.6

300

Frequency ratio

2stress σ (N/mm )dot:
exampleof test result
dot: anexampleof experimentline: bar analysis

200△ : model for beam analysis

1000

measured by caliper
analysis model

0.5
0.4
0.3
0.2
0.1
0.006000

12000

0.5

18000

0.7

0.8

1.01.2strain ε (µ)

Area remaining ratio

Fig.11 Stress-strain relationship of the corroded
reinforcing bar

Fig.12 Distribution of net cross sectionof the corroded reinforcing bar

analysis

800

600

load P (kN)load P (kN)

800

400200analysis600
400200experimentexperiment
00020

4060

deflection
δ (mm)
Deflection
δ (mm)(a) Gloval view

8002468deflection δδ(mm)Deflection(mm)(b) Yield point

Fig.13 Load deflection curve of the beam specimen

To determine the stress-strain relationships of the corroded reinforcing bar, tensile strength test andanalysis has been carried out following the previous procedure (Oyado, 2006). In the tensile strength
test, targets were attached to both ends of the test span to measure the elongation of the bar. The
dotted line in Fig.11 indicates the test result of the stress-strain relationship. The stress is the applied
load divided by the nominal cross-sectional area without corrosion, and the strain is the measured
elongation divided by the gauge length (352mm) between the targets. The solid line indicatesanalytical result calculated by the infinite element method. In this analysis, the reinforcing bar ismodeled dividing into infinite elements along the axial direction, and the stress-strain relationshipobtained from the non-corroded bar is adopted for each element. The cross sectional area for eachelement is assumed to have the distribution shown in Fig.12, which corresponds to the observed
results shown in Fig.9.
Fig.13 indicates the load-deflection curve of the beam specimen obtained by the loading test and thefiber analysis. The assumption that the fiber strain is proportional to the distance from the neutral axis
is adopted for the analysis. According to Fig.13, the test result after the yield point and ultimate
strength is almost equal to the analysis result. However, the test result of the load-deflection curve
before the yield point is different from the analysis result. It is assumed that the difference was causedby the crack pattern and the reduction of the bond strength between the reinforcing bar and theconcrete.

7. CONCLUSION
In order to clarify the effects of the corrosion on the mechanical performance of the RC beams, theloading test and analysis were carried out for the old RC beam specimen that was in service for 70years. When a bending force was applied to the beam, the main crack at the mid-span shows about 50mm width at ultimate because of the debonding of the longitudinal bar due to corrosion and failure ofcover concrete. The corroded reinforcing bar is modeled by tensile test results and fiber analysis forthe beam specimen is carried out. It was also indicated that the bending strength of the corroded RC
beam could be estimated to a satisfactory accuracy if the distribution of net cross section of reinforcing
bar excluding the corroded portions was taken into account in its calculation.
REFERENCES
Oyado,M., Kanakubo,T., Yamamoto,Y., Sato,T. (2006). “Influence of Corrosion of Reinforcing Bars
on Bending Performance of Reinforced Concrete Members.” JSCE Journal of Materials, Concrete
Structures and Pavements, Division E, V.62, No.3, August, pp.542-554. (in Japanese)