10th International Conference on Short and Medium Span Bridges

Quebec City, Quebec, Canada, July 31 – August 3, 2018

EFFECTS OF TENDON RUPTURE AND RE-GROUTING ON THE FLEXURAL BEHAVIOR OF PRESTRESSED CONCRETE BEAMS

Wijayawardane, Isuru S.K.1,4, Mutsuyoshi, Hiroshi2 and Yokota, Toshihiro3

1 Saitama University, Japan2 Saitama University, Japan3 Saitama University, Japan4 isuruwijayawardane@gmail.com

Abstract: Since 1960, a large number of prestressed concrete (PC) bridges constructed in Japan and some of them located in severe corrosive environments undergo corrosion and break of the PC tendons due to insufficient filling of grout inside the sheath. This paper describes the flexural behavior of the PC beams after rupturing the prestressing tendons, and the effect of re-grouting of the PC beams, which are having ruptured tendons in the insufficiently grouted region. This type of a study will be useful to evaluate the load carrying capacity of the PC beams and repair them effectively. In addition to that, use of a non-destructive inspection method to detect the ruptured tendon location of the PC beams is also described in the paper. In this study, large-scale bending tests were carried out for PC beams. It was found that the ultimate flexural capacity of the PC beams varies according to the rupture location of the tendons, even though they had the same ruptured PC tendon area. This is because the prestressing force in the ruptured PC tendons is recovered within the bonding length. The experiment results revealed that the ultimate flexural capacity of the PC beams having ruptured tendons in the insufficient grout filling region can be improved significantly after re-grouting.

1 INTRODUCTION

Post-tensioned concrete is extensively being used for construction of bridges. However, the steel tendons inside the prestressed concrete (PC) beams can be corroded due to insufficient grout filling inside the sheath (Figure 1). Therefore, both durability and the flexural capacity of the PC beams reduce. Corrosion of tendons of improperly grouted PC beams can occur due to contact of rainwater, moisture in atmosphere, seawater, de-icing salt or any other corrosive agent. Many researchers have carried out surveys and case studies on PC bridges and reported about this issue (Miyashita 2011, Woodward et al. 2001). Depending on the residual flexural capacity of the PC beams after tendon rupture, some PC bridges can be further used with or without minor renovations, whereas some of them may need costly renovations. However, there are comparatively less studies carried out to investigate the flexural behavior of the PC beams having ruptured tendons.

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(a) (b)

Figure 1: Corrosion of tendons due to insufficient grout filling; (a) PC beam, (b) PC box girder

It is very important to identify the location of rupture of the tendons in PC beams for evaluation of the flexural capacity. There are many non-destructive evaluation techniques described in literature (Muldoon et al. 2007, Abraham and Cote 2002). Magnetic flux leakage (MFL) test is one of the non-destructive test methods which can be easily applied for simple beams. Fernandes et al. (2012) used this technique for a PC box-beam bridge and reported that, the MFL method can detect hidden corrosion and strand breaks with a sufficient amount of accuracy. In this method, the ferromagnetic property of the steel tendon is used and it detects the formation of magnetic dipoles due to tendon rupture.

The objectives of this is study are, investigate the flexural behavior of the PC beams having ruptured tendons at different locations, and use of magnetic flux leakage test method to detect the location of rupture of the tendons in PC beams.

2 EXPERIMENT PROCEDURE OF BEAM FLEXURAL TEST

2.1 Details of Materials and Test Specimens

The PC beams used in this study are 2,400 mm in length and 200 mm in width and height. The cross-sectional details and elevation of a specimen are shown in Figure 2 and Figure 3, respectively. All the test specimens were post-tensioned with three of 7 mm diameter (7) steel tendons (Figure 2). The mechanical properties of the steel tendons complied with the Japanese Industrial Standards (JIS) G-3536 (2014). The PC tendons consist of indented single wires and according to JIS G-3536, they belong to the category of SWPD1L. The cross-sectional area of the tendon and the material properties are given in Table 1. In all the specimens, 18 mm diameter steel sheaths were used and the gap between the tendon and the sheath was filled with cement grout, after introducing the prestressing force. Altogether there were eight PC beam specimens (CASE0 to CASE7) used in this study and the compressive strength of cement grout and the concrete of each beam are given in Table 2. The compressive strength of the concrete and cement grout was tested on the same day of the bending test. Prestress was introduced to all the beams until each tendon gained a strain of 5,000 μ. Details of prestressing of each specimen are given in Table 2. All the reinforcing steel used in the beams were 6 mm in diameter (D6) and they belong to the category of SD345, according to the Japanese standards. The material properties of the reinforcing bars are given in Table 1.

Insufficient grout filling

Corroded tendon

Rebar

Concrete

RebarCorroded tendons

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Figure 2: Cross-section of a PC beam (units: mm)

Figure 3: Elevation of a PC beam (units: mm)

Table 1: Material properties of steel tendons and reinforcing bars

Steel tendon (SWPD1L - 7)

Reinforcement(SD345 - D6)

Yield strength

Ultimatestrength Area Yield

strengthYoung’s modulus

σpy

(MPa)σpu

(MPa)Ap

(mm2)σsy

(MPa)Es

(GPa)

1,435 1,653 38.48 475 215

Table 2: Details of PC beams

Specimen name

Concrete strength f’

c

(MPa)

Strength of grout f'

g

(MPa)

Stress in PC tendon* σp

(MPa)

CASE0 54.9 74.4 758CASE1 56.5 91.0 809CASE2 56.6 89.8 721CASE3 57.4 93.5 782CASE4 56.3 94.1 830CASE5 57.7 89.1 743CASE6 42.7 64.4 813CASE7 42.1 65.8 794

* Prestress in the steel tendons immediately after post-tensioning.

Tendon

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2.2 Test Variables

In this study, large-scale four-point and three-point static bending tests were conducted to investigate the flexural capacity of the PC beams with the ruptured location of the PC tendons and the loading type. Figure 4 shows the loading types and the location of ruptured PC tendons in each specimen. There was no tendon rupture in CASE0 beam and it was the control specimen. CASE0, CASE1, CASE2 and CASE3 beams were tested under four-point bending, whereas CASE4, CASE5, CASE6 and CASE7 beams were tested under three-point bending. As shown in Figure 4, the flexural span and the shear span of the four-point bending setup were 1,000 mm and 500 mm, respectively. The shear span in the three-point bending setup was 1,000 mm. To simulate the rupture of the PC tendons, the tendons were mechanically cut by core-drilling from the beam’s sideways, with a 15 mm diameter cutter. After cutting the tendons, the drill-holes were filled with cement mortar. The influence of the ruptured area on the flexural capacity of the PC beams was studied by changing the number of ruptured PC tendons. CASE6 and CASE7 specimens were tested in order to study the effect of re-grouting on the flexural capacity of the PC beams. As illustrated in Figure 4, both CASE6 and CASE7 beams had an un-grouted length of 1 m and in CASE7, grout was re-injected into this region after cutting the tendons. During the bending test the load was applied using a manually operated hydraulic jack. Applied load, midspan deflection and midspan tensile and compressive strains were recorded until beam failure.

<CASE0>No Rupture(Basic Case)

<CASE1>Center-One wire

<CASE2>Center-Two wires

<CASE3>500mm from center- Two wires

<CASE4>Concentrated loading of CASE3

<CASE5>Concentrated loading between ruptured point

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Figure 4: Details of the loading and locations of the PC tendon rupture in the specimens (units: mm)

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3 RESULTS AND DISCUSSION

3.1 Strain Variation in the Ruptured PC Tendon

The difference between the strains before and after rupture of the tendons (Δεp) is described using CASE2 beam. As shown in Figure 4, in CASE2 beam, two PC tendons were cut at the center of the span. Strain gauges were attached on the PC tendons at 200 mm intervals from the ruptured location, and the strain was measured before and after the rupture (after core-drilling) of the tendons. The strain gauge data of one of the ruptured tendons of CASE2 beam is given in Table 3. Distance to each strain gauge was measured from the beam center. There were no strain gauges attached to the tendons at the ruptured location (X = 0 mm). Therefore, the strain of the tendon (before the rupture) at the rupture location was calculated based on the other strain gauges attached to the tendon at X = 200 mm, X = 400 mm and X = 600 mm.

Table 3: Strain of the tendon in CASE2 beam - before and after rupture

Distance from center

X (mm)

Strain of steel tendon (μ)

Before rupture εpbefore

After rupture εpafter

Difference Δεp

0 3,303 0 3,303200 3,219 2,228 992

400 3,009 2,931 78

600 3,682 3,672 10

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Figure 5 shows the variation of the Δεp along half length of the beam. It can be noted that the Δεp is very small (78 μ) at 400 mm, which means the prestressing force in the tendon has fully recovered at a length of approximately 57 × diameter of the tendon. In order to confirm this, a bond test was conducted. According to the bonding test results, the length of indented steel tendon for fully-bond was around 700 mm when the grout strength was 70 MPa. In CASE2 beam, when one of the three tendons was ruptured, the elastic deformation of the beam was increased due to the release of prestressing force and that caused increase of the strains in the other tendons. The increment of the strain in tendons was calculated and that was very small (about 35 μ), which is approximately 1% of the total strain after introducing prestressing force.

3.2 Effects of Tendon Rupture Location and Grout Re-injection

Figure 6 shows the relationship between the load and displacement of the test specimens (CASE0 to CASE5) and Figure 7 shows the relationship between the bending moment and displacement at the tendon ruptured location (CASE0 to CASE5). As the number of ruptured tendons increases the ultimate bending moment of PC beams (in CASE0, CASE1 and CASE2) was reduced (Figure 7). However, in CASE3 (beam with two tendons ruptured under one of the loading points), the moment capacity was higher than that of the CASE2 (beam with two tendons ruptured at midspan). This confirms that the rupture location of the tendon affects the flexural capacity of the PC beams. The moment capacity of CASE4 and CASE5 beams was significantly higher than all the other beams having ruptured tendons. This is because of the difference between the loading types, where a comparatively low bending moment was applied at the cross-section of the tendon rupture in CASE4 and CASE5 beams.

The effect of grout re-injection on the flexural capacity of the PC beams can be described using Figure 8. The flexural capacity of CASE4 is about 88% compared to that of the control specimen (CASE0), whereas those of CASE6 and CASE7 are much lower than that. This happened because the tendons in CASE4 were cut after grouting the sheath and hence, the prestressing force recovered along the bonding length.

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In CASE6 and CASE 7, the tendons were cut in the un-grouted length of the beams. Therefore, the prestressing force in the un-grouted length was lost. CASE7 beam was re-grouted after tendon cutting. Experiment results show that the flexural capacities of CASE6 and CASE7 beams are 50% and 68% in comparison to CASE0, respectively (Figure 8). Therefore, the grout re-injection is very effective if the grout unfilled length is longer, and the flexural capacity of the PC beams can be significantly improved.

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Figure 5: Variation of Δεp along half-length of the CASE2 beam (X=0 mm refers to the ruptured location of the tendon)

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Figure 7: Bending moment and displacement at tendon ruptured location (CASE0 to CASE5)

Figure 8: Bending moment and displacement at rupture location (CASE0, CASE4, CASE6 and CASE7)

3.3 Experimental and Calculated Ultimate Bending Moment of the Beams

The failure of all the specimens was due to crushing of the concrete at the beam center or near the loading point, which occurred after yielding of the PC tendons. Experimental ultimate bending moment and calculated ultimate bending moment at the ruptured section for CASE0 to CASE5 beams are given in Table 4. The ultimate bending moment of the ruptured section was calculated based on the loss of PC tendon area (due to rupture) in that particular section. At failure, the ultimate bending moment of CASE1 (beam with one tendon ruptured at center) was reduced by 72% compared to that of CASE0 (control specimen). In CASE2 (beam with two tendons ruptured at center), the reduction of the ultimate bending moment was 48.4% compared to CASE0. The calculated ultimate bending moment at ruptured section of CASE1, CASE2, and CASE3 beams is approximately same as the experimental values (Table 4). This is because, the rupture locations of the tendons in those three beams were in the bending span, where the maximum bending moment of the beams exists.

In CASE4 and CASE5 beams (tested under three-point bending setup), the bending moment at the ruptured section from the experiment (half of the ultimate bending moment) was 16.2 kN-m and 16.4 kN-m, respectively. This was agreed with the calculated bending moment at the ruptured section of CASE4. However, in CASE5, there were two rupture locations in one tendon (Figure 4). In the calculation of ultimate bending moment at ruptured section, loss of PC tendon at one location was taken into account. Therefore, the calculated ultimate bending moment of CASE5 at the ruptured section showed a significantly higher value than that of the experimental value (Table 4).

Table 4: Experimental and calculated ultimate bending moment of the beams

Specimen name

Maximum load (experiment)

Ultimate bending moment (experiment)

Ultimate bending moment at ruptured section (calculated)

P (kN) M (kN-m) M/MCASE0 (%) Mu (kN-m) Mu/MuCASE0 (%)

CASE0 145.7 36.4 － 30.8 －

CASE1 104.6 26.2 72.0 23.7 76.9

CASE2 70.6 17.6 48.4 16.5 53.6

CASE3 76.6 19.2 52.7 16.5 53.6

CASE4 64.6 16.2 44.5 16.5 53.6

CASE5 65.6 16.4 45.1 23.7 76.9

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4 MAGNETIC FLUX LEAKAGE (MFL) TEST

4.1 Test Procedure

Magnetic flux leakage test is one of the non-destructive tests used to detect the damaged steel tendons in PC beams. It utilizes the ferromagnetic properties of the PC tendons. Initially, the steel tendons of the PC beam need to be magnetized and this method can detect the N-S magnetic dipole formation at the tendon rupture location. This test is carried out using two devices 1) device with a permanent magnet and 2) magnetic flux density measuring device. The permanent magnet is used to magnetize the internal PC tendons from the beam surface. Since there are three tendons in all specimens, as shown in Figure 9(a), each tendon was magnetized by moving the permanent magnet along the beam surface. Thereafter, the magnetic flux density of each tendon was measured by moving the MFL measuring device on the beam, along the tendons (Figure 9(b)). In order to have smooth movement of the MFL measuring device, as shown in Figure 9(b), an aluminum guide rail was used. The MFL test was done prior to the beam flexural test.

The method of determining the broken point of a PC tendon using MFL measuring device is explained in Figure 10. In a permanent magnet, from south-pole to north-pole, the magnetic flux varies from a negative value to a positive value. The MFL measuring device records the magnetic flux density along a particular tendon, in which the device is being moved on. If the PC tendon is not broken, the variation of the magnetic flux density along the tendon length is similar to that of a permanent magnet (Figure 10). But when the PC tendon is broken, new north and south poles will be created at the broken point. This creates a sudden change in the magnetic flux density at that location and the graph between the magnetic flux density and distance becomes S-shape at the broken point.

(a) (b)

Figure 9: Magnetic flux leakage test method. (a) Magnetizing, (b) Measuring

4.2 Test Results

The test results of CASE2, CASE3, and CASE4 are shown in Figure 11(a) and those of CASE1 and CASE5 are shown in Figure 11(b). The distance to the rupture location of the tendons X was measured from the beam support. In CASE1 and CASE2, the X is 1000 mm, in CASE3 and CASE4, X equals to 500 mm, in CASE5, X is 500 mm and 1500 mm.

Permanent magnet

PC beam

MFL measuring

device

Aluminum rail

PC tendon

PC beam

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Figure 10: Variation of magnetic flux density in a normal PC tendon and in a ruptured PC tendon

As shown in Figure 11(a), the S-shape curve at the broken point of the tendon was observed in CASE2, CASE3, and CASE4. However, in CASE1 and CASE5, it was difficult to observe S-shape curve clearly (Figure 11(b)). The main reason for this is, in CASE1 and CASE5, there was only one PC tendon was broken whereas in the other specimens two tendons were broken. Therefore, the change in magnetic flux density in CASE1 and CASE5 beams are significantly lower than that of the other beams.

As shown in Figure 11(a), the magnetic flux density in CASE3 is lower than that in CASE2 and CASE4. In fact, the CASE3 beam was magnetized before the rupture of the PC tendons by core-drilling and all the other beams were magnetized after core-drilling. During core-drilling, the magnetic effect in the PC tendon of CASE3 beam might have weakened by the friction between the drill and the tendon and due to the rupture of the PC tendon.

Figure 11: Variation of magnetic flux density. (a) CASE2, CASE3, and CASE4, (b) CASE1 and CASE5

5 CONCLUSIONS

Based on the test results, the following conclusions are drawn.

1. The ultimate flexural capacity of the PC beams varies according to the rupture location of the tendons, even though the ruptured tendon area is the same.

2. The moment capacity of the PC beams having ruptured tendons can be approximately predicted by the loss of PC tendon area at the ruptured section. As the bonding between the PC tendon and the

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grout become weaker due the rust, the flexural behavior of the PC beams having ruptured tendons by corrosion may be different than this experiment.

3. The ultimate flexural capacity of the PC beams having ruptured tendons in the insufficiently grout filled region can be improved significantly by grout re-injection if the grout un-filled length is longer.

4. The magnetic flux leakage test method is effective for the PC beams with smaller diameter tendons. However, the area of ruptured tendons should be adequate and the concrete cover should not be very large in order to measure the magnetic flux in the PC tendons.

Acknowledgement

This study was supported by Grants-in-aid for Scientific Research (foundation - B). Shinko Wire Company Ltd. and Nippon Expressway Research Institute Company Ltd. supplied steel tendons and magnetic flux leakage testing instruments, respectively. Authors would like to acknowledge those supporters. Special thank should be given to Mr. Toshihiro Yokota of Yachiyo Engineering Co. Ltd., Mr. Makoto Hirose of Shikoku Research Institute Inc. and all the PC group members in Saitama University, Japan.

References

Abraham, O. and Cote, P. 2002. Impact-echo Thickness Frequency Profiles for Detection of Voids in Tendon Ducts. ACI Structural Journal, ACI, 99(3): 239–47.

Fernandes, B, Titus, M. Karl Nims, D. Ghorbanpoor, A. and Devabhaktuni, V. 2012. Field Test of Magnetic Methods for Corrosion Detection in Prestressing Strands in Adjacent Box-beam Bridges. Journal of Bridge Engineering, 17(6): 984-8.

Japan Industrial Standards (JIS). 2014. Steel Wires and Strands for Prestressed Concrete (JIS G-3536). Japan Industrial Standards Committee, Japan.

Miyashita, T. 2011. The Survey Result and Future Correspondence about the PC Cable Damage on Myoko Oohashi Bridge. Annual meeting for reading research papers of HOKURIKU Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism, Nigata, Japan (In Japanese).

Muldoon, R. Chalker, A. Forde, M.C. Ohtsu, M. and Kunisue, F. 2007. Identifying Voids in Plastic Ducts in Post-tensioning Prestressed Concrete Members by Resonant Frequency of Impact–echo, SIBIE and tomography. Construction and Building Materials, 21(3): 527-37.

Woodward, R.J. Cullington, D.W. and Lane, J.S. 2001. Strategies for the Management of Post-tensioned Concrete Bridges. Current and future trends in bridge design, construction, and maintenance 2: Safety, economy, sustainability, and aesthetics, 23-32.

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