long term stress monitoring on pc beam …...long term stress monitoring on pc beam by using...

LONG TERM STRESS MONITORING ON PC BEAM BY USING ELASTO-MAGNETIC SENSOR

Shoji KUROKAWA Sunaryo SUMITRO Department of Civil Engineering Research and Development Div.

Kisarazu National College of Technology Keisoku Research Consultant JAPAN JAPAN

Keiji SHIMANO Ming L. WANG

Department of Civil Engineering Dept. of Civil and Materials Eng. Kisarazu National College of Technology University of Illinois at Chicago

JAPAN USA Keywords: maintenance, EM sensor, actual stress, fatigue, monitoring,

1 INTRODUCTION Steel together with concrete is the main structural material of the infrastructures. Along with social

development, steel materials have been used for a variety of structures, such as, highway bridges, railroad bridges and buildings. In recent years, many structures have been deteriorating. In order to carry out maintenance management of the structures economically and rationally, it is necessary to evaluate their life cycle cost (LCC) based on the following stages, i.e., construction, inspection, repair, maintenance management, destruction and/or dismantlement [1]. Research on the optimization of maintenance plan has been reported by taking into account inspection expense and inspection accuracy on a description model of the LCC of civil engineering structure [2]. In considering a maintenance plan for Pre-stressed Concrete (PC) structure which aims to reduce its LCC, it is requested that the measurement system should fulfill “AtoE” characteristics, i.e., (A)ccuracy, (B)enefit , (C)ompact, (D)urable and (E)asy to operate, besides enable to measure the actual stress of the PC tendon, non-destructive to the PC tendon itself and no damage to its protection sheath [3]. However, the stress measurement method that fully satisfies such characteristics was not found.

Elasto-Magnetic (EM) actual stress measurement method by utilizing the sensitivity of incremental magnetic permeability due to stress change has been being developed [4,5,6]. Numerous tests of various steel materials by applying this measurement method in which satisfy above-mentioned characteristics have been conducted [7]. It has been proved that EM measurement method enable to measure actual stress of steel wire, PC bar and PC strand precisely without destroying their polyethylene covering sheath [8].

For a PC structure member, although pre-stressing measurement by the load-cell installed at a jack end can be performed before fixation, but the problem becomes difficult after fixation of the PC tendon. Therefore, in order to solve the problem, the steel tendon stress measurement test on outer cable PC beam was conducted by utilizing EM sensor. By comparing with load-cell and strain gage measurement results, the actual stresses of the outer PC tendon in the following conditions are clearly investigated: actual mechanical properties of PC wire under tension test, pre-stress change due to set-loss at PC tendon fixation stage, pre-stress change due to PC tendon relaxation, concrete creep and shrinkage at long-term pre-stressing stage, PC tendon pre-stress change of PC beam under cyclic fatigue loading and actual pre-stress change due to re-pre-stress process. As the result of the test, it is confirmed that EM stress measurement technology can provide adequate accuracy and reliability to monitor actual stresses of PC tendons during a life cycle of PC structures.

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MonitoringSession 15

2 MEASUREMENT TECHNIQUE The material magnetization is typically described

by the relationship between the magnetic field strength, H (Amp-turns/m), and the flux density, B (Webers/m2), and for any material can be expressed by the general constitutive equation HB

rrrµ=

where µr is the magnetic permeability tensor.

However if the material is macroscopically homogeneous and isotropic, the relationship can be reduced to its scalar form and μ is a scalar. Fig.1 shows a typical magnetization curve for a ferromagnetic material. It is evident that the permeability is not constant, but is dependent on the field strength. It should be noted that μ is not the slope of the magnetization curve, but simply represents the ratio B/H. One of the easiest ways to magnetize a material and study its magnetic characteristics can be carried out using the principle of magnetic induction and two solenoids a primary coil and a secondary coil with the material whose magnetic characteristics are to be investigated as the core as shown in Fig. 2. If a DC current is applied across the primary coil it produces a magnetic field (H) and the magnetic flux density (B) within the specimen. Amplitude permeability is defined as the ratio B/H, and incremental permeability defined as the ratio HB ∆∆ . In both cases permeability depends also on “working point” in which it is measured.

3 SPECIMEN AND EXPERIMENT SET UP Fig. 3 shows the specimen and experiment set up. The loading system is conducted by a Pulsator

Fatigue Machine with maximum load 200kN. The movable rotation supports at both sides were set in the span of 2000mm. The outer cable PC beam specimen was constructed in the length of 2100mm, the width of 150mm, and the height of 180mm. The end of the PC beam was made as a rectangular form to fix PC tendon and a ditch was formed inside the PC beam to enable to install PC tendon and

Fig. 3 Specimens and loading system

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Proceedings of the 1st fib Congress

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EM sensor. Fig. 4 shows the cross sectional plans of the PC beam, the width and the height of the ditch is 40mm and 120mm, respectively. The PC tendon is JIS G3536 SWPR7BN wire with diameter 15.17mm, cross section area 138.41mm2, ultimate tensile force 264kN, yielding tensile force 235kN, and Young’s modulus 191kN/mm2. SD295A diameter 6mm deformed steel bars were used for stirrup and arranged in interval of 100mm. The material used for concrete is ordinary Portland cement, pit sand and pit gravel produced by Chiba prefecture. Table 1 shows the mix proportion and Table 2 shows the concrete compression and tension strengths, and Young’s modulus.

Three outer cable PC beam specimens, i.e., PCS1, PCF1 and PCS2 were constructed as shown in Table 2. PCS1 and PCS2 are for static loading tests and PCF1 is for fatigue tests. Pre-stressing was introduced by using center hole jack for PCS1 and PCF1 in the age of 71 days, and for PCS2 and PCF2 in the age of 57 days. Pre-stressing was introduced up to 70% of ultimate tensile force and fixed by applying wedge anchor. For specimens PCS1 and PCS2, load cell was installed in between anchor plate and wedge anchor as shown in Fig.3. EM sensor was installed for all specimens in the middle of PC tendon, strain gage was also attached in the middle of PC tendon in the longitudinal direction. The measured initial pre-stressing force by load cell and EM sensor are shown in Table 2.

Max size Slump Air Ｗ／ＣＳ／ａ Unit amount (kg/m3)

（ｍｍ）（ｃｍ）（％）（％）（％）ＷＣＳＧＡE（ml)

20 7.5 5.0 47 40.9 160 340 720 1060 34

Table 1. Mix proportions

Table 2. Test specimens

Concrete strength and Young's modulus (N/mm2)

Compression Tension Young's modulus Load cell EMsensor

PCS1 35.4 2.96 28900 - 134

PCF1 35.4 2.96 28900 128 139

PCS2 37.1 3.08 32500 123 139

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4 STRESS ACCESSMENT, ACCURACY AND RELIABILITY 4.1 PC wire under tension test The strain measurement of PC wire under tension test was conducted by applying strain gage and strain type extenso-meter. In order to study the actual mechanical properties of PC wire, a strain gage was attached in the longitudinal direction as shown in Fig.5. It maybe easy to understand that the measured strain direction differs from the actual axial tension direction. Fig.6 shows the test results obtained by two kinds of strain sensors. Based on the test results, the re-calculated Young’s moduli E can be expressed as follows:

(i) strain gage (referred as Straingage W condition) : E = 210kN/mm2 (ii) strain extenso-meter (referred as Straingage A condition) : E = 191kN/mm2

However, the certified mechanical properties of PC wire written on an inspection certificate is conducted under tension test with Straingage A condition, therefore, conventional stress measurement method by multiplying strain (measured by strain gage) with certified Young’s modulus (Straingage A condition) would just have 91% of the result by multiplying with Young’s modulus obtained in Straingage W condition. Stress measurement by EM sensor was utilized from zero stress condition to yielding load at 235kN by applying 19.6kN incremental loads, and continued with precise incremental loads up to rupture stage. Fig. 7 shows the comparison between EM sensor and load-cell results. By considering that the average error is 1.74%, it is confirmed that EM is capable to measure stress of PC wire in at all loading stage from zero stress stage to the total breakage stage. 4.2 Pre-stress introducing stage Pre-stress introduced by a jack in the 19.6kN incremental load. The pre-stress in PC tendon was measured by load cell (only PCF1 and PCS2), EM sensor and strain gage. The fixation was done at 70% of ultimate tensile force. Test results of each specimen are shown in Fig. 8 to Fig.10. Measurement no.12 was measured after removing load-cell at the jack.

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Measured pre-stressing force on PCS1 is shown in Fig. 8. EM sensor measurement results are the closest values to load-cell at jack and Straingage W. Measured set-loss by EM sensor, Straingage W, and Straingage A are 25.8%, 28.4% and 28.4%, respectively. It is observed that set-loss measured by EM sensor is the smallest one. Measured pre-stressing force on PCF1 is shown in Fig. 9. EM sensor measurement results are the closest values to load-cell at jack and Straingage W. It is observed that set-loss measured by EM sensor is the smallest one. Fig.10 shows the measured pre-stress force on PCS2, results recorded by EM sensor have the nearest value with the values recorded by load-cell at jack. Largest set-loss was measured by load-cell at jack, on the other hand, smallest set-loss was measured by EM sensor. As the general characteristic, the EM sensor measurement value has a closest value to the load-cell at the jack and has a tendency to record smallest set loss measurement. 4.3 Long term pre-stress measurement Figs. 11 and 12 show the pre-stress loss ratio in percentage in a function of time. Since the pre-stress loss ratio recorded by Straingage A equals to Straingage W, for convenience, it will refer to ‘Straingage’. Loss of pre-stress force on PCF1 is shown in Fig. 11. The data measured by EM sensor has the same tendency (about 1% error) with the data measured by load-cell at jack from the immediately after pre-stressed stage to 500 hours stage. However, the data measured by Straingage differs in 3% error in the beginning and have a tendency to become larger due to pre-stressing time. Fig. 12 shows the loss of pre-stress force on PCS1. The data measured by EM sensor has the same tendency with the data measured on PCF1 as shown in Fig.11. As the conclusive feature, stress measurement by utilizing EM sensor has a good agreement with the data recorded by load-cell at jack and it is confirmed that EM sensor is a suitable measurement device to monitor pre-stress change due to PC tendon relaxation, concrete creep and shrinkage at long-term pre-stressing stage of PC structures.

Fig.10 Measured pre-stress force on PCS2

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4.4 PC beam under static loading PC beam specimens used for PC tendon stress measurement under static loading are PCS1 and PCS2. Pre-stress was introduced up to 70% of ultimate tension strength. For PCS1, immediately before applying static loading, pre-stressing was released, and then re-pre-stress was introduced. Loading system as a simple beam with the span of 2000mm was loaded in two points in a distance of 200mm as shown in Fig.3. Loading was increased gradually up to first crack occurred, continued with unloading, then, reloading up to the crack reached the width of 0.2mm, continued with unloading, and then, reloading up to the crack reached the width of 0.4mm, continued with unloading, and then, reloading up to the rupture stage of the beam. Measurement of PC tendon stress was recorded by EM sensor and strain gage in the center of span. Stress at end portion was measured load-cell installed at fixation end. For PCS1, the loads at the first crack occurrence stage, crack width of 0.2mm stage, crack width of 0.4mm stage and beam rupture stage are 27.5kN, 30.8kN, 31.4kN and 36.5kN. The structure failure type was the concrete compressive bending failure. Loads and PC tendon tensile forces relationship is shown in Fig. 13. Before loading, the measured effective pre-stress tendon by Straingage has more than 10% difference from data measured by EM sensor. It is considered that the difference of recorded data by Straingage W and Straingage A are effected by re-pre-stress introducing process. For all sensor, load and tendon force relationship due to load increasing shows the same tendency.

For PCS2, the loads at the first crack occurrence stage, crack width of 0.2mm stage, crack width of 0.4mm stage and beam rupture stage are 28.9kN, 29.4kN, 31.1kN and 36.6kN. The structure failure type was the concrete compressive bending failure. Loads and PC tendon tensile forces relationship is shown in Fig. 14. Compare to PCS1 test results, the measured effective pre-stress tendon forces of PCS2 measured by Straingage has less difference from data measured by EM sensor. Measured result by Straingage W is close to EM sensor measurement but differ from Straingage A about 12% (it may be influenced by different Young’s modulus). The measurement result by EM sensor has good agreement with PCS1, therefore, if the material properties of concrete and PC tendon for both specimens are equal, it is confirmed that stress measurement by EM sensor is the most reasonable measurement result. Furthermore, it is verified that regardless to previous loading history, the easy install EM sensor enable to perform accurate and reliable stress measurement for PC structures at various loading stages. 4.5 PC beam under cyclic fatigue loading Before fatigue failure of a PC beam under cyclic loading, pre-stress force changes due to concrete crack occurs and propagates, reinforcement debonds and concrete separates. In order to consider EM sensor as one of the possible measurement method to monitor the pre-stress force change of the PC

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Fig.14 Load and tendon force relationship on PCS2


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beam under cyclic fatigue loading, static loading test to maximum load was performed in the middle period of fatigue test and the pre-stress force of PC tendon was measured.

The specimen for the cyclic fatigue loading test is PCF1. The sinusoidal loading wave (see Fig. 15) was applied to the specimen by a Pulsator Fatigue Machine with maximum load 200kN. The load frequency is 4.2Hz with the minimum load of 10kN. The maximum load was set to 28.4kN up to 1.8 million cycles, afterwards it was set to 29.4kN. The fatigue test was terminated when the loading achieved 4.5 million cycles, however, there was no fatigue failure observed. Therefore, the specimen was loaded statically up to PC beam rupture at the rupture load of 33.7kN. The structure failure type was the concrete compressive bending failure.

Furthermore, the fatigue test machine was stopped timely during fatigue test period to perform static loading test from zero stress to the maximum load. The PC tendon force was measured by EM sensor and load-cell installed in the fixation end. Pre-stress tendon force and load cycle relationships for zero stress (no-load), minimum load and maximum load are shown in Fig. 16. The sudden change of tendon force at 1.8 million cycles stage is influenced by maximum load setting from 28.4kN to 29.4kN. Generally, the measurement value by EM sensor is larger than the measurement value by the load-cell, it may be influenced by the restraint of the load-cell setting under severe cyclic load. In the case of load-cell installment, a rational measurement result cannot be recorded without precise load-cell setting. On the other hand, EM sensor is set-free and does not receive any external force, therefore, it is considered that EM sensor enables to record more rational measurement.

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Fig.16 Change of pre-stress tendon force due to cyclic loading

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5 CONCLUDING REMARKS In the case of the life cycle cost evaluation on PC structures, reliable and rational assessment of

pre-stress force is pointed out to be the most important problem. The long term stress monitoring on the PC tendon pre-stress force by utilizing EM sensor was conducted on outer cable PC beam in a model experiment. The concluding remarks of the stress measurement by utilizing EM sensor technology can be summarized as follows: 1. For PC wire under tension loading, EM sensor can perform highly precise stress measurement in

the range from zero stress stage to yielding stress stage. 2. As the result of stress measurement at the pre-stress introducing stage, comparing to strain gage

and load-cell install at fixation end, EM sensor measurement is the nearest result to the result showed by the load-cell at the jack.

3. In the case of long term pre-stress measurement, stress measurement by utilizing EM sensor has a good agreement with the data recorded by load-cell at jack, therefore, it is confirmed that EM sensor is a suitable measurement device to monitor long-term pre-stress changes.

4. By observing the result of PC beam under static loading test, it is verified that regardless to previous loading history, the easy install EM sensor enable to perform accurate and reliable stress measurement for PC structures at various loading stages.

5. Stress measurement on the pre-stress tendon of PC beam under cyclic fatigue loading test was performed by load-cell and EM sensor. All sensors data shows the same tendency of the stress change which respect to number of loading cycles, however, there is a large difference in pre-stress force quantity. It may be influenced by the restraint of the load-cell setting under severe cyclic load. On the other hand, EM sensor is set-free and does not receive any external force, therefore, it is considered that EM sensor can perform more rational measurement on PC structures under cyclic loading.

REFERENCES [1] Tominaga, M., Sumitro, S., Okamoto, S., Kato, Y., and Kurokawa, S. : Development of monitoring

technology for steel and composite structures, J. of Constructional Steel, Vol.9, Nov, pp.575-582, 2001 (in Japanese)

[2] Honjo, Y., Ueki, J., Sumitro, S., Matsui, Y., and Kato, T. : Influences of quality and frequency of inspection on maintenance of civil engineering structures, Proc. of JSCE Annual Conference, Vol.56, CS6-004, Oct, pp.262-263, 2001 (in Japanese)

[3] Sumitro, S, Okamoto, T., Matsui, Y. and Fujii, K. : Long span bridge health monitoring system in Japan, Proc. SPIE 8th Annual International Symposium on Smart Structures and Material, Health Monitoring and Management of Civil Infrastructure Systems, Newport Beach CA, Vol. 4337-67, pp.517-524, 2001

[4] Chen, Z., Wang, M.L., Okamoto, T., and Sumitro, S. : A new magnetoelastic stress/corrosion sensor for cables in cable-stayed bridges using measurement of anhysteretic curve, 2nd Workshop on ATUEDM, Kyoto, July 11-13, 2000

[5] Wang, M.L., and Chen, Z.: Magneto-elastic permeability measurement for stress monitoring in steel tendons and cables. Proc. of the SPIE 7th Annual Symposium on Smart Structures and Materials, Health Monitoring of the Highway Transportation Infrastructure, Vol. 3995, pp. 492-500, 2000

[6] Wang, M.L., Lloyd, G. M., and Hovorka, O.: Development of a remote coil magneto-elastic stress sensor for steel cables, Proc. SPIE 8th Annual International Symposium on Smart Structures and Material, Health Monitoring and Management of Civil Infrastructure Systems, Newport Beach CA, Vol. 4337, pp. 122-128, 2001

[7] Kurokawa, S, Sumitro, S., Wang, M.L., and Shimano, K. : Stress measurement of various cables by EM sensor, Proc. of Japan Prestressed Concrete Engineering Association 10th Annual Conference, pp.101-106, 2001 (in Japanese)

[8] Sumitro, S. : True-stress measurement of PC steels by EM sensor, J. of Pre-stressed Concrete Japan (Japan Prestressed Concrete Engineering Association), Vol.43, No.6, Nov, pp.99-103, 2001 (in Japanese)


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