Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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THE EXPERIMENTAL RESEARCH ON THE HEALTH MONITORING OF THE CONCRETE STRUCTURES USING OPTICAL FIBER SENSOR

F. Katsuki, T. Yajima
Dept. of Civil Engineering, Shibaura Institute of Technology
3-9-14, Shibaura, Minato-ku, Tokyo, 108-8548, Japan

ABSTRACT

The optical fiber sensing technology is noticed as the technique which can measure strain and displacement of the concrete structure widely and real-time. In this study, the result of measuring the reinforcement strain in the concrete beam using BOTDR (Brillouin Optical Time Domain Reflectometer) sensor that could continuously measure the distribution strain in optical fiber is reported in this paper. The result of the experiment showed that bending crack width of the reinforced concrete beam could be evaluated by the BOTDR sensor.

KEYWORDS
Optical fiber, Bending crack, Strain, Monitoring, Strand, AFRP

INTRODUCTION

It is very important to urgently detect degree and position of the damage concerning afterwards maintenance and safety, when the concrete structure received the damage by some causes. The visual observation inspection causes the difficulty in the inspection widely crossed, though there is the visual observation inspection in the representative inspection technique. And, the electric strain gauge is utilized in order to measure strain and deformation which arise in the structure. However, the measurement system is very complicated so that electric strain gauge may need 1 lead wire in 1 observation point, when continuous strain distribution of the structure is widely measured using electric strain gauge. Then, the optical fiber sensor is noticed as the technique which measures the strain which arises in the concrete structure. It is examined that the optical fiber sensor is utilized for the health monitoring of the structure, because the strain distribution which arises in the structure can be measured in 1 optical fiber smartly. In this study, it was clarified experimentally that optical fiber which was united with the concrete reinforcing material could monitor strain distribution of the reinforcement and bending crack width of concrete beam.

MEASURING DEVICE OF THE STRAIN

In the strain measurement by optical fiber, BOTDR [1] was used. BOTDR detects frequency shift quantity of the Brillouin backward scattering light, and the strain is measured (Figure 1). That is to say, the strain in the optical fiber is measured using the property in which the frequency distribution of the Brillouin scattering light shifts in proportion to the strain (Figure 2). The generation position of the strain is required by measuring the time taken from the incidence of the pulsed laser to the detection of the Brillouin scattering light. Therefore, the strain distribution along optical fiber is measured by continuously detecting the Brillouin scattering light. However, the strain of some measuring point monitored in BOTDR is a mean value of the strain which is arising from the measuring point to the 1.0m interval, because pulse duration of emitting light is 1.0m. This 1.0m is called the range resolution. The specification for BOTDR equipment used in this study is shown in Table 1 [1].

Range of the measurable distance 2Km-10Km
Range of the measurable strain3%
Pulse duration of light source10ns
Dynamic range4dB
Range resolution1.0m
Accuracy of measured strain0.01%
Interval of the measuring point5cm
Minimum value of displayed strain0.0001%
Table 1: Specification of BOTDR.

Fig 1: Frequency distribution of the Brillouin scattering light. Fig 2: Relationship between strain and frequency shift.

TENSILE TEST OF REINFORCEMENT

Reinforcements
Dimension and dynamic characteristics of the reinforcement used by the experiment are shown in Table 2. Each characteristic value in this table is experimental value by tensile test method for following for JSCE-E531. The optical fiber used for strain measurement is a single-mode, and it is a wire of Æ1.0mm covered by the nylon.

  Strand AFRP
Outer diameter (mm)15.27.88
Cross section (mm2)140.745.7
Maximum tensile load (kN)27281.2
Elongation (%)7.53.2
Yield load (KN)245-
Elastic modulus (KN/mm2)194.155.8
Table 2: Characteristics of the reinforcement.

Installation Method of Optical Fiber
The installation of the optical fiber to the prestressinng strand is shown in Figure 3. The optical fiber which added some tensile force is bonded to the concave part of the strand by epoxy resin. Therefore, it is necessary to correct the strain measured by the optical fiber in the strain of the axis of member direction in order to bond the optical fiber together in the spiral along the groove of the strand.

Fig 3: Prestressing strand type. Fig 4: AFRP rod type.

The installation of the optical fiber to the AFRP rod is shown in Figure 4. The stainless SUS tube is placed in the rod production, and the optical fiber is bonded to the inside of the SUS tube by epoxy resin. .

Test Method
The outline of the direct tensile test is shown in Figures 5. The reinforcement installed in the frame produced in H steel and steel plate was pulled by the center hole jack. Considering the range resolution of BOTDR being 1.0m, the tension interval of the reinforcement ensured 3.0m in order to measure the tensile strain in the reinforcement by BOTDR.

Fig 5: Outline of the tensil test.

Results of Direct Tensile Test
The experimental result of the strand and AFRP rod is respectively shown in Figure 6, 7. The distance from the center in the frame is shown in the quadrature axis of the graph. And, the strain monitored in BOTDR is shown in the axis of ordinate. The strain distribution monitored in BOTDR is underestimated, because range resolution is 1.0m, when the region including the part in which tensile force does not affect optical fiber and part in which tensile force works is measured. Therefore, the strain distribution of the accurate reinforcement detected by optical fiber seems to be strain region shown in the figure as uniform distribution. Then, the relationship between the value which averaged the strain of the uniform distribution region in the each every load stage and the stress is shown in Figure 8, 9. The stress is the value which divided the tensile force measured by load indicator by cross section of the reinforcement shown in Table 2. In addition, the approximate straight line for measured value is shown in each Figure. From each figure, it is proven that the strain of the reinforcement detected by the optical fiber shows the increase which is almost rectilinear in the elasticity range. The ratio (Eopt/E) between elastic modulus got from the gradient of the approximate curve in the figure (Eopt) and elastic modulus got from the alternative test shown in Table 2 (E) is 1.06 in the strand, and it is 1.07 in AFRP rod. Therefore, it was proven that strain distribution in the reinforcement could be measured at the high accuracy by the installation method of optical fiber shown at figure 3, 4.

Fig 6: Strain distribution of the strand. Fig 7: Strain distribution of AFRP rod.

Fig 8: Relationship between stress and strain of the strand. Fig 9: Relationship between stress and strain of AFRP rod.

BENDING TEST OF BEAM

Test Specimens
The outline of the test specimen used in bending test is shown in Figures 10. The configuration of the reinforcement that the optical fiber was bonded together is shown in Figures 11. The installation method of optical fiber to the reinforcement is shown in Figure 3, 4. The failure mode of the beam is flexural failure in the design.

Fig 10: Reinforced concrete speciemens in flexural test. Fig 11: Cross section of test specimens.

Test Method
The span length of the beam was 2.3m, and the static load for 2 points of the beam simply supported was added to 60kN. Still, the loading point interval of the beam which the bending moment of the uniform distribution affected was defined as 1.0m, because the range resolution of BOTDR was 1.0m.

Analysis
In this study, measuring precision of reinforcement strain by optical fiber was confirmed using nonlinear analysis program (ATENA) of the marketing.

Results of Bending Test
The strain distribution of the strand measured by optical fiber is shown in Figure12. In addition, the strain distribution of the strand required by nonlinear analysis is shown in this figure. The distance from the span center is shown in the quadrature axis of this graph. And, the crack distribution of the beam at the 60kN loading load is shown in Figures 13. From Figure 12, it is proven that the measuring result by optical fiber comparatively agrees with analytical result. However, the uniform distribution region does not exist for strain distribution measured by optical fiber unlike the analytical result, because range resolution of BOTDR is 1.0m. In the 60kN load, it seems to cause the rapid increase of the strain distribution near the loading point of left side by the rapid increase in the crack width near the loading point showing in Figures 13.

Fig 12: Strain distribution of the strand placed in the beam Fig 13: Crack distribution (Load=60kN)

The relationship between strain and loading load in the span center is shown in figures of 14. However, the strain of span center measured by optical fiber in the figure has been evaluated as a mean value of the loading point interval, because range resolution of BOTDR is 1.0m. It is proven that the strain of the strand placed in concrete beam which receives the bending can be measured by the optical fiber, because the measured value agrees with analytic value almost.

Fig 14: Relationship between strain measured by the optical fiber and load Fig 15: Strain distribution of AFRP rod paced in the beam Fig 16: Crack distribution (Load=60kN)

The strain distribution of the AFRP rod measured by optical fiber is shown in Figure15. In addition, the strain distribution of the strand required by nonlinear analysis is shown in this figure. And, the crack distribution of the beam at the 60kN loading load is shown in Figures 16. With the increase in the loading load, the strain distribution of AFRP rod measured by optical fiber increases. However, the measured value by optical fiber is smaller than analytic value in each load stage shown in continuous line. The slippage had been confirmed after the test end between rod and SUS tube, and adhesion cutting of the SUS tube placed in AFRP might occur under loading. Though such phenomenon did not occur in the direct tensile test, the adhesion between rod and SUS tube will have to be improved.

Evaluation of Bending Crack Width

Fig 17: Relationship between increase stress and crack width
The relationship between bending crack width in the beam which placed the strand and increase stress of the strand is shown in Figures 17. The bending crack width is average width of the flexural cracking in loading point interval measured by the crack kale. Bending crack width shown as the experimental value averaged the flexural cracking width in the loading point interval measured by the crack-scale. The increase stress of the strand shown as the experimental value was required by multiplying the elastic modulus (Eopt) in the strain of span center shown in Figures 12. In addition, the relationship between increase stress of the reinforcement and bending crack width calculated from equation (1) regulated in JSCE [2] was shown in the continuous line. From this figure, it is proven that the experimental value agrees with calculated value. Therefore, it was proven that the increase stress of the strand and bending crack width could be accurately evaluated from the strain measured by the optical fiber.

(1)

Where k is a constant considering adhesive properties of the reinforcement, c is the cover of the reinforcement, cs is a central interval of the reinforcement, Æ is a diameter of the reinforcement,s is the increase stress of the reinforcement, Es is an elastic modulus of the reinforcement, e'cs is a constant considering drying shrinkage and creep of the concrete.

CONCLUSION

  1. By the optical fiber which bonded to the strand, it is possible to measure strain distribution which arises in the strand at the high accuracy.
  2. It is possible to evaluate increase stress which arises in the strand placed in the beam and bending crack width which arises at the underside of the beam by the optical fiber.
  3. It is not possible to accurately measure the strain of AFRP rod by the optical fiber, if the adhesion of the rod and SUS tube is not strengthened, because the adhesion cutting was confirmed in bending test between the rod and SUS tube.

REFERENCES

  1. ANDO ELECTRIC Co.,Ltd. : The Manual of BOTDR (in Japanese)
  2. Japan Society of Civil Engineers (JSCE) : Concrete standard specifications (the design edition), 1996 (in Japanese)
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