Feasibility Study of Measuring the Stress in Prestressing Tendons Using Induced Magnetic Field

Transcription

1 11th European Conference on Non-Destructive Testing (ECNDT 2014), October 6-10, 2014, Prague, Czech Republic Feasibility Study of Measuring the Stress in Prestressing Tendons Using Induced Magnetic Field Changbin JOH 1, Jungwoo LEE 1, Imjong KWAHK 1 1 Structural Eng. Division, Korea Institute of Civil Eng. and Building Technology; Goyang, Korea Phone: , Fax: ; cjoh@kict.re.kr, duckhawk@kict.re.kr, kwakim@kict.re.kr Abstract This paper investigated experimentally the feasibility of the method to estimate the stress in the prestressing tendons of prestressed concrete bridges using the magnetic field induced by a moving electromagnet. This method is based on the Villari effect in which the magnetic permeability of prestressing tendons changes when subjected to a mechanical stress. The test results show high linearity between the stress in the prestressing tendon and the induced magnetic flux density within the practical stress range of the tendons. In addition, the induced magnetic flux density in the tendon depends on the intensity of the electromagnet, the distance between the electromagnet and the tendon, the presence of other ferromagnetic materials such as rebar and metal sheath. Keywords: Prestressing Tendon, stress measurement, Villari effect, induced magnetic field 1. Introduction Diversified nondestructive test (NDT) methods have been studied to estimate the prestress force of the prestressing tendons during the lifespan of prestressed concrete bridges but without successful application on site. Especially, there is practically no example in which economically efficient estimation of the prestress force has been realized for the bonded prestressing tendons applied in existing prestressed concrete bridges [1]. The research dedicated to the prestress measurement in the bonded tendon can be subdivided with regard to the use of guided wave or the use of stress wave. The guided wave method is widely used for the inspection of cracks or corrosion in pipelines but presents limitation in being applied to bonded PS steel in a tube filled with grout. Concretely, in case of the grouted tendon in a PSC bridge, the guided wave can practically not be measured at a distance far by about 1.5 m from the anchorage because of its attenuation in concrete [2][3]. Kim et al. [4] applied the phenomenon in which the velocity of the longitudinal stress wave in the PS tendon increases with respect to the prestress force for the measurement of the stress in the tendon. However, the sensitivity of the stress to the stress wave velocity appeared to reduce significantly at the practical stress level of the PS tendon superior to 40% of the yield stress. As an alternative to overcome the drawbacks of the ultrasonic wave and stress wave, a technique using the magnetic field may constitute an interesting option. This paper derives a method measuring the prestress force of prestressing tendons in prestressed concrete bridge using the Villari effect [5], also known as the inverse magnetostriction, and the induced magnetic field (IMF) generated by an electromagnet. The feasibility of the method is verified experimentally through model and PSC beam tests. 2. Villari effect and induced magnetic field The relation between the intensity, H, of the magnetic field and the magnetic flux density, B, can be defined by the magnetic permeability, µ, as expressed in Equation (1):

2 B = µh (1) The Villari effect is the change of the magnetic permeability, µ, of a magnetic material when subjected to a mechanical stress [6]. Fig. 1 describes conceptually the Villari effect. (a) Positive (b) Negative Figure 1. B-H hysteresis and concept of Villari effect An IMF is generated in the prestressing tendon when a yoke-shaped electromagnet is disposed below the prestressed concrete structure (Fig. 2). This layout forms a magnetic circuit in which a magnetic flux, Φ, flows through the electromagnet and the prestressing tendon. The magnetic flux density, B, representing the size of this IMF in the prestressing tendon depends on the permeability, µ, and the cross sectional area of the magnetized prestressing tendon, and the continuity of the magnetic circuit (closed magnetic circuit). The magnetic flux density, B, crossing the prestressing tendon is measured by a hall sensor installed on the pole face. Figure 2. IMF in tendons of different stresses The accurate measurement necessitates near magnetic saturation of the prestressing tendon. The continuity of the magnetic circuit depends on the separation between the prestressing tendon and the pole face. Since larger separation results in larger loss of the magnetic flux forming the closed magnetic circuit, a relatively strong electromagnet is required to induce magnetic field in the prestressing tendon. Considering that concrete is a non-magnetic material, concrete does not disperse the magnetic flux flowing in the prestressing tendon and the concrete cover has practically no effect. Fernandes et al. [7][8] exploited the fact that the

3 magnetic flux density of the magnetic field is influenced by the cross sectional area of the prestressing tendon to measure the degree of corrosion of the tendon in a prestressed concrete structure. Since the permeability, µ, of the prestressing tendon subjected to stress varies according to the stress when the tendon is magnetized by means of a permanent magnet or an electromagnet with a magnetic field intensity, H, the density, B, of the magnetic flux flowing in the tendon will vary with respect to the stress owing to the Villari effect. Even if the cross sectional area of the tendon decreases under larger stress due to the Poisson s effect, this effect can be assumed to be relatively insignificant. Consequently, the measurement of the magnetic flux density, B, flowing in the tendon will enable to estimate the stress of the tendon by comparison with the stress of the tendon that has been preliminary measured and the magnetic hysteresis curve B-H. To that goal, need is to collect a database by measuring in advance the B-H hysteresis curve according to the stress of the prestressing tendon used in the prestressed concrete structure. In this process, the effects of the intensity of an electromagnet, the concrete cover, the separation and temperature must be considered. 3. Model test 3.1 Test The feasibility of the method measuring the stress of the prestressing tendon by means of the Villari effect and the induced magnetic field is examined experimentally by measuring the relation between the stress of the tendon and the IMF. Figure 3. Layout for the test Figure 4. Model test setup Table 1. Test variables Variable Values Remarks Input current of electromagnet (A) 3.67, 5.47 Stress of prestressing tendon (f y ) 0.14, 0.27, 0.41, 0.54, 0.68, 0.81 f y = 1600 MPa (SWPC7B 15.2mm 7-strand) Separation (mm) 25, 50 Fig. 2 Figs. 3 & 4 show the layout and the realization of a model test. A loading device and a frame are prepared so as to vary the stress of the tendon during the test, and an electromagnet is

4 disposed at a position below the tendon to secure definite separation. Two hall sensors are installed on the face poles of the electromagnet to measure the magnetic flux density, B. The variables used in the test are arranged in Table Result and analysis In Fig. 5a, a highly linear relationship can be observed between the stress of the tendon and the induced magnetic induction B for both input currents. Besides, the size of the magnetic flux density induced in the tendon increases when the input current rises from 3.67 A to 5.47 A. Figure 5. Stress of prestressing tendon vs. magnetic induction B Fig. 6 plots the results of the tests investigating the relation between the concrete cover and the induced magnetic induction for the input current of 5.47 A and separation of mm. It appears that the cover made of non-magnetic concrete has practically no effect on the magnetic flux density induced in the tendon. Despite of slight change in the magnetic induction B due to the concrete cover, this change remained within the measurement error occurring during the measurement of the voltage of the hall sensors, which means that the concrete cover has practically no effect on the magnetic flux density. Figure 6. Relation between the concrete cover and the induced magnetic induction B

5 4. PSC beam test 4.1 Test As listed in Table 2, four types of specimens presenting different magnetic characteristics were designed to examine the relationship between the induced magnetic field and stress of the PS tendon in the PSC beam (Fig. 7). SWPC7B tendons with diameter of 15.2 mm and nominal tensile strength of 1,900 MPa were used as PS tendons in the tests. The nominal yield strength of this tendon is 1,600 MPa and the cross-sectional area is mm 2. Table 2. Characteristics of PSC beam specimens. Specimens Reinforcing bar Stirrup Sheath Remarks P-FRP-S FRP mm PVC PVC tube fabricated to have P-D16-S Steel mm PVC thickness identical to steel M-FRP-S FRP mm Steel tube (diameter 20 mm) M-D16-S Steel mm Steel (a) Sections with and without stirrups (b) Longitidinal section Figure 7. PSC beam specimens. The test was conducted by moving the electromagnet considering the site conditions (Fig. 8 & 9). The electromagnet was displaced at speed of 55.5 mm/s and the intensity of the induced magnetic field at the N pole face of the electromagnet was measured by the hall sensor. Figure 8. Test setup concept for PSC beam specimens

6 (a) Test setup (b) Moving unit with measuring devices Figure 9. Test setup for PSC beam specimens. 3.2 Result and analysis Figure 10 illustrates the variation of the magnetic flux density generated in the PS tendon according to the prestress force of specimen P-FRP-S which minimizes the effect of the magnetic body in the section. The size of the induced magnetic field in the PS tendon shows uniform reduction according to the increase of the prestress force introduced in the PS tendon from about 30 kn to 180 kn by step of 30 kn. The three peaks occurring in the longitudinal direction of the specimen correspond to the increase of the magnetic flux density provoked by the enlargement of the cross-sectional area of the magnetic body resulting from the combination of the area of the steel reinforcement and the area of the PS tendon at the locations of the stirrups. Figure 10. Stress and magnetic flux density of PS tendon (P-FRP-S) In Fig. 11, the relation between the stress of the PS tendon and the mean value of the size of the magnetic flux density at the 2,000 mm point is seen to reproduce the linear relationship between the prestress and intensity of the magnetic flux density observed in the model test. Note that a difference appears in the absolute value of the size of the induced magnetic field but this difference can be attributed to the neighboring magnetic bodies chosen as one of the test conditions. For the model test, it seems that the relative proximity of the steel frame contributed to the intensity of the induced magnetic field.

7 Figure 11. Stress of PS tendon and size of magnetic flux density (PSC beams and model tests) 5. Conclusions This paper investigated the feasibility of the method estimating the stress in the bonded tendon of prestressed concrete bridge using the Villari effect and the induced magnetic field. To that goal, a magnetic field was generated in the prestressing tendon by means of an electromagnet and the relation between the stress of the tendon and the so-induced magnetic flux density was observed. The both test results revealed that, within the stress range (from 0.14f y to 0.81f y ) of the prestressing tendon used in field, there is a linear relationship between the stress of the tendon and the induced magnetic induction. Moreover, the magnetic flux density induced in the tendon depended on the intensity of the electromagnet and the separation but was not affected by the concrete cover. Accordingly, this linear relation between the stress of the tendon and the induced magnetic flux density can be used to estimate the stress of the bonded prestressing tendon in existing prestressed concrete bridges. This means that, if magnetic sensors are used to measure the magnetic flux density induced in the prestressing tendon by an external electromagnet attached to the prestressed concrete bridge, the stress of the tendon can be estimated by the Villari effect in which the magnetic permeability varies according to the stress of the magnetic body. However, additional studies should be conducted for further practical application of the method. Concretely, the magnetic B-H hysteresis curves should be constructed per stress of the prestressing tendon, and study should be performed to examine systematically the effect of the separation and the intensity of the electromagnet on the magnetic flux density induced and the sign of magneticstriction in the prestressing tendon. In addition, methods enabling to consider the effects of the diverse details in actual prestressed concrete bridges like the effect of longitudinal and transverse reinforcements, the case of prestressing tendons arranged in bundles, and the ordinary use of metallic sheath shall also be investigated.

8 Acknowledgements This research was supported by a grant from a Strategic Research Project (Diagnostic Technology for the Prestressing Tendons and Grouting in Concrete Bridges) funded by the Korea Institute of Civil Eng. And Building Technology. References 1. C Joh, J W Lee, and I Kwahk, 'Feasibility Study of Stress Measurement in Prestressing Tendons using Villari Effect and Induced Magnetic Field', International Journal of Distributed Sensor Networks, Article ID , M D Beard, M J S Lowe and P Cawley, 'Ultra Guided Waves for Inspection of Grouted Tendons and Bolts', Journal of Materials in Civil Engineering, ASCE, Vol. 15, No. 3, pp , S Salamone, I Bartoli, R Phillips, C Nucera and F L Scalea, 'Health Monitoring of Prestressing Tendons in Posttensioned Concrete Bridges', Journal of the Transportation Research Board, No. 2220, pp 21 27, B H Kim, J B Jang, H P Lee and I K Lee, 'Estimation of Prestressed Tension on Grouted PSC Tendon Using Measured Elastic Wave Velocity', Journal of Korean Society of Civil Engineers, Vol. 32, issue 5A, pp , 2012 (in Korean). 5. E Villari, 'Change of Magnetization by Tension and by Electric Current', Ann. Phys. Chem., p. 126, pp , B D Cullity and C D Graham, Introduction to Magnetic Materials, IEEE Press, pp , B Fernandes, J D Wade, D K Nims and V K Devabhaktuni, 'A New Magnetic Sensor Concept for Nondestructive Evaluation of Deteriorated Prestressing Strand', Research in Nondestructive Evaluation, Vol. 23, No. 1, pp 46-48, B Fernandes, M D Titus, D K Nims, A Ghorbanpoor and V K Devabhaktuni, 'Practical Assessment of Magnetic Methods for Corrosion Detection in an Adjacent Precast Prestressed Concrete Box-Beam Bridge', Nondestructive Testing and Evaluation, ifirst article, pp 1-20, 2012.