Application of ultrasonic testing to steel-concrete composite structures

Transcription

1 More Info at Open Access Database Application of ultrasonic testing to steel-concrete composite structures Arisa YANAGIHARA 1, Hiroaki HATANAKA 2, Minoru TAGAMI 2, Katsuya TODA 3, Yoshihiko NAKAMURA 4 1 Production Engineering Dept., Manufacturing Division, Aero-Engine & Space Operations, IHI Corporation; Soma-shi, Fukushima, Japan, Phone: , Fax: Production Technology Dept., Production Engineering Center, Corporate Research & Development, IHI Corporation; Yokohama-shi, Kanagawa, Japan 3 Research & Development Dept., Engineering Headquarter, IHI Infrastructure Systems Co., Ltd.; Minato-ku, Tokyo, Japan 4 Construction Dept., Engineering Headquarter, IHI Infrastructure Systems Co., Ltd.; Minato-ku, Tokyo, Japan Abstract This paper presents the method to examine the boundary between steel plates and concrete for steel-concrete composite structures such as slabs and rigid connections between piers and beams by using ultrasonic testing. The ultrasound with a frequency of hundreds khz is employed. The multiple reflections in thin steels and the multiple boundary reflections for thick steels are studied. In case that the concrete is filled enough, the multiple reflections are attenuated because ultrasound goes partly through into concrete. On the other hand, if large unfilled or delaminated parts exist at the boundary, the reflections are observed with the same echo heights as only steel plates. This method has been successfully used in field conditions and could be applied to detect rain infiltration parts for in-service slabs. Keywords: Steel-concrete composite structures, Low-frequency ultrasound, Multiple reflections, Lamb wave, Fresh concrete, Multiple boundary reflections, Distance-Gain-Size (DGS) diagram 1. Introduction In recent years, steel-concrete composite structures have been widely applied in bridge construction due to their durability and cost efficiency. Examples include steel-concrete composite slabs and rigid connections between piers and beams. The steel-concrete composite structures must be completely filled with concrete in order for them to exhibit their required strength and to enhance the durability of the bridge. However, steel plates make it impossible to visually verify how evenly the concrete casting has performed. With reference to the steel-concrete composite slabs (hereinafter called composite slabs [1] ) shown in Figure 1, any parts left unfilled after construction, including gaps and honeycombing, would allow rain infiltration into slabs, posing the risk of diminished durability as a result of corrosion of reinforcing members (e.g., ribs, studs, and channels) or steel bottom plates. As for rigid connections between a reinforced concrete pier and a beam shown in Figure 2, the

2 direction of concrete casting tends to cause gaps on the bottom surface of the steel plate flange. In addition, the thickness of steel plates make it difficult to perform simple testing methods such as hammering inspections [2], [3]. To solve these issues we focused our attention on ultrasonic testing and developed the method with low-frequency ultrasound for detecting unfilled or delaminated parts at steel-concrete boundary, which is able to apply not only to hardened concrete but also to fresh concrete. Moreover, by applying to this method, the rain infiltration parts for in-service composite slabs could be detected. Figure 1. Schematic view of steel-concrete composite slabs Figure 2. Schematic view of rigid connections between a reinforced concrete pier and a steel 2. Overview of the method by using ultrasonic testing 2.1 Ultrasonic testing for concrete Ultrasonic testing (UT), one type of nondestructive testing methods, is suited to inspecting the inner condition of materials. It has been widely applied in industrial fields for steel weld inspection and steel quality examination [4]. For concrete structures, UT is used to measure sound velocity, slab thickness, and depth of cracking, as well as for estimating strength. Although 2-5 MHz ultrasound is normally used in UT for metallic materials, khz ultrasound is used for concrete because it is a porous composite material made of cement paste and aggregates [5]. Acoustic properties of concrete which depend on the material properties and the age of the concrete should also be taken into consideration in the case of estimation by UT. 2.2 Development of testing with the use of multiple reflections in thin steels Principle of the developed method When the sound in medium 1 reflects at the boundary of medium 2, the reflection coefficient

3 of sound pressure ( ) is considered as; = + (1) where, Z : acoustic impedance calculated as the multiplication of density by sound velocity The examples of reflection coefficients are shown in Table 1. Table 1. Reflection coefficients for longitudinal straight-beam incidence Material Density 10 3 kg/m 3 Sound velocity m/s Reflection coefficient for steel % Hardened concrete 2.3 4, Fresh concrete 2.3 1, Self-compacting mortar 2.3 4, Air Water (20 ) In the case of hardened concrete, it is easy to distinguish concrete from unfilled or delaminated parts (i.e. air) at the boundary by echo height [6]. However, it is difficult to detect unfilled parts when the concrete is not hardened (i.e. fresh concrete) and to distinguish air from water for the investigation of the rain infiltration parts at in-service composite slabs. Steel bottom plates used for composite slabs are commonly as thin as 6-9 mm. When the lowfrequency longitudinal ultrasound with a frequency of hundreds khz order is transmitted from the steel plate, ultrasound propagate mainly as Lamb waves in the plate while repeating multiple reflections, mode conversion, and interference. Lamb waves occur when the thickness of the medium is smaller than wave length and the complicated oscillations of waves are generated. The same probe is used as a receiver because the back wall echo of the thin steel plate would be in the dead zone if single-probe straight-beam technique is used with low-frequency ultrasound. The reflection signals contain several frequency contents. The frequency content of transmitted ultrasound ft is strongly observed, and the interval of longitudinal multiple reflections fm is also observed and considered as; = (2) where, v : sound velocity of steel, t : thickness of a steel plate

4 Multiple reflections tend to strongly occur for when a steel plate oscillate freely due to the existence of unfilled or delaminated parts. On the other hands, multiple reflections are attenuated when the fresh concrete or rain infiltration exist on a steel plate because ultrasound goes partly through into them. Therefore, unfilled parts in fresh concrete and the rain infiltration at in-service composite slabs could be recognized based on the intensity of fm Demonstration test The property of fresh concrete depends on the concrete mixture which is influenced by temperature. So, tests were performed in constant temperature and humidity room. The several concrete temperatures included in the available range of the concrete temperature for casting normal concrete (concrete temperature: 5 to 35 ) [7] were adopted. As for concrete, Ordinary Portland Cement to provide a designed compressive strength of 30 MPa by 28 days, a slump of 10 cm and the maximum aggregate diameter of 20 mm were used. Figure 3. Testing equipment (left) / Specimen, ultrasound probes and testing aid (right) The schematic views of the setup are shown in Figure 3. Normal longitudinal wave probes with a frequency of 250 khz including a transducer with a diameter of 38 mm were used and settled to the steel plate with testing aid. The thickness of the steel bottom plate was 8mm and each sides of the plate was surrounded by the steel formwork. Concrete was filled to make the height of fresh concrete 100mm while ultrasonic signals were continuously acquired. The examples of test results are shown in Figure 4. The intensity of fm decreased more than 6 db right after filling and vibrating concrete on the steel plate as compared to the intensity acquired before filling concrete. In addition, the same results were acquired in the cases of the temperature 7 and 33. It was cleared that this method was applicable to detect unfilled parts during concrete casting for the available range of the concrete temperature for casting normal concrete.

5 Figure 4. Test results at concrete temperature Field test Small probes with a frequency of 250 khz including a transducer with a diameter of 20 mm were used for the easy handling in fields. Gel sheets with a thickness of 5mm were also used as the alternative coupling medium. Ultrasonic signals were continuously acquired during casting and vibrating concrete at 4 different points. The intensity of fm at each points decreased about 4 db right after casting and vibrating concrete as compared to the intensity acquired before casting concrete. Considering that the conditions of vibrating concrete have an influence on the decrease of fm, fresh concrete was fully casted from the test results. The setup for field test is shown in Figure 5. Figure 5. The setup for field tests

6 Generally, hammering testing has been performed to confirm that concrete has been filled on the bottom steel plate. This method is easy but the testing results depend on the inspector because the judgement is performed on the basis of listening sound. On the other hands, the developed method enables to perform the inspection with the judgement standard and to record the results Application to detect rain infiltration into in-service composite slabs In the case of in-service composite slabs, a slightly thin gap (i.e. adhesion fracture) between a steel bottom plate and hardened concrete is sometimes occurred due to the shrinkage of concrete and repeated wheel loads by passing cars. As mentioned before, rain infiltration into the slabs causes the risk of diminished durability. Therefore, we tried to apply the method developed for the testing during casting to the detection of the rain infiltration parts at the boundary for in-service slabs. Demonstration test was performed by using the specimen which had adhesion fracture at the boundary caused by repeated wheel loads test. The through holes for pouring water were drilled in the steel plate. Ultrasonic signals were continuously acquired during pouring water. The same equipment as shown in Figure 5 was used. The schematic views of the setup for this test are shown in Figure 6. Figure 6. Specimen (left) / Holes for pouring water (center) / Setting of the probes (right) The examples of test results are shown in Figure 7. The intensity of fm decreased more than 6 db after the boundary was filled with water (i.e. water gap was created) as compared to the intensity acquired before pouring water. Generally, visual testing has been performed by checking the water leakage from the defined places such as monitoring holes or joints. On the other hands, this method enables to investigate anywhere. To perform the investigation of rain infiltration efficiently, it is recommended that the measuring points should be decided before applying this method based on the judgement of echo height because the intensity of echo is strong when any gaps exist at the boundary.

7 Figure 7. Test results for the detection of water infiltration 2.3 Study of the testing with the use of echo heights of multiple reflections at the boundary between thick steel plates and concrete Principle of the developed method The thicknesses of steel plates used in rigid connections are often mm depending on the required strength. The ultrasound beam spreads with the increasing distance from transducers. In far field, the flaw echo height bear a proportionate relationship to the area of the flaw when the flaws are smaller than the beam width. Figure 8. Schematic views of the difference in the number of the boundary reflections

8 Figure 8 shows how the number of reflections at the steel-concrete boundary influences the ultrasonic echo height. The echo height of reflected ultrasound gradually diminishes with increasing the number of the boundary reflections for the reflection coefficient shown in formula (1). It could be expected that the echo height of multiple boundary reflections is useful for detecting unfilled parts under thick steel plates Logic verification As for the logic verification, a normal longitudinal wave probe with a frequency of 500 khz including a transducer with a diameter of 25.4 mm was employed. The thickness of steel plates was 75mm, and self-compacting mortar was chosen as the casting material. As experimental values, B3 echoes for several artificial unfilled parts were acquired For this case, the correlation between the echo height of a flaw and the propagation distance of the ultrasound could be calculated based on Distance-Gain-Size (DGS) diagram [8]. In addition, the scattering caused by aggregates should be made consideration. Fine aggregates are contained as the scattering sources in self-compacting mortar. Assuming the shape of a scattering source is sphere and the multiple scattering between sources might be ignored, the total energy of scattering is considered as [9] ; _ = ( + ) (3) where, : amplitude of the incident wave e: ratio of the scattering sources for total volume, V : total volume, D: diameter of the scattering source, λ: wave length, K: elastic modulus of the medium, ρ: density of the medium : average of the difference in elastic modulus between the medium and the scattering source, : average of the difference in density between the medium and the scattering source The value of _ was calculated using the figures shown in Table 2. The value of V was considered as 1, and the value of λ was calculated as 9 mm by using the frequency of ultrasound and the sound velocity of self-compacting mortar shown in Table 1. Then, the amplitude of scattering sources is able to be calculated by taking the square root of formula (3). Therefore, the theoretical value for the amplitude of scattering sources was equal to 0.24.

9 Table 2. Values used for the calculation of _ in the case of self-compacting mortar Material ρ, 10 3 kg/m 3 K, kn/mm 2 D, mm e Fine aggregates Cement paste Assuming the total amplitude as at the boundary between steel and self-compacting mortar, the amplitudes of reflected waves and transmitted waves (i.e. incident waves into self-compacting mortar) were considered as 0.64 and 0.36, respectively by using the reflection coefficient shown in Table 1. The actual amplitude of transmitted waves with considering the scattering waves was given as To keep the energy balance at the boundary, the apparent amplitude of reflected waves was given as , i.e Thus, the apparent reflection coefficient 73% was applied to the calculation of the theoretical values based on DGS diagram. The results of both experiment and theoretical calculation are shown in Figure 9. The experimental result provided good agreement with theoretical values. Further study will be made to apply this method to the cases of steel plates with the different thicknesses. Figure 9. Correlation between echo heights and artificial flaws in the specimens representing rigid connections between the RC pier and the steel beam 3. Conclusion In this paper, ultrasonic testing with low-frequency ultrasound is studied to examine the boundary between steel and concrete for steel-concrete composite structures. As for composite slabs, the interval of longitudinal multiple reflections fm in the thin steel

10 plate is applicable to distinguish unfilled parts from filled parts during concrete casting. Moreover, this method could be applied to detect rain infiltration into in-service slabs. As for rigid connections between the RC pier and the steel beam, the echo height of multiple boundary reflections is useful for detecting unfilled parts under thick steel plates. The theoretical values considering the influence of scattering by aggregates showed the good agreement with experimental values. For the future, it is needed that quantitative testing which is applicable to different material age, concrete mix, compacting condition etc. Then, it will be performed to make more practical application to the structures in real fields and to expand applicable objects of developed method. 4. References 1. O Suzuki, Y Watanabe, A Fukui, N Uno and J Ogawa, 'Study on Development of Channel Beam Composite Slabs', IHI Engineering Review, Vol 37, No 3, pp 81-89, October K Mukae and M Fujita, 'A Fundamental Study on Concrete Defect Detection by Tapping', Reports of Technical Research Institute of Sumitomo Construction Co., Ltd., No18, pp , October K Toda, H Hatanaka, Y Hayashi, Y Kawano and M Iketani, 'Diagnosis of Concrete Deterioration by Nondestructive Evaluation', IHI Engineering Review, Vol 37, No 3, pp , October Y Nakanishi, A Yanagihara and Y Yamaguchi, 'Plain Welding Course for Bridge Fabrication and Construction Vol. 9 Nondestructive Inspection from the aspect of welding Bridge and Foundation Engineering', Vol 44, No 11, pp , November H Hatanaka, Y Kawano, N Ido, M Hato and M Tagami, 'Ultrasonic testing with advanced signal processing for concrete structures', Nondestructive Testing and Evaluation, Vol 20, No 2, pp , June A Yanagihara, H Hatanaka, M Tagami, K Toda and Y Nakamura, 'Development and Application of Non-Destructive Inspection for Steel-Concrete Composite Structures', IHI Engineering Review, Vol 46, No 1, pp 15-21, January Japan Society of Civil Engineers, 'Standard Specification for Concrete Structures [Construction guide]', pp , March Y Nagai, A Yanagihara, H Hatanaka, K Toda and Y Nakamura, 'Ultrasonic Examination to Detect Concrete Unfilled Parts for Thick Steel-Concrete Composite Structures', Proceeding of the 4th Symposium on Nondestructive Testing Method for concrete Structure, Vol 4, pp , August Japan Society for the Promotion of Science (GAKUSHIN), 'Ultrasonic Material Testing (Revised Edition)', pp , 1974