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Non-Destructive Testing Method for the Detection of the Internal Defects Inside ConcreteS. K. Woo, Korea Electric Power Research Institute
Y. C. Song, Korea Electric Power Research Institute
In this study, the non-destructive test was conducted in order to detect the internal defects of concrete by using the electromagnetic method and the impact echo method. For deriving characteristics and detectable limitations of each method, a specimen was manufactured. For the non-destructive test, one radar system and one impact echo method system were used. A piece of styrofoam (2´10cm), a piece of wood (2´10cm), a piece of 50mm sheath, a piece of 30mm PVC pipe and a piece of 25mm diameter steel bar were embedded in the concrete specimen with their respective covering thickness being different, and then they were measured for detecting internal defects in the concrete specimen. As a result of conducting this study, it has been demonstrated that the electromagnetic method is effective to detect steel bars embedded in concrete but it is difficult to detect internal defects in concrete by using the electromagnetic method, while it is difficult to detect steel bars by using the impact echo method, but the impact echo method is the non-destructive test method which is effective to detect a cavity, a cavity in the rear of a steel bar in concrete and the thickness of a member.
Keywords: Non-Destructive Test, Radar System, Impact Echo, Concrete, Internal efects
As various unexpected large accidents have recently taken place with civil and architectural structures, it is emerging as an important challenge to ensure effective maintenance and safety of existing structures. In order to maintain a structure to be safe and available at all times, the structure should be periodically inspected and evaluated, and proper and necessary measures should be taken before any problem takes place. It is, therefore, very important to inspect the structure through a precise checking, assess such inspection results correctly and further establish a proper measure depending upon such assessing results. This study is to identify applicability of the non-destructive test method to assessment of the soundness of the structure through measurement of internal defects in the concrete structure.
In this study, the non-destructive test was, therefore, conducted in order to detect internal defects in concrete by using the electromagnetic method and the impact echo method. Meanwhile, for deriving characteristics and detectable limitations of each method, a specimen was manufactured.
2.1 Principle and Characteristics of the Electromagnetic Method
The measurement principle of the radar using the electromagnetic wave is based on the fact that when the electromagnetic wave belonging to the microwave band penetrates any object or medium, penetration and reflection of the electromagnetic wave gets to be affected by electromagnetic characteristics and shape of the object or medium. When the electromagnetic wave is radiated from the transmitting antenna to the concrete test specimen of which internal defects steel bars to be detected are embedded in the actual measuring process, this electromagnetic wave gets to be reflected from the boundary surface of any matter having any different electromagnetic characteristics, such as a steel bar, a cavity or an internal defect, and further to reach the receiving antenna. The shape and material of the embedded object can be discerned through the reflected waveform of the electromagnetic wave. Fig. 1 depicts briefly the measuring process using the radar system.
Wherein, the velocity (n) of the electromagnetic wave in the concrete specimen varies depending upon the dielectric constant, er which is the electromagnetic material characteristic of concrete.
Wherein, C is the velocity of the electromagnetic wave (3´108 m/s) in a vacuum condition, and er is the dielectric constant of concrete.
|Fig 1: Schematic Diagram of Radar System.|
The distance (D) from the transmitting antenna to the embedded object can be obtained by using Equation 2 on the basis of the propagating time (T) while the incident wave goes from the transmitting antenna to the object and further the reflected wave returns from the object to the receiving antenna.
2.2 Principle and Characteristics of the Impact Echo Method
In a structure of which component particles are large, such as a concrete structure, the component particles cause the wave to disperse greatly. Thus, it is difficult to use the ultrasonic wave method, which is widely used in the metal field, in such structure. This is why it is preferable to use the method using the elastic wave therein. The impact echo method using the elastic wave is a method which is used to assess quality of, measure thickness of, detect a crack and a cavity in a structure by inflicting an impact on the surface of a test specimen to generate the body wave, and then looking into the waveform of the body wave that is reflected from any discontinuity or any boundary face between heterogeneous medium layers and comes back to the surface.
2.3 Principle and Characteristics of the Surface Wave Method
The surface wave method is to identify characteristics of any object (the ground or a structure) by using the dispersive characteristic that the propagating speed varies depending upon any change in the wavelength of the Rayleigh wave among elastic waves which propagates along the surface of the object. The surface method is based on measurement and interpretation of the surface wave dispersed in the bedded structure. Dispersion of the surface wave means that the propagating speed of the wave varies in the medium having the bedded structure, depending upon the frequency (or wavelength) of the wave. This dispersive characteristic of the surface wave is attributable to the motional distribution of the medium particle depending upon the depth thereof.
2.4 Impact Echo-Surface Wave Method
In order to estimate the thickness and defects of any structure by using the impact echo method, the P-wave speed in the material thereof should be known beforehand. The P-wave speed can be known by using the impact echo method on the part of which the boundary condition and the thickness are already known. However, such structure as a tunnel or a floor slab has typically only the one face exposed, and therefore, it has no local part of which the exact thickness is known. This is why the core is collected therefrom, the P-wave speed is obtained from it, and then such P-wave speed is applied to the impact echo method as a representative value of the overall structure. However, as the case may be, from the characteristics of such structure as the tunnel, the core can not be collected from the structure for the reason that any waterproofing problem or any damage should be prevented from taking place with it, or the physical property value of concrete may be different depending upon positions in the same structure. Therefore, it cannot be deemed that the wave speed obtained from the core represents the entire structure.
As another method used for obtaining the P-wave speed in the structure, there is a direct measurement method to obtain the P-wave speed by installing two detectors on the surface of the structure and further using the difference in the time that P-wave reaches the detector and the distance between the two detectors. This direct measurement method has an advantage to obtain the P-wave speed relatively reliably in the position where the experiment is carried out. But it requires a sophisticated high-sensitive equipment, and through it, only the wave speed propagating along the surface is measured, and it may be, therefore, unreliable to estimate the P-wave speed corresponding to the whole thickness of the member from the part of which the surface is defective.
Thus, the impact echo-surface wave method (IE-SASW Method), which obtained the P-wave speed by using the surface method and further applied it to the impact echo method, was used in this study. This method has a demerit to assume the Poisson's ratio of the material in its application, but it enables the full non-destructive test to be conducted in the position where the experiment is to be carried out, and further with it, the wave speed corresponding to the median part or the whole thickness of the structure, not the wave speed propagating along the surface, can be obtained. Therefore, it can overcome the demerit that may take place with the direct measurement method.
Fig. 2 illustrates the specimen manufactured for the experiment. As illustrated in Fig. 2, the specimen has a piece of styrofoam (2´10cm), a piece of wood (2´10cm), a piece of sheath tube ( 30 mm) and a piece of PVC pipe ( 30 mm ) and two pieces of steel bars (25 mm) embedded in concrete, and the experiment was conducted just over such objects.
|Fig 2: Section of Specimen.|
4.1 Electromagnetic Method
|Fig 3: Experimental Results using the Electromagnetic Method in Specimen.|
4.2 Impact Echo-Surface Wave Method
(a), (b), (c) and (d) of Fig. 4 illustrate the experimental results using the impact echo-surface wave method in the specimen. (a) illustrates the experimental result between the wood and the sheath tube, showing that when the P-wave speed of the experimental member is assumed to be 4000m/sec, the member's thickness is obtained as "depth = VP/2f = 4000/(2´10752) = 0.186m", which is similar to the actual thickness of the member.
|(a) Between Wood and Sheath Tube|
|(d) PVC Pipe|
|Fig 4: Experimental Results using the Impact Echo-Surface Wave Method in Specimen.|
If respective positions of the embedded styrofoam and the wood are calculated by this method, their depths are obtained as 6.8cm and 6.4cm respectively, which are approximate to 6cm, the actual depth of the embedded styrofoam and wood. Meanwhile, if the position of the embedded PVC pipe is calculated, it is obtained as 9.2cm, which is a little different from 8cm, the actual depth of the embedded PVC pipe. It is thought that this is because some wave returns along the PVC pipe of which the sectional shape is round.
This study is to derive characteristics and detectable limitations of the electromagnetic method and the impact echo method for detecting internal defects in concrete. Through experiments, the following conclusions have been yielded;
This study has been performed under the support from the mid to long term nuclear research and development program initiated by Ministry of Science and Technology. We acknowledge such support with a warm heart.
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