Abstract

The amount of concrete structures has increased tremendously in Japan since 1960. Due to the increase of the structures in use for more than 30 years, evaluation of the structures is becoming an important task for the civil engineers to specify when and how to repair or to strengthen the structures. To deal with the problem, non-destructive inspection technique is becoming more and more important, both for overall inspection and detailed inspection. The paper describes the present situation in Japan how the inspection is being done using NDI, and also introduces some of the newest techniques, such as thermograph, multi-spectral image sensing, ultra sonic inspection, etc., developed in Japan. These techniques may also be valuable in other countries.

1. Introduction

Recently, spalling of concrete from concrete structures, such as bridges, tunnels, etc., has become a big problem in the mass media. Although the spalling of concrete may not be a big problem considering the load carrying capacity of the structure, it may cause traffic accident when cars and trains run underneath at a high speed, such as in the case of the Shinkansen, which runs at a speed above 250km/hr. In 1999, a block of concrete hit a running Shinkansen in Fukuoka tunnel, and the top of several cars were partly damaged. The Ministry of transportation and ports and harbors organized a special committee to investigate the causes, and concluded that a part of the concrete lining spalled off due to insufficient consolidation at the time of construction and formation of inner cracks from early on.

The inspections of these structures were done mostly by visual inspection, and could not predict such an accident may occur. After the accident, all the tunnels were inspected not only by visual inspection but also by non-destructive inspection (NDI). The results of NDI, such as sonic inspection, thermograph, etc., gave more information about the deterioration of structures than visual inspection, showed that a large number of similar defects exist in concrete structures. Since then, many civil engineers and researchers are interested in NDI to inspect existing structures.

During the inspections, the problems encountered by the civil engineers who have to evaluate the structures are as follows: 1) It is difficult to select an appropriate NDI method for a specific cause of deterioration. 2) It is difficult to understand the measured results without enough knowledge about the NDI technique. To deal with the problem, this paper explains briefly the accuracy of the individual NDI method to inspect internal voids in reinforced concrete based on committee work of IIS [1].

2. Methods for Inspection and Evaluation of Concrete Structures in General

Maintenance of concrete structures is done by the engineers who take care of the structures. In case of public structures, the ministries, etc., maintain the structures as soon as they are completed. For the time being, the methods for the maintenance differ according to the owners of the structures. Although there are some differences, the main concept of the maintenance being used can be summarized as follows.

2.1 Periodic inspection
Periodic inspection is essential in the most cases. The inspectors inspect the structures visually, sometimes with the help of binoculars, once a month, once a year, or once in several years, according to the importance and the time after the structure is completed. In some cases, sonic inspection is carried out along with hammers. The inspectors are mostly trained engineers with experiences.

The periodic inspection covers not only the determination of the degree of deterioration but also to estimate the main cause of the deterioration. For example, when a crack was found at the corner of openings, the inspector may estimate the main cause as drying shrinkage with restraint. The periodic inspection is also used to decide whether further detailed inspection is needed or not. The decision is made, in most cases, based on the visual information data such as cracks, stains and spallings.

2.2 Detail inspection
The detailed inspection is done when the estimated degree of deterioration exceeds certain limits, or when some new phenomenon is found during the periodic inspection. The inspection is done normally by using non-destructive tests or taking core samples out from the inspected structure. The purpose of the inspection is to decide the cause of the deterioration and also to evaluate whether repair and/or strengthening is needed or not.

Fig 1: Void detected by thermograph.

In case of large concrete structures, such as bridges, tunnels, dams, buildings, etc., the structures are too large to be inspected in detail. To overcome the problem, the following methods are being used. 1) Overall inspection techniques, such as using digital still camera, thermograph, see Fig. 1, radar, sonic and laser technique are often used to sweep the whole area to be inspected and find out the distribution of defects within the structure. 2) Other techniques, such as X-ray, ultra-sonic, natural potential, and acoustic emission are often applied to get more detailed information.

Some movable inspection cars and trains have been developed for the overall inspection. In case of tunnel linings, an inspection train mounted by both a heating facility and a thermograph has been developed to obtain the crack distribution and possible spalling portions for the subways. To inspect the voids at the back of tunnel linings or pavements, radar mounted cars have been developed for railways, waterways and highways. To digitize the surface cracks of reinforced concrete slabs and tunnel linings, laser mounted cars have been developed. These techniques are important to obtain overall data of the structure automatically without any personal errors, and the data can be used to check whether the deterioration advanced since the previous inspection.

3. Non-Destructive Inspection of Internal Voids in Reinforced Concrete[2]

To repair a structure, one of the most important works is to determine the defects within a structure. Even when NDI is used, it is not an easy task to determine the location and size of an internal defect accurately in reinforced concrete. It has already been clarified that we can detect internal voids and defects in case of non-reinforced concrete structures [2]. But it has not yet been clarified to what extent the NDI can be applied in case of steel reinforced concrete. It is said that steel reinforcements hinder the capability of NDI in a large extent.

3.1 Outline of experiment

Fig 2: General view of specimens for inspection.

To check the applicability of NDI to concrete structures, 9 special reinforced concrete specimens (size:1000x1000x400-500mm each) were made and tested by several NDI methods. The test specimens used are summarized in Table 1 and the view of the specimens is shown in Figure 2. The tested NDI are as follows: thermograph (passive method and active method), sonic method, impact echo method, ultrasonic method, radar, etc. Among the tests being performed, results of the tests on Specimen No.7 are explained in the following paragraphs.

The size of the specimen is 1000X1000X400mm with a void in a shape of "A" embedded in the concrete specimen with 10 steel bars of D16. The concrete used for the test specimen is W/C ratio of 0.558, slump of 12cm, and air content of 4.8%. The compressive strength at the age of 28days is 33.5MPa, and the Young's modulus is 4400MPa.

Fig 3: Details of the specimen No. 7.

3.2 Tested results using thermograph
Three test results are explained in the following. Figure 5 shows the result of the thermograph by active method. The specimen was heated by artificial heat for 40 minutes and data was taken every 10 minutes. As shown in the figure, a slight figure of "A" can be seen after the specimen was cooled for 40 minutes.

Fig 4: Appearance of test specimen before concrete casting.

Fig 5: Result of thermograph by active method.

Fig 6: Result of thermograph by passive method.

Fig 7: Result of thermograph by time subtraction.

The result of the thermograph by passive method is shown in Figure 6. The specimen was heated by sunlight in the outdoors. Although a special technique was not used to heat up the specimen, the void can be detected as a triangle at the center of the concrete block. Although the data were obtained at daytime without considering the time of measurement, the result shows a good agreement with Figure 4.

Figure 7 shows the difference of thermograph data obtained at 10:00 and 13:00. Although the original data contains scattering of temperature from place to place, due to uneven heating, the temperature difference gives better results as shown in the figure.

3.3 Test results using sonic measurement

Fig 6: Result of sonic test.

Result of the sonic test is shown in Figure 6. The data shows the ratio of responded sonic wave amplitude to the original applied sonic amplitude by pulse hammer. There is a good correlation between the responded amplitude ratio and thickness of concrete, and using this relation, depth from the surface to the void can be estimated accurately. As shown in the figure, most of the void can be detected except the portion at the middle of "A".

3.4 Test results using impact echo measurement
The impactor used for applying impact at the surface of concrete was a 10mm steel ball. The impact was applied at each 50mm point and the reflected waves were monitored. The result shown in Figure 7 shows the shallower depth from the surface by dark color. As shown in the figure, the impact echo can detect the internal void quite accurately even when steel bars exist between the void and the surface. In order to detect the void accurately, selection of impactor and filter to eliminate low frequency waves is important. The result shows that the accuracy of the measurement was 71%.

Fig 7: Result of impact echo test.

3.5 Test results using ultra sonic measurement

Fig 8: Result of ultra sonic measurement.

The ultra sonic measurement (100 kHz) was done by measuring the depth from the surface at each 50mm point. Although measurement was done in the same manner, the reflected waves were affected by reinforcing steel bars. To eliminate the effect, average values were used to the measured points which were affected by the bars. The result is shown in Figure 8. As shown in the figure, the result coincides well with the actual depth of the void. To show the accuracy of the measurement, Figure 9 is shown in 2 dimensional expression. The figure clearly shows that the measured data coincides well with the actual void at an accuracy of more than 80%. The measured error of the depth was 10mm in case of the bottom surface of the specimen and 5mm in case of the surface of void (100mm depth).

Fig 9: Result shown in 2-Dimensional form.

4. Conclusions

As shown in the experimental works explained above, the individual NDI has its own characteristics. The methods described here are only a part of the work being done by the research committee of NDT in IIS, University of Tokyo. There may be other methods for detailed inspection, but it is important to recognize the difference and possibility of the NDI even in case of reinforced concrete.

Acknowledgement

The author would like to send sincere thanks to the committee members of NDT of IIS allowing me to use a part of their works for this paper.

References

1. Committee on NDT: Committee report on detail inspection using Non-Destructive Inspection, 2003, 4, IIS, University of Tokyo (in Japanese)
2. T. Hirata, T. Uomoto: Inspection of Concrete by Ultrasonic Technique (2) - Measurement Accuracy in Internal Void Detection in Concrete -, SEISAN-KENKYU, Vol.53,No.3 (in Japanese)