GB2309077A

GB2309077A - Detecting defects in a structure by temperature distribution measurement

Info

Publication number: GB2309077A
Application number: GB9616322A
Authority: GB
Inventors: Kenichiro Yamazaki; Kiyotaka Kawase; Toshio Koike; Susumu Harashima
Original assignee: Nitto Chemical Industry Co Ltd
Current assignee: Nitto Chemical Industry Co Ltd
Priority date: 1996-01-11
Filing date: 1996-08-02
Publication date: 1997-07-16
Anticipated expiration: 2016-08-02
Also published as: GB9616322D0; GB2309077B; FR2743633A1; FR2743633B1

Abstract

In order to detect a defect in a structure (31), such as a concrete bridge, the radiation energy from the surface of the structure is measured by means of an infrared radiometric thermometer (2). This thermometer is used to detect areas of the surface having a temperature which is different by 0.3{C from the temperature of the surrounding areas. It is determined that there is a defect present if the area detected is not less than 400 cm 2 . Furthermore, the radiation energy in both day time and at night time is measured and the temperature distributions of the structure's surface at these various times are obtained. Since defected areas have a higher temperature than non-defected areas during the day, and a lower temperature during the night, defects in the structure can be more accurately detected by determining the differential temperature distributions between these various times, since the difference between defected and non-defected areas then becomes more enhanced.

Description

SPECIFICATION TITLE OF THE INVENTION METHOD OF DETECTING DEFECTS OF STRUCTURE DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method of detecting defects of a structure.

Prior Art Defects such as cracks of external wall, separation of mortar or lifting of tiles take place in structures after the lapse of extended period of time. Since leaving of these defect without treating them may cause leakage of water or stripping of tiles, it is necessary to detect and repair them as prematurely and reliably as possible. A tapping method has heretofore been known in which an operator taps the surface of a structure with, for example, a wooden hammer and determines the presence of defects by listening to the sound generated from the tapped surface. However, this method is inferior in operation efficiency and reliability.

Hence, defect detecting methods which are excellent in operation efficiency and reliability has been demanded. As one of these methods, there is a method in which presence and/or position of a defect in the structure is determined by measuring the radiation energy from the structure surface by means of an infrared radiometer for identifying an area which has a higher different temperature than that of the other area.

Now, the detecting method of this type which has heretofore been carried out will be briefly explained. The radiation energy from the structure surface is measured by an infrared radiometer.

The distribution of the temperature on the structure surface is determined from the results of the measurement and is image processed so that it is displayed on a CRT screen as a thermal image. An operator observed the image on the CRT to determine based upon his experience that there is a defect in an area where a different temperature distribution is exhibited. Although this method is essentially excellent, various improvements in it have been proposed.

In order to detect the defect by this type of method, defected and non-defected areas should exhibit different temperature distributions, which are displayed as thermal images. It is preferable that difference in temperature distribution between the defected and non-defected areas be distinct since the operator visually determine the presence or absence of the defect.

However, the environmental conditions around actual structures are different. Due to difference in season or time when the measurement is carried out, or difference in amount of sun light with which the structure is exposed or shadow of the other structure, defected and non-defected areas may exhibit identical temperature distributions, or different defected or non-defected areas may exhibit different temperature distributions.

In such a case, it is very difficult to detect the presence or absence of defect only by measuring the temperature distribution on the structure surface by means of an infrared radiometer and by the operator visually observing the measurement results displayed on the CRT.

The accuracy of the detection of the defect is inevitably low since the operator who visually observes the CRT screen should determine whether or not there is a defect on the basis of his or her experience.

On the other hand, steel plates are bonded to concrete floor slab of the road or bridge for reinforcement thereof. If any defect such as incomplete bonding to the reinforcing steel plates occurs, the strength of the bridge is not only lowered, but also there is the risk of falling of the reinforcing plates.

In order to detect such defect, the above-mentioned tapping method has been used in which an operator taps the surface of the reinforcing steel plates with a wooden hammer to determine the defect by listening to the sound.

However, this tapping method does not have only a low reliability, but also is inferior in operation efficiency and economy since the scaffolds for the operators should be provided below the bridge.

The method of detecting the defect of such steel plate reinforced concrete structure may further includes impact method, repulsion method and ultrasonic method in addition to the tapping method. However, any of these method is not satisfactory in operation efficiency, reliability and economy.

In order to detect the defect of the steel plate reinforced structure, the method of detecting the defect of the steel plates reinforced structure may be considered in which the radiation energy from the external wall is measured by means of an infrared radiometric thermometer. However, this method ha snot been used for the steel plates reinforced concrete structure to which the present invention is applied. The reason why this method has not been used for the steel plate reinforced concrete structure is that it has been presumed that the temperature difference is so low that it is not possible to detect the defect although the radiation energy from its surface is detected by the infrared radiometer since the size of the steel plate per se is so remarkably greater than that of tile that sufficient energy is not actually provided to the steel reinforced structure.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an enhanced detection accuracy of a defect on the structure surface by making it possible to distinctly determine the temperature difference between the defected and non-defected areas on the structure surface.

It is another object of the present invention to make it possible to detect a defect of the steel plate reinforced structure by measuring the radiation energy from the structure surface by means of an infrared radiometer.

A method of detecting a defect of a structure by detecting the radiation energy from the surface of the structure by means of an infrared radiometric thermometer to determine the distribution of the temperature on the surface of the structure, comprises the steps of: detecting an area having a temperature which is different by i0.3 C or more from that of the surrounding area based upon a signal of the temperature from said infrared radiometric thermometer; and determining that the area has a defect if the area is not less than 400cm: In this case, measurements are conducted at different plural times and the temperature difference is measured at every times to determine that there is a defect in an area having a temperature difference which is not less than +0.3 C.

A determination can be made that there is a defect in an area having a temperature difference which is not less than t0.3 C and -0.3 C when measurement is conducted in the daytime and night, respectively.

A method of detecting a defect of a structure by detecting the radiation energy from the surface of the structure to determine the distribution of the temperature on the surface of the structure, comprises the steps of:detecting the radiation energy in the periods of the daytime and night to obtain the distributions of the temperature on the surface of the structure in respective periods of time, and determining the differential temperature distribution by differentiating these temperature distributions to determine the defect of the structure based upon the differential temperature distribution.

In this case, an integrated temperature distribution may be obtained by integrating said differential temperature distribution for time and in which the defect of the structure is detected based upon the integrated temperature distribution.

A determination may be made that there is a defect at an area having a temperature difference not less than t0.3 C in the differential or integrated temperature distribution.

The present inventio is applicable to the structure which is reinforced with steel plates which are integral with a concrete slab on the surface thereof.

When the temperature on the surface of a structure is measured, various disturbances affect a captured thermal image signal although the temperature difference between the area in interest and the surrounding area is high. Accordingly, the present invention makes a determination that there is a defect at an area if the area is not less than 400cm: when the area has a temperature difference of 0.3 C or more in comparison with the other area. Conversely, the present invention makes determination that there is no defect at an area having a temperature difference of not less than 0.3 C if the area is not larger than 400cm: , or neglects the defect in view of strength of the structure.

In case in which a determination whether there is a defect is made with reference to a temperature difference between an area in interest and the surrounding area, the temperature difference is often not less than 0.3 C due to disturbances if the area in interest is not less than 400cm2 . If a determination that there is a defect in all areas having a temperature difference of 0.3 C or more is equally made, non-defected area may be determined as defected area. It is practical and no problematic to determine that there is no defect if the area in interest having a temperature difference of 0.3 C or more is not less than 400cm: In such a manner, determination of defect is not made with reference to absolute level of the temperature, but is made with reference to the temperature difference between the area in interest and the surrounding area. It is possible to objectify the determination.

In order to further enhance the detection accuracy, it is preferable to conduct measurement at plural different times to determine that there is a defect at an area having a temperature difference of 0.3 C or more at each of measuring times.

In the daytime, the temperature on the surface of a structure is elevated due to sun light exposure. Part of the solar energy which is incident upon the surface is reflected thereon while the other is absorbed by this structure. The absorbed thermal energy is transmitted in a depth direction. The temperature on the surface of the structure is determined depending upon the relation between the thermal energy and the heat transmission rate. If there is a defect such as lifting of tiles, heat transmission is hindered at the lifting area so that transmission of the absorbed thermal energy is delayed. The temperature on the surface of the defected area becomes higher than that of the other non-defected area.

In the night, a reversed heat transmission in an opposite direction occurs in which the heat energy which is absorbed by the structure dissipates to an atmosphere having g a lower temperature.

Accordingly, a reversal phenomenon in which the temperature on the surface of the structure is lower than that inside of the structure. "Reversal phenomenon of the temperature difference" in which the defected area is lower in temperature than non-defected area occurs under this reversed phenomenon.

Therefore, the present invention determines that there is a defect in an area having a temperature difference which is 0.3 C or more and -0.3 C or more and when measurement is carried out in day time and night, respectively.

On the other hand, in the conventional detecting method, the distribution of the temperature on the surface of the structure is displayed on a CRT as an infrared image. An operator visually observes the image on the CRT to determine the level of temperature to make a decision relating to the defect. Since this determination is not objective, but is based upon the subjectivity of the operator. Determinations are different among persons and wrong determination of defect is liable to be made.

In the present invention, the temperature difference between the defected and non-defected areas is enhanced by differential operation, preferably further integration operation. Even if sufficient energy is not provided to the structure so that the temperature difference, which is measured by the infrared radiometer is small, the defect can be positively detected.

Further, in the present invention, the radiation energy is detected by the infrared radiometer in the daytime and night, to obtain the distributions of the temperature on the surface of the structure in respective times. A differential temperature distribution is obtained by conducting a differential operation between both temperature distributions. Since "reversal phenomenon" occurs between the daytime and night, the temperature difference between the defected and non-defected areas is enhanced by obtaining the differential temperature distribution, so that it become more clear. It becomes easier for operator to detect the defect when he visually observes the differential temperature distribution displayed on the CRT.

The temperature difference between the defected and nondefected areas can be enhanced by integrating the differential temperature distribution for time. It becomes easier to detect the defect.

Determination that there is a defect in an area having a temperature difference of 0.3 C or more in the differential or integrated temperature distribution is made based upon numerals.

Higher objective detection of defect can be carried out in comparison with a method in which an operator make a determination based upon his experience and perception.

On the other hand, in the present invention, in order to detect a defect of a structure which is reinforced with steel plates which are integral therewith, the radiation energy from the reinforced surface of the structure is measured by means of the infrared radiometric thermometer to detect the defect based upon a signal from the thermometer.

This kind of method has been used for detecting a defect of an external tile wall face. In such a conventional detection method, the distribution of the temperature on the structure surface is displayed on a CRT as an infrared image and an operator visually observes the image on the CRT to determine the level of the temperature to to make a decision relating to the defect. This determination is not objective, but is based upon operator's subjectivity. Determinations are different among persons. Wrong detection of the defect is liable to be made.

In contrast to this, the detection method of the present invention is not based upon the operator's subjectivity so that it is applicable to the steel plate reinforced structure.

"Reversal phenomenon of the temperature difference" occurs even in the structure which is reinforced with steel plates on the surface thereof. In other words, the temperature of the reinforcing steel plate is elevated on exposure to sun light in the daytime. Part of the solar energy which is incident upon the surface is reflected thereon while the other is absorbed by the structure. The absorbed thermal energy is transmitted in a depth direction of the concrete slab. The heat transmission is hindered by the defect such as air gap which is formed due to insufficient filler and separation of steel plate due to vibration. The transmission of the absorbed thermal energy is delayed, so that the temperature on the surface of the defected area is higher than that of the non-defected area.

In the night, heat transmission in a reversed direction occurs in which the thermal energy which is absorbed by the structure is dissipated to atmosphere having a lower temperature. Accordingly, reversal phenomenon of the temperature difference occurs similarly to a structure having no reinforcing steel plate, which has a lower temperature on the surface thereof, and a higher temperature inside thereof. Under this this condition ,reversal phenomenon of the temperature difference in which the defected area is in temperature than non-defected area occurs. It is deemed the reason of this phenomenon is that it takes longer time for the non-defected area to be cooled since heat transmission is conducted via the nondefected area while the steel plate surface at the defected area is cooled in a short period of time and thereafter heat transmission is hardly conducted.

Therefore, the present invention is preferably applicable to steel plate reinforced structure.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing a model structure which is used in the EXPERIMENT 1 of the present invention; Fig. 2 is a bottom view showing a steel plate reinforced floor slab which is formed with a simulated defect; Figs. 3 and 4 are graphs showing the results of EXPERIMENT 1; Fig. 5 is a schematic view showing a bridge which is used for applied EXPERIMENT of the present invention; Fig. 6 is a bottom view showing the steel plated reinforced floor slab of its bridge; Fig. 7 is a graph showing the result of EXPERIMENT 1 of the present invention; Figs. 10 and 11 are graphs showing the results of EXPERIMENT of the present invention; Fig. 12 is a schematic view showing the thermal image of the wall surface of the structure in interest; Fig. 13 is a schematic view showing the thermal image of the wall surface of the structure in interest which is measured at the other time; Fig. 14 is a schematic view showing the subtracted images; and Fig. 15 is a graph showing the results of the EXPERIMENT 2.

DESCRIPTION OF MODE OF PREFERRED EMBODIMENT Now, modes of the preferred embodiments of the present invention will be described in detail.

The basic method of detecting defect of the present invention is conducted as follows; An infrared radiometer thermometer (thermal image sensor) is provided to face the surface of the structure to be measured. The infrared radiation energy from the surface is detected. The detected signal is fed to an image analyzing device in which a contour map representing temperature difference is prepared based upon the measured temperatures at respective areas.

In this case, as a reference temperature which is a reference for a temperature difference, an average temperature in a given imaged area at a measuring time is adopted, or a minimum temperature (excluding a temperature which is remarkably different from the average temperature) in a group of unit areas which are segmented from an imaged area at a measuring time, or the temperature at an area where the temperature does not change over a given length in a graph showing the temperature difference since the area of the defect portion is smaller than that of the nondefected area. These adopted reference temperatures are not substantially different.

On the other hand, it is necessary to obtain the correlation between the area in the contour map and the actually measured surface (reinforcing steel plate surface) since they are relevant in the contour map of the ' temperature difference. Hence, in practice, it is possible to determine whether or not the area having a temperature difference of 0.3 'C or more is not less than 400cm: of the measured area by measuring the distance between the position of the infrared radiometric thermometer and the measured steel place surface by means of measuring instrument or by making the area of the current image proportional to the area of the steel plate by also considering the angle of the depression or elevation if the position of the infrared radiometric thermometer has been known on the map. In this case, the magnification of a used lens should be also considered.

Thus, if the area of the detected surface is more than 400 cm2 , it is determined that there is a defect in this area. The limitation of the area of the measured surface and the temperature difference is determined by the following model experiment which is described in detail.

EXPERIMENT 1 (model experiment) A model of a bridge is made as a steel plate reinforced structure as shown in Fig. 1. A steel plate reinforced floor slab 31 comprising a floor slab 31A made of precast concrete having a width of 1 m, a length of 2.5 m and a thickness of 15 cm, to which a reinforcing steel plate 31B having a width of 40 cm, a length of m and a thickness of 6 mm is bonded with epoxy resin is disposed on scaffolds 32. Concrete panels 33 are suspended from the opposite sides of the floor slab 31, for simulating a main girder of an actual bridge.

The steel plate reinforced floor slab 31 is disposed so that the reinforcing plate 31B is located on the lower side of slab 31.

An infrared radiometric thermometer 2 is mounted on a tripod 2a so that it is below the reinforcing steel plate 31B and its sight line is aligned with the steel plate reinforcing floor slab 31. The thickness used steel plate 31B is a standard thickness of the plate which is practically used in the bridges.

The steel plate reinforced floor slab 31 is intentionally formed with a portion which is not filled with epoxy resin as a defect. The simulated defect comprises four simulated defects E30, K20, K10, KS which are 30 cm square, 20 cm square, 10 cm square and 5 cm square, respectively as shown in Fig. 2.

A signal from the infrared radiometric thermometer 2 is analyzed by an image analyzing device 20 and is displayed on a CRT display 21 if necessary.

In thus formed system, the difference in temperature between the simulated defects and non-defected area (which is filled with an epoxy resin) is determined by detecting the infrared radiation energy from the reinforcing steel plate 31B. The measurement is carried out above every 10 minutes for 23 hours. The results of the temperature differences of the 10 cm square simulated defects K10 and non-defected area are shown in Fig. 3 and the results of the temperature differences of 20 square simulated defected area K20 and non-defected area are shown in Fig. 4.

Referring now to Fig. 3, the temperature difference is relatively stably no less than 0.3 C about in a period of 1 hour of 6 to 7 a.m., 1 hour of 11 to 12 a.m., 1 hour of 14 to 15 p.m.

and 1.5 hour of 15.30 to 17 p.m., if the area of the defect is 100cm2. Temperature difference is less in the other hours. The period of time in which the temperature difference is not less than 0.3 C is short.

In contrast to this, the result shown in Fig. 4 in which the area of the defected portion in 400cm2 shows that the temperature difference is not less than -0.3 C for 8 hours between 15 to 23 p.m. and for 2 hours between 7 to 9 a.m.

It is found from these results that the temperature difference is unstably not less than 0.3 C or not higher than 0.3 C depending upon the measuring hours if the area of the defected portion is 100cm: . This tendency is more remarkable if the area of the defected portion is 25cm' . Therefore, considering the practical detection whether or not any defect is present upon basis of the temperature difference, this method is not suitable for detection of a defect having a small area.

In contrast to this, the presence or absence of a defect can be stably determined with reference to the temperature difference of 0.3 'C provided that the measurement is conducted in these long period of time when the temperature difference is not less than 0.3 'C if the area of the defect is 400cm: It is found that the same is applied to the case in which the area of the defect is 900cm: Referring to Fig. 4 again, the temperature of the non-defected area is lower than that of the separated area and the temperature difference is large for 8 hours between 15 to 23 p.m. It is deemed that this is due to the fact that the sunlight exposure and the change in the temperature of the external atmosphere is relatively low and the disturbance is less.

In contrast to this, Fig. 3 shows the tendency that the temperature difference is less for one day and the change in temperature difference is random. This tendency is due to the fact that sunlight exposure and the change in the temperature of the external atmosphere is influenced so that the temperature difference and the change in the temperature difference is vague since the area of the defected portion is less.

Therefore, in accordance with the present invention, presence of a defect is determined with reference to the temperature difference not less than 0.3 C, which is measured during preferably 15 to 23 p.m., more preferably 18 to 23 p.m. when the amount of sunlight is less. It is preferable to totally determine thaw presence or absence of a defect by measuring the temperature difference at different times. It is found from the many various experiments that the possibility of wrong determination of defect is higher when the determination of the presence of the defect is made with reference to the temperature difference not higher than 0.3 'C.

(Applied Experiment) In order to confirm whether the result of the above-mentioned model experiment is applicable to an actual bridge, an applied experiment was conducted in a bridge which is practically used.

This applied experiment was conducted for a bridge in a highway across a first class river in Tokyo area.

As shown in Fig. 5, the bridge 50 comprises a floor slab 51 having a concrete floor slab 51A to which a reinforcing steel plates 51B is bonded on the lower side thereof. A pavement road 51 is formed on the floor slab 51. The floor slab 51 is supported on a main girder 53. Water supply main tubes 54 extend under the floor slab 51. The bottom of the bridge in a given longitudinal area having two lanes is schematically shown in Fig. 6.

At the area of Fig. 6, tapping method was preliminarily conducted in which the reinforcing steel plates are tapped with a test hammer while using a boom vehicle. Detected area K was marked with a chaulk. It was presumed that this defected portion K is a portion in which bonding agent is separated at about 500cm: In the experiment as shown in Fig. 5, an infrared radiometric thermometer 2 is placed on the ground below the bridge. The thermal radiation energy from X portion is computed by the thermometer 2. Similarly to the model experiment, the signal from the infrared radiometric thermometer 2 is analyzed by an image analyzing device 20 and is displayed on a CRT display 21 if necessary.

Then, the temperature difference between the detected portion and non-detected portion is determined by the image analyzing device 20. The measuring is conducted about every 10 minutes for 24 hours. The result is shown in Fig. 7.

It is found from Fig. 7 that the temperature difference between defected portion and non-defected portion is not less than -0.3 C at 1 to 10 a.m. Accordingly, it is found from this experiment that the defect in a steel plate reinforced structure can be properly detected with reference to the temperature difference.

Now, a basic method of detecting a defect of the present invention using the differential temperature distribution will be described. An infrared radiometric thermometer (thermal image sensor) is provided in such a manner it faces the surface of a structure to be measured. The thermal energy radiated from the surface of the structure to be measured is detected by the infrared radiometric thermometer. A signal from the thermometer is fed to an image analyzing device. The temperature in an image range and the temperature distribution image is obtained. A portion having a temperature which is different from that of the other area is extracted so that the portion is determined as a defected portion.

In the present invention, measuring of the thermal energy is conducted by means of the infrared radiometric thermometer during daytime and night so that thermal images during both times. Then, in the image analyzing device, substraction-processed image is obtained by subtracting the temperature distribution image which is obtained during the night from the temperature distribution image which is obtained during the daytime. A portion having a higher temperature than that of the other portion is extracted and determined as a defected portion by the subtraction processing.

Since the temperature distribution between the defected portion and the non-defected portion is enhanced by the subtraction processing so that detection of the defected portion is easy.

Furthermore, an integration processed image is obtained by integrating the subtraction-processed image. A portion having a higher temperature than that of the other portion in the integration-processed image is extracted and determined as a defected portion. The temperature difference between the defected portion and the non-defected portion is further enhanced so that detection of the defected portion becomes easier.

Processing such as the above-mentioned subtraction-processing and the time-integration processing can be easily conducted by using a thermal image processing software "TVS-2000" commercially available from Nippon Avio is a defect at an area having g a higher temperature. It is found from the other many experiments that the possibility of wrong determination of the defect which is made with reference to not higher than t0.3 C becomes higher.

EXPERIMENT 2 (Basic Experiment) A model structure 1 with sides, each having a width of 5 m and a height of 1.8 m on which new small tiles are applied is built on the roof of building existing at a rural city as shown in Fig. 8.

As shown in Fig. 9, a simulated defected portion (tile separated portion) having a 7 % area is intentionally formed on an external wall 1A of the model structure 1. The simulated defected portion comprises an area of 32 tiles, an area of 16 tiles, as area of 8 tiles and an area of 4 tiles in order to enable to determine whether there is a temperature difference due to difference in area of the defected area. The area of 7 % is an average value which is found by actual tile defect investigation.

In order to detect the defect of the external wall, an infrared radiometric thermometer (thermal image sensor) 2 which faces the external wall is placed on the roof.

A signal from the thermometer 2 is input to an image analyzing device 20. The analyzed result is displayed on a CRT display 21 or is recorded on a floppy disk and the like.

In the thus formed system, the infrared radiation energy from the external wall 1A is detected by the infrared radiation thermometer 2. An average temperature of each of unit are is obtained by dividing the measured area and presence or absence of a defect is determined based upon the temperature difference between adjacent unit areas.

In the experiment, measurements were conducted by means of an infrared radiometric thermometer successively every 10 minutes for about 25 hours since the sunrise on a clear day in spring.

The changes in surface temperature of the defect having an area corresponding to 32 tiles is shown in Fig. 10. The changes in surface temperature of the defect having an area corresponding to 8 tiles is shown in Fig. 11.

As is apparent from Fig. 11, the defect having an area corresponding to 32 tiles exhibits a temperature which is different by not higher than t0.3 C from that of surrounding area in a short period of time between 4:30 to 6:00, 10:00 to 11:30, 15:00 to 17:30. Detection of the defect can be conducted during the other time, specifically almost all times, such as for 20 hours a day.

However, the defect having an area corresponding to 8 tiles exhibits a temperature which is different by not less than +0.3 C from that of the surrounding area in every shortened period of time, for example, 8:30 to 9:30, 13:00 to 15:30, 0:00 to 3:00.

Detection of the defect can be conducted for 9.5 hours a day, the period of time of which is shorter than that when the detection of the defect can be made, unlike the detection of the defect in 32 tile area.

Therefore, subtraction operation is conducted between the temperature distributions of the structure surface, which are detected during the daytime and night in accordance with the present invention. The process of the subtraction operation will be described with reference to an thermal image displayed on the CRT display 21A which are schematically shown in Figs. 12 to 14.

The thermal images representative of the temperature distributions of the surface of the model structure 1 which were detected at 10:00 in the daytime and 23:00 in night are shown in Figs. 12 and 13, respectively.

The area corresponding to 8 tiles can not be determined as a defect from the temperature distributions shown in Figs. 12 and 13 in consideration of an error since it is different in temperature by +0.2 C and -0.2 C, respectively than that of the surrounding area. If the subtraction operation is conducted by subtracting the thermal image shown in Fig. 13 from the thermal image shown in Fig.

5 by means of an image analyzing device, the temperature difference of the area corresponding to 8 tiles is amplified to +0.4 C, so that the defect can be detected at a higher accuracy.

The temperature difference of the area corresponding to 8 tiles is enhanced to +0.6 C by integrating the image shown in Fig. 12 for the image shown in Fig. 14. The detection accuracy can be further enhanced.

On the other hand, the temperature difference of the are corresponding to 32 tiles is amplified to +0.8 C. The detection accuracy can be enhanced.

(Applied Experiment) In order to confirm whether the result of the above-mentioned basic experiment be applicable to the steel plate reinforced structure, an applied experiment was conducted in an actually used bridge, which is a steel plate reinforced structure. The experiment was conducted for the bridge as shown in Fig. 5 similarly to EXPERIMENT 1.

In the experiment, an infrared radiometric thermometer 2 is placed on the ground so that it is located below the bridge as shown in Fig. 5. The thermal radiation energy from X area is measured by the thermometer 2. A signal from the thermometer 2 was analyzed in the image analyzing device 20 and was displayed on a CRT display 21.

A temperature difference between the defect area which was detected by the tapping method and non-defected area was determined. The measurement was conducted about every 10 minutes for about 24 hours. The result is shown in Fig. 15.

It is found from Fig. 15 that the temperature difference between the defected and non-defected areas is not less than 0.3 C during 1:00 to 10:00. It is relatively easy to detect the defect for the period of time, since it is possible to clearly make difference between the defected and non-defected areas. If the measurement is conducted at 22:00 when the temperature difference is t0.0 C, it is impossible to detect the defect.

Accordingly, subtraction operation between the thermal images which were detected at 12:00 in day time and at 23:00 in night is conducted. The temperature difference between the defected area and the surrounding area is about 1.0 C so that the defected area can be detected at a higher accuracy. If the thermal image which was differential-operated was integrated for time, the temperature difference between the defected area and the surrounding area is 1.5 C. Defect detection can be conducted at a higher accuracy.

The result of the above-mentioned applied experiment shows that the method of detecting the defect of the structure which was conducted in the basic experiment can be used for the detection of the defect of the steel plate reinforced structure and is applicable to the actual bridge.

The present invention can be applied to the diagnosis of roof, fume stack ,dam sight, bank, road (high level road) as well as the above-mentioned diagnosis of the external wall of building.

Defects such as cracks, separation and lifting can be detected.

In case in which the structure to be measured is steel plate reinforced structure, the method of the present invention can be used to building, roof, fume stack ,dam sight, bank, tunnel, subway, road (high level road) which is reinforced with steel plate in addition to the above-mentioned diagnosis of the bridge.

Claims

What is claimed is:

1. A method of detecting a defect of a structure by detecting the radiation energy from the surface of the structure by means of an infrared radiometric thermometer to determine the distribution of the temperature on the surface of the structure, comprising the steps of: detecting an area having a temperature which is different by t0.3 C or more from that of the surrounding area based upon a signal of the temperature from said infrared radiometric thermometer; and determining that the area has a defect if the area is not less than 400cm2 .

2. A method of detecting a defect of a structure as defined in Claim 1 in which measurements are conducted at different plural times and in which the temperature difference is measured at every times to determine that there is a defect in an area having a temperature difference which is not less than +0.3 DC.

3. A method of detecting a defect of a structure as defined in Claim 1 or 2 in which a determination is made that there is a defect in an area having a temperature difference which is not less than +0.3 "C and -0.3 DC when measurement is conducted in the daytime and night, respectively.

4. A method of detecting a defect of a structure by detecting the radiation energy from the surface of the structure to determine the distribution of the temperature on the surface of the structure, comprising the steps of: detecting the radiation energy in the periods of the daytime and night to obtain the distributions of the temperature on the surface of the structure in respective periods of time, and determining the differential temperature distribution by differentiating these temperature distributions to determine the defect of the structure based upon the differential temperature distribution.

5. A method of detecting a defect of a structure as defined in Claim 4 in which an integrated temperature distribution is obtained by integrating said differential temperature distribution for time and in which the defect of the structure is detected based upon the integrated temperature distribution.

6. A method of detecting a defect of a structure as defined in Claim 4 or 5 in which a determination is made that there is a defect at an area having a temperature difference not less than +0.3 'C in the differential or integrated temperature distribution.

7. A method of detecting a defect of a structure as defined in any one of Claims 1 to 6 in which said structure is reinforced with steel plates which are integral with a concrete slab on the surface thereof.

8. A method of detecting a defect of a structure substantially as herein described with reference to the accompanying drawings.