Abstract

The committee on nondestructive inspection of steel reinforced concrete structures in the Federation of Construction Material Industries, Japan, has published a specified standard on identification of reinforcement by electro-magnetic methods. It deals with two techniques. One is an application of a ground-penetration radar system for determination of the location and cover depth of rebar in concrete. The other is an electro-magnetic method to identify the location, cover-depth and diameter of rebars. Based on results of bench-mark tests, practical procedures are studied. Then, proposed standards and the results of benchmark test are reviewed and discussed.

Introduction

In order to secure the structural performance and durability of steel-reinforced concrete, the arrangement of rebars in the concrete and cover depth are critical variables. It is also important to assess these variables when evaluating the integrity of existing structures.

When nondestructive inspections are used to determine the in-place locations and diameters of rebars in concrete, the electromagnetic method is often applied, the ground-penetration radar method may also be adopted. Although there are test standards such as BS 1881 Part 204¹ for the electromagnetic method, Japan does not have such a test standard. The Committee for Nondestructive Inspection of Steel Reinforced Concrete Structures, which was set up by the Federation of Construction Material Industries of Japan, therefore conducted a study for compiling a standard related to technologies for identifying the in-place locations of rebars in concrete using the radar method and the electromagnetic method. The Committee has proposed two draft test standards: "Method for Locating of Rebars in Reinforced Concrete by Radar, JCMS-III B5707-2003"² and "Method for Locating Rebars and Determining the Diameters of Rebars in Reinforced Concrete by Electromagnetic-Induction, JCMS-III B5708-2003."²

In order to ensure widespread applicability, test standards should show minimal variations between different testing devices and testing persons. The study therefore gathered the opinions of instrument manufacturers and instrumentation service firms, and amended the standards accordingly. The study also used skilled technicians and unskilled ones to check the accuracy of measurements conducted as specified by the proposed test standards.

This paper outlines the test standards, as well as the results on the accuracy of measurements conducted according to the test standards.

Outline of proposed standards

Table 1 shows the specified standards for the radar method and the electromagnetic method. Each standard comprises the main body and Appendixes 1 through 6. The standards stipulate the standardized applicable range, performance of the measuring device, measurement procedure, precautions, and other issues when using commercial measuring devices.

The applicable range of the cover depth of target rebars is set to be 100 mm or less. Generally, the actual cover depth is not more than 100 mm, and so all commercial measuring devices are applicable. Measurable pitch is not specified because the pitch differs with the kind of bar arrangement (single, double, or zigzag). The standards do not deal with steel fiber reinforced concrete. The steel bars that are within the scope of the test standards are ribbed bars having nominal diameters of 13 mm to 38 mm. Since the range is an experimented range, in practice plain bars and bars having diameters of 6 mm or larger can be measured by the measuring devices.

Table 2 shows the required performance of the measuring device for each method. The device is regularly calibrated using a test piece and a spacer specified by Appendix 5, applying individual specimens prepared under the respective conditions of three sizes of reinforcing bar (diameters of 13, 25, and 38 mm) and three cover depths (30, 60, and 100 mm). Errors of determined in-place positions and cover depth are derived to assess whether the required performance given in Table 2 is achieved.

Regarding other precautions, Appendix 6 gives a method for determining the influence of in-depth double-layered reinforcing bar arrangement on the measurement results. The rebars are arranged in-depth double layers, the errors of in-plane positions and cover depth of rebars are measured, and thus the distance between centers of the upper and lower bars satisfying the accuracy required of the device is determined in advance.

Radar Method
The principle of the radar method is that electromagnetic waves having very little pulse width are transmitted from an antenna into the target concrete, the electromagnetic waves reflected from a substance (such as reinforcing bar or underground pipe) having different electrical properties from those of the concrete are received by a receiving antenna, and then the distance from the reflecting body is determined based on the time taken between outgoing and received electromagnetic wave signals. By moving the antennas along the surface of the concrete, the three-dimensional shape of the reflecting body can be determined.

Variables that affect the accuracy of measurement include the. This constant stabilizes within a range of 7 to 8 after 3 to 4 weeks have passed since concrete placing. For a younger concrete or for a concrete having many voids, however, the constant significantly varies according to variations in the water content. Thus, the electric permittivity of concrete should be confirmed using the same kind and conditions of concrete.

Notes: (1) Only electro-magnetic method

Notes: (1) Actual cover depth of concrete in target structure (mm)

Electro-magnetic method
The principle of the electromagnetic method is that an electromagnetic field excited by a probe is detected using a search coil, then the in-place locations, directions, and diameters of the rebars are determined based on the variations of the electro-magnetic field. Standards for this method include BS 1881 Part 204¹.

Variables that affect the accuracy of measurement include the kind and cross sectional shape of reinforcing bar and the presence of adjacent multiple rebars. When measuring special rebars such as high-tension steel bars, an error of ± 5% or more may occur, in which case, the test piece for calibration shall be the same kind as the target rebars. Generally, the presence of multiple rebars within the range of probe detection degrades the accuracy of determining the cover depth. In such a case, a test piece of a similar kind as these target rebars shall be used, or on-site calibration shall be performed. Other causes of measurement errors include an aggregate or finishing material having magnetism, and temperature variations during measurement. Methods for adjusting individual devices are used to counter the influence of these other causes.

Accuracy of measurement based on bench-mark test

Specimens
Fig. 1 and Fig. 2 show examples of the dimensions, shape, and bar arrangement of each specimen. Test 1 was performed for one kind of columnar specimen and two kinds of wall specimens, giving four measurement faces for both types of specimen. Test 2 was performed for one columnar specimen and one slab specimen. The bar arrangements were prepared by combining various cover depth and rebar diameter of rebars. The applied rebars were those specified by JIS G3112, having nominal diameters ranging from 13 mm to 38 mm. The arrangement of the rebars and the real value of cover depth for individual specimens were confirmed in advance by the X-ray penetration method.

Fig 1: Examples of rebar arrangement (Test 1) (dimension unit: mm).

Fig 2: Examples of rebar arrangement (Test 2) (dimension unit: mm).

Table 3 shows the mix proportion of concrete used for the specimens.

Measuring devices and operators
The skilled technicians for Test 1 were dispatched from four firms for the radar method and six firms for the electro-magnetic method. The measuring devices were those usually used by the technicians concerned. As for the unskilled technicians for Test 2, university students were trained to operate the devices for individual methods for a period of one week by supplying respective devices, operational manuals, and test standards. The measuring devices were two kinds for the radar method and three kinds for the electro-magnetic method. For all the tests, no information about the arrangement of rebars was given to the operators.

Results and discussions
Table 4 shows the measurement errors on in-place location of rebars. The range of measurement errors is expressed by three grades: ±10 mm, ± 30 mm, and over ±30mm.

There was no significant difference in the accuracy between these methods. Regarding the in-place locations of rebars, the skilled technicians achieved the level of within ± 10 mm at the 85% significance level for both the radar method and the electro-magnetic method. For the unskilled technicians, measurement errors were concentrated on the edge portions of the test pieces, showing that inspection at the edge portions of materials requires skilled use of the measuring device.

Table 5 shows the measurement errors on diameter of rebars. The range of measurement errors is expressed by three grades: ± 3 mm, ± 6 mm, and over ± 6 mm. Fig. 3 shows the distribution of measurement error on diameter.

Fig 3: Distribution of measurement error on diameter (electro-magnetic method).

As for the rebar diameter, the range of error within ± 3 mm was about 86% for the skilled technicians and about 69% for the unskilled technicians. Although most of the outermost rebars such as hoop-bars in the column could be measured to an accuracy of within ± 3 mm, measurements of the internal rebars and large-diameter rebars showed lower accuracy. At present, therefore, the diameter of rebars should be determined only for the outermost ones.

Table 6 shows the measurement errors on cover depth. The range of measurement errors is expressed by three grades: ± 5 mm, ± 10mm, and over ± 10 mm. Figures 4 and 5 show the distribution of measurement error on cover depth.

Fig 4: Distribution of measurement error on cover depth (Test 1).

Fig 5: Distribution of measurement error on cover depth (Test 1).

Regarding Test 1, since the average of errors is close to zero, and since the root mean square is within 10 mm, the cover depth can be determined within an approximate error range of ± 10 mm. Overall, the electro-magnetic method gives higher measurement accuracy than the radar method. As for Test 2, the radar method yields a high accuracy, but its mean of errors is on the positive side presumably because of unskilled identification of the point used to determine the cover depth on the screen of the measurement device.

Conclusions

We proposed test standards to identify the in-place locations, diameter, and cover depth of rebars for each of the radar method and the electro-magnetic method. We conducted a bench-mark test according to the proposed test standards to check the measurement accuracy. The results are summarized as follows.

1. The in-place location of rebars is determined within an approximate range of ±10 mm for both methods. Since, however, errors might become significant at the edges of the target materials, the measuring device should be used carefully at such parts.

2. The diameter of rebars is determined within an approximate range of ±5 mm. However, errors are larger for internal rebars and large-diameter rebars.

3. The cover depth is determined within an approximate range of ±10 mm. With the radar method, however, skill in identifying the point used to determine the cover depth on the screen of the measurement device is required.

Acknowledgement

We sincerely thank the members of the Committee for Nondestructive Inspection of Steel Reinforced Concrete Structures of the Federation of Construction Material Industries of Japan, and the staff who assisted the bench-mark test. This study was conducted with a grant from the Ministry of Economy, Trade and Industry, Japan.

References

1. British Standard Institution, "BS 1881 Testing concrete : Part 204 Recommendations on the use of electromagnetic covermeters : 1988", Dec. 1989
2. Federation of Construction Material Industries Japan, "Nondestructive Evaluation of Concrete Properties", Mar. 2003