Special Feature

Non-destructive Inspection Technologyfor Concrete Structures
Shigeru Yamaguchi, Hiroshi Irie†, Shinobu Tsutsumi,and Yasukatsu Yoshida
Abstract
The need to develop non-destructive technologies that allow safe and effective inspection and analysis of infrastructure facilities is increasing with the degradation of old infrastructure facilities such as
cable tunnels and utility holes and the aging of professional engineers. This article describes the importance and direction of research and development of non-destructive inspection technology for testing
concrete structures as well as macroscopic ultrasonic and electromagnetic methods that are under development.

1. Introduction
NTT inspects cable tunnels, which are concrete
structures, through a process of regular inspections atintervals of 3 to 5 years to identify the status of surface degradation quantitatively in terms of a rank
indicating the degree of degradation. The need for
repair is judged based on the degree of degradation.
When there is degradation that cannot be identified

by visual methods or for which the cause needs to bepursued, detailed inspections are carried out, the needto take action is determined, and appropriate repair
measures applicable to the cause of degradation areselected. The inspection procedure is described inTable 1.
Regular inspections are usually performed visually
to check the degradation status of the concrete surface
such as cracks, leaks, or exposed reinforcing steel.

Table 1. Procedure for inspecting cable tunnels.
Regular inspection

Detailed inspection

Description

Surface degradation phenomena arechecked to create basic quantitative
data for equipment management.
Usually performed at 3–5-year intervals.
Interval can be shortened depending on
degradation status.

Carried out when there is degradationthat cannot be identified by visualinspection or when the cause ofdegradation must be clarified to judge
whether countermeasures are needed
and to select the optimum method.

Techniques

Looking and hammering• Destructive methods (surface removaland core sampling)• Non-destructive inspection

Typicalinspection
items

Cracks, water leakage, exposed steel

Cracks, flaking, cavities, strength,
corrosion, neutralization, depth, salt
concentration, etc.

Evaluation
criteria

Classified into six levels according todegree of degradation

Judged individually according todegradation status† NTT Network Access Service Systems Laboratories
Tsukuba-shi, 305-0805 Japan
E-mail: h.irie@ansl.ntt.co.jp

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NTT Technical Review

Special Feature

Concrete
Reinforcing steel
Crack

Measure cracks

Internal crack
Corrosion ofreinforcing steel

Measure position ofreinforcing steel

Measure thicknessof concrete

Measure internalcracks and cavities
Measure corrosion

Fig. 1. Main items in a detailed cable tunnel inspection.

Detailed inspections, on the other hand, check fordegradation occurring inside the concrete, such as
corrosion of reinforcing steel or internal cracks, bydestructive testing of a concrete sample and by chemical analysis of a core sample to determine the degreeof degradation.
Both types of inspection are important, but detailed
inspection is especially important for evaluating thesafety and long-term reliability of facilities. The main
items of a detailed inspection are shown in Fig. 1. It
is important to develop ways to perform these inspections non-destructively to assure the safety of aged
facilities. Thus, we need diagnostic methods as well
as visual inspection and non-destructive inspectiontechnology is necessary to reduce the strain on aged
structures and enable inspections to be performed
safely.
2. Current status and issues for non-destructive
inspection technology
Non-destructive inspection technologies commercially utilized at worksites can be categorized as
impact acoustic methods, impact-generated elastic
wave methods and ultrasonic methods using soundwaves, electromagnetic wave methods, electromagnetic induction methods using electromagneticwaves, test hammer methods utilizing repulsive
force, and half-cell potential methods that measureelectrical characteristics. Figure 2 shows the items ofconcrete structures that can be inspected using theseVol. 4 No. 6 June 2006

general inspection technologies as well as the position of each method in terms of performance andaccuracy. It shows that we must select the optimum
method to suit the degradation phenomena to be analyzed when inspecting structures because these general inspection methods have limited applicability
because of their technical characteristics. Moreover,the measurement conditions may be limited or accuracy may be unstable depending on the environment.
We are currently focusing on research and development for inspection technologies using ultrasonic andelectromagnetic waves as the most promising technologies for resolving such problems and we arefocusing on situations where we can apply the knowhow accumulated by NTT Laboratories and Group
companies. As shown in Fig. 2, these ultrasonic- andelectromagnetic-wave-based non-destructive methods approach the performance and accuracy ofdestructive methods.
3. Direction of research and development of
non-destructive inspection
Figure 2 also shows targets for research and development of non-destructive inspection technologies.
With ultrasonic inspection, we are focusing on measuring the compression strength and corrosion ofreinforcing steel in concrete as an important researchand development theme for upgrading macroscopic
ultrasonic inspection technology that we have provided to business in FY 2005. With this technology, we25

Special Feature

Major inspection items(concrete structures)

Status ofdegradation

Break-off,cavities

CracksCompression
strength

Steel corrosion

Size, etc

Position ofreinforcement

Low
Impactacoustic
methods

Performance, accuracy

Infraredthermography

ElectroImpactmagnetic
wave methods generated
elasticwave
methods

Test hammer

Ultrasonicmethods

Half-cell potential method
Polarization resistance method
Macroscopic
ultrasonicmethods

Overlapping
steel
Concretethickness

Electromagnetic
induction
methods

Impactgenerated elastic
wave methods

High

Ultrasonicmethods

Macroscopic
ultrasonicmethods

Electromagnetic
wave
methods

Macroscopic
ultrasonicmethods
Electromagnetic
wave
methods

Electromagnetic
wave
methods

Ultrasonicmethods

Macroscopic
ultrasonicmethods: Under development: Developed (as of 2004): Conventional technology

Fig. 2. Types of non-destructive inspection and future development targets. For reference, the range of destructive
methods (surface removal and core sampling) is shown in pink.

are aiming to develop a flexible system that can measure various inspection specimens with high accuracy using a single piece of equipment without requiring other measurement equipment to be brought in.
Electromagnetic inspection technology is a new
development theme that utilizes high-frequency gigahertz waves not currently commercially available.
With this new technology we aim to achieve moreaccurate diagnosis of defects in two dimensions (2D).
Those technologies will allow us to eliminate
destructive inspection from the worksite and will
greatly contribute to reducing inspection costs andrepair costs (repetitive repairing) and will result inimprovements in inspection accuracy, as well as
improvements in the safety of facilities. They areones that can be applied to making various measurements made by a single system, allow measurementwith accuracy equivalent to that of destructive inspection, allow the continuous capture of 2D information,and ensure that cables are unaffected by high humidity and high temperature inside underground facilities.

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4. Macroscopic ultrasonic method
4.1 Overview
The conventional ultrasonic inspection method foranalyzing concrete structures injects ultrasonic waves
into the concrete to analyze the state within it fromthe reflected waves or waves transmitted from or
through substances having different elastic properties
from those of the concrete. When ultrasonic waves
are injected into the concrete, the received waves contain a lot of noise because they are scattered by water,
air bubbles, or aggregate in the concrete. Therefore,
accurate diagnosis has been impossible because of
the difficulty of distinguishing between the target
waves reflected from the reinforcing steel and waves
reflected from the bottom of the concrete structure.
On the other hand, macroscopic ultrasonic inspection
allows us to detect only the target reflected wave in ashort time of just about 10 seconds by averaging themeasurements thousands of times while moving an
ultrasonic probe and using a noise filter to eliminate
unwanted frequency components (Fig. 3). As shownby the example of received ultrasonic waves in Fig. 4,
we can clearly identify the reflected waveform fromthe bottom of the concrete by performing averaging
NTT Technical Review

Special Feature

Measuring the thickness of concrete

ProbeThickness

Scattered waves

Concrete

Reflected
waves

Reflected wavestClarifiedt

Averaged

00

Receive all the reflected
waves (scattered waves)

Perform averaging
and frequency filtering

Clarify
waveform

Fig. 3. Concept of macroscopic ultrasonic inspection technology.

Wave reflectedfrom bottomof concrete

000

0.0
100.0
200.0
Averaged a few times

0.0
100.0
200.0
Averaged 1000 times

0.0
100.0
200.0 depth
Averaged 3000 times(with frequency filtering)

Fig. 4. Example of averaging processing (measuring thickness of concrete).

(in this case, 3000 times) and filtering operations.
Here, “macroscopic” means that all the reflected
waves including noise caused by scattering and so on
are received instead of only the reflected wave being
microscopically detected in a pinpoint manner.
4.2 Degradation analysis system for reinforced
concrete (RC)The RC degradation analysis system that we provided as a practical inspection system to business in
FY 2005 is a non-destructive inspection system utilizing macroscopic ultrasonic inspection technology.
Photographs of the system’s appearance and mannerof use for measurement are shown in Figs. 5 and 6,respectively. Measurement items and applications areshown in Table 2. For this device, we developed
functions that enable initial settings of voltage, gain,
axis scale, and frequency filter setting values required
for the operation of averaging received waveforms to
be automatically set to the optimum values for theitems to be measured. As shown in Fig. 7, we haveVol. 4 No. 6 June 2006

Main device

Probe

Fig. 5. RC degradation diagnosis system.

Probe

Fig. 6. Example of use for measurement.

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Special Feature

also developed a one-click function that can automatically recognize the measured waveform and display
the measurement results as numerical values. These
functions can shorten the work time and minimize
measurement errors caused by differences in the skill
levels of inspectors because high-level specialist
knowledge is not needed. Furthermore, the system
can be used in small spaces such as cable tunnels or
utility holes and in high-humidity environments
because it can be made small, waterproof, and battery
driven.
4.3 Attempting application to new measurement
items
When judging the durability (reduction in strength)of concrete structures, we need to know the corrosion
status of the reinforcing steel and the compression
strength of the concrete itself. We are now developTable 2. Items to be measured and applications.

Items to bemeasured• Depth of crack• Place and depth of break-off and cavity• Concrete thickness• Overlapping depth of surface
reinforcing steel• Overlapping depth of duplicatedreinforcing steel

Applications• Internal degradation diagnosis forexisting equipment• Confirmation of material dimensions
for existing equipment• Confirmation of material dimensions
when construction was completed

ing ways of measuring these values using the macroscopic ultrasonic inspection method. In this section,
we describe the progress of this development for thistechnology.
4.3.1 Measuring the degree of corrosion ofreinforcing steel
We prepared concrete specimens with different
degrees of reinforcing steel corrosion and recorded
the waveforms and frequency spectra of waves
reflected from the reinforcing steel using several
measurement methods such as changing the probe
distance. We found that the frequency characteristics
in the high-frequency region differed depending on
the degree of corrosion (Fig. 8). This difference isprobably caused by fine cracks near the reinforcing
steel created by corrosion and by the pressure ofexpanding gas generated by corroded products. We
will accumulate more data through basic experiments
to establish inspection and analysis technology.
4.3.2 Measuring the compression strength ofconcrete
We are developing technology for estimating thecompression strength of concrete from the velocity ofultrasonic waves traveling inside the concrete, utilizing the characteristic of ultrasonic waves that theelastic modulus changes in proportion to the velocityof ultrasonic waves. We tried to measure this phenomenon using an actual structure on site. However,
although we could recognize a positive correlation
between sound velocity and compression strength,
we concluded that it is difficult to estimate the compression strength from only the velocity of ultrasonic

Waveform of concrete thickness measurement

Measured result below isdisplayed automatically when
the button on the right isclicked.

Fig. 7. Display of values of results of automatic recognition.

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Special Feature

Corrosion A: Corroded half as much as B
Corrosion B: Lightly corroded where only initial
cracking of concrete occurs

Transmitting
antenna

Receiving
antenna

Waveform without
corrosion

High

Strength

Reflected

Transmitted

Waveform forcorrosion A
Waveform forcorrosion B

ObjectLow

Fig. 10. Concept of electromagnetic inspection.
Low

Frequency

HighFig. 8. Frequency spectrum of wave reflected fromreinforcement.

Compression
strengthHighFig. 11. Light Espar.

Low
Slow

Sound velocity

FastFig. 9. Compression strength of concrete versus velocity
of ultrasonic waves.

waves because the correlation (= 0.72) was not verystrong (Fig. 9). We will further study and analyze factors that affect ultrasonic wave velocity to improvethe measurement accuracy by developing an error
compensation algorithm, for example.
5. Electromagnetic wave methods
5.1 Overview
The usual electromagnetic wave method used forinvestigating a concrete structure measures the distance to a reflective object (for example, reinforcing
steel in concrete) from the round-trip time of reflected electromagnetic waves using the property thatelectromagnetic waves are reflected by any substancewith different electrical characteristics (relativedielectric constant) from the medium. The concept of
this inspection method is shown in Fig. 10. The main
problems with this conventional method are that themeasurement depth is limited by attenuation of theVol. 4 No. 6 June 2006

Fig. 12. Measurement in progress.

electromagnetic waves and environmental noise andthat the depth measurement accuracy is insufficient.
5.2 Light Espar
Light Espar [1] is a non-destructive inspection system using electromagnetic waves provided by AirecEngineering Corp., a company in the NTT Group. Its
external appearance is shown in Fig. 11, and an
example of an actual measurement in progress isshown in Fig. 12. An example of the resulting search
image is shown in Fig. 13. Measurement items andapplications are shown in Table 3. Improvements inthe amplification of reflected waves and data analysis
software in this system resolve the problems with the29Special Feature

Reinforcing steel

istics of concrete and reinforcing steel. Our aim is toenable quantitative diagnosis of the degree of degradation inside concrete structures.
6. Future topics

Fig. 13. Example image of search result.
Table 3. Items to be measured and applications.
Items to bemeasured• Arrangement of reinforcing steel• Position of break-off and cavity inconcrete

Applications• Searching for break-off and cavity
in existing equipment• Checking the arrangement ofreinforcing steel in existing equipment• Checking the arrangement ofreinforcing steel after construction

conventional technology. This system is currently
utilized in the field for investigating reinforcing steel
and cavities inside concrete structures.
5.3 Development of new measurement methods
Focusing on the characteristics of electromagnetic
wave methods, which allow non-contact and continuous searching, we have started development of nondestructive inspection technology that can measure
the degree of degradation inside concrete very accurately and efficiently. Although the normal electromagnetic inspection method for investigating concrete structures uses the 1–2-GHz band, we are investigating higher frequencies to improve the resolution
and capture more precise electromagnetic character-30

After an accident involving concrete falling fromthe walls of a Shinkansen tunnel in 1999, major construction companies and inspection facilities manufacturers conducted a lot of research on non-destructive inspection technologies utilizing various methods. However, no systems have been introduced at
actual work sites for various reasons such as physicallimits (because most are based only on results
obtained from test specimens), measurement accuracy, and worksite applicability. We believe that it isimportant to establish an optimum measurement
method by accumulating experimental data so that it
will be available in the field, as well as to implement
field tests to confirm worksite applicability. In thefuture, we will identify suitable applications and create manuals for facilities inspection using nondestructive inspection technologies.
In conclusion, the technology we are now developing can be used to diagnose the degree of degradationof general concrete structures or confirm the dimensions of structural materials. We believe this technology can contribute to society through the maintenance and management of Japan’s social infrastructure because it can be applied to facilities besides
those in NTT’s infrastructure, such as private buildings and structures and public buildings and structures such as bridges and tunnels. From this standpoint, we will continue to work to advance the development of technology.
Reference[1]

http://www.airec.co.jp/products/html/000021.html

NTT Technical Review

Special Feature

Shigeru Yamaguchi

Shinobu Tsutsumi

Senior Research Engineer, Supervisor, Civil
Engineering Promotion Development Project,
Second Promotion Project, NTT Access Network Service Systems Laboratories.
He received the B.E. degree in civil engineering and the M.E. degree in science and engineering from Kagoshima University, Kagoshima, in1982 and 1984, respectively. He joined Nippon
Telegraph and Telephone Public Corporation(now NTT) in 1984. He is currently engaged inR&D of non-destructive exploration technology
and maintenance management technology forcommunication infrastructure facilities. He is amember of the Japan Society of Civil Engineers(JSCE).

Research Engineer, Civil Engineering Promotion Development Project, Second Promotion
Project, NTT Access Network Service Systems
Laboratories.
He received the B.E. degree in civil engineering from Gunma National College of Technology, Gunma, in 1987. He joined NTT in 1987. He
is currently engaged in R&D of non-destructive
inspection technology for communication infrastructure facilities. He is a member of JSCE.

Hiroshi Irie

Katsuyasu Yoshida

Senior Research Engineer, Civil Engineering
Project, NTT Access Network Service SystemsLaboratories.
He received the B.E. degree in civil engineering from Saga University, Saga, in 1988 and theM.E. degree in civil engineering from Kyushu
University, Fukuoka, in 1990. He joined NTT in1990. He is currently engaged in R&D of nondestructive inspection technology and maintenance management technology for communication infrastructure facilities. He is a member ofJSCE.

Research Engineer, Civil Engineering Project,NTT Access Network Service Systems Laboratories.
He received the B.E. in civil engineering fromTohoku Gakuin University, Miyagi, in 1989. He
joined NTT in 1989. He is currently engaged in
R&D of non-destructive inspection technology
and maintenance management technology forcommunication infrastructure facilities.

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