1 - Development of Concrete Spalling Inspection Device

Development of Concrete Spalling Inspection Device Incorporating Non-Contact Laser
Measurement Technology
Norikazu Misaki (West Japan Railway Company)
Yoshinori Shimada (Institute for Laser Technology)
Oleg KOTYAEV (Institute for Laser Technology)
Masahiro Shinoda (Railway Technical Research Institute)
Shigeo Emoto (Unirock Corporation)
1.
Background of development
The primary method for inspecting concrete is the hammering test. However, the
hammering test requires inspectors to use vehicles with aerial inspection platforms in
order to access the concrete surface. Because of the number of regions that must be
inspected, the hammering test requires considerable time, which imposes a great burden
on inspectors and is a constraining factor for railway tunnel lining inspections.
Inspections are not possible during a railway’s service hours. Moreover, arrangements
must be made to transport to the site aerial-platform vehicles that can operate on railway
tracks. Finally, electrical power for the railway service must be suspended during the
inspections. To handle these conditions, we developed a concrete spalling inspection
device that incorporates non-contact laser measurement technology. The device can
detect problematic regions in the concrete before the concrete detaches. The device also
minimizes the use of aerial platform vehicles and the suspension of electrical power. With
this device, concrete structures can be inspected from a distance of a few meters.
2.
Overview of the technology
2.1 Principle of non-contact laser measurement technology
The hammering test and the non-contact laser measurement technology both rely on the
same principle for detecting defects: exciting the concrete surface to determine the
surface vibration (Fig. 1). Figure 2 shows the technology’s detection principle. The
device irradiates the concrete surface with a high-power excitation laser, which converts
solid materials (cement, etc.) on the surface to a gas that is then released into the air. The
irradiation causes impact waves to propagate inside the concrete, exciting surface
vibrations. The generated vibrations are measured by means of laser beam interference
using a measurement laser capable of continuous oscillation. Because concrete surfaces
are extremely rough, conventional laser interferometers produce lower measurement
sensitivity. Our device, however, employs phase conjugation technology that uses
dynamic holographic crystals. The laser beam is separated using a beam splitter into
signal light and reference light. The signal light reflects off the concrete surface, carrying
information about the surface geometry, and enters the dynamic holographic crystals,
causing interference between the signal light and the reference light. This forms a
hologram of the concrete surface geometry within the dynamic holographic crystals,
which compensates for the roughness of the concrete surface. Free from influence of the
concrete’s surface roughness, the signal light and reference light then enter a detector,
resulting in excellent detection sensitivity.
-1-
Hammering testing
Non-contact laser measurement
レーザーリモートセンシング法
打音法
Use a hammer
ハンマーで
to excite
振動を励起
vibration.
Detect
the
耳で聴き
vibration as a
音として
sound.
An excitation laser
打撃の振動
加振レーザーで
excites vibration.
振動を励起
欠陥
欠陥
Detect
the vibration
検出レーザーの
by
means of beam 表面振動
干渉により
interference.
振動を把握
コンクリート
Concrete
表面振動
振動を把握
振動
コンクリート
Concrete
Fig.1. Comparison of hammering test and non-contact laser measurement
Concrete
コンクリート
Excitation
laser
加振用レーザー
振動
計測用レーザー
Measurement
laser
Detector
検出器
Dynamic holographic
ダイナミック
crystals
ホログラム結晶
Reference
参照光
light
振動
Signal
light
信号光
ビーム
Beam splitter
スプリッター
欠陥
欠陥
表面振動
Fig. 2 Principle of non-contact laser measurement technology
2.3 Verification test using test device in a Sanyo bullet train tunnel
A verification test using the test device was conducted at night in a Shinkansen bullet
train tunnel. Because the vibration measurements were hampered by noise and vibration
from maintenance vehicles and power generators, a vibration isolation bench was
employed and sound protection measures implemented. Figure 6 is an example of the
testing results. The spectrum results show good agreement between the proposed
technology and the hammering method (surface excitation test).
-2-
Normalized
vibration amplitude
正規化した振幅
1.5
No-sound
region
不健全箇所
レーザー
(loosening)
Non-contact laser measurement
technology
（浮き）
表面加振試験
Hammering method
1.0
0.5
0.0
0
5000
10000
Frequency
(Hz)
周波数(Hz)
15000
20000
Fig.-6 Verification test results in bullet train tunnel
using improved version of test device
2.4 Development of practical device
The results of the verification test show that ambient vibration and noise in the bullet
train tunnel were reduced, allowing the device and its technology to successfully take
measurements. For practical applications, the device should travel along the inspection
passage provided in the center of a bullet train tunnel. Therefore, a smaller device having
the same functions as the larger test device was developed (Fig. 7). The smaller device
significantly reduces the time and labor required for inspection work.
Fig. 7. Device in operation in tunnel’s central passage
-3-

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