Concrete Surface Strain Measurement using Moiré Fringes
S.Tanoue1*, S.Umemoto1, N.Miyamoto1, T.Takaki2 , I.Ishii2 and T.Aoyama2
1Keisoku Research Consultant Co., Japan
2
Hiroshima University, Japan
*
1-665-1, Fukuda, Higashiku, Hiroshima, 732-0029, Japane-mail: tanoue@krcnet.co.jp
e-mail: umemoto@krcnet.co.jp
e-mail: miyamoto@krcnet.co.jp
e-mail: takaki@robotics.hiroshima-u.ac.jp
e-mail: iishii@robotics.hiroshima-u.ac.jp
e-mail: aoyama@robotics.hiroshima-u.ac.jp

ABSTRACT
Assessment of the stress state of concrete in prestressed concrete (PC) structures is
extremely important for construction management and operation and maintenance(O&M). The authors developed a strain visualization sticker using the principle ofmoiré fringes, which makes it possible to read the rough strain value of the concretesurface directly by visual inspection and enables remote, noncontact measurement ofstrain using a digital camera or similar device. This paper presents an outline of thestrain visualization sticker and reports the results of verification of its performance.
Keywords. Moiré Fringes, Strain Visualization Sticker, Remote Measurement, NoncontactMeasurement, Image processing

1. INTRODUCTION
Assessment of the stress state of concrete in prestressed concrete (PC) structures is extremely
important for construction management and operation and maintenance. As a strain
measurement method, the authors have been engaged in development of strain visualization
stickers using the principle of moiré fringes, which enables remote, noncontact measurementof strain using a general digital camera or similar device (Figure 1) (Takaki, T. et al. 2012)(Umemoto, S. et al. 2012). As advantages of this technique, i) It is possible to read strain
values directly by visual inspection, and ii) the strain visualization sticker does not use
electrical elements such as a strain gauge, amplifier, or signal cable. In previous work, wedeveloped and verified the performance of strain visualization stickers for use with steel.
Recently, we developed a new strain visualization sticker for strain measurement of concretesurfaces in order to apply this technique to strain measurement of concrete, which requires
higher measurement accuracy. This paper presents an outline of the newly-developed strain
visualization sticker and reports the results of verification of its performance.

Strain Visualization Sticker

Figure 1. Concept of a proposed Strain Visualization Sticker
2. OUTLINE OF STRAIN VISUALIZATION STICKER
2.1 Principle of Moiré Fringes
As shown in Figure 2(1), when a line grating 1 having a pitch p is overlaid on a second line
grating 2 having a pitch p+Δp which is Δp (<<p) larger than the pitch of grating 1, a patterncalled a “moiré fringe” appears, which has a pitch W that is larger than the pitches ofgratings 1 and 2. Moiré fringes are produced by overlapping two line gratings. Theirrelationship is expressed by Eq. (1) (Takaki, T. et al. 2012).

W=(p+Δp)/Δp・p(1)

As shown in Figure 2(2), when line grating 1 is moved in direction (A) by pitch p, the moiré
fringes move in direction (A) by pitch W. In other words, displacement Δx can be displayed
visually as enlargement by a ratio of (p+Δp)/Δp. This enlargement ratio is defined as M. If
the amount of movement of the moiré fringes is Δxm, this relationship is given by Eq. (2).

Δxm=M・Δx(2)2.2 Structure of Strain Visualization Sticker
Figure 3 shows the structure of the strain visualization sticker. The sticker consists of twofilms, in which line gratings are printed on the bottom side of the upper film and the top sideof the lower film. To obtain clear moiré fringes, the bottom side of the lower film is painted
white. The space between the two films is filled with oil so that the two films will notseparate.

Figure 2. Example of a Moiré Fringe

Figure 3. Construct of Strain Visualization Sticker
2.3 Performance of Strain Visualization Sticker
The displacement measurement resolution of the strain visualization sticker is 1 μm. Thestrain measurement resolution is determined depending on the reference length. Previouslydeveloped strain visualization stickers (Umemoto, S. et al. 2012) are shown in Figure 4, andtheir performance with steel was verified as shown in Table 1. The visualization resolutionof both Sticker I and Sticker II is 200 με (Figure 5). The strain measurement accuracy of theacquired strain visualization stickers obtained by image processing is <10 με for referencelength L=200 mm and 32 με for reference length L=50 mm. Thus, measurement accuracy
approximately on the order of measurement resolution can be obtained.

Table 1. Performance of Strain Visualization Sticker
Test

Sticker Name

Tensile Test

StickerⅠ

Bending Test

StickerⅠ
StickerⅡ

Reference Length
L=200 mm(using jig)

L=50 mm
L=50 mmVisualization
resolution

Strain measurement
accuracy-----< 10 με

200 με
200 με

32 με
-----

StickerⅡ
StickerⅠ

Reference length
L=50 mmFigure 4. Strain Visualization Sticker
0 με200 με

400 με

600 με

800 μεFigure 5. Images of Strain Visualization Sticker

【Front side】

10【Back side】
100unit:mm

Figure 6. The Developed Strain Visualization Sticker
3. NEWLY-DEVELOPEDSTRAINVISUALIZATIONSTICKERFORCONCRETE SURFACE STRAIN MEASUREMENT
Figure 6 shows the newly-developed strain visualization sticker. The length, width, andthickness of the strain visualization sticker are 100 mm, 10 mm, and 0.44 mm, respectively,
and the reference length is 100 mm. The top side of this sticker shows fringe characters witha scale of 100 με, making it possible to read the strain values directly with the naked eye.
The bottom side shows simple moiré fringes for use in image processing. The enlargement
ratio M of the characters is 731x, The enlargement ratio M of the moiré fringes is 321x.

4. COMPRESSION TEST
4.1 Specimen
The specimen for the compression test was a cylindrical specimen of shrinkagecompensating mortar with a diameter of 100 mm and height of 200 mm. The properties ofthe specimen material are shown in Table 2. After the specimen was prepared, it was cured
in water for 7 days, after which the test was performed. As shown in Figure 7, a strain gauge
for use with concrete was attached to the center of the specimen for comparison purposes,
and the strain visualization sticker was attached adjacent to the strain gauge using an instantadhesive.

4.2 Outline of apparatus for compression test
As shown in Figure 8, the specimen was set in a compression test machine (rated capacity:1000 kN), and a digital video camera for measurement of the strain visualization sticker wasset up directly facing the sticker at a distance of 0.7 m from the compression test machine.
The digital video camera was a commercially-available USB type product with resolution of
2 million pixels. The digital video camera was connected to a real-time image processing
device, and the strain gauges were connected to a strain measuring device.

Table 2. Properties of specimen material
Water binder ratio Compressive strength
3 days28 days%
22N/mmN/mm
Shrinkage-compensating mortar
1825 or more 45 or more

50100200100unit : mm
：ひずみ可視化シート：Strain Gauge
50：Strain Visualization：ひずみゲージ
Sticker

Strain Gauge
for Concrete

Strain Visualization
Sticker

Figure 7. Specimen for compression test

Test piece
specimencameraData Logger

Figure 8. Apparatus for compression test
4.3 Test method
As the test method, loading up to 200 kN was applied at a constant speed so as not to give
impact to the specimen, and on reaching 200 kN, unloading was performed by the same
method. During the test, the strain visualization sticker was photographed continuously withthe digital video camera, and the amount of strain was calculated in real time by image
processing. Strain at the strain gauge was also measured continuously using the strain
measuring device.

4.4 Result of compression test
Figure 9 shows the relationship of time and strain; Figure 10 shows the relationship of stainand load. Figure 11 shows the difference (error) between the strain values of the strain

visualization sticker calculated by image processing and those of the strain gauge. At thestart of loading, an error of approximately 30 με occurred with the strain visualization sticker,
which is thought to have been caused by buckling of the sticker, as illustrated in Figure 12.
However, with the exception of this error, the measurement accuracy of the strain
visualization sticker was roughly ±20 με during both loading and unloading. This test result
confirmed that the strain visualization sticker can measure strain with comparatively good
accuracy by a noncontact method, even in the case of strain generated at a concrete surface.
Next, Figure 13 shows images of the strain visualization sticker at 100 με intervals. Focusing
on the part where display of the scale begins, display of the scale (characters) inscribed at
100 μ intervals began at each 100 με increment of strain, demonstrating that generated 100μεstrain can be read with the naked eye.
1400

250

Strain Visualization Sticker
Strain Gauge

1200

Strain Visualization Sticker
Strain Gauge
200

Load (kN）Strain ε (x10 -6）

1000
800
600

150

10040050200000501001502002503000200400600800

1000

1200

1400

Strain ε (x10-6）

Time (s)Figure 9. Relationship of time and strain

Figure 10. Relationship of strain
and load

Error for Strain Gauge ε (x10-6）100

Loading
Unloading

8060
4020

Buckling

0-20-40-60-80
-100
0200

400

600

800

1000

1200

1400

Strain ε (x10-6）

Figure 11. The error for strain gauge

Figure 12. Schematic diagram offilm buckling phenomenon

0με

700με

100με

800με

200με

900με

300με

1000μ400με

1100μ500με

1200μ600με

Figure 13. Change in character display with increasing strain
5. CONCLUSION
The authors developed a new strain visualization sticker for use with concrete which makes
it possible to read strain values directly with the naked eye and enables measurement ofstrain by a remote, noncontact method using a general digital camera or similar device,without using electrical elements such as a stain gauge, amplifier, or signal cable. The resultsof verification of its performance showed that visualization resolution was improved to 100με. Although some problems remain, it was possible to obtain the amount of strain withcomparatively good accuracy by performing image processing of sticker images obtained bya noncontact method. Based on the knowledge obtained in this research, the authors will
work to improve strain measurement accuracy, and plan to expand this technique tomonitoring of concrete stress when tensile force is introduced during construction ofprestressed concrete (PC) structures, understanding of the change of prestress after
introduction in the maintenance, and similar issues.

REFERENCES
Takaki, T. et al. (2012). Strain visualization sticker using moire fringe for remote sensing,Proceeding of the Sixth International Conference on bridge maintenance, Safety andManagement, Stresa, Lake Maggiore, Italy, 8-12 July 2012
Umemoto, S. et al. (2012). Strain measurement of bridge members using Strain Visualization
Sticker, Proceeding of the Sixth International Conference on bridge maintenance,Safety and Management, Stresa, Lake Maggiore, Italy, 8-12 July 2012