IN-SITU NON-DESTRUCTIVE EVALUATION OF CONCRETE STRUCTURES
USING SHEAROGRAPHY
Y. Y. Hung, L. Liu and Y.H. Huang
Dept. of Manufacturing Engineering and Engineering Management
City University of Hong Kong
Edward C.Y. Yiu
Department of Real Estate and Construction
The University of Hong Kong
ABSTRACT
At present the core test is commonly employed for evaluating concrete strength of aging structures. The core test, which
requires coring concrete samples for subsequent off-site laboratory test, is somewhat destructive and only limited to taking
samples from a few locations. This paper reports a novel optical technique based on shearography for non-destructive
evaluation of concrete strength. The technique does not require coring, and therefore allows test on many structural locations
to be performed. The preliminary results have demonstrated that the technique has a great potential for development into a
practical tool for evaluation of concrete strength.
Introduction
Assessing and monitoring the strength of concrete structures in existing, especially aging, buildings, is an important task that
can never be over emphasized. In comparison with the core test, non-destructive testing (NDT) for assessing in-situ strength
of concrete structures, as defined in [1], has the main advantage of not impairing the intended performance of the element or
member under investigation. Table 1 lists some Standards Specifications that describe procedures for testing concrete
strength. Upon reviewing various NDT methods for concrete strength, including surface hardness test, penetration resistance
test and ultrasonic pulse velocity test, Tam [2] reported great limitations in each of these techniques. In the light of these
limitations, the compressive core test is commonly adopted to establish a correlation with NDT results.
Table 1: American (ASTM) and British (BS) Standards for NDT of concrete strengths
Type of NDT
American Standards
British Standards
Surface hardness test (Rebound hardness)
C805-02
BS EN 12504-2:2001
Penetration resistance test (Windsor probe)
C803/803M-97e1
BS 1881-207:1992
Pull-off test
BS 1881-207:1992
Pull-out test
C900-01
BS 1881-207:1992
Break-off test
BS 1881-207:1992
Ultrasonic pulse velocity
C597-97
BS 1881-203:1986
The surface hardness test, which measures the surface hardness by means of the Schmidt rebound hammer, is most
commonly used. This test is based on the principle that the rebound of an elastic mass is related to the hardness of the
surface upon which it strikes [3]. However, Kolek [4] and Tam [5] reported that there was no unique correlation between the
Rebound Number and the strength, as the data showed wide scatter from the mean curve. The penetration resistance test, on
the other hand, is based on the depth of penetration of a standard probe (Windsor) driven into the concrete. Similar methods
applied near the concrete surface have included pull-off, pull-out and break-off tests. However, these tests focus on the
surface hardness of concrete at the point of test. Therefore, proximity of steel reinforcement, depth of carbonation and
volumetric fraction of coarse aggregates are found to have an influence on the test results [6]. Ultrasonic pulse velocity
measurement, which involves sending an electro-acoustic pulse through a concrete path, correlates pulse velocity and
concrete strength. However, through modelling, Tam [2] pointed out that a universal correlation could not be found as the
major factors influencing strength and velocity were not necessarily the same, and that an attempt to correlate pulse velocity
and concrete strength was questionable. There are other NDT techniques, such as recording the maximum speed of change in
the contact force in the magnetoelastic transducer [7], but they are not commonly used by the industries.
The core test, which is described in detail in [8] and [9], is often used together with NDT techniques. However, the core test is
not NDT; thus the number of cores extracted must be limited, and the position of the core, such as the tension zones of a
structure, would greatly affect the measured strength. A combination of these NDT techniques is often suggested, but each of
these tests is not sufficiently understood.
NDT USING SHEAROGRAPHY
NDT using optical interferometry such as moiré, holography, and shearography have gained rapid recognition since the
invention of the laser about 5 decades ago [10]. Their main advantages lie in being full-field, non-contacting, and nonradioactive. During testing, data acquisition, retrieval, analysis and interpretation can be achieved very rapidly with digital
processing techniques. Furthermore, the use of optical fibre for light transmission allows inspection of areas that are nonaccessible by other conventional methods, thereby making optical NDT more attractive for industrial applications. Moiré and
holography have been applied to assessing concrete structures; however, a main limitation of these techniques is the need for
environmental stability (e.g., vibration and air movement) during testing. Shearography, which was invented by the first author
of this paper, has overcome this concern and has been adopted for aircraft tyre inspection as well as in various industrial
sectors for in-situ inspection [11, 12]. Shearography is also a technique that yields direct strain- or surface slope- related
results, hence making it an excellent tool for full-field flaw detection, surface profiling, and stress/strain measurements.
Figure 1. Optical layout for digital shearography.
Figure 1 illustrates an optical layout for digital shearography. The image of a laser-illuminated test object is recorded using a
digital camera fitted with a doubly-refractive prism, which is an image-shearing device. This device splits the image of the test
object into two partially overlapping images, thereby causing light scattered from two neighbouring points B and M on the test
specimen to interfere at the image plane of the camera after exiting from the polarizer. It is worth noting that the distance
between B and M is equal to the amount of image-shearing. The camera is connected to a personal computer via a frame
grabber so that the recorded image is digitized and stored in the computer memory for subsequent processing. Shearographybased NDT involves two recordings, one before and one after the test object is slightly deformed. Comparison between the
two images yields phase-change (ΔΦ) of the object surface and is subsequently displayed on the computer monitor as fringelines representing constant slope- or strain- distribution along the direction of image-shearing (Fig. 2). If the direction of imageshearing is aligned to the reference x-axis and the amount of image-shearing is δx , the phase-fringes are mathematically
described by the following expression.
Δφ =
2π ⎡ ∂u
∂w ⎤
∂v
δx
+ A3
+ A2
A1
∂x ⎥⎦
λ ⎢⎣ ∂x
∂x
(1)
where λ is the wavelength of the laser used; (A1, A2, A3) are system parameters related to the optical arrangement; (u, v) are
in-plane displacement components along the reference x- and y- axis, respectively; w is the out-of-plane displacement
component along the reference z-axis; and (∂u ⁄ ∂x, ∂v ⁄ ∂x, ∂w ⁄ ∂x) are displacement-gradients along the direction of imageshearing x. If, on the other hand, the direction of image-shearing is aligned to the reference y-axis, all the x-terms in Equation 1
will be replaced by y. Furthermore, if the directions of illumination and recording are both normal to the surface of the test
object (i.e., along the z-axis in Fig. 1), the fringe-lines will depict only out-of-plane displacement gradients (i.e., either ∂w ⁄ ∂x or
∂w ⁄ ∂y) with the maximum fringe-visibility.
(a)
(b)
Figure 2. (a) Typical shearographic fringes depicting x-derivative of deflection of a clamped square plate under central point
load. (b) The fringe phase distribution obtained by a phase-shift technique.
The phase-change (ΔΦ) can be deduced from the recorded images using methods such as Fast Fourier Transform or Multiple
Phase-Shifting. Fig. 2(b) shows the phase distribution of the fringe pattern of Fig. 2(a) obtained by a four-frame phase-shift
algorithm. Thus, from the extracted, or the measurements of, phase-change, displacement-gradients and surface strains can
be calculated.
It is worth noting that Equation 1 is derived on the basis of small image-shearing, in which B and M are closely spaced. For
large image-shearing, however, the space between B and M is not too close, the phase-change (ΔΦ) is now described by the
following expression, which is similar to Equation 1 [13].
Δφ =
2π
λ
[ A1 (u M − u B ) + A2 (vM − vB ) + A3 ( wM − wB )]
(2)
Equation 2 clearly shows that the phase-change (ΔΦ) is now related to the relative displacement between points B and M, and
it is similar to that of holography. Similar to shearography with small image-shearing, if the directions of illumination and
recording are both normal to the surface of the test object (i.e., along the z-axis), the fringe-lines obtained using large imageshearing will depict only relative out-of-plane displacement gradient (wM - wB). The large-shear setup can be used to measure
the displacement of a deformed region relative to an undeformed region. In this case, it measures the absolute displacement
distribution of the deformed region. The result is similar to holographic interferometry. Figure 3 shows a fringe pattern
obtained by this setup which depicts the deflection of a plate.
Figure 3. Example of a fringe pattern obtained by the large-shear setup, which depicts the absolute deflection of a plate fixed
along its boundaries and loaded transversely at its center.
In this paper, the technique of shearography with large image-shearing is used for the assessment of concrete strength. A
concrete block is subjected to a concentrated load-increment P applied at point O (see Fig. 4) between the two recordings.
Treating the concrete block as a straight semi-infinite boundary, the relative out-of-plane displacement
(η ≡ | wM – wB | ) along the direction of loading between two points M and B on the block is given by the following expression
[14].
η=
2P
πE
log e
s
r
(3)
where E is the Young’s modulus of elasticity; s is the distance between points O and B; and r is the distance between points O
and M. With large-image shearing, the displacement wB may be treated as negligibly small compared to wM so that the relative
displacement η in Equation 3 is essentially the displacement of point M under the load-increment. The displacement η is a
mathematical function of the measured phase-change (ΔΦ) through a system constant that is related to the optical
arrangement. Thus, combining Equations 2 and 3 for the case of normal illumination and recording, the following expression is
obtained.
s⎞
⎛P
Δφ = K ⎜ log e ⎟
E
r
⎝
⎠
(4)
where K may be treated as a system constant.
Equation 3, as well as Equation 4, also suggests that, corresponding to a given value of P and s ⁄ r, the displacement η is
inversely proportional to the Young’s modules E. For a given value of s ⁄ r, the displacement η is linearly proportional to the
applied load-increment P.
Figure 4. Determination of out-of-plane displacement of a semi-infinite straight edge under concentrated load.
EXPERIMENTAL RESULTS AND DISCUSSION
In this investigation, both uniaxial compression tests and shearographic tests were conducted on specimens prepared from 3
different concrete mix grades. The Young’s modulus E and compressive strength ξ obtained from uniaxial compression tests
are shown in Table 2.
Table 2. Values of Young’s modulus E and compressive strength ξ of specimens used.
TEST SAMPLE
C20
C40
C60
UNIAXIAL COMPRESSION TEST RESULTS
ξ
(MPa)
42.4
64.0
72.8
SHEAROGRAPHY RESULTS
E (GPa)
E (GPa)
29.6
34.3
35.6
28.7
33.2
36.4
Figure 5 shows a typical phase-fringe distribution. The left-border nearly passes through the centre of the applied load, the
right-border corresponds to the point B in Figure 4, and the position where the first left-fringe is located corresponds to point M
in Figure 4. Thus, the fringe-lines depict the relative out-of-plane displacements between M and B, as are described by
Equations 2 and 3.
Figure 5. Typical shearographic fringe pattern of a concrete block subjected to point load-increment.
Figure 6 shows the distribution of out-of-plane displacement η along the length of the concrete block (r) when an incremental
load P of 2.7 kN is applied between recordings. Figure 7 shows the linear relationship between η and P for a constant value of
r. The trend of these experimental results in Figures 6 and 7 is consistent with theory (see Eqs. 3 & 4). The shearographic test
results also enabled determination of the Young’s modulus E; the values have compared well with those obtained from the
uniaxial compression test (Table 2).
As the compressive strength of concrete ξ is found to increase with increasing Young’s modulus E [15], Equation 3 further
suggests that the displacement η increases with decreasing values of concrete compressive strength ξ. This trend is also
observed in the shearographic test results shown in Figures 6 and 7.
Figure 6. Average displacement distribution of three types of concrete samples
Figure 7. Relationship between average displacement and applied load for three types of concrete samples.
CONCLUSIONS
In-situ evaluation of concrete strength is one of the most important tasks in condition appraisal of concrete structures. At
present, the core test is commonly used for testing concrete strength; but this involves coring concrete samples for subsequent
laboratory testing, thus resulting in a slow and destructive process. Furthermore, the number of cores has to be limited and
their positions, such as the tension zones of a structure, would greatly affect the measured strength. This paper reports the
novel use of shearography for non-destructive evaluation of concrete strength that does not require coring. The preliminary
results have demonstrated that the technique has great potential for development into a practical tool for in-situ evaluation of
concrete strength.
ACKNOWLEDGEMENT
The work described in this paper has been supported by the grant (CityU 1/01C) and (CityU 1196/02E) of the Research Grants
Council of Hong Kong, China.
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