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Nondestructive identification of delaminations in concrete floor toppings with acoustic methods
Automation in Construction 20 (2011) 799–807
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Automation in Construction j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a u t c o n
Nondestructive identification of delaminations in concrete floor toppings with acoustic methods Jerzy Hola, Lukasz Sadowski, Krzysztof Schabowicz ⁎ Institute of Building Engineering, Wroclaw University of Technology, Plac Grunwaldzki 11, 50-377 Wroclaw, Poland
a r t i c l e
i n f o
Article history: Accepted 26 February 2011 Available online 31 March 2011 Keywords: Concrete Nondestructive tests Acoustic methods Impulse-response Impact-echo
a b s t r a c t This paper presents an original methodology for the nondestructive identification of delaminations in concrete floor toppings by means of the combined impulse-response and impact-echo acoustic methods. It is demonstrated that the impulse-response method is highly suitable for the fast exploration of large stretches of concrete floor and rough location of defective areas while the impact-echo method is ideal for the precise location of the boundaries of the areas. If the surface area of the tested floor topping is large, the nondestructive tests can be automated by mounting the equipment on a special scanner or robot. An example of the practical use of the proposed methodology is presented. It confirms the usefulness of the methodology for the nondestructive identification of delaminations in large-area concrete floor toppings. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The durability of concrete floor toppings is to a large degree determined by their pull-off (from the concrete base) strength. In practice there are cases in which, because of serious errors made during the laying of the topping, this strength may be equal to zero in some areas. This is tantamount to delamination at the concrete base/ floor topping interface. The defective areas reduce the durability of the floor topping whereby the latter is shortly put out of service. For this reason (among others), prior to accepting and putting floor toppings (particularly ones covering large areas and heavily loaded) into service, tests are carried out to early detect any areas in which delamination may have occurred. Drill cores are pulled off the concrete base in the way described in American standard ASTM D 7234 [1] and European standard EN 12504-3:2006 [2]. The pulling off force equal to zero indicates that a delamination is present in the tested place. The effectiveness of the pull-off method in such tests significantly depends on the number of drill cores. In order to precisely determine the size and boundaries of a detected faulty area for repair planning purposes, a denser test grid should be used (by increasing the number of boreholes). But then the labour intensity of such tests increases. In addition, the areas in which boreholes are drilled need to be repaired. These drawbacks become particularly apparent when floor toppings covering large areas (from a few to tens of square meters) are tested.
⁎ Corresponding author. E-mail addresses: [email protected] (J. Hola), [email protected] (L. Sadowski), [email protected] (K. Schabowicz). 0926-5805/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.autcon.2011.02.002
An interesting alternative is the use of nondestructive test methods to test floor toppings (particularly large ones). In order to precisely locate delaminated areas in the topping it is recommended to use jointly two nondestructive test methods: the impulse-response method and the impact-echo method. These state-of-the art acoustic methods were described by Davis [4], Sansalone and Strett [5] and in the ACI 228.28–98 report[6]. 2. State of the art The identification of delaminations in concrete toppings was investigated by Delatte et al. [7]. They proposed a way for making a map of delaminations determined by the pull-off method. Also Garbacz et al. [8] proposed the use of the pull-off method to produce a delamination map on the surface of layered concrete elements, including floor toppings with an overlaid repair layer. Davis et al. [9] and Hertlein and Davis [10] recommended the nondestructive impulse-response method to search for delaminations in concrete floor toppings. Ottosen et al. [11] and Garbacz [12] proposed the use of the nondestructive impact-echo for this purpose. They successfully applied the impact-echo method to small-area floors. Nevertheless, cases of applying combined impulse-response and impact-echo methods to delamination identification are hard to find in the literature on the subject. It was Oh at al. [13] who came up with this observation. On the basis of their own experience Hola et al. [14] concluded that the nondestructive impulse-response and impact-echo methods combined are complementary and highly useful in identifying delaminations, particularly in large-area floor toppings. From amongst the arguments for this conclusion one should mention the fact that in the
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Fig. 1. Idea of impulse-response method: a) measuring system, b) typical mobility N versus frequency curve, c) typical trace of elastic force F generated by hammer, d) typical trace of elastic wave velocity w recorded by geophone.
impulse-response method test points can be as far apart as 2000 m whereby this method is not very accurate. However, it is suitable for quick searching of large flat surfaces and for the approximate identification of areas in which delaminations occurred. As regards the
impact-echo method, the measuring points are closely spaced (a few tens of millimetres apart). For this reason, in the case of larger floors this method is more labour-intensive. But it is ideal for the precise identification of the boundaries of the area previously detected by the
Fig. 2. Idea of impact-echo method: a) measuring kit, b) exciters and measuring probes, c) typical amplitude-frequency spectrum for floor topping thickness measurement, d) typical amplitude-frequency spectrum indicating defect in floor topping, e) typical amplitude-frequency spectrum indicating delamination in floor topping (when topping and base are made of different materials).
J. Hola et al. / Automation in Construction 20 (2011) 799–807
Fig. 3. Graphic illustration of nondestructive identification of delaminations in concrete floor topping by means of impulse-response and impact-echo methods.
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Fig. 4. Typical crack in floor topping.
impulse-response method. Nondestructive tests conducted by the two methods can be easily automated by mounting the test equipment on a special scanner or robot [3]. Considering the above, this paper presents a methodology for the nondestructive identification of delaminations in concrete floor toppings by means of the combined impulse-response and impactecho methods.
3. Brief description of the methods 3.1. Impulse-response method The nondestructive impulse-response method consists of generating an elastic wave in the tested concrete element by striking its surface with a calibrated rubber-ended hammer. The tested element is
Fig. 5. Fragment of tested floor topping: fields marked with letters A–F and faulty area approximately located by impulse-response method, b) exemplary arrangement of measuring points in field C, c) way of testing by impulse-response method.
J. Hola et al. / Automation in Construction 20 (2011) 799–807
struck at selected measuring points. The elastic wave propagating in the element is registered by a geophone and simultaneously amplified by an amplifier. The registered signals are further processed by a dedicated software installed in a laptop (Fig. 1). First the value of elastic force F generated by the hammer is analyzed. According to [11], it depends on element location x and it is an integral of the product of time-dependent function t and exponential function e dependent on the product of imaginary number i, dominant frequency ϖ and time t: ∞
−iϖt
F ðx; ϖÞ = ∫ f ðt Þe
Table 1 Mobility and stiffness values in tested fragment of floor, determined by impulseresponse method. Segment no.
Point no.
A
1 2 3 4 5 6a 7 8 9a 1 2 3a 4 5 6a 7 8 9a 1 2 3 4 5 6 7 8 9
ð1Þ
dt:
−∞
In the case of concrete and reinforced concrete structures, the value of this force should be in the range of 4–6 kN at a frequency of 200 Hz (as shown in Fig. 1c). The trace of elastic wave velocity w registered by the geophone should be without interference. According to the relation given in [11], the value of velocity w is defined as an integral of function w, dependent on element location x and time t, and exponential function e dependent on the product of imaginary number i, dominant frequency ϖ and time t: ∞
wðx; ϖÞ = ∫ wðx; t Þe
−iϖt
dt:
C
E
ð2Þ
−∞
This means that the geophone should register only one elastic wave velocity maximum in 200 μs, while the rest of the trace should oscillate around an average value as shown in Fig. 1d. The mobility trace is subjected to a preliminary analysis. According to [11], mobility N is defined as the ratio of maximum velocity wmax and maximum elastic force Fmax generated by the hammer: N=
wmax : Fmax
ð3Þ
A mobility trace is considered useful if mobility increases linearly in the range of 0–80 Hz, whereas in the range of 100–800 Hz it oscillates around average Nav, as shown in Fig. 1b. If the above conditions are satisfied, the obtained results should be processed using the dedicated software. As a result, the values of five characteristic parameters in the individual points, and ultimately maps of the distribution of the parameters on the surface of the tested floor topping, will be obtained. The parameters are: average mobility Nav, stiffness Kd, mobility slope Mp, mobility times mobility slope Nav Mp and voids index v. In practice, the most useful guide for the location of delaminations in concrete floor toppings are maps of the distribution of parameters Nav and Kd. For the sake of clarity, it is worth adding that: - average mobility Nav is an average value of this parameter in the frequency range of 100–800 Hz, as a function of element thickness (Fig. 1d). If this value increases locally on the mobility map, this means that the material is susceptible to deflection, which may be indicative of a smaller element thickness, delamination, honeycombing in the concrete layer or a surface crack at the particular measuring point; - stiffness Kd is understood as the cotangent of angle α of the mobility curve slope in the frequency range of 0–80 Hz (Fig. 1d). Knowing the stiffness value one can infer the quality of the bond between the layers, and in particular the presence or absence of delamination at the interface. A local decrease in stiffness on the map may indicate the lack of interaction between the layers or a smaller slab thickness.
803
a
Mobility
Stiffness
Nav [m/(s·N)]
Kd [m/N]
5.23 6.91 6.97 8.06 8.99 16.01 7.03 7.59 16.11 5.11 6.12 15.91 7.23 6.01 17.99 4.23 6.90 16.93 7.51 7.43 7.81 5.32 6.47 6.91 8.19 6.22 6.74
0.27 0.25 0.21 0.22 0.22 0.04 0.21 0.21 0.05 0.21 0.22 0.11 0.22 0.23 0.09 0.21 0.22 0.10 0.22 0.21 0.22 0.23 0.26 0.22 0.27 0.24 0.22
Segment no.
Point no.
B
1 2 3 4 5 6 7a 8a 9 1a 2a 3 4a 5a 6 7a 8a 9 1a 2a 3 4 5 6 7 8 9
D
F
Mobility
Stiffness
Nav [m/(s·N)]
Kd [m/N]
6.21 5.99 6.78 6.72 8.11 7.89 16.89 17.82 8.02 17.95 18.82 7.01 18.91 16.99 7.10 17.95 15.86 7.14 17.87 16.11 5.61 5.65 6.88 5.12 6.78 7.95 7.13
0.21 0.22 0.22 0.21 0.23 0.22 0.09 0.11 0.23 0.09 0.10 0.21 0.11 0.10 0.22 0.09 0.11 0.21 0.09 0.09 0.21 0.22 0.25 0.21 0.21 0.22 0.21
Points at which mobility is high while stiffness is low.
3.2. Impact-echo method The measuring kit used in the nondestructive impact-echo method is shown in Fig. 2. The kit includes measuring probes with exciters in the form of a set of steel balls of different diameters and a laptop. The method consists of exciting an elastic wave in the tested element by striking its surface with an exciter. The specialized software enables the recording of the graphic image of the elastic wave propagating in the tested element in the amplitude-time system and the conversion of this image into an amplitude-frequency spectrum by means of the fast Fourier transform or artificial neural networks [15]. The spectrum is subjected to further analysis. According to [5], this method uses the dependence (described by relation (4)) between frequency fD, elastic wave velocity in concrete Cp and the depth at which the defect occurs or element thickness D: fT =
0:96⋅Cp : 2⋅D
ð4Þ
In the amplitude-frequency one can distinguish dominant frequency fD1 corresponding to element thickness, and frequency fD2 corresponding to the reflection of the elastic wave from the defect. If no defect (delamination) occurs in the tested element, one can determine element thickness D1 using the amplitude-frequency spectrum and transforming relation (4) into relation (5), as shown in Fig. 2c: D1 =
0:96⋅Cp : 2⋅fD1
ð5Þ
If delamination occurs, the amplitude-frequency spectrum has the form shown in Fig. 2d. It includes a dominant frequency corresponding to the reflection ( fD2) of the elastic wave from the
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Fig. 6. Area tested by impact-echo method: a) precisely located area and delamination boundary, b) field B with marked grid of measuring points (crosses mark points for which results are shown in Table 2).
defect. Using relation (4) one can calculate the depth (from the surface of the floor topping) at which the defect occurs.
D2 =
0:96⋅Cp : 2⋅fD2
ð6Þ
4. Methodology for nondestructive identification of delaminations The proposed methodology for identifying delaminations in concrete floor toppings by means of a combination of two nondestructive acoustic methods, i.e. the impulse-response method and the impact-echo method, is shown graphically and described in Fig. 3. The nondestructive identification of delaminations should be conducted in two stages.
In stage 1, floor topping areas in which there is no adhesion at the topping/base interface are approximately identified by the impulseresponse method. For this purpose a grid of n measuring points spaced at every 1000 mm (keeping a minimum distance of 500 mm from the edge) should be marked on the floor topping to be tested. If the surface area of the floor topping is considerable, it is recommended that the spacing of measuring points be increased. The test can be automated by mounting the equipment on a special scanner. Then an elastic wave is generated at each of the measuring points in the measuring grid by means of the calibrated hammer and each time the value of elastic force F is generated by the hammer, the trace of elastic wave velocity w and the trace of mobility N are analyzed. The conditions which should be satisfied in order for the results to be acceptable are given in Section 3.1. If the results are satisfactory, they should be processed using the specialized software. As a result, the values of five characteristic parameters: average mobility Nav, stiffness Kd, mobility slope Mp,
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Table 2 Exemplary frequency (corresponding to element thickness) and elastic wave velocity measurements in defective area. Segment no.
No. of points on measuring grid (column/row)
Frequency corresponding to element thickness [kHz]
Frequency in place where defect (delamination) occurs [kHz]
No. of points on measuring grid (column/row)
Frequency corresponding to element thickness [kHz]
Frequency in place where defect (delamination) occurs [kHz]
A
38/26 38/23 35/23 35/21 29/20 29/18 27/33 25/27 28/33 28/27 28/26 25/25a 28/25a 26/19
2.10 2.11 2.09 2.14 2.08 2.08 2.09 2.10 2.08 2.09 2.11 2.11 2.11 2.09
– 34.55 – 34.50 – 34.41 – – 34.43 34.45 – 34.44 – –
4/18 4/16 9/18 9/16 15/15 15/13 20/15 4/12 4/10 9/13 9/10 15/15 15/13 19/15 5/14 5/13 13/36 13/29 11/25 10/21 5/19
2.07 2.14 2.07 2.12 2.08 2.13 2.06 2.11 2.14 2.13 2.08 2.12 2.10 2.11 2.12 2.14 2.10 2.11 2.13 2.07 2.12
– 34.44 – 34.42 – 34.41 – 34.42 – 34.45 – 34.41 – 34.39 – 34.41 34.42 34.43 34.41 34.43 34.41
22/18 20/18 22/12 24/12 24/8 27/8 25/14 29/23 28/10 27/19 26/14 31/11 31/12 36/12 36/11 20/13 26/13 26/12 28/15 28/13 33/14 33/12 19/12 27/14 17/13 30/15 30/13 39/18 39/17 11/11 11/10 15/36 15/29 13/25 12/21 7/19
2.11 2.12 2.11 2.13 2.10 2.11 2.09 2.10 2.08 2.12 2.14 2.13 2.06 2.11 2.11 2.12 2.11 2.13 2.11 2.10 2.08 2.13 2.08 2.09 2.08 2.06 2.10 2.14 2.07 2.11 2.12 2.11 2.06 2.11 2.15 2.11
34.42 – – 34.41 – 34.41 – 34.40 – 34.44 34.46 – 34.46 34.41 – 34.49 – 34.43 34.45 34.46 – 34.44 – 34.39 – 34.41 – 34.44 – – 34.40 – – – – –
B
C
D
E F
a
Points at which exploratory boreholes were drilled.
mobility times mobility slope Nav·Mp voids index w are determined for each point in the measuring grid. Then maps of the distribution of the parameter values on the floor topping surface are produced. By closely examining the maps one can identify approximately the areas in which delamination occurs. In stage 2, defective areas (particularly their boundaries) are more precisely identified. The impact-echo method is used for this purpose. First, a grid of k measuring points (at a 100 × 100 mm spacing) is marked in the area detected and approximately identified in stage 1. Then in each of the points an elastic wave is excited by means of an exciter and the amplitude-time spectrum is recorded. Subsequently, the spectrum is converted into an amplitude-frequency by means of the dedicated software using the fast Fourier transform algorithm. Finally, the amplitude-frequency spectrum obtained in each of the points (Section 3.2) should be analyzed to determine whether delamination occurs.
The results of the nondestructive identification of delamination can be practically verified through test pits made in a randomly selected place(s).
Fig. 7. Elastic wave amplitude-frequency spectrum for measuring point 28/25 in field B (no delamination present in floor).
Fig. 8. Elastic wave amplitude-frequency spectrum for measuring point 25/25 in field B (delamination present in floor).
5. Example of using the methodology 5.1. Short description of tested floor topping A faulty concrete floor topping with an area of 2000 m2 in a multistorey car park was subjected to testing. The topping was 55 mm thick. On the lowest storey the topping had been laid on a base in the form of an 850 mm thick concrete foundation slab and on the higher storeys it had been laid on a 250 mm thick concrete floor slab. Expansion gaps dividing the surface into 2200× 4000 to 4000 × 4000 mm fields had been made in the floor topping. After about one year of service, defects, such as cracks (Fig. 4) and buckling of some field corners, appeared in the concrete floor topping. The concrete floor topping would curl under
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Table 3 Elastic wave velocities in tested floor, frequencies corresponding to element thickness and frequencies at places where delamination occurs, for points 28/25 and 25/25 (where test pits were later made) in field B. No. of points on measuring grid (column/row)
Velocity of elastic wave in floor
Velocity of elastic wave in groundslab
Mean velocity of elastic wave
Frequency corresponding to element thickness
Frequency in defective place
Element thickness
Depth at which defect is located
Cp1 [m/s]
Cp2 [m/s]
Cp [m/s]
fD1 [kHz]
fD2 [kHz]
D1 [mm]
D2 [mm]
28/25 (test pit 2) 25/25 (test pit 3)
3200 3200
4000 4000
3947 3947
2.11 2.11
34.44
900 900
55
moving vehicles. In winter, water from melting snow carried by the tyres of cars would penetrate into the cracks. As cars drove onto the floor topping the water would be squeezed out and then would penetrate into the cracks. Hence it was suspected that there was extensive damage (lack of cohesion at the topping/underlay interface) in the floor. In order to locate defective areas and their boundaries a decision was made to carry out nondestructive acoustic tests. The impulse-response method and the impact-echo method were used for this purpose.
5.2. Exemplary test results and their analysis The whole area of the floor topping was subjected to testing, but here only the tests carried out on a 100 m2 fragment of the floor topping laid on the lowest storey are presented. The tested fragment covered six 3800 × 4000 mm fields denoted by the letters A–E (Fig. 5). In accordance with the proposed methodology, first the floor was tested by the impulse-response method. Measuring points were marked on a grid of squares with an 800– 1000 mm side, as shown in Fig. 5. An elastic wave was generated at each of the points by striking the floor topping's surface with the hammer (Fig. 5c). Exemplary test results for this fragment of the floor topping are shown in Table 1. It is apparent that mobility is high (15–20) and stiffness is low (0–0.2) in the measuring points: 6 and 9 in field A, 7 and 8 in field B, 3, 6 and 9 in field C, 1, 2, 4, 5, 7 and 8 in field D and 1 and 2 in field F. At the other measuring points mobility is low (5–10) and stiffness is above 0.2. It is highly probable that delamination is present in the places where mobility is high while stiffness is low. In Table 1 the points are marked with an asterisk. This was verified by a test pit made in field D (Fig. 5): a delamination was found to be present at the topping/base interface. On the basis of the test results the defective area was approximately located, as illustrated in Fig. 5a. In order to precisely determine the boundaries of the defective area located by the impulse-response method, tests were carried out using the impact-echo method. The P-wave speed in the concrete was measured according to [15]. A grid of k measuring points (at a 100 × 100 mm spacing) was marked in this area, as shown in Fig. 6b for field B. An elastic wave was excited (by means of an exciter placed in the impact-echo apparatus head) at the measuring points adjacent to the approximately located boundary of the faulty area (Fig. 2b). Then using the dedicated software with the fast Fourier transform, the signals were converted to obtain an amplitude-frequency spectrum of the registered elastic wave. Exemplary test results are shown in Table 2. Two different amplitude-frequency spectra (Figs. 7 and 8) were obtained during the tests. It appears from the analysis of the amplitude-frequency spectrum shown in Fig. 7 that dominant frequency fD1 corresponding to floor topping thickness is 2.11 kHz (Table 2). Using relation (4) the topping + base (foundation slab) thickness was calculated. The thickness amounted to about 900 mm which means that at the measuring points for which such a spectrum was obtained a defect in the form of a delamination is not present. Also acoustic signals whose amplitude-frequency spectrum is shown
in Fig. 8 were obtained during the tests. Such a signal occurs when the excited ultrasonic wave is reflected from the bottom and from a defect. Consequently, two characteristic frequency values are subjected to analysis. The first dominant frequency fD1 (corresponding to the element thickness) is 2.11 kHz while the second value corresponding to the frequency (fD2) at the place where the defect is present is 34.44 kHz (Table 2). It was calculated from Eq. (3) that the defect occurs at a depth of about 55 mm, i.e. at the topping/base (foundation slab) interface. This means that delamination is present at the measuring points for which the spectrum shown in Fig. 8 was obtained. The values of the elastic wave velocity, the frequency corresponding to element thickness and the frequency in the place where delamination occurs, for the two different amplitude-frequency spectra obtained while testing the floor in measuring points 28/25 and 25/25 (in which test pits were later made) in field B are compiled in Table 3. In addition, in order to verify the nondestructive test results, two test pits (marked in Fig. 6) were made. No delamination was found to be present in test pit 2, whereas in test pit 3 delamination was found to be present at the topping/base interface (Table 3). 6. Conclusion An original methodology for the nondestructive identification of delaminations in concrete floor toppings by means of state-of-the-art acoustic test methods: the impulse-response method and the impactecho method has been presented. The nondestructive test methods are not commonly used and so they are less known. Therefore a short description of them was included to facilitate the understanding of the proposed methodology. Two stages are distinguished in the proposed procedure. In stage 1, in which a floor topping is investigated by the impulse-response method, delaminations are approximately identified. In stage 2, in which the floor topping is investigated by the impact-echo method, the delamination areas and boundaries are precisely identified. If the surface area of the tested floor topping is large, the nondestructive tests can be automated by mounting the equipment on a special scanner or robot. An example of the practical use of the proposed methodology was presented. It confirmed the usefulness of the methodology for the nondestructive identification of delaminations in large-area concrete floor toppings. References [1] ASTM D7234-05, Standard test method for pull-off adhesion strength of coatings on concrete using portable pull-off adhesion testers. [2] EN 12504–3:2006, Testing concrete in structures, Part 3: nondestructive tests, determination of pull-off force (in Polish). [3] J. Hola, K. Schabowicz, State-of-the-art nondestructive methods for diagnostics testing of building structures—anticipated development trends, Archives of Civil and Mechanical Engineering 11 (2010). [4] A. Davis, The non-destructive impulse response test in North America: 1985– 2001, NDT&E International 36 (2003) 185–193. [5] M. Sansalone, W. Streett, Impact-echo: Nondestructive Evaluation of Concrete and Masonry, Bullbrier Press, Ithaca, 1997.
J. Hola et al. / Automation in Construction 20 (2011) 799–807 [6] American Concrete Institute Report ACI 228.2R-98, Nondestructive Test Methods for Evaluation of Concrete in Structures, ACI, Farmington Hills, Michigan, 1998. [7] N. Delatte, D. Fowler, B. McCullough, Full-Scale Test of High Early Strength Bonded Concrete Overlay Design and Construction Methods, Transportation Research Board of the National Academies 1544 (1996) 9–16. [8] A. Garbacz, M. Gorka, L. Courard, Effect of concrete surface treatment on adhesion in repair systems, Magazine of Concrete Research 57 (2005) 49–60. [9] A. Davis, B. Hertlein, K. Lim, K. Michols, Impact-echo and impulse response stress wave methods: advantages and limitations for the evaluation of highway pavement concrete overlays, Conference on Nondestructive Evaluation of Bridges and Highways, Scottsdale, 88, 1996. [10] B. Hertlein, A. Davis, Locating Concrete Consolidation Problems Using the Nondestructive Impulse Response Test, American Concrete Institute Fall Convention, Los Angeles, 1998.
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[11] N. Ottosen, M. Ristinmmaa, A. Davis, Theoretical interpretation of impulse response tests of embedded concrete structures, Journal of Engineering Mechanics 130 (2004) 1062–1071. [12] A. Garbacz, Non-destructive assessment of repair efficiency with impact-echo and ultrasonic methods—an overview, Concrete Repair, Rehabilitation and Retrofitting – Alexander, 2006. [13] S. Oh, B. Suh, M. Noh, S. Han, K. Kim, E. Cho, et al., Non-destructive test for the assessment of concrete structure safety applied to full-scale test model, American Geophysical Union, Fall Meeting, 12, 2009. [14] J. Hola, L. Sadowski, K. Schabowicz, Nondestructive evaluation of the concrete floor quality using impulse response method and impact-echo method, e-Journal of Nondestructive Testing & Ultrasonics 14 (2009) 55–62. [15] Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-echo Method, American Society for Testing And Materials, 1998.
Contents lists available at ScienceDirect
Automation in Construction j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / a u t c o n
Nondestructive identification of delaminations in concrete floor toppings with acoustic methods Jerzy Hola, Lukasz Sadowski, Krzysztof Schabowicz ⁎ Institute of Building Engineering, Wroclaw University of Technology, Plac Grunwaldzki 11, 50-377 Wroclaw, Poland
a r t i c l e
i n f o
Article history: Accepted 26 February 2011 Available online 31 March 2011 Keywords: Concrete Nondestructive tests Acoustic methods Impulse-response Impact-echo
a b s t r a c t This paper presents an original methodology for the nondestructive identification of delaminations in concrete floor toppings by means of the combined impulse-response and impact-echo acoustic methods. It is demonstrated that the impulse-response method is highly suitable for the fast exploration of large stretches of concrete floor and rough location of defective areas while the impact-echo method is ideal for the precise location of the boundaries of the areas. If the surface area of the tested floor topping is large, the nondestructive tests can be automated by mounting the equipment on a special scanner or robot. An example of the practical use of the proposed methodology is presented. It confirms the usefulness of the methodology for the nondestructive identification of delaminations in large-area concrete floor toppings. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The durability of concrete floor toppings is to a large degree determined by their pull-off (from the concrete base) strength. In practice there are cases in which, because of serious errors made during the laying of the topping, this strength may be equal to zero in some areas. This is tantamount to delamination at the concrete base/ floor topping interface. The defective areas reduce the durability of the floor topping whereby the latter is shortly put out of service. For this reason (among others), prior to accepting and putting floor toppings (particularly ones covering large areas and heavily loaded) into service, tests are carried out to early detect any areas in which delamination may have occurred. Drill cores are pulled off the concrete base in the way described in American standard ASTM D 7234 [1] and European standard EN 12504-3:2006 [2]. The pulling off force equal to zero indicates that a delamination is present in the tested place. The effectiveness of the pull-off method in such tests significantly depends on the number of drill cores. In order to precisely determine the size and boundaries of a detected faulty area for repair planning purposes, a denser test grid should be used (by increasing the number of boreholes). But then the labour intensity of such tests increases. In addition, the areas in which boreholes are drilled need to be repaired. These drawbacks become particularly apparent when floor toppings covering large areas (from a few to tens of square meters) are tested.
⁎ Corresponding author. E-mail addresses: [email protected] (J. Hola), [email protected] (L. Sadowski), [email protected] (K. Schabowicz). 0926-5805/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.autcon.2011.02.002
An interesting alternative is the use of nondestructive test methods to test floor toppings (particularly large ones). In order to precisely locate delaminated areas in the topping it is recommended to use jointly two nondestructive test methods: the impulse-response method and the impact-echo method. These state-of-the art acoustic methods were described by Davis [4], Sansalone and Strett [5] and in the ACI 228.28–98 report[6]. 2. State of the art The identification of delaminations in concrete toppings was investigated by Delatte et al. [7]. They proposed a way for making a map of delaminations determined by the pull-off method. Also Garbacz et al. [8] proposed the use of the pull-off method to produce a delamination map on the surface of layered concrete elements, including floor toppings with an overlaid repair layer. Davis et al. [9] and Hertlein and Davis [10] recommended the nondestructive impulse-response method to search for delaminations in concrete floor toppings. Ottosen et al. [11] and Garbacz [12] proposed the use of the nondestructive impact-echo for this purpose. They successfully applied the impact-echo method to small-area floors. Nevertheless, cases of applying combined impulse-response and impact-echo methods to delamination identification are hard to find in the literature on the subject. It was Oh at al. [13] who came up with this observation. On the basis of their own experience Hola et al. [14] concluded that the nondestructive impulse-response and impact-echo methods combined are complementary and highly useful in identifying delaminations, particularly in large-area floor toppings. From amongst the arguments for this conclusion one should mention the fact that in the
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Fig. 1. Idea of impulse-response method: a) measuring system, b) typical mobility N versus frequency curve, c) typical trace of elastic force F generated by hammer, d) typical trace of elastic wave velocity w recorded by geophone.
impulse-response method test points can be as far apart as 2000 m whereby this method is not very accurate. However, it is suitable for quick searching of large flat surfaces and for the approximate identification of areas in which delaminations occurred. As regards the
impact-echo method, the measuring points are closely spaced (a few tens of millimetres apart). For this reason, in the case of larger floors this method is more labour-intensive. But it is ideal for the precise identification of the boundaries of the area previously detected by the
Fig. 2. Idea of impact-echo method: a) measuring kit, b) exciters and measuring probes, c) typical amplitude-frequency spectrum for floor topping thickness measurement, d) typical amplitude-frequency spectrum indicating defect in floor topping, e) typical amplitude-frequency spectrum indicating delamination in floor topping (when topping and base are made of different materials).
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Fig. 3. Graphic illustration of nondestructive identification of delaminations in concrete floor topping by means of impulse-response and impact-echo methods.
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Fig. 4. Typical crack in floor topping.
impulse-response method. Nondestructive tests conducted by the two methods can be easily automated by mounting the test equipment on a special scanner or robot [3]. Considering the above, this paper presents a methodology for the nondestructive identification of delaminations in concrete floor toppings by means of the combined impulse-response and impactecho methods.
3. Brief description of the methods 3.1. Impulse-response method The nondestructive impulse-response method consists of generating an elastic wave in the tested concrete element by striking its surface with a calibrated rubber-ended hammer. The tested element is
Fig. 5. Fragment of tested floor topping: fields marked with letters A–F and faulty area approximately located by impulse-response method, b) exemplary arrangement of measuring points in field C, c) way of testing by impulse-response method.
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struck at selected measuring points. The elastic wave propagating in the element is registered by a geophone and simultaneously amplified by an amplifier. The registered signals are further processed by a dedicated software installed in a laptop (Fig. 1). First the value of elastic force F generated by the hammer is analyzed. According to [11], it depends on element location x and it is an integral of the product of time-dependent function t and exponential function e dependent on the product of imaginary number i, dominant frequency ϖ and time t: ∞
−iϖt
F ðx; ϖÞ = ∫ f ðt Þe
Table 1 Mobility and stiffness values in tested fragment of floor, determined by impulseresponse method. Segment no.
Point no.
A
1 2 3 4 5 6a 7 8 9a 1 2 3a 4 5 6a 7 8 9a 1 2 3 4 5 6 7 8 9
ð1Þ
dt:
−∞
In the case of concrete and reinforced concrete structures, the value of this force should be in the range of 4–6 kN at a frequency of 200 Hz (as shown in Fig. 1c). The trace of elastic wave velocity w registered by the geophone should be without interference. According to the relation given in [11], the value of velocity w is defined as an integral of function w, dependent on element location x and time t, and exponential function e dependent on the product of imaginary number i, dominant frequency ϖ and time t: ∞
wðx; ϖÞ = ∫ wðx; t Þe
−iϖt
dt:
C
E
ð2Þ
−∞
This means that the geophone should register only one elastic wave velocity maximum in 200 μs, while the rest of the trace should oscillate around an average value as shown in Fig. 1d. The mobility trace is subjected to a preliminary analysis. According to [11], mobility N is defined as the ratio of maximum velocity wmax and maximum elastic force Fmax generated by the hammer: N=
wmax : Fmax
ð3Þ
A mobility trace is considered useful if mobility increases linearly in the range of 0–80 Hz, whereas in the range of 100–800 Hz it oscillates around average Nav, as shown in Fig. 1b. If the above conditions are satisfied, the obtained results should be processed using the dedicated software. As a result, the values of five characteristic parameters in the individual points, and ultimately maps of the distribution of the parameters on the surface of the tested floor topping, will be obtained. The parameters are: average mobility Nav, stiffness Kd, mobility slope Mp, mobility times mobility slope Nav Mp and voids index v. In practice, the most useful guide for the location of delaminations in concrete floor toppings are maps of the distribution of parameters Nav and Kd. For the sake of clarity, it is worth adding that: - average mobility Nav is an average value of this parameter in the frequency range of 100–800 Hz, as a function of element thickness (Fig. 1d). If this value increases locally on the mobility map, this means that the material is susceptible to deflection, which may be indicative of a smaller element thickness, delamination, honeycombing in the concrete layer or a surface crack at the particular measuring point; - stiffness Kd is understood as the cotangent of angle α of the mobility curve slope in the frequency range of 0–80 Hz (Fig. 1d). Knowing the stiffness value one can infer the quality of the bond between the layers, and in particular the presence or absence of delamination at the interface. A local decrease in stiffness on the map may indicate the lack of interaction between the layers or a smaller slab thickness.
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a
Mobility
Stiffness
Nav [m/(s·N)]
Kd [m/N]
5.23 6.91 6.97 8.06 8.99 16.01 7.03 7.59 16.11 5.11 6.12 15.91 7.23 6.01 17.99 4.23 6.90 16.93 7.51 7.43 7.81 5.32 6.47 6.91 8.19 6.22 6.74
0.27 0.25 0.21 0.22 0.22 0.04 0.21 0.21 0.05 0.21 0.22 0.11 0.22 0.23 0.09 0.21 0.22 0.10 0.22 0.21 0.22 0.23 0.26 0.22 0.27 0.24 0.22
Segment no.
Point no.
B
1 2 3 4 5 6 7a 8a 9 1a 2a 3 4a 5a 6 7a 8a 9 1a 2a 3 4 5 6 7 8 9
D
F
Mobility
Stiffness
Nav [m/(s·N)]
Kd [m/N]
6.21 5.99 6.78 6.72 8.11 7.89 16.89 17.82 8.02 17.95 18.82 7.01 18.91 16.99 7.10 17.95 15.86 7.14 17.87 16.11 5.61 5.65 6.88 5.12 6.78 7.95 7.13
0.21 0.22 0.22 0.21 0.23 0.22 0.09 0.11 0.23 0.09 0.10 0.21 0.11 0.10 0.22 0.09 0.11 0.21 0.09 0.09 0.21 0.22 0.25 0.21 0.21 0.22 0.21
Points at which mobility is high while stiffness is low.
3.2. Impact-echo method The measuring kit used in the nondestructive impact-echo method is shown in Fig. 2. The kit includes measuring probes with exciters in the form of a set of steel balls of different diameters and a laptop. The method consists of exciting an elastic wave in the tested element by striking its surface with an exciter. The specialized software enables the recording of the graphic image of the elastic wave propagating in the tested element in the amplitude-time system and the conversion of this image into an amplitude-frequency spectrum by means of the fast Fourier transform or artificial neural networks [15]. The spectrum is subjected to further analysis. According to [5], this method uses the dependence (described by relation (4)) between frequency fD, elastic wave velocity in concrete Cp and the depth at which the defect occurs or element thickness D: fT =
0:96⋅Cp : 2⋅D
ð4Þ
In the amplitude-frequency one can distinguish dominant frequency fD1 corresponding to element thickness, and frequency fD2 corresponding to the reflection of the elastic wave from the defect. If no defect (delamination) occurs in the tested element, one can determine element thickness D1 using the amplitude-frequency spectrum and transforming relation (4) into relation (5), as shown in Fig. 2c: D1 =
0:96⋅Cp : 2⋅fD1
ð5Þ
If delamination occurs, the amplitude-frequency spectrum has the form shown in Fig. 2d. It includes a dominant frequency corresponding to the reflection ( fD2) of the elastic wave from the
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Fig. 6. Area tested by impact-echo method: a) precisely located area and delamination boundary, b) field B with marked grid of measuring points (crosses mark points for which results are shown in Table 2).
defect. Using relation (4) one can calculate the depth (from the surface of the floor topping) at which the defect occurs.
D2 =
0:96⋅Cp : 2⋅fD2
ð6Þ
4. Methodology for nondestructive identification of delaminations The proposed methodology for identifying delaminations in concrete floor toppings by means of a combination of two nondestructive acoustic methods, i.e. the impulse-response method and the impact-echo method, is shown graphically and described in Fig. 3. The nondestructive identification of delaminations should be conducted in two stages.
In stage 1, floor topping areas in which there is no adhesion at the topping/base interface are approximately identified by the impulseresponse method. For this purpose a grid of n measuring points spaced at every 1000 mm (keeping a minimum distance of 500 mm from the edge) should be marked on the floor topping to be tested. If the surface area of the floor topping is considerable, it is recommended that the spacing of measuring points be increased. The test can be automated by mounting the equipment on a special scanner. Then an elastic wave is generated at each of the measuring points in the measuring grid by means of the calibrated hammer and each time the value of elastic force F is generated by the hammer, the trace of elastic wave velocity w and the trace of mobility N are analyzed. The conditions which should be satisfied in order for the results to be acceptable are given in Section 3.1. If the results are satisfactory, they should be processed using the specialized software. As a result, the values of five characteristic parameters: average mobility Nav, stiffness Kd, mobility slope Mp,
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Table 2 Exemplary frequency (corresponding to element thickness) and elastic wave velocity measurements in defective area. Segment no.
No. of points on measuring grid (column/row)
Frequency corresponding to element thickness [kHz]
Frequency in place where defect (delamination) occurs [kHz]
No. of points on measuring grid (column/row)
Frequency corresponding to element thickness [kHz]
Frequency in place where defect (delamination) occurs [kHz]
A
38/26 38/23 35/23 35/21 29/20 29/18 27/33 25/27 28/33 28/27 28/26 25/25a 28/25a 26/19
2.10 2.11 2.09 2.14 2.08 2.08 2.09 2.10 2.08 2.09 2.11 2.11 2.11 2.09
– 34.55 – 34.50 – 34.41 – – 34.43 34.45 – 34.44 – –
4/18 4/16 9/18 9/16 15/15 15/13 20/15 4/12 4/10 9/13 9/10 15/15 15/13 19/15 5/14 5/13 13/36 13/29 11/25 10/21 5/19
2.07 2.14 2.07 2.12 2.08 2.13 2.06 2.11 2.14 2.13 2.08 2.12 2.10 2.11 2.12 2.14 2.10 2.11 2.13 2.07 2.12
– 34.44 – 34.42 – 34.41 – 34.42 – 34.45 – 34.41 – 34.39 – 34.41 34.42 34.43 34.41 34.43 34.41
22/18 20/18 22/12 24/12 24/8 27/8 25/14 29/23 28/10 27/19 26/14 31/11 31/12 36/12 36/11 20/13 26/13 26/12 28/15 28/13 33/14 33/12 19/12 27/14 17/13 30/15 30/13 39/18 39/17 11/11 11/10 15/36 15/29 13/25 12/21 7/19
2.11 2.12 2.11 2.13 2.10 2.11 2.09 2.10 2.08 2.12 2.14 2.13 2.06 2.11 2.11 2.12 2.11 2.13 2.11 2.10 2.08 2.13 2.08 2.09 2.08 2.06 2.10 2.14 2.07 2.11 2.12 2.11 2.06 2.11 2.15 2.11
34.42 – – 34.41 – 34.41 – 34.40 – 34.44 34.46 – 34.46 34.41 – 34.49 – 34.43 34.45 34.46 – 34.44 – 34.39 – 34.41 – 34.44 – – 34.40 – – – – –
B
C
D
E F
a
Points at which exploratory boreholes were drilled.
mobility times mobility slope Nav·Mp voids index w are determined for each point in the measuring grid. Then maps of the distribution of the parameter values on the floor topping surface are produced. By closely examining the maps one can identify approximately the areas in which delamination occurs. In stage 2, defective areas (particularly their boundaries) are more precisely identified. The impact-echo method is used for this purpose. First, a grid of k measuring points (at a 100 × 100 mm spacing) is marked in the area detected and approximately identified in stage 1. Then in each of the points an elastic wave is excited by means of an exciter and the amplitude-time spectrum is recorded. Subsequently, the spectrum is converted into an amplitude-frequency by means of the dedicated software using the fast Fourier transform algorithm. Finally, the amplitude-frequency spectrum obtained in each of the points (Section 3.2) should be analyzed to determine whether delamination occurs.
The results of the nondestructive identification of delamination can be practically verified through test pits made in a randomly selected place(s).
Fig. 7. Elastic wave amplitude-frequency spectrum for measuring point 28/25 in field B (no delamination present in floor).
Fig. 8. Elastic wave amplitude-frequency spectrum for measuring point 25/25 in field B (delamination present in floor).
5. Example of using the methodology 5.1. Short description of tested floor topping A faulty concrete floor topping with an area of 2000 m2 in a multistorey car park was subjected to testing. The topping was 55 mm thick. On the lowest storey the topping had been laid on a base in the form of an 850 mm thick concrete foundation slab and on the higher storeys it had been laid on a 250 mm thick concrete floor slab. Expansion gaps dividing the surface into 2200× 4000 to 4000 × 4000 mm fields had been made in the floor topping. After about one year of service, defects, such as cracks (Fig. 4) and buckling of some field corners, appeared in the concrete floor topping. The concrete floor topping would curl under
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Table 3 Elastic wave velocities in tested floor, frequencies corresponding to element thickness and frequencies at places where delamination occurs, for points 28/25 and 25/25 (where test pits were later made) in field B. No. of points on measuring grid (column/row)
Velocity of elastic wave in floor
Velocity of elastic wave in groundslab
Mean velocity of elastic wave
Frequency corresponding to element thickness
Frequency in defective place
Element thickness
Depth at which defect is located
Cp1 [m/s]
Cp2 [m/s]
Cp [m/s]
fD1 [kHz]
fD2 [kHz]
D1 [mm]
D2 [mm]
28/25 (test pit 2) 25/25 (test pit 3)
3200 3200
4000 4000
3947 3947
2.11 2.11
34.44
900 900
55
moving vehicles. In winter, water from melting snow carried by the tyres of cars would penetrate into the cracks. As cars drove onto the floor topping the water would be squeezed out and then would penetrate into the cracks. Hence it was suspected that there was extensive damage (lack of cohesion at the topping/underlay interface) in the floor. In order to locate defective areas and their boundaries a decision was made to carry out nondestructive acoustic tests. The impulse-response method and the impact-echo method were used for this purpose.
5.2. Exemplary test results and their analysis The whole area of the floor topping was subjected to testing, but here only the tests carried out on a 100 m2 fragment of the floor topping laid on the lowest storey are presented. The tested fragment covered six 3800 × 4000 mm fields denoted by the letters A–E (Fig. 5). In accordance with the proposed methodology, first the floor was tested by the impulse-response method. Measuring points were marked on a grid of squares with an 800– 1000 mm side, as shown in Fig. 5. An elastic wave was generated at each of the points by striking the floor topping's surface with the hammer (Fig. 5c). Exemplary test results for this fragment of the floor topping are shown in Table 1. It is apparent that mobility is high (15–20) and stiffness is low (0–0.2) in the measuring points: 6 and 9 in field A, 7 and 8 in field B, 3, 6 and 9 in field C, 1, 2, 4, 5, 7 and 8 in field D and 1 and 2 in field F. At the other measuring points mobility is low (5–10) and stiffness is above 0.2. It is highly probable that delamination is present in the places where mobility is high while stiffness is low. In Table 1 the points are marked with an asterisk. This was verified by a test pit made in field D (Fig. 5): a delamination was found to be present at the topping/base interface. On the basis of the test results the defective area was approximately located, as illustrated in Fig. 5a. In order to precisely determine the boundaries of the defective area located by the impulse-response method, tests were carried out using the impact-echo method. The P-wave speed in the concrete was measured according to [15]. A grid of k measuring points (at a 100 × 100 mm spacing) was marked in this area, as shown in Fig. 6b for field B. An elastic wave was excited (by means of an exciter placed in the impact-echo apparatus head) at the measuring points adjacent to the approximately located boundary of the faulty area (Fig. 2b). Then using the dedicated software with the fast Fourier transform, the signals were converted to obtain an amplitude-frequency spectrum of the registered elastic wave. Exemplary test results are shown in Table 2. Two different amplitude-frequency spectra (Figs. 7 and 8) were obtained during the tests. It appears from the analysis of the amplitude-frequency spectrum shown in Fig. 7 that dominant frequency fD1 corresponding to floor topping thickness is 2.11 kHz (Table 2). Using relation (4) the topping + base (foundation slab) thickness was calculated. The thickness amounted to about 900 mm which means that at the measuring points for which such a spectrum was obtained a defect in the form of a delamination is not present. Also acoustic signals whose amplitude-frequency spectrum is shown
in Fig. 8 were obtained during the tests. Such a signal occurs when the excited ultrasonic wave is reflected from the bottom and from a defect. Consequently, two characteristic frequency values are subjected to analysis. The first dominant frequency fD1 (corresponding to the element thickness) is 2.11 kHz while the second value corresponding to the frequency (fD2) at the place where the defect is present is 34.44 kHz (Table 2). It was calculated from Eq. (3) that the defect occurs at a depth of about 55 mm, i.e. at the topping/base (foundation slab) interface. This means that delamination is present at the measuring points for which the spectrum shown in Fig. 8 was obtained. The values of the elastic wave velocity, the frequency corresponding to element thickness and the frequency in the place where delamination occurs, for the two different amplitude-frequency spectra obtained while testing the floor in measuring points 28/25 and 25/25 (in which test pits were later made) in field B are compiled in Table 3. In addition, in order to verify the nondestructive test results, two test pits (marked in Fig. 6) were made. No delamination was found to be present in test pit 2, whereas in test pit 3 delamination was found to be present at the topping/base interface (Table 3). 6. Conclusion An original methodology for the nondestructive identification of delaminations in concrete floor toppings by means of state-of-the-art acoustic test methods: the impulse-response method and the impactecho method has been presented. The nondestructive test methods are not commonly used and so they are less known. Therefore a short description of them was included to facilitate the understanding of the proposed methodology. Two stages are distinguished in the proposed procedure. In stage 1, in which a floor topping is investigated by the impulse-response method, delaminations are approximately identified. In stage 2, in which the floor topping is investigated by the impact-echo method, the delamination areas and boundaries are precisely identified. If the surface area of the tested floor topping is large, the nondestructive tests can be automated by mounting the equipment on a special scanner or robot. An example of the practical use of the proposed methodology was presented. It confirmed the usefulness of the methodology for the nondestructive identification of delaminations in large-area concrete floor toppings. References [1] ASTM D7234-05, Standard test method for pull-off adhesion strength of coatings on concrete using portable pull-off adhesion testers. [2] EN 12504–3:2006, Testing concrete in structures, Part 3: nondestructive tests, determination of pull-off force (in Polish). [3] J. Hola, K. Schabowicz, State-of-the-art nondestructive methods for diagnostics testing of building structures—anticipated development trends, Archives of Civil and Mechanical Engineering 11 (2010). [4] A. Davis, The non-destructive impulse response test in North America: 1985– 2001, NDT&E International 36 (2003) 185–193. [5] M. Sansalone, W. Streett, Impact-echo: Nondestructive Evaluation of Concrete and Masonry, Bullbrier Press, Ithaca, 1997.
J. Hola et al. / Automation in Construction 20 (2011) 799–807 [6] American Concrete Institute Report ACI 228.2R-98, Nondestructive Test Methods for Evaluation of Concrete in Structures, ACI, Farmington Hills, Michigan, 1998. [7] N. Delatte, D. Fowler, B. McCullough, Full-Scale Test of High Early Strength Bonded Concrete Overlay Design and Construction Methods, Transportation Research Board of the National Academies 1544 (1996) 9–16. [8] A. Garbacz, M. Gorka, L. Courard, Effect of concrete surface treatment on adhesion in repair systems, Magazine of Concrete Research 57 (2005) 49–60. [9] A. Davis, B. Hertlein, K. Lim, K. Michols, Impact-echo and impulse response stress wave methods: advantages and limitations for the evaluation of highway pavement concrete overlays, Conference on Nondestructive Evaluation of Bridges and Highways, Scottsdale, 88, 1996. [10] B. Hertlein, A. Davis, Locating Concrete Consolidation Problems Using the Nondestructive Impulse Response Test, American Concrete Institute Fall Convention, Los Angeles, 1998.
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[11] N. Ottosen, M. Ristinmmaa, A. Davis, Theoretical interpretation of impulse response tests of embedded concrete structures, Journal of Engineering Mechanics 130 (2004) 1062–1071. [12] A. Garbacz, Non-destructive assessment of repair efficiency with impact-echo and ultrasonic methods—an overview, Concrete Repair, Rehabilitation and Retrofitting – Alexander, 2006. [13] S. Oh, B. Suh, M. Noh, S. Han, K. Kim, E. Cho, et al., Non-destructive test for the assessment of concrete structure safety applied to full-scale test model, American Geophysical Union, Fall Meeting, 12, 2009. [14] J. Hola, L. Sadowski, K. Schabowicz, Nondestructive evaluation of the concrete floor quality using impulse response method and impact-echo method, e-Journal of Nondestructive Testing & Ultrasonics 14 (2009) 55–62. [15] Standard Test Method for Measuring the P-Wave Speed and the Thickness of Concrete Plates Using the Impact-echo Method, American Society for Testing And Materials, 1998.