Acoustic emission monitoring of CFRP reinforced concrete slabs

Construction and Building Materials 23 (2009) 2016–2026

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Acoustic emission monitoring of CFRP reinforced concrete slabs Sandeep Degala, Piervincenzo Rizzo *, Karthik Ramanathan, Kent A. Harries Laboratory for NDE and SHM Studies, Department of Civil and Environmental Engineering, University of Pittsburgh, 3700 O’Hara Street, 949 Benedum Hall, Pittsburgh, PA 15261, United States

a r t i c l e

i n f o

Article history: Received 28 November 2007 Received in revised form 19 August 2008 Accepted 27 August 2008 Available online 5 October 2008 Keywords: Acoustic emission CFRP Reinforced concrete Nondestructive evaluation Intensity analysis Principal component analysis

a b s t r a c t Debonding of externally bonded carbon fiber reinforced polymer (CFRP) materials used for repair of reinforced concrete elements is commonly observed and is often the critical limit state for such systems. This paper presents an acoustic emission (AE) study performed during laboratory tests of concrete slab specimens strengthened with CFRP strips. Several specimens having different CFRP details were monitored. An AE paradigm to monitor damage initiation, progression, and location in the test specimens is demonstrated. An algorithm to classify the cracks in concrete, the disbond of the CFRP strips from the soffit of the slab, and the eventual failure (debonding or concrete shear) is also presented. The proposed general approach can be applied to large scale CFRP–concrete systems. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The use of fiber reinforced polymer (FRP) composite laminates for the seismic retrofit of columns and piers and flexural strengthening of beams and girders, is increasing. CFRP laminates offer, for instance, superior performance in terms of corrosion resistance, environmental durability, and stiffness-to-weight ratio over steel plates or alternate means of retrofitting. Moreover, the ease of application makes CFRP extremely attractive for use in civil infrastructure applications, especially in cases where dead weight, space, or time limitations exist. In structural rehabilitation applications, FRP laminates are externally bonded to the substrate concrete using the wet lay-up method or by direct adhesive application of preformed strips. Although FRP retrofitting has been established on a structural basis, aspects related to materials selection and use, design detailing, fracture and failure mechanisms, and durability are still not well understood [1]. The critical issue in FRP retrofit applications is bond, or rather the mechanism of debonding: the most commonly observed mode of failure in externally bonded FRP applications [2]. Because the laminates represent the component that provides the additional capability to resist flexural or shear stress, proper bond between the FRP and the concrete is critical. In reinforced concrete (RC) beams retrofitted with FRP laminates, defects such as voids and/or delamination between the laminate and the substrate may affect the integrity, performance, and life expectancy * Corresponding author. Tel.: +1 412 624 9575; fax: +1 412 624 0135. E-mail address: [email protected] (P. Rizzo). 0950-0618/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2008.08.026

[3] as well as the flexural strength [4] of the resulting retrofit. Other types of defects such as delamination, which can be caused by moisture at the bond interface, high temperature gradient during curing, and improper application, may also cause failure of the laminates themselves [5]. Thus, an efficient and accurate nondestructive testing (NDT) technique is necessary to detect damage from a variety of mechanisms at the earliest possible stage in the life of the structure. In the last decade different approaches have been proposed to inspect the bond between CFRP and concrete. Infrared thermography has been used to monitor FRP strengthened RC bridge columns [6], bridge decks [7,8], AASHTO Type II girders [9], and FRP strengthened RC beams [4]. Microwave testing has been used to detect artificially induced disbonds and delaminations within the FRP and at the FRP-to-concrete interface [10,11], and delaminations in RC beams fabricated and strengthened with CFRP [12]. Methods based on the propagation of stress waves have been also used. Such methods can be classified as ‘‘active” (ultrasonic testing) and ‘‘passive” (acoustic emission). An ultrasonic pulse echo technique was used by Bastianini et al. [13] to inspect CFRP and GFRP composite materials applied to different substrates (concrete, masonry, and polyurethane). Mirmiran and Wei [14] used ultrasonic pulse velocity (UPV) testing to quantify the extent and progression of damage in concrete filled FRP-tubes. Giurgiutiu et al. [15] and Kim et al. [16] demonstrated a monitoring scheme using piezoelectric transducers and the CFRP strip as wave guide to identifying CFRP debonding. Mirmiran et al. [17] investigated the applicability of acoustic emission (AE) technology to inspect hybrid CFRP tubes filled with concrete (hybrid columns) and to correlate

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the AE parameters to the state of stress in concrete. Gostautas et al. [18] used AE to monitor GFRP bridge deck panels. Carpinteri et al. [19] numerically and experimentally studied RC beam retrofitted with CFRP sheets using AE techniques. The present paper describes an AE approach to monitor the failure mechanism in RC slabs retrofitted with CFRP strips. Nine slabs were tested in a monotonic load-to-failure protocol under displacement control [20] and monitored using an AE instrumentation suite. Each slab possessed a different CFRP strip geometry described by the ratio of CFRP width-to-CFRP spacing (bf/s). The four slabs discussed in the present work had different FRP reinforcing ratios and therefore exhibited two distinct failure modes: shear failure of the concrete slab or CFRP strip debonding. AE data were analyzed using three approaches: parameter analysis, intensity analysis (IA), and principal component analysis (PCA), with the aim of predicting and characterizing the failure mode. Details of the slab test program described herein can be found in [20]. 2. Acoustic emission For the sake of completeness, brief overviews of the principles underlying AE and the algorithms used for the analysis are provided. An acoustic emission is defined as the transient elastic wave generated by the rapid release of energy from a localized source or sources within a material. The elastic energy propagates as a stress wave (AE event) in the structure and is detected by one or more AE sensors. AE events may be generated by moving dislocations, crack onset, growth and propagation, fiber breaks, disbonds, plastic deformation, etc. AE differs from other NDT methods in two key respects. First, the signal has its origin in the material itself and is not introduced from an external source. Second, AE detects movement or strain, whereas most other methods detect existing geometric discontinuities or breaks [21]. One of the main objectives of AE techniques is to discriminate among different sources of damage; thereby attributing each emission to a particular source type or failure mode. The parameter analysis of an AE event evaluates and correlates AE features such as counts, amplitude, rise time, energy, etc. The classification of these parameters drives the investigator toward the correlation of the AE with its source. In the last few years advanced signal processing linked to wavelet transforms [22,23], neural networks [24,25], ‘‘b-value analysis” [26], and the moment tensor inversion method [27] have been used for automatic pattern recognition of AE sources. Moreover, once failures are identified, their location and size may be determined based on the same technology. 2.1. AE in concrete slabs retrofitted with CFRP Both RC and fiber reinforced polymer (FRP) materials emit discrete bursts of AE energy when undergoing stress. AE sources in concrete include microcracking, cracking, friction associated with aggregate interlock, and debonding of aggregate and mortar [24]. AE sources in FRP include debonding between the matrix and the fibers, matrix cracking, delamination, and fiber breakage [28]. Another AE source in hybrid concrete–FRP systems is debonding along the interface between FRP and concrete. The heterogeneous composition of RC is a great challenge for all ultrasound-based testing methods [29]. The detection of AE in RC slabs retrofitted with CFRP strips is even more complex. The AE event may originate in one of the following: the internal steel reinforcement, the inner concrete, the concrete surface, the CFRP strip (where the AE sensors were attached in this study), or the adhesive layer. A stress wave originating in the concrete slab propagates as a bulk wave with velocity independent of the wave frequency. Upon reaching the interface between the concrete and CFRP, the bulk

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wave is partially reflected back into the slab and is partially refracted into the CFRP. If, instead, the AE signal is generated on the slab surface, the wave travels as a surface (Rayleigh) wave and is partially converted into a guided wave at the interface with the FRP. If the AE event is generated in the steel reinforcement, the CFRP strip, or the adhesive layer it propagates as a guided wave in that medium. Wave motions in a waveguide are dispersive (wave velocity and attenuation is dependent on frequency), and can propagate in symmetric and anti-symmetric modes. Moreover, the characteristics of the wave propagating in the CFRP strip are also dependent on the direction of the wave propagation with respect to the fiber orientation. 2.2. Intensity analysis (IA) IA evaluates the structural significance of an AE event as well as the level of deterioration of a structure by calculating two values called the historic index (HI) and severity (Sr) [18,30,31]. The HI compares the signal strength of the most recent emissions to the signal strength of all emissions. This requires estimating the slope changes of the cumulative signal strength (CSS) plotted as a function of time. The presence of one or more peaks may reveal the occurrence of new damage or the propagation of damage, respectively. The severity is the average of the J largest signal strength emissions received at a sensor. As the severity is a measure of structural damage, an increase in severity often corresponds to new structural damage. Analytically, the HI and the Sr are defined as

HI ¼

N NK

PN

i¼Kþ1 Soi PN i¼1 Soi

J 1 X Sr ¼ Som J m¼1

! ð1Þ

!

ð2Þ

where N is number of AE emissions (referred to as ‘hits’) up to time t; Soi is the signal strength of the ith event; K and J are empirical constants based on the material under investigation [31]. In the present paper the following values for K and J are used: for N 6 100, K is not applicable; for 101 < N < 500, K = 0.8N; and for N > 500, K = N  100; for N 6 20, J is not applicable whereas for N > 20, J = 20. 2.3. Principal component analysis (PCA) PCA is a mathematical algorithm used to reduce the dimensionality of a data set for compression, pattern recognition and data interpretation. The algorithm projects, by a linear transformation, a p-dimensional data vector X into a new q-dimensional data vector Z, containing what is referred to as the data’s ‘principal components’ [32]. Given the data Xi = (x1i, x2i, . . ., xpi) with i = 1, . . ., N, the new data vector Zi = (z1i, z2i, . . ., zqi) where: z1 is the linear combination of the original xj (j = 1, . . ., p) with maximal variance; z2 is the linear combination which explains most of the remaining variance and so on. If the p-coordinates are a linear combination of q < p variables, the first q principal components will completely characterize the data and the remaining p–q components will be zero. For the analytical formulation, the interested reader may refer to Refs. [33–35]. 3. Test protocol and experimental setup Of the nine small-scale RC slabs retrofitted with CFRP strips tested [20], four are discussed in the present work. The slab dimensions and internal reinforcement arrangement are shown in Fig. 1a. The CFRP system used consisted of high modulus preformed CFRP

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Fig. 1. Reinforced concrete slab retrofitted with CFRP. (a) Details of the steel reinforcement. (b) Details of the CFRP trip monitored with AE. (c) Positioning of the AE transducers. Drawings not to scale.

strips bonded to the concrete substrate with an appropriate structural adhesive. All materials’ mechanical properties are summarized in Table 1. Selected reinforcing bars were instrumented with strain gauges located 8 in. on either side of the slab midspan. Similar gages were located on the CFRP strips to capture the strain in the CFRP strips and help to identify the onset of debonding [20]. The slabs considered here had four different CFRP width-tospacing (bf/s) ratios (Fig. 1b). The following notation is used to denote the slabs: the first number indicates the number of CFRP strips bonded to the slab and the second number represents the width (in inches) of each strip. Therefore Specimen 2  4 has two 100 mm wide strips bonded to its soffit. Each slab was tested in four point bending over a simple span of 1220 mm with a 152 mm constant moment region and two 533 mm shear spans (Fig. 2a). Load was applied in a monotonic manner to failure. The AE instrumentation suite consisted of: (1) broadband AE piezoelectric transducers (Physical Acoustics PICO transducers) used in conjunction with preamplifiers set at a 40 dB gain; (2) a

four-channel high-speed Physical Acoustics lDiSP data acquisition board; (3) laptop with dedicated AEwin v2.11 software for signal processing and storage. Prior to the tests, transducer positioning, signal threshold settings, and sensor sensitivity were determined using the traditional pencil lead break test. AE sensors were attached to a single CFRP strip about 203 mm (8 in.) from either end of slab (Fig. 1c) using hot melting glue. The threshold level was set at 40 dB. The sampling rate was chosen as 5 MHz. 3.1. Retrofit slab behaviour The typical failure modes of RC flexural members (beams or slabs) retrofit with externally bonded CFRP strips can be classified into seven categories [2]: (a) flexural failure by CFRP rupture; (b) flexural failure by crushing of compressive concrete; (c) shear failure; (d) concrete cover separation; (e) plate end interfacial debonding; (f) intermediate flexural crack-induced interfacial debonding; (g) intermediate flexure–shear crack-induced interfacial

S. Degala et al. / Construction and Building Materials 23 (2009) 2016–2026 Table 1 Mechanical and geometrical properties of concrete, steel reinforcement, CFRP, and adhesive Property

Concrete

Reinforcing steel

Preformed strip

Adhesive

Compressive strength (MPa) Density (kg/m3) Entrained air (%) Modulus of elasticity (GPa) Shear modulus (MPa) Tensile yield strength (MPa) Tensile strength (MPa) Elongation at rupture (strain) Thickness (mm) Volumetric fiber content (%)

33.5

n.d.

n.d.

n.d.

2230 4.5–7 n.d.

n.d. n.a. 200

1810 n.a. 155

n.d. n.a. n.d.

n.d. n.d.

n.d. 414

n.d. n.a.

3960 n.a.

n.d. n.d.

621 0.018

2800 0.018

31 0.025

n.a. n.a.

n.a. n.a.

1.4 62

2.54 n.a.

n.a. – Not applicable or not defined for material. n.d. – Not determined in present study.

debonding. Generally, failure modes (d) and (e) are referred to as plate end debonding failures. Mode (f), although theoretically possible is not observed in practice; Mode (g), occurring in the shear span is always observed to dominate over mode (f) [2]. Plate end debonding failures occur due to high interfacial shear and normal stresses that result near the end of the retrofitting plate. When these stresses exceed the strength of concrete, debonding propagates through the concrete initiating at the plate end. Often, the failure plane will migrate to the first layer of internal reinforcing steel resulting in complete cover concrete delamination reminiscent of a splitting failure.

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When a flexure or flexure–shear crack bridged by FRP strips is formed in the concrete, the tensile stresses released by the cracked concrete are transferred to the strips. As a result, high local interfacial Mode-II (shear) stresses between the FRP strips and the concrete are induced adjacent to the crack. At a flexure–shear crack in the shear span, additional Mode-I (peeling) stresses are developed at the crack opening (tensile on the side of the crack having ‘lower moment’ and compressive on the other side). As the applied loading increases, the tensile stresses in the FRP strip bridging the crack and the interfacial Mode-I and II stresses between the FRP strip and the concrete near the crack also increase. When these stresses reach critical values, debonding initiates at the crack and propagates in the direction of decreasing moment gradient, i.e. towards the nearest support in a simply supported member. Fig. 2 shows typical behaviour of flexural members having bonded FRP reinforcement on their soffit. In the present study, specimens 1  4 and 4  1 were relatively lightly reinforced with additional CFRP. These specimens exhibited flexure–shear crack-induced debonding (g). Specimens 2  4 and 8  1 had twice the CFRP reinforcement. This additional flexural reinforcement resulted in the slabs failing in a concrete shear dominated mode initiating at the end of the CFRP plates (similar to (d) when observed in a slab). Details of specimen behaviour are provided in Ref. [20].

4. Experimental results This section presents the AE results from the four slabs tested. The first part of this section describes the results from the parameter analyses. Then, the outcomes from the IA are discussed. Finally, the results from the PCA algorithm are illustrated.

Fig. 2. (a) Behaviour of flexural member having bonded reinforcement on soffit. (b) Midspan debond initiated by flexural and/or shear cracks. (c) End peeling initiated high bond shear stresses at the end of bonded reinforcement. Figure adapted from [41].

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4.1. Parameter analysis 4.1.1. Specimen 1  4 Counts, cumulative energy, amplitude, and rise time of the acoustic activity are plotted as functions of time in Fig. 4a–d, respectively. The total applied load (twice the shear carried by the slab) is superimposed on the plot of acoustic count history (Fig. 3a). During the first 50 s, up to an applied load of 5.8 kN, the slab is settling onto its supports; emissions during this initial portion of the test are artifacts of the test setup and disregarded. Significant acoustic activity was detected in two instances around 8.90 kN and 22.2 kN, respectively. This activity was associated with the initiation of flexural cracks in the concrete matrix and is confirmed by applied load–displacement data and observation [20]. Fig. 3c and d shows that high-amplitude AE events do not always correspond to high-rise time AE events. Such emissions are impulsive phenomena that originate short events of high energy. At 53.4 kN the density of AE events (number of AE events per unit of time or load) increased. Fig. 3b reveals that much of the AE energy was detected by channel 2, which was located on the south side of the slab. At 59.8 kN the strip debonded from the concrete substrate. The intermediate crack-induced debonding failure initiated along the southern part (Fig. 1c) of the slab, and is confirmed by the higher AE activities observed in sensor 2, in comparison with those of sensor 1. At the instant of failure, AE events of amplitudes above 70 db and having rise times greater than 400 ls were recorded. The residual load capacity observed following debonding is associated with the ability of the concrete slab to sustain the displacement rate. Fig. 4 shows the locations of the AE sources as determined from the two installed sensors using a linear algorithm. Based on preliFig. 4. AE location time as a function of time for slab 1  4 AE location. (a) AE amplitudes in the range 40–59 and 60–100 dB are distinguished. (b) AE peak frequencies in the range 10–75, 76–300, >300 kHz are discriminated.

Fig. 3. Acoustic emission results during quasi-static loading to failure for slab 1  4. Counts and applied load (a), cumulative energy (b), amplitude (c), and rise time (d) as a function of time.

minary pencil lead break tests, the wave velocity was set equal to 1 km/s. The time occurrence of the event is plotted as a function of the X position along the CFRP strip in Fig. 4a. Event amplitudes falling between 40–59 and 60–100 db are distinguished. Higher amplitude events occurring prior to final failure were localized closer to channel 2. This validates that the debonding initiated toward the south end of slab. Fig. 4b shows the location of the AE events as a function of time clustered by frequency content. It is observed that higher frequency events are mainly localized in the shear span of the south side of the slab. It must be noted that the linear location algorithm employed depends on the selection of the wave velocity. Based upon the considerations discussed in Section 2.1, any AE event originating inside the slab, or in the steel reinforcement may only be localized qualitatively. For instance, it is known, that the bulk wave velocity in concrete is approximately 3.1 km/s but varies depending upon the material properties of concrete [36]. Fig. 5 shows the peak frequency and the centroidal frequency as a function of time. The plot of the applied load is also superimposed. The peak frequency is concentrated mainly in two distinct clusters: between 100 and 200 kHz and between 400 and 500 kHz. The centroidal frequency is concentrated in a single cluster between 200 and 350 kHz. Lower values were observed especially during the first portion of the test, when cracks on the concrete surface were observed. When evaluating AE events by spectral analysis, which determines the frequency content of individual events, parameters such as the location of the AE source with respect to the transducer and the characteristics of the transducer itself must be considered [17]. In Refs. [17, 37] it was shown that the frequency content of an AE

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Fig. 5. Spectral analysis of AE monitoring during quasi-static loading-to-failure for slab S1  4. (a) Peak frequency and applied load, (b) centroidal frequency as a function of time. The load history is superimposed.

signal in a concrete beam is a function of the transducer’s frequency response. The authors, in these cases, used narrowband transducers resonant at 150 kHz. The PICO transducers used in the present work possess broader sensitivity: in the range 250– 750 kHz, which allows extension of the analysis up to 800 kHz. The peak frequency, rather than being uniformly distributed over the broad spectrum of the PICO, is concentrated in the two ranges discussed above. This is most likely associated to the type of event that generated the AE and the characteristics of wave propagation in the multi-layer system composed of the concrete substrate, adhesive, and CFRP strip. (Ref. [38] discusses guided waves in multiple layers.) Moreover, as the debonding failure progressed, the sensitivity of the sensing system to detect AEs in concrete decreased while that for AE sources in the adhesive and CFRP strip increased. The progression of debonding reduced the area of acoustic coupling between concrete and CFRP. 4.1.2. Specimen 2  4 Counts, cumulative energy, amplitude and rise time of the acoustic activity are plotted as functions of time in Fig. 6a–d, respectively. The values of total applied load are also superimposed. The load history shows a discontinuity around 70 s. This discontinuity was probably related to the formation of flexural cracks. Moreover, at the same instance, a hairline crack was visually observed on the top slab surface. The nature of this crack and its distance from the AE transducers suggests that no significant acoustic activity is expected to be detected by the AE transducers. The variation of the applied load–time plot observed around 180 s was associated with a manual variation of the loading rate. This more heavily reinforced slab (twice the CFRP of previously described 1  4) failed at 75.6 kN due to a concrete shear failure occurring at the north support, i.e.: close to sensor 1 but not between sensors 1 and 2. Despite the preliminary pencil lead break tests having determined that the sensitivity of the attached AE transducers was similar to the sensitivity observed for Specimen 1  4, the signals obtained from this slab were weaker. This is most likely associated with the fact that many AE events were related to the onset and propagation of cracks in concrete. Except for two isolated events, all AE amplitudes were below 60 dB (Fig. 6c). The outcomes from the source location identification, not presented here, did not highlight the occurrence of shear failure above the north support. This is most likely associated with the choice of the wave speed in the location algorithm and the fact that the failure occurred outside the region enclosed by sensors 1 and 2. More interesting data are collected from the spectral

Fig. 6. Acoustic emission results during quasi-static loading to failure for slab 2  4. Counts and applied load (a), cumulative energy (b), amplitude (c), and rise time (d) as a function of time.

Fig. 7. Spectral analysis of AE monitoring during quasi-static loading to failure for slab 2  4. (a) Peak frequency and applied load, (b) centroidal frequency as a function of time. The load history is superimposed.

analysis (Fig. 7). The peak frequency (Fig. 7a) and centroidal frequency (Fig. 7b), both plotted as a function of time, show clusters below 100 kHz and between 100 and 150 kHz, respectively. Such lower values in comparison with the frequencies observed in Specimen 1  4 (Fig. 5) are expected due to (a) the dominance of concrete behaviour in this specimen and (b) the greater attenuation of higher frequencies in concrete.

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Fig. 8. Acoustic emission results during quasi-static loading to failure for slab 4  1. Counts and applied load (a), cumulative energy (b), amplitude (c), and rise time (d) as a function of time.

4.1.3. Specimen 4  1 The area of CFRP reinforcement in this slab was equal to that in the 1  4 slab. Two PICO transducers were glued onto the second strip (S2) from west (noted with an ‘A’ in Fig. 1b). Counts and total applied load, cumulative energy, amplitude and rise time of the acoustic activity are plotted as functions of time in Fig. 10a–d respectively. The step-like shape of the load history at failure is due to the progression of individual strips debonding from the slab. The load profile shows a small discontinuity around 40.0 kN, which based upon strain data is due to flexural cracks in the slab [20]. The third strip (S3) from west debonded at 74.2 kN, followed by strips S2, S1, and S4. All strips exhibited intermediate crack-induced debonding propagating toward the south end of the slab in the area close to the channel 1 transducer; the north end of the strips remained bonded to the concrete in all cases. The results from the parameter analysis (Fig. 8) show that sensor 1 detected greater activity than sensor 2. Additionally, the AE system was clearly able to detect emissions from both debonding events: CFRP strip S3 failing at 550 s and S2 failing 30 s later. Due to their proximity to the channel 1 transducer, the AE parameters (counts, energy, and rise time) associated with both debonding failures have similar values. The other two strips (S1 and S4) failed shortly afterward, at 590 and 598 s, respectively. It is not surprising that the AE transducers were able to detect the AE originating in strips 1 and 4. The fact that only a portion of strip S2 is bonded from the concrete substrate allowed the continued acoustic transmission from concrete into the laminate. As the acoustic energy enters the laminate, the stress wave undergoes less attenuation as it travels along the debonded strip since no diffraction into surrounding material is possible. As such, AE parameters recorded during the failure of both strips S1 and S4 have comparable or even

Fig. 9. Acoustic emission results during quasi-static loading to failure for slab S8  1. Counts and applied load (a), amplitude (b) as a function of time. (c) Spectral analysis: peak frequency, and (d) centroidal frequency as a function of time.

greater values than the initial debonding of S3 and S2. While no relevant information was revealed by studying the location history, the analysis in the frequency domain provided results very similar to those discussed for slab 1  4 (Fig. 5) and are not shown here. 4.1.4. Specimen 8  1 Four PICO transducers were placed on the 3rd and 6th strips from the west side of the slab (‘A’ in Fig. 1b). Counts and total applied load, and amplitude of the acoustic activity are plotted as functions of time in Fig. 9a and b, respectively. Before 80 s the load profile seems to be non-linear indicating some settlement of the slab in the test frame. At 86 and 260 s, the load profile shows two small discontinuities associated with flexural cracks in the tensile zone of the slab. While the first event created enough energy to be detected by sensor 2 (count number above 100), the energy released during the second event was attenuated prior to reaching the AE transducers. It is argued that this second event was the propagation of the initial crack toward the neutral axis of the slab. Shear failure of the slab occurred at 97.9 kN at the south support in the area closer to the channel 1 transducer. This was confirmed by the parameter analysis which shows a significant event at the instance of the failure with number of counts close to 2000 and signal amplitudes up to 78 dB. The rise time provided similar indications and is not shown here. Fig. 9c–d shows the results from the spectral analysis. The frequency ranges are similar to those observed during debonding failures (specimens 1  4, 4  1). It can be argued that such high frequencies are the results of flexural cracks and/or damage onset in the adhesive layer in one or more of the eight bonded strips. Such occurrences generated high frequency events having low amplitude and counts and/or were

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Fig. 10. Intensity analysis for slab 1  4. (a) Historic index as a function of time. The plot of the cumulative signal strength is superimposed. (b) Magnified portion of plot (a). (c) Severity index as a function of time. The plot of the cumulative signal strength is superimposed. (d) Intensity chart.

attenuated prior reaching the pairs of sensors glued on strips S3 and S6. 4.2. Intensity analysis (IA) Fig. 10a shows the historic index (HI) as a function of time for Specimen 1  4. The plot of the cumulative signal strength (CSS) is superimposed. For clarity, a magnified portion of Fig. 10a is presented in Fig. 10b. The presence of an AE ‘knee’ defined as a point of significant change in the slope of the CSS, is highlighted in the time history of the CSS. The AE knees may be used to identify possible damage mechanisms and to locate the onset of failure [18]. Prior to debonding, in the time interval 580–650 s, a higher density of peaks was observed. The values of these peaks however are not necessarily the greatest. This is because the value of a HI peak is associated with the variation in slope rather than to the slope itself. Fig. 10c shows the severity, Sr, as a function of time with a plot of CSS superimposed. Both plots are qualitatively very similar. Moreover, the plot of the severity index is similar to the plot of the cumulative energy (Fig. 3b). This indicates that the large amount of energy being released from high energy events generates a significant increase in the slope of the severity line. Considering the plot of the cumulative energy (Fig 3b), there is a significant increase in slope prior to the CFRP strip debonding. The increase is larger in channel 2 which was closer to the area of debonding. Using the maximum value from each channel for both HI and severity, the intensity chart can be plotted (Fig 10d). Generally, this chart may be divided into intensity zones that identify the structural significance of a given sequence of AE events. Intensity values clustered toward the top right-hand corner are associated with phenomena of high structural significance, while less structurally significant events concentrate near the bottom left [18]. In Fig. 10d the fact that the value from sensor 2 is in the upperright corner while the value from sensor 1 is in the lower-left confirms that the greater amount of damage occurred in the area of sensor 2. Fig. 11 shows the intensity charts for all slabs investigated in this paper plus an additional slab (2  2), which failed due to deb-

onding [20]. Specimens that failed due to CFRP debonding are grouped in Fig. 11a and the remaining slabs (having concrete-dominated failure modes) are grouped in Fig. 11b. In order to divide the intensity chart into zones that can be used for providing a precursor to the onset of permanent damage, a larger number of samples needs to be tested. Although no generalization can be made at this stage, a trend is readily observed. The values associated with CFRP strip debonding are localized in right side of the chart whereas the values associated with shear failure are localized in the lower-left side of these charts. Rather than plotting the final maximum value of HI and severity for each sensor, the maximum values observed during the loadingto-failure test are plotted in Fig. 11c for two slabs that experienced CFRP debonding (1  4 and 4  1) and two slabs that experienced shear failure (2  4 and 8  1). The progression of the intensity values moving from lower-left of the chart to the upper-right can be used in real-time to flag combinations of HI and Sr which enter an intensity zone associated with severe damage. 4.3. Principal component analysis (PCA) As mentioned in Section 2.3, PCA is a mathematical procedure that transforms a number of possibly correlated variables into a smaller number of linearly uncorrelated variables called principal components. In this study the AE features from the time domain analysis (AE amplitudes, energy, counts, duration and rise time) were separated from those of the spectral analysis (average frequency, centroidal frequency, and peak frequency). Before transformation to principal components, the data were standardised by dividing the normalized data for each parameter by its respective standard deviation. Interested readers are referred to [34] for an extensive discussion on the effect of standardization prior to the transformation of AE data into principal components. Each principal component is a linear combination of the original variables and is represented as a single axis in the new coordinate space. Fig. 12 shows the PCA visualization of the AE data for slabs 1  4 (Fig. 12a and b) and 2  4 (Fig. 12c and d), respectively. The analyses reduced the dimension from three (Fig. 12a and c) and from

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are visible. The cluster at the top of the figure is associated with the highest values of the average frequency. The cluster in the bottom left part is originated from data having average frequency below 400 kHz and peak frequency around 150 kHz. The third group clusters data having average and peak frequency about 500 and 450 kHz, respectively. The last two clusters reproduce the behaviour discussed in Figs. 5 and 7. As expected, Fig. 13b shows a single cluster associated with the low AE activities discussed in Figs. 6 and 7. Not shown here, the PCA analysis from the other three slabs provided similar results. In general, it was observed that the PCA components scattered towards quadrants 1 and 4 (positive values of the first principal component) for slabs that experienced shear failure whereas the clustering of AE data from the slabs experiencing debonding scattered towards quadrants 2 and 3. The reason for this kind of clustering concentrated at a single location and scattering towards different quadrants with respect to the damage can be associated with the type of damage observed in these specimens: flexural cracking in the concrete matrix, cracks in the adhesive layer, and CFRP debonding. 5. Discussion and conclusions

Fig. 11. Intensity charts for all specimens discussed in the present paper. (a) Slabs that failed due to CFRP debond. (b) Slabs that failed due to shear. (c) Progression of the intensity chart during quasi-static loading to failure of four slabs. In the legend (D) indicates slabs that failed due to debonding and (S) identifies those slabs that failed due to shear.

five AE parameters (Fig. 12b and d) to two principal components. For AE events associated with damage onset or propagation, no normal condition data can be identified. However all low signature AE events are clustered at the origin (0, 0). The results from slab 1  4 (Fig. 12a and b) show that data moves away from the origin toward negative values of the first principal component. All data from the instant prior to failure are located away from the main cluster. The fact that such data are associated with the channel 2 transducer confirms that the CFRP disbond was localized in the area close to sensor 2. Moreover, the same plots suggests that the ‘‘movable pattern” [39] varies by the selection of the input vector. As such, a parametric study may be employed to maximize the PCA outputs in terms of clustering. Fig. 13 shows the PCA visualization of the AE data for slabs 1  4 (Fig. 13a) and 2  4 (Fig. 13b), respectively using three parameters in the frequency domain: the average frequency, the peak frequency and the centroidal frequency. In Fig. 13a three clusters

Results of AE monitoring of reinforced concrete slabs retrofitted with CFRP strips are presented in this paper. The slab tests were performed to assess the effects of CFRP geometry and placement [20]. In the present study AE monitoring complements the structural analysis of the slabs. Three different approaches to analyze AE data, namely, parameter analysis, intensity analysis, and principal component analysis were performed with the aim of identifying the different modes of structural failure and to provide a means to predict the onset of failure. Monitoring of the acoustic activities in the slabs subjected to monotonic load-to-failure allowed the authors to follow the occurrence and progression of early damage that was not necessarily detectable as macroscopic stiffness changes or visually observable phenomena. Amplitude, energy, and frequency components of the acoustic emissions were used to provide a qualitative correlation with the type of damage observed. The parametric analysis demonstrated that flexural cracks near the laminates and debonding produce AE activities much higher in terms of signal amplitude and frequency range than eventual shear cracking. Although initially counterintuitive (shear cracks release greater energy than flexural cracks), this observation is explained by the crack locations. Flexural cracks are expressed at the CFRP–concrete interface and thus their emissions are transmitted directly by the CFRP strip on which the AE sensors are mounted. Shear cracks, on the other hand, occur in the body of the concrete and do not affect the CFRP–concrete interface as significantly. This study showed that, as should be expected, cumulative energy correlates well with the degree of damage sustained by the specimens. A comparative study of failure of the slabs conducted with the IA demonstrated that such an approach can be used to provide warning of severe structural events in real-time. Although the slabs discussed in this paper were slightly different to each other, the results from the intensity charts show promise for the application to provide quantitative information on the identification of areas of large damage and on the ability to classify different sources of damage. The application of PCA to reduce the dimensionality of AE data in CFRP-retrofitted RC slabs is a novelty of this study. This approach showed some degree of correlation between the type of failure and the location of the cluster in the plot of the first two principal components. However the reason of such behaviour needs to be fully understood prior to practical implementation of this approach being realized.

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Fig. 12. PCA reduction of standardised traditional AE features from AE monitoring of slab S1  4 (a, b) and slab S2  4 (c, d). (a, c) AE amplitude, counts, and rise time in the input vector. (b, d) Amplitude, energy, counts, duration, and rise time in the input data vector.

In summary, the results presented showed that: (1) AE is wellsuited for failure monitoring of RC retrofitted with CFRP; (2) accurate AE data analyses identify different sources of damage (i.e. concrete cracking vs. CFRP debonding); (3) parameter analysis and IA can be used in-situ for real-time health monitoring of structural systems provided a higher number of sensors are deployed; and (4) PCA shows promise but requires further study in this application. To be useful in application and to address a significant cause of structural deterioration [40], future studies should be oriented toward the laboratory testing of RC elements subject to fatigue loads. In such studies the Kaiser effect, which could not be studied here because of the load-to-failure protocol, may be monitored. Additionally, the application of newer signal-based techniques may be studied for their applicability to this practical application. Acknowledgments The fourth author acknowledges the in-kind support of Fyfe Company Inc., SIKA Corporation and Fox Industries. All testing was conducted in the Watkins Haggart Structural Engineering Laboratory at the University of Pittsburgh. References Fig. 13. PCA reduction of standardised AE features: average frequency, peak frequency, and centroidal frequency. (a) Slab 1  4. (b) Slab S2  4. Note that in the PCA reduction all frequency activities in the very low frequency range were removed.

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